Because rocket and air-breathing propulsion systems are the foundation on which planning for future aerospace systems rests, the Deputy Assistant Secretary of the Air Force for Science, Technology, and Engineering (SAF/AQR), with support from the Director, Defense Research and Engineering (DDR&E), asked the Air Force Studies Board (AFSB) of the National Research Council (NRC) to review and comment on the planning for propulsion development that is under way at the Department of Defense (DoD) and the commercial technical base for these air-breathing and rocket engines that allow access to space and for in-space propulsion systems (see Box 1-1 for the study’s statement of task). This full-spectrum propulsion study assesses the existing technical base in these areas and examines the future Air Force capabilities the base will be expected to support; it also defines gaps and recommends where future warfighter capabilities not yet fully defined could be met by current science and technology (S&T) development plans. The recommendations in this report may shape DoD engine development planning for the next 15 years and, possibly, military capabilities beyond that.
Statement of Task
The NRC will:
The report will discuss (1) the potential range of warfighter capabilities that may need to be addressed by the propulsion technical base; (2) the programs/activities under way in the technical base; and (3) an initial discussion of perceived gaps, inadequacies, disconnects, and/or omissions. The report will provide the NRC recommendations for dealing with these issues and a rough order of magnitude estimate of the investment needed to address them.
The committee soon discovered that the capabilities-based planning currently in use throughout DoD is still quite immature and does not allow propulsion needs to be easily determined (Bexfield and Disbrow, 2005).1
Further, DoD’s S&T Reliance planning process has been modified to adapt to the capabilities-based planning process, interrupting the technology area reviews and assessments for 1 year.
In addition, capabilities-based planning, which involves analyzing the alternatives for any given capability, does not specify system solutions until after the analysis is complete and a decision has been made. This methodology requires that the technical base programs funded within the S&T budget cover a broad spectrum of alternatives to enable potential future capabilities. Clearly, a technical base manager would seek maximum definition of the capabilities likely to be needed, while a capability definer, usually the warfighter, would like the technical base to be flexible and general enough to support any defined need. Unfortunately, at the present time, neither group can attain the clarity it would like, for a number of reasons.
With this reality in mind, the committee sought to determine the extent to which future capabilities can be clearly defined and how the propulsion technical base can be structured to realize them. One would expect to see a propulsion technical base consisting of numerous programs at technology readiness levels (TRLs) of 2 or 3 awaiting orders to be matured to TRL 6 or 7. To do this successfully, realistic development roadmaps and funding profiles for crossing the gulf between TRLs 3 and 6 would be developed and kept up-to-date to create a basis for program objectives memorandum (POM) development. If this were the real-world case, the committee’s data-gathering efforts would have been relatively straightforward. However, the committee members assigned this task found, with a few exceptions, neither well-defined capabilities nor realistic technology transition planning.
For the purpose of this report and in lieu of clearly stated needed capabilities, the committee’s analysis must be based on a great deal of informal and anecdotal information gathered through contacts with both capabilities planners and the technical base managers attempting to satisfy the planners and on briefings provided to the committee from the Services, DDR&E, the Defense Advanced Research Projects Agency (DARPA), the National Aeronautics and Space Administration (NASA), and various academic and
requirements and thereby reduce the tendency to fixate on a certain threat, location, or set of conditions. DoD has shifted during the past 4 years from the threat-based model that dominated defense planning in the past to a model focused on capabilities—a model that places emphasis more on how an adversary might fight rather than on who the adversary might be or where the war might occur. This model is designed to plan for uncertainty—the defining characteristic of today’s strategic environment. A more detailed explanation of capabilities-based planning and processes is found in Chapter 2.
industry representatives. This is not to say that there are no documents that help define potential future technology developments. There are. In fact, one excellent example is the Department of Defense Space Science and Technology Strategy, cosigned by the Air Force Under Secretary and the DDR&E and discussed in Chapter 4 (DoD, 2004). However, in informal discussions with Air Force Research Laboratory (AFRL) and Air Force Space Command (AFSPC) personnel, it became obvious that this strategy was not the determinant of their marching orders and was considered to be only one out of several strategies in play. The committee concluded that even when clear direction exists, it is not always followed.
Air Force Vision 2020 Capabilities Review and Risk Assessments reviews the global strike, homeland security, global mobility, global persistent attack, nuclear response, and space and C4ISR tasks or missions. The capabilities required are (1) command and control; (2) intelligence, surveillance, and reconnaissance; and (3) force application and force projection. Based on its review of the Air Force Master Capabilities Library (USAF, 2005) and presentations made to it, the committee highlighted some of the missions for which propulsion capabilities will be needed in the 2020 time frame, along with particular requirements:
Global or Long-Range Strike
Achieve desired effect(s) rapidly and/or persistently, on any target; With little or no warning time;
In any environment, including those where weapons of mass destruction may be located; and
Having been denied contiguous areas from which to operate.
Global Mobility and Airborne C4ISR
Global mobility (airlift and air refueling)
Robust, sustained, adverse weather capabilities for deployment, employment, and redeployment and
Survivable in radio frequency, infrared, and directed-energy environments.
Ability to shorten the kill chain by achieving better situational awareness;
Platforms that can persist for hours/days;
Ability to operate from long distances; and
Platforms that are globally connected (node in a sensors, datalink, and fused intelligence network).
Reliability and Maintainability
Consider that 60 percent of present military aircraft inventories will still be in service in 2018;
Use the Component Improvement Program (CIP) to replace low-reliability parts;
Utilize evolutionary programs to develop reliable, efficient, low-cost turbine engines; and
Minimize total life cycle cost.
Inlet/exhaust integration and flow control
Increased ranges and payloads;
Increased size of operational envelopes (missiles and air-breathing);
Address power densities for sensor suites, directed-energy weapons; and
Environmental factors (noise, emissions).
Military uses 1960s and 1970s technology
DoD has elected to use commercial technology and
Commercial rotor technology not designed for DoD operating environments.
Future needs for
Air Force: rapidly deployable, highly reliable, survivable, all-weather, long-range platforms;
Special operations, combat search and rescue, medium lift (e.g., noncombatant evacuation operation);
Miscellaneous support and force protection;
Army: responsive, deployable, agile, versatile, lethal, survivable, sustainable, and dominant anywhere;
More range and payload;
Reduced logistics footprint; and
Low operational and support costs.
Small Unmanned Aerial Systems
Hovering/perching small unmanned aircraft systems (UASs) and micro-UASs
Micro: 1-3 nm range/day/fair wx, 0.5 lb payload, and field supportable;
Man-portable: 1-2 hr endurance, 1-2 lb payload, and field supportable; and
Tactical class: 10-12 hr endurance, 100 lb payload, and multimission.
Range of requirements from low speed, long endurance to high speed, long range;
Small size, low observable;
Long shelf life; and
Increased power requirements (USAF, 2005).
Finding 2-1. Space strategy is clearly defined by the DoD Space Science and Technology Strategy. However, it is not being followed except as part of a broader set of strategies deriving from other considerations. There appear to be no similar documents at the level of the Office of the Secretary of Defense (OSD) that set forth the strategies for developing capabilities in other important areas such as aircraft systems and UASs.
Recommendation 2-1. DoD should develop strategy documents containing clear guidance on future required capabilities in all system development areas and should pursue funding to achieve those capabilities.
With a clearer understanding of the capabilities that are needed, technical base managers can reassess their programs to identify the program elements that will allow them to draw up the necessary technology transition roadmaps and funding profiles. Service leaders would then be in a much better position to assess alternative capabilities. The committee’s judgments are based specifically on data gathered about propulsion technology devel-
opment and not on the broader aspects of technology development and transition. For example, specified propulsion capabilities for assured access to space—survivable, low-cost, and reliable launch systems to enable on-demand launch of payloads to any orbit and altitude required—would be sufficient to focus propulsion research and development. However, the committee fears that basing important analyses of alternatives on less than fully considered cost and schedule realities does not serve decision makers well.
It is clear to the committee that unless additional emphasis is placed on propulsion, the technological lead the United States has enjoyed for so long, and perhaps taken for granted, will cease to exist. This very complex issue will become obvious to anyone who reads this report, and the decrease in funding for propulsion research in this country and the narrowing of the technological lead this country enjoys over other countries cannot be ignored. The culmination of a number of events, some interdependent and others independent, has gotten us to this point. The demands for resources for everything from fighting the war on terror; responding to natural disasters like the tsunami in Indonesia and Hurricane Katrina at home; to modernizing our military forces, health care, and education have made it very difficult to maintain funding for propulsion research. That fact, coupled with reductions in industry investments in propulsion research, has created a near crisis.
Engine original equipment manufacturers (OEMs) historically invested a percentage of their engine sales in research for the next-generation engine. Significant reductions in military engine acquisitions for many years and the downturn in commercial sales since 9/11 have contributed greatly to this situation. Compounding the situation is the above-mentioned change in how DoD determines its requirements, from a threat-based system to a capabilities-based system. Although this change was made in response to the uncertainty surrounding any given threat and the need to be able to respond to an increasing variety of threats, it makes the job of the research and development (R&D) community much more difficult. In the past, this community could develop a propulsion system for a given threat, or at least devote its resources to such an undertaking. Today, it must spread its resources over a number of systems that have the potential to provide a given capability. As a result, none of these systems may get the priority or funding they need to reach maturity and provide the capability needed.
The Deputy Assistant Secretary of the Air Force for Science, Technology, and Engineering (SAF/AQR) told the committee at its first meeting that the Air Force annual S&T investment in propulsion and power (PR) was about $300 million and was not likely to change much in future years.2 This number reflects funding for 6.2 and 6.3 research only. Any 6.1 propulsion-related funding is accounted for separately as part of basic research, which is administered by the Air Force Office of Scientific Research (AFOSR). The overall DoD funding includes Army, Navy, and DARPA funding as well as funding for programs emanating directly from DDR&E. The Air Force funding in PR makes up between 66 and 75 percent of the overall DoD investment in this area. In FY04 and FY05, Air Force funding for PR was $291 million and $297 million, respectively, as the result of congressional add-ons to the Presidential Budget Requests (PBRs) of $251 million and $234 million. The Air Force projects increases in PBRs from $252 million in FY06 to $305 million in FY11. The overall DoD investment in PR from FY04 to FY07 remains fairly flat at around $400 million to $410 million except for a spike to $440 million in FY05.3
The flatness of the numbers does not give a true picture, however, particularly for 6.2 work, where the budgets in gas turbine technology from FY02 to FY06 were fairly flat but cover AFRL payroll and administration costs. While the amounts for in-house R&D remained fairly flat, the amounts for industrial R&D fell precipitously over the FY02-FY06 period. The committee finds that there has already been significant erosion of the U.S. lead in propulsion technology and that a flat budget will lead to further erosion.
Because the investments projected through FY07 are flat—they do not even cover inflation—one can expect only incremental improvements in technology over this period. Anything revolutionary would have to be funded at the expense of existing programs or would require additional
funding. Funding for 6.1 and 6.2 research is vital to the warfighter since such research is the source of new ideas and technologies and educates new engineers and scientists in aerospace propulsion.
The Committee’s Judgment
The study statement of task required the committee to “identify technical gaps and suggest rough order of magnitude (ROM), specifically applied S&T investments in these areas of propulsion.” The committee based its ROM estimates on its collective judgment, which, in turn, was based on the members’ extensive experience and expertise in aerospace propulsion. By definition, these estimates are rough; however, the committee believes each is reasonably within the correct order of magnitude. In its recent budget requests, the Air Force did not project changing its investment in propulsion S&T in future years much from its current level. The committee believes this investment needs to be increased if technical gaps are to be filled.
The Reliance Program
The DoD Science and Technology Reliance program (the Reliance Program) was originally mandated by the Service Assistant Secretaries to focus resources on propulsion requirements and capabilities (DMR 922) as part of the 1989 Deputy Secretary of Defense challenge to the Services to increase efficiency in research, development, testing, and evaluation (RDT&E). As discussed in Chapter 2, the committee heard anecdotally from informed sources that the Air Force was the lead service in work on propulsion for the Reliance Program. Since the Air Force has been by far the largest investor in the S&T arena in air-breathing propulsion and rocket propulsion, the committee felt this information made sense. However, when the committee reviewed the Reliance Program it found no mention of a lead service for propulsion. In fact, the Defense Technology Area Plan (DTAP) divided responsibility for propulsion among four panels: air platforms, nuclear technology, space platforms, and weapons.
The committee believes that having propulsion spread over different DTAP panels results in overlapping, unfocused efforts on the part of the various Services as well as the panel subject areas. Further, the panels’ efforts do not set any priorities for DoD technology objectives. The Reliance Program, as presently structured, also does not give the panel chairs the authority they need to achieve cooperation and discipline in Reliance
Program execution. Overall, the present Reliance Program organizational construct tends to inhibit the maturation of propulsion efforts, from basic research (6.1) to applied R&D (6.2, 6.3) and demonstrations (6.2, 6.3), and the coordination of their funding across DoD.
AIR-BREATHING PROPULSION SYSTEMS
Challenges Facing Air-Breathing Propulsion Systems
The Air Force is transitioning to a capabilities-based planning model. DDR&E presented capabilities requirements for the 2020 time frame for four warfighting scenarios: traditional, irregular, catastrophic, and disruptive (Sega, 2005). The propulsion challenges are discussed in Box 1-2. The committee looked at DDR&E’s capabilities requirements and the capabilities of the current operational systems that will still be in service in the 2020 time frame to define the technology requirements, the opportunities for technology improvement, and opportunities to leverage different business approaches to meet the needs of 2020 warfighters.
Characteristics of Aircraft Needed by Warfighters in 2020
From the information it had gathered, the committee was able to paint an overall picture of the 2020 warfighter’s fleet powered by air-breathing propulsion systems:
First, over 60 percent of the aircraft that will be used by the 2020 warfighter are in service today. If the Joint Strike Fighter (JSF) F-35 is included, then it would mean that over 80 percent of the 2018 fleet exists or is under development today. Additionally, the costs of sustaining and fueling this fleet are the two most expensive items in the budget and are growing rapidly.
Second, time-critical targets and the expected decrease in the number of forward bases make high-speed aircraft a requirement for the 2020 warfighter. Two main types of propulsion systems not in operation today will be required to provide this speed capability: gas turbine engines (GTEs) that operate in the Mach 3.6 to 4.2 range and ramjets/scramjets that operate in the Mach 4.0 to 16 range. In fact, some capabilities may require combining cycle propulsion
Future U.S. armed forces must address an array of challenges that far surpass those faced in the past. Numerous security studies have described the evolving and transformational nature of the U.S. security environment. In response to these challenges, aircraft propulsion systems must evolve more rapidly than ever before. The four propulsion challenges are traditional, irregular, catastrophic, and disruptive:
systems where both gas turbine and ramjets/scramjets are integrated into an overall propulsion system.
Third, mobility and the ability to remain over the target or staging areas for an extended time require propulsion systems much more fuel-efficient than those in service today.
These three overarching characteristics of the 2018 warfighter’s aircraft guided the committee as it reviewed existing and proposed programs and business models. The committee divided the gas turbine requirements into three classes of engine: large (more than 10,000 lb thrust), small (rotorcraft-type engines), and expendable (missile-type engines). It reviewed technology and business opportunities for existing propulsion systems, propulsion systems under development, and future propulsion systems for each engine class. It also reviewed ramjet and scramjet propulsion systems for both aircraft and missile applications.
The aircraft GTE has been continually evolving and improving since its introduction during World War II. Although significant advancements in fundamental engine performance parameters have been realized, there remains substantial potential for improvement beyond the state of the art. The efficiency of fielded military GTEs was improved by increasing compressor outlet temperature (T3) and turbine inlet temperature (T41). For example, the fuel efficiency of large turbofan engines has been improved, but only 38 percent of the gap between the first jet engines and the theoretical Brayton cycle limit has been eliminated. An additional 15 percent gain in fuel efficiency could be realized in large GTEs between now and the end of the 2020 planning horizon. Similarly, today’s small turboshaft, turbojet, and expendable engines have increased specific horsepower, but by only 33 percent of the theoretical Brayton cycle limit. Between now and the end of the 2020 planning horizon, small GTE efficiency could be further improved by 30 percent. The committee believes that five technologies are critical to obtaining the improvements: (1) high-temperature compressor disk materials, (2) high-temperature turbine blade materials, (3) thermal management systems utilizing high-temperature, high-heat-sink fuels, (4) lightweight hot structures, and (5) signature controls.
Finding 3-1. Gas turbine engines will continue to play a dominant role in propulsion in future warfare. Their performance can be improved enough (15-20 percent) to meet a wide range of future warfighter needs if they are given adequate funding during the planning horizon. The FY06 President’s
planned budget funding levels for gas turbine S&T in FY06 are one-half to one-third pre-FY00 levels. This level of funding will not produce engine technology that allows U.S. aircraft to dominate future air wars.
Recommendation 3-1. To accelerate the development of new engine technologies, the Air Force gas turbine S&T funding should be increased significantly, from approximately $100 million annually to a level that reflects buying power at the time when the F-15 and F-16 engines were being developed. Top priority should be given to overcoming the technology barriers that will have the largest impact on future weapons systems:
Compressor discharge temperature limits,
Turbine inlet temperature limits,
High-temperature, high-heat-sink fuels for thermal management,
Lightweight structures, and
Some of these barriers apply as well to ramjet/scramjet systems.
Recommendation 3-2. The Air Force and DoD should execute a total system engineering process starting with a preliminary design to establish project feasibility when undertaking any new propulsion development program.
Large Gas Turbine Engine Programs
Large GTEs are the backbone of the military aviation force that guards U.S. interests at home and abroad, and they play an enormous role in establishing U.S. air dominance at the battlefront. Owing to the technological superiority gained from programs such as IHPTET, current turbine engines have enabled U.S. forces to achieve air dominance in all recent conflicts. To maintain this edge, however, the United States must meet the increasing demand by the armed forces for more efficient, survivable, and lethal weapon systems. At the same time, the military needs to make those systems more affordable to minimize the impact of military demands on the federal budget. This may be accomplished through continual R&D in the turbine engine field.
A new generation of aircraft and propulsion systems technology is introduced to warfighters roughly every 25 years. Today the United States is
fielding state-of-the-art large GTEs for the F-22 and the F-35. The propulsion technologies in these engines are the result of roughly two decades of efforts by the IHPTET program,4 Manufacturing Technology (ManTech),5 other DoD programs, and NASA aeronautics programs (DSB, 2006). The resulting propulsion systems are technically, in the committee’s view, approximately 10 years ahead of competing systems such as that in the Euro-Fighter, which does not allow it to supercruise or have thrust vectoring or stealth features. The level of Euro-Fighter engine technology is roughly equivalent to the technology levels in the most advanced F-15 engines (F100-PW-229 and F110-GE-129). This 10-year technology advantage is much smaller than the 20-year advantage in the 1970s, when the F-15 and F-16 were launched. Current DoD funding for gas turbine S&T is much less than in the 1990s, and if it is not increased, the United States will probably lose its GTE technical advantage, as has happened in civil aviation.
IHPTET and VAATE Demonstrator and Research Programs
Since turbine engines are so critical to the capabilities of military aircraft, DoD has pioneered many advances through demonstrator and research programs such as its preeminent turbine engine research programs IHPTET and VAATE. IHPTET, begun in 1988, reached its conclusion in 2005; VAATE, begun in 1999, extends to 2017.
See, for example, the IHPTET Web site at http://www.pr.afrl.af.mil/divisions/prt/ihptet/ihptet.html. Last accessed on March 27, 2006.
See, for example, the ManTech Web site at https://www.dodmantech.com/. Last accessed on March 27, 2006. ManTech is a program that develops new manufacturing processes for new technologies (DSB, 2006). During large system design and development (SDD) programs, there are usually new technologies proposed to save weight, cost, and performance or improve durability. For example, some recent programs have proposed integrally bladed rotors (IBRs), ceramic matrix composite (CMC) materials, and low-observable coatings. In addition, ManTech has been working on methodologies and manufacturing techniques to replace IBR blades in overhaul. During the prototype phase, CMCs and low-observable coatings are usually developed in the laboratory experimentally. ManTech is then chartered to take these laboratory experimental processes and transform them into a feasible production manufacturing process with demonstrated cost reductions and acceptable quality for production. During the planning process for SDDs, there are usually insufficient ManTech funds to properly develop and verify new manufacturing technologies. For example, the Air Force still has not developed a cost-effective process to replace blades on an IBR. Therefore, the burden will be on the overhaul facilities to develop this process, which will be very time-consuming and expensive and raise sustainment costs considerably.
Because IHPTET pervades the turbine engine S&T community, the committee examined its origin, organization, goals, products, and lessons learned to gain insight into the structure and likely success of VAATE. Based on this examination, the committee believes VAATE may achieve extremely efficient turbine engines that will double range or halve aircraft size, providing the Air Force with truly transformational capabilities that would affordably maintain U.S. military air superiority.
The key technologies include these:
High-speed, expendable turbine engines can be carried by all bomber and fighter aircraft in the fleet. They provide tremendous standoff capability yet can strike targets at more than four times the speed of sound. This work provides a foundation for future man-rated, responsive propulsion to gain access to space.
Adaptive cycle engines optimize performance across the aircraft flight envelope. In essence, this propulsion capability will have variable features that allow both responsive supersonic strike and persistent subsonic loiter in a single air vehicle. This propulsion concept offers benefits to power generation and thermal management.
Compact, efficient, direct-lift engines enable short takeoff and landing (STOL) and short takeoff and vertical landing (STOVL) capabilities on future large transports. This will result in long-range, high subsonic cruise and short/vertical takeoff operations capability for future multimission mobility.
Technologies developed under the three VAATE focus areas will have direct commercial impact. The versatile core focus area will allow greater hardware commonality between military and commercial applications, reducing costs through economies of scale. The prognostics and health maintenance concepts developed in the intelligent engine focus area and many products of the durability focus area will directly benefit almost all commercial applications. Conversely, VAATE will pull from the commercial sector to leverage NASA work on minimizing the emissions and noise impacts of military aircraft. VAATE will also have similar spin-off benefits for turbine engines used in marine, ground transportation, and power-generation applications.
The original VAATE program, which would have allowed robust tech development, demonstration, and transition capability, was scheduled to be funded at $145 million in FY06 and $149 million in FY07. Turbine
engine S&T funding was cut drastically, to just under $90 million in the FY06 PBR, a reduction of $48 million from the FY03 PBR. The FY07 PBR is expected to remain at $90 million. At that level, it will be difficult to demonstrate and transition turbine engine technology.
Finding 3-3. The committee concludes that the IHPTET program demonstrated several marked strengths that form a foundation for the continued success of VAATE. IHPTET transitioned performance, durability, and cost-reduction technologies for both fielded and developmental engines—in particular, the F119 engine for the F/A-22 and the F135 and F136 engines for the F-35 Joint Strike Fighter. VAATE’s focus on (1) optimization of the propulsion system at the air vehicle system level, (2) an “affordable capability” goal that includes both performance and cost metrics, and (3) planned synergy with civil aeronautics requirements and attention to dual-use goes beyond the IHPTET approach and is highly appropriate for the demanding yet uncertain requirements of the future.
On a very positive note, each VAATE contractor reviewed by the committee appeared to have a portfolio of advanced technologies planned for development and transition, all of them essential for giving the U.S. armed forces the ability to conduct their missions in an effective, timely, and affordable way with minimal human cost. VAATE’s payoffs are designed to be realized in both the long term, for new air systems now on the drawing board, and the near term, for systems currently fielded (e.g., the F-16 and F-18), in production (e.g. F/A-22), or in the system development phase (e.g., the F-35 Joint Strike Fighter).
While turbine engine requirements have increased considerably, DoD funding for new turbine engine research under VAATE has been dramatically cut. It is unlikely that VAATE’s goals—and along with them, the driving military capability—can be accomplished in the program time frame at the envisioned reduced funding level. Figure 1-1 shows the increase in turbine engine requirements over time.
Recommendation 3-3. DoD should restore gas turbine S&T funding under the VAATE program to the original planned level. VAATE should address the primary risk areas necessary to advance jet engine technology, which include a robust engine demonstrator program and key producibility challenges.
Component Improvement Programs
All of the military services are faced with huge and growing costs for sustaining the current fleet of aircraft. Over 60 percent of the expected warfighter’s fleet of aircraft in 2020 is in existence or under development today. Near-term opportunities exist to incorporate existing GTE technologies into this fleet to significantly reduce the cost of sustainment and decrease the amount of fuel burned.
The committee saw many examples where CIPs, derivative engine programs, and engine capability enhancement programs (ECEPs) could yield sizable reductions in fuel burned, significantly improve performance, and greatly increase the time between shop visits.
History has shown that CIPs decrease Class A mishaps, increase time on the wing, decrease fuel burn, and extend service life. In most instances it can be readily shown that the costs of incorporating the existing technologies into the legacy fleet are rapidly recouped. In fact, programs of this type have historically returned $8 to $10 for every dollar invested in the sustainment and fuel burn of existing engines.
Finding 3-4a. The costs of fueling and sustaining the legacy fleet are the two highest annual propulsion costs faced by DoD.
Recommendation 3-4. DoD should sustain the current funding for the Component Improvement Program to ensure solutions to operational problems and safety issues and the development of future upgrades.
Derivative Engine Programs
DoD aircraft systems are continually modernized to remain viable and responsive to the warfighter’s need. These upgrades address identified performance deficiencies or provide new mission capabilities. As aircraft capabilities grow, weight, drag, and electrical and mechanical power loads typically increase, which translates, in turn, to increased demands on the propulsion system. To accommodate these increased loads, the propulsion system must become more capable.
A proven, cost-effective, efficient approach to improving propulsion capability is to develop derivative versions of existing engines. This is done by transitioning newer technology into legacy propulsion systems to improve performance and power.
There is currently no active program for increasing the performance of legacy propulsion systems to replace the engine model derivative programs (EMDPs), which were canceled in 1998, resulting in two significant gaps in the DoD engine development process. The first gap is the inability to conduct timely propulsion system enhancement studies or to develop technology transition roadmaps to support and complement studies of aircraft modernization and capability growth prior to Acquisition Milestone A. The second gap is the lack of a demonstration process to mature the propulsion technology from technology readiness level (TRL) 6 (demonstration in a relevant system) to TRL 7 (demonstration through initial flight test). This gap increases the risk of incorporating new technology into the propulsion system or makes it impossible to do so after Acquisition Milestone B.
EMDPs were a cost-effective way to improve capabilities and decrease the cost of support. Funded EMDPs would ensure that the propulsion capability requirements are met in a timely and cost-effective way. New centerline engine developments cost billions of dollars and require more than 10 years to complete. Derivative engines often cost only hundreds of millions of dollars and require only 3 to 5 years to complete. This reduced cost and shorter time for development mitigates the cost and schedule risks of weapons system development.
Small Gas Turbine Engine Programs
This section reflects the committee’s views on the status, requirements, and anticipated plans for small (500-15,000 shaft horsepower (SHP)) GTEs—turboprop and turboshaft—intended for use by DoD from now until 2020. The comments apply to propulsion systems for UASs, helicopters, and compound helicopters and tilt rotors. Turboshaft engines have broad applicability to a wide variety of systems used or anticipated by the Army, the Navy, the Air Force, the Marine Corps, and the Coast Guard.
Small Engine Requirements and Development
All of the helicopter and UAS turboshaft engines used by DoD were designed in the 1960s and 1970s. Since that time, few of the technologies developed by the joint turbine advanced gas generator (JTAGG), IHPTET, and other programs in materials, electronics, software, computational design tools, and network-centric warfare have found their way into turboshaft propulsion systems.
An important finding of the committee is that DoD could immediately benefit from technology that has been completed over the last 30 years by developing a new 3,000-SHP class turboshaft engine and a new 10,000-SHP class turboshaft engine. A modern 3,000-SHP class turboshaft engine could be applied to the Air Force’s plan for a personnel recovery vehicle system, the Army’s Apache AH-64 Block III and Blackhawk UH-60M helicopters, the Navy’s SH-60 Sea Hawk, and the Marine Corps’s UH-1Y and AH-1Z vehicles. Additionally, a modern 10,000- to 15,000-SHP class turboshaft engine would have an enormous positive impact on the design, performance, and cost of the emerging joint heavy lift (JHL) vehicle and on the Marine Corps’s improved CH-53X helicopter, capable of performing well at high altitudes and high temperatures.
The 3,000- and 10,000-SHP engines could fulfill the 2020 requirements of all military services. Most of these requirements will be for rotary-wing manned helicopters and UASs.
Successful, affordable, and enduring warfighting performance for these systems will depend on modern turboshaft engines to satisfy the required capabilities for attack, reconnaissance, utility, and medium-cargo missions. All of these aviation systems incorporate engines that will be 50 to 60 years old in 2020 unless proactive steps are taken today.
Past science and technology programs have demonstrated significant advances in the capabilities of turboshaft engines. Specific improvements are expected to lead to smaller, lighter, more affordable rotorcraft and UASs. The resulting systems will be far more capable, with significant improvements in the cost of operation and sustainment. It is clear that an incremental change in potential is available today and that continued investment is justified.
One of the last elements in the IHPTET program (expected to have run its course in 2005) is the JTAGG III demonstrator, which is being designed and built jointly by Honeywell and GE. (The goals of the IHPTET program are shown in Table 3-1 for Phases I, II, and III.) The technologies demonstrated are planned to directly support the planned Affordable Advanced Turbine Engine (AATE) and the Improved Turbine Engine Program (ITEP). The payoff for implementing JTAGG technology rather than that available with a simple derivative engine is huge. An
improvement of 30 percent in specific fuel consumption is shown for the relatively large turboshaft engine required to power the JHL vehicle.
Finding 3-6. Two new small military gas turbine engines—3,000-SHP and 10,000-SHP class—are needed to meet the mission requirements of all the military services. The U.S. military has not developed a new centerline engine in these classes since 1972. The technology level of the U.S. military GTEs in the small and expendable classes is roughly on a par with that of competing nations. This equivalence is driven not by available technology, however, but by the fact that no new military engines in these power classes have been fielded since the early 1970s. The technology and need exist to field new 3,000- and 10,000-SHP class gas turbines for helicopters and UASs.
Recommendation 3-6a. The Army should consider combining its AATE demonstration program and its unfunded ITEP, also targeted at 3,000 SHP.
Recommendation 3-6b. The Army should ensure that the size of the Future Affordable Turbine Engine (FATE) program, which remains undecided, is suitable for the demonstration of a 10,000-SHP class small gas turbine. The FATE demonstration could then form the basis for a new engine for a future heavy-lift helicopter mission or the Joint Unmanned Combat Air System mission.
Recommendation 3-6c. In addition to developing two new small gas turbines, DoD should carefully investigate innovative ways to integrate advanced engines and advanced vehicle propulsion systems. Examples here include novel inlets, exhausts, IR suppression systems, particle separators, integrated flight/engine controls, and systems to manage component health.
Expendable Turbine Engine Programs
Expendable GTEs are the prime propulsion systems for many cruise missile systems and some unmanned aircraft systems. Currently, expendable GTEs must be storable for a long time, fit into very small volumes, be characterized by high T/W ratios, operate over a wide range of Mach numbers,
and provide good fuel economy. In the committee’s expert opinion, expendable engines used by DoD are as good as or better than those known to be used by other nations. Over the past two decades, IHPTET has been working on technology for expendable engines. Some of these technologies have been incorporated in the derivative engines that will power the planned joint air-to-surface standoff missile (JASSM). The better fuel economy of the expendable engine powering the JASSM is a major contributor to its longer range. However, “irregular” and “catastrophic” scenarios require a large increase in the Mach number range over which expendable engines must be able to operate. Standoff cruise missiles with operational Mach numbers of 4.0 or so are required for warfighter missions until 2020. After reviewing the program for demonstrating engines at high Mach numbers, the committee believes the demonstrations are well planned. However, given the criticality of high Mach number missiles to the 2020 warfighter, the committee suggests that DoD might wish to use ManTech funding to develop the high-temperature materials with consistent properties that will allow GTEs to operate at a Mach of about 4.25 and to ensure that the United States can supply these materials. This need is not unique to expendable GTEs but extends to all types of GTEs. The committee also notes that the high Mach demonstration program is success-oriented; that is, the program assumes success at all milestones, with little or no allowance for problems that might arise.
Finding 3-7. High Mach number cruise missiles will be critical to the 2020 warfighter. The VAATE and DARPA programs plan to demonstrate a Mach 4.25 expendable engine in 2008.
Recommendation 3-7. Given the criticality of the high Mach number cruise missile, DoD should support the success of these system demonstrations by funding programs to ensure the availability of high-temperature materials.
Other Technology Programs for Aerospace Propulsion
There is a general perception that aeropropulsion is a mature, plateau technology. This section on alternatives addresses several nascent and compelling revolutions in aeropropulsion. Beyond conventional rocket and GTE aeropropulsion in concept and timescale lies an emerging array of alternative aeropropulsion cycles, including hybrid rockets using highly
energetic fuels and oxidizers and propulsors applicable to in-atmosphere cruise and in some cases to Earth-to-orbit (ETO) and in-space propulsion. These frontier concepts are included in the NRC report Materials Research to Meet 21st Century Defense Needs: “In this case, the most important contribution of the panel may have been to identify opportunities that are not being pursued aggressively due to limited budgets and a current focus on immediate needs and near-term payoff” (NRC, 2003, p. 58). They satisfy in various ways DoD needs in 2018, which include on the one hand, increased range, loiter, timeliness, reliability, small-system performance, and flight envelope and, on the other, reduced observables and cost.
Alternative propulsion concepts are at various levels of maturity and application. The pulsed detonation engine (PDE) approach is in the laboratory and exploratory stage. Variants of electric propulsion are currently being applied to a subset of small air vehicles in an attempt to increase vehicle size and performance. Also applicable are back-to-the future approaches for significantly improving internal combustion engines using, for example, free piston devices or highly refined Wankel engines.
One exceedingly interesting, even revolutionary, possible source of energy for aeropropulsion, in addition to the ongoing research in high-energy-density materials (HEDM), is positrons. Positrons do not require the heavy shielding, power sources, or magnets associated with most earlier nuclear propulsion schemes. Their projected favorable safety characteristics would allow their application to in-atmosphere operation for both cruise and ETO. Isomers are similar in terms of radiation safety but have a somewhat lower energy density.
Ramjet and Scramjet Engine Programs
Scramjet propulsion is crucial for standoff strikes on time-critical and hardened targets, boost-phase intercept, and flexible access to space using airplanelike operations. Although the first patent on ramjet propulsion (René Lorin) dates as far back as 1913 and scramjet research started nearly 50 years ago, flight tests have occurred mainly in the last 15 years. Serious application efforts are under way in various countries.
Scramjet propulsion systems are applicable to missiles and to strike, reconnaissance, and ETO missions. Scramjets can fly at up to Mach 8 using hydrocarbon fuels and can far exceed Mach 8 using hydrogen fuels. Volume/ weight/length-constrained, air-launched missiles with scramjet propulsion can fly twice as far as missiles using existing technology. Scramjets offer
(1) global reach (they can fly to anywhere on the globe in approximately 2 hours), (2) time-critical strike (they can reach targets hundreds of miles away in minutes), (3) enough kinetic energy to penetrate hardened targets, and (4) flexible access to space using airplanelike operations.
Air-breathing hypersonic propulsion currently faces a number of challenges: (1) combined cycle transition; (2) system thermal management (engine/vehicle); (3) high-temperature/lightweight materials; (4) flow/ combustion numerical simulation and design sensitivity; (5) cooled leading edge for Mach numbers larger than 7; (6) miniaturization of the control and fuel system; and (7) need for a dual-mode scramjet flow path. Another challenge is the integration of the turbine-based combined cycle system and the rocket-based combined cycle system.
The Air Force is working on two scramjet programs: (1) the hypersonic technology (HyTech) single-engine demonstrator (SED), to be used on a liquid hydrocarbon scramjet engine with a solid booster, and (2) the Falcon, to be used on a hydrogen scramjet engine with a rocket motor. The Navy is working on the HyFly program, which is developing a hydrocarbon scramjet with a solid booster, and on the RATTLRS program, for a ramjet based on a turbojet engine. The Army is developing a Mach 12 hydrogen scramjet engine with a terminal high-altitude area defense (THAAD) missile booster.
To address the new threats, which include a hypersonic glide vehicle, a hypersonic powered vehicle, and a container-launched cruise missile, the Army is currently developing Mach 12 interceptor hypersonic projectiles in its Scramfire program. These systems are capable of variable velocity operation, are maneuverable, and can serve as an accelerator and/or a cruiser. The propulsion system is a scramjet engine that uses hydrogen fuels. Extensive experimental and numerical simulations are under way. The committee believes that this project is suitably funded to achieve its goals by 2009-2010.
Finding 3-8. Consistent with the National Aerospace Initiative (NAI), DoD has an active scramjet technology development effort. In the committee’s opinion, the level of U.S. technology is on a par with or ahead of the competition. It is also the committee’s opinion that more synergism among DoD’s several scramjet efforts would allow DoD to meet the country’s needs more economically and quickly.
The existing scramjet programs are well focused and address the DoD S&T strategy. NASA scramjet propulsion programs are being replanned. There is a need for a focused government-sponsored program like IHPTET/ IHPRPT to develop scramjets. Such a program is needed to maintain the U.S. technology base, including both people and infrastructure. It also appears that maturation funding is inadequate to drive the implementation of scramjet propulsion.
Recommendation 3-8. DoD should develop a strategy to exploit the synergies between the hypersonics programs in each of the services for the benefit of DoD, in the form of a common technology and cost savings. There are alternative solutions for both time-critical, hardened targets and flexible space warfare, and these should also be studied and compared with the scramjet solution.
ROCKET PROPULSION SYSTEMS FOR ACCESS TO SPACE
Anticipated Military Spacelift Propulsion Needs and Identification of Critical Technologies
In early 2005 the U.S. Space Transportation Policy was signed by the President (NSPD, 2005). This policy reinforced the goals originally stated in the AFSPC Strategic Master Plan FY06 and Beyond: “AFSPC will sustain and modernize its current Satellite and Launch Operations into the Far-Term when it will transition to advanced capabilities” (AFSPC, 2003, p. 29).
The Air Force’s overarching need to have responsive access to space and to operate effectively in space under all realistic scenarios demands the establishment of requirements for (1) strategic and responsive spacelift total systems, (2) responsive on-board propulsion systems in space, and (3) return from space. Transformation in access-to-space or in-space operations will require using a total systems engineering process, with “mission success over the committed life of the system” as the primary criterion for selection among options for the system’s architecture and elements. The evolution of such a system engineering program, together with the validation of trade-off parameters using the supercomputing capabilities available today, would provide a powerful and objective quantitative tool for defining and evaluating low-risk, cost-effective total system concepts for strategic
and operationally responsive spacelift and in-space operations. For example, when carrying out systems engineering for access-to-space missions, the total system for accomplishing the mission must consider launch vehicle configuration (number of stages, reuse), launch locations (fixed or mobile, including from high-altitude aircraft), facility and logistical requirements, operations concepts (payload integration on launch-stand or preintegrated, attachable payload modules), technology validations that remain to be carried out, overall development schedules, life-cycle cost, industrial support viability, and so on.
Only the unbiased application of such a tool can provide a credible basis for justifying specific system requirements such as the number of stages, the choice of propellant, the extent of reusability, and the flexibility of launch locations. The process would allow establishing the quantitative risk assessment profiles needed for selecting total system options. The process would also allow setting propulsion-system-specific requirements and subsystem basic configurations. Then, design criteria can be specified that assure a subsystem option or function will serve its purpose. Identifying missing or unvalidated design criteria associated with propulsion systems for operationally responsive spacelife (ORS) would define critical gaps in the available technology base.
Finding 4-1. The committee does not believe that the Air Force will be able to reliably and cost-effectively transform U.S. military space transportation capabilities by focusing on pushing high-thrust rocket propulsion technologies to their limits. Even if the total systems optimization process is objectively carried out, the technologies it selects are unlikely to be (and need not be) transformational in themselves. It is more likely that any transformational access to space achieved during the planning period will be the result of creative total system architectures. Focusing Air Force resources on identifying the gaps in the critical design criteria for total systems-defined rocket propulsion elements will be crucial to success of the AFSPC Strategic Master Plan FY06 and Beyond.6
Recommendation 4-1. The Air Force should place a high priority on developing an integrated total system engineering process using quantita-
There are a number of new propulsion technologies that do in fact have the potential to directly enable transformation of in-space rocket propulsion systems performance. They are discussed in Chapter 5, Rocket Propulsion Systems for In-Space Operations and Missiles.
tive life-cycle mission success as the selection criterion for near-term, highly leveraged engineering technology funded by the Air Force. This process is crucial to defining justifiable total system architectures, rocket propulsion systems requirements, and critical technologies for military space transportation to support the AFSPC Strategic Master Plan FY06 and Beyond.
Current Technology for Large, First-Stage (Core), Liquid Propellant Booster Engines
RS-68 Engine for Delta IV Launch Vehicle
In the early 1990s, Rocketdyne initiated development of the first new indigenous booster-class engine in the United States in more than 25 years, the RS-68. The RS-68 was ultimately selected to power the Delta family of evolved, expendable launch vehicles (EELVs) developed for the Air Force by the Boeing Space Systems Company.
The RS-68 is a conventional bell-nozzle booster engine that develops 650,000 lb of thrust at sea level and is the largest liquid oxygen/liquid hydrogen (LOx/LH2) engine in the world today. It uses a simple, open gas generator cycle with a regeneratively cooled main chamber. It can be throttled to 60 percent of full power.
During the design and development phases, this engine was based on a simplified design philosophy that reduced parts count and production costs below those of the contemporary space shuttle main engine (SSME). The RS-68 engine has only 11 major components, which amounts to a reduction in parts compared to the SSME of over 80 percent and a reduction in hand-touched labor of 92 percent. The development cycle time was also much reduced, and nonrecurring costs were said to be one-fifth those for previous cryogenic engines. The engine was designed, developed, and certified in a little over 5 years and flew on the first Delta IV launch in late 2002.
RD-180 Engine for Atlas V Launch Vehicle
The engine that powers the first stage of the Atlas V EELV is the RD-180. The RD-180 is a two-thrust-chamber version of the original Russian RD-170 (four chambers) and offers the performance, operability, and reliability of the RD-170 in a size suited to the booster needs of the Atlas V EELVs.
The RD-180 is a total propulsion unit/engine system with hydraulics for control valve actuation and thrust vector gimbaling, pneumatics for valve actuation and system purging, and a thrust frame to distribute loads, all self-contained as part of the engine. The engine, which employs a LOx lead start, a staged combustion cycle, and a LOx-rich turbine drive, delivers 10 percent better performance than current kerosene (RP-1)-fueled operational U.S. booster engines and can provide relatively clean reusable operation (beyond one mission duty cycle).
Finding 4-2. The current family of U.S. EELV boosters does not need to be replaced for the next 15 to 20 years, nor are there plans to do so. Nevertheless several candidate designs were started under NASA’s Space Launch Initiative (SLI) program in 2001.
Recommendation 4-2. DoD should begin work relatively slowly, investing about $5 million per year, in the committee’s judgment, in technology development for an advanced-cycle booster engine that could provide the basis for a new far-term access-to-space vehicle.
Current Technology for Large, First-Stage, Strap-on, Solid Propellant Motors
GEM 60 Rocket Motor for Delta IV M+ Launch Vehicle
ATK Thiokol (now Alliant Techsystems) originally developed the graphite epoxy motor (GEM) for the Delta II launch vehicle for the U.S. Air Force and Boeing. The GEM 40 is a highly reliable motor used on Delta II. The GEM 46 is a larger derivative—with increased length and diameter and with vectorable nozzles on three of the six ground-start motors—for use on the Delta III. The motor has also been used on the Delta II heavy-lift vehicle. The 70-ft GEM 60 provides auxiliary liftoff capability (in two or four strap-on motor configurations) for the Delta IV medium-plus-lift (M+) vehicles.
Aerojet Rocket Booster for Atlas V Launch Vehicle
The solid rocket strap-on booster motor for the Lockheed Martin Astronautics Atlas V EELV has been developed, flight qualified, and produced by Aerojet. This new generation of solid rocket motors provides
reliable, high-performance boosting power for the Atlas V medium- to heavy-lift expendable launch vehicle used by U.S. civil and military spacecraft launch programs as well as those of other countries.
The Aerojet solid rocket motor design for the Atlas builds on decades of flight design, testing, and real mission experience accrued by, among others, the Minuteman, Peacekeeper, and small intercontinental ballistic missile (ICBM) motors, as well as by Aerojet’s extensive work on other propulsion and space systems and a wealth of associated flight-proven technologies.
The Atlas V family of launch vehicles will use from one to five strap-on solid rocket motors depending on the mission and the launch trajectory requirements. The solid rocket motors are ignited at liftoff and burn for over 90 sec; each motor provides a thrust in excess of 250,000 lbf. At about 94 sec into the flight, the solid rocket boosters are jettisoned sequentially.
Current Technology for Delta IV and Atlas V Second Stages: RL-10 Family of Engines
The RL-10 has evolved significantly over more than four decades. It began with a vacuum thrust of approximately 15,000 lb for the RL-10A-1. Through a series of modifications, the average thrust became 24,750 lb in the RL-10B-2. This engine has probably had every possible ounce of thrust wrung out of it, but that has reduced the safety margins for some of the failure modes. Significant improvements in performance and reliability could be achieved with a new engine-cycle design.
Currently, EELVs have only one basic second stage, the RL-10. The Delta IV of Boeing uses the RL-10B-2, while the Atlas V of Lockheed Martin uses the RL-10A-4-1 or -2. The basic RL-10 engine, developed by Pratt & Whitney in the late 1950s, was the world’s first LOx/LH2-fueled rocket engine operated in space. Since the first successful launch of an Atlas/ Centaur RL-10, Pratt & Whitney has developed nine different models of the RL-10 engine family. The RL-10 earned the reputation of being a reliable, safe, and high-performance cryogenic upper-stage engine for a wide variety of U.S. EELVs.
Finding 4-3. The technology for the RL-10A and RL-10B family of upper-stage engines is now more than 40 years old. Although numerous upgrades have been incorporated over the life of the engine, much of the design is now outdated. Because the second-stage engine for both EELVs comes from a single supplier, Pratt & Whitney, the Air Force is totally dependent on this
single contractor and engine for all large payload launches. Should a failure occur that involves the second-stage engine, all launches with these systems would probably be frozen until the root cause was identified and corrected, which could take a year or more. While the probability of such an event is not high, it is not zero. In a time of crisis, this could be extremely debilitating for the nation. The number of failures in recent years (and their cost) would seem to be another good reason for developing and qualifying a new engine that would be supplied by more than one manufacturer.
In addition, to make full use of the Delta and Atlas heavy vehicles, a higher-thrust engine is needed. To develop a new upper-stage engine for the nation’s critical strategic launch vehicle fleet requires a major effort and an extended qualification program. The extremely high reliability demanded by a strategic launch capability means that a new engine development program cannot skimp on hardware or testing.
Recommendation 4-3. DoD should place a high priority on development of a new medium-thrust (50,000-80,000 lb) upper-stage LOx/H2 engine to assure the nation’s strategic access to space. The cost of developing such an engine through its initial operational capability (IOC) is estimated by the committee at $150 million to $250 million, providing the design does not try to push new technologies to their limits.
Propulsion Needs and Propulsion Technologies for Responsive Spacelift
An important element of a transformed total access-to-space architecture would be the introduction of ORS vehicles early in the far term of the AFSPC Strategic Master Plan for FY06 and Beyond (AFSPC, 2003), sometimes referred to in this report as SMP FY06. Responsive spacelift is shown in the DoD space transportation roadmap, Figure 4-1. It was thought that two or three small launch vehicles would be flown in the demonstration phase, 2004 through 2009. Those small vehicles were under competitive demonstration within DARPA’s FALCON program in 2005. Flight demonstrations were expected for only two concepts in 2006. Either or both of those concepts (SpaceX and AirLaunch) could evolve into small vehicle families able to satisfy early DoD needs for responsive access to space until subscale and full-scale ORS vehicles can be developed and qualified in the medium term and far term of the SMP FY06. Some of the vehicles initiated
under FALCON are expected to transition into cost-effective commercial launchers that could replace high-cost small vehicles.
FALCON Small Launch Vehicles
The DARPA/Air Force/NASA FALCON program started in August 2003. Its overall goal is to develop and validate in-flight technologies that will enable both near-term and far-term capabilities to execute time-critical, prompt, global-reach missions while at the same time demonstrating affordable and responsive spacelift. The technical underpinning of the FALCON program was that a common set of technologies could be matured in an evolutionary manner that would provide a near-term (circa 2007-2010) operational capability for responsive, affordable small-satellite spacelift and prompt global reach from the continental United States (or equivalent reach from an alternative U.S. base). These technologies might also enable the development of a reusable hypersonic cruise vehicle (HCV) in the far term (circa 2025).7,8
There are two tasks in this program. Task 1 involves a small launch vehicle (SLV) and Task 2 involves a hypersonic technology vehicle (HTV). Two capabilities—placing small satellites or payloads into low Earth orbit (LEO) and performing HTV missions in a responsive manner together— are an important step in the evolution of ORS vehicles for the Air Force (DARPA, 2004).9
After the FALCON program is completed, DARPA will hand over the demonstration vehicle systems aspects to the AFSPC for operational system development and implementation. The winning vehicles may be allowed to contract directly with NASA or private entities (e.g., academia, industry, and other government agencies) to implement commercial launches.10
Finding 4-5. The FALCON program is an initial response to the need for low-cost, operationally responsive access to space. This program plans to perform in-flight validations of technologies leading to highly responsive
For additional information, see http://www.darpa.mil/body/news/2003/falcon_ph_1.pdf. Last accessed on March 30, 2006.
David Weeks, NASA Marshall Space Flight Center (MSFC), personal communication to committee member Ivett Leyva on May 18, 2005.
vehicles that can carry out time-critical, global-reach missions. The cost goal for FALCON-technology-based designs is $5 million (2003 dollars) per launch. Current costs for similar payloads using available small and medium-size vehicles are $20 million to $30 million. Successful FALCON demonstration vehicles and, later, production vehicles would open the door to a larger market for commercial space payloads. An increased launch rate would allow for the increased production of SLVs, which in turn would lower the cost of the vehicles through true mass manufacturing. Also, if more satellites could be launched each year, they would not need to be designed for a 5- to 10-year lifespan but could instead be updated or replaced more often. In FALCON, cost is prized over performance.
Expendable vehicles using low-parts-count, pressure-fed liquid propulsion systems such as systems used for the AirLaunch FALCON demonstrator and the SpaceX vehicle can be developed for much less money than reusable ones. Depending on the annual flight rate, they can also cost less per flight.
Recommendation 4-5. In September 2005, DARPA downselected to just one company for Phase 2B. DARPA should continue to fund and monitor this company to completion of the FALCON program objectives. The Air Force should evaluate the propulsion technologies to be demonstrated for the air-launched FALCON vehicle and include them in total system studies of options for ORS vehicles.
As stated above, the overall goal of the FALCON program is to develop and validate, in flight, technologies that could provide both near-term and far-term capabilities to execute time-critical, prompt, global-reach missions from the continental United States (or equivalent reach from an alternative U.S. base) while also demonstrating affordable and responsive spacelift for a variety of small satellites. Achieving these capabilities is important for achieving the Air Force’s overall goal of ORS vehicles and global precision strike.
Air-Based Vertical Launch
In the fall of 2005, another vehicle launch concept was disclosed that appears to have good potential for enabling the achievement of the above
capabilities for very fast and precise global and tactical strike and for responsive, cost-effective launch of satellites at the low end of the small satellite spectrum11 into various LEOs (Smith, 2005). The idea of air-based vertical launch (ABVL), which resulted from trying to find solutions to the severe time and geographic constraints associated with ground-based, boost-phase ballistic missile defense, is to install a vertical launching system in a large-body aircraft. Such aircraft could be on station anywhere in the world where it is desired to optimize the chances of total mission success for the various applications.
The basic feasibility of an ABVL is being studied by BAE Systems and ATK Thiokol under a small DARPA contract. Beyond that, an integrated total systems engineering program is necessary to establish propulsion requirements that can exploit the responsiveness and potential for low-cost ABVL spacelift for small satellites. The concept is also of interest for prompt-reach missile applications (see Chapter 5). New technologies for modifications of existing designs for stage or missile boosters critical to meeting those propulsion requirements can then be defined and demonstrated.
Finding 4-6. Configurations for candidate launch vehicles (including parallel boosters or strap-on combinations), along with propulsion technologies such as propellant combinations (solids, storable liquids, gelled combinations, storable-oxidizer hybrids) and operating characteristics (including assured start-up profiles, thrust vector control, and rocket plume impingement patterns) need to be optimized to take full advantage of the potential new operationally responsive mission capabilities of aircraft-based vertical launch for small satellites, satellite arrays, and near-space military applications (see also Chapter 5).
Recommendation 4-6. The Air Force and DoD should sponsor a detailed system engineering study to fully understand the transformational potential of cost-effective, operationally responsive launch of small, micro-, and nanosatellites (particularly for large-number satellite arrays) utilizing ABVL concepts. The propulsion technologies that are needed to take full advantage of such launch platforms should be identified and developed.
Operationally Responsive Spacelift Requirements
Department of Defense Space Science and Technology Strategy (DoD, 2004) states that assured access to space is the highest priority within the space support mission area and that a responsive space capability is directly coupled to both the space support and space force enhancement mission areas. An important element of a transformed total access-to-space architecture is the introduction of ORS vehicles early in the medium term (FY12 to FY17) of SMP FY06.
In the Air Force’s proposed roadmap of ORS spirals (Figure 4-1), selected vehicles from the FALCON program described above would continue developmental and operational flights as part of the Air Force’s SLV fleet into the far term (James, 2005). Each of the selected concepts would probably evolve into a family of fast-response, expendable vehicles having payload capabilities from 2,000 to 10,000 lb to LEO. The proposed roadmap also shows the start of full-scale development of an ORS vehicle in 2010.
Meeting the demanding objectives for ORS vehicles may necessitate a number of new propulsion subsystem technologies in addition to existing qualified subsystems. As discussed in some detail in the introduction to Chapter 4, an integrated total systems engineering process, whereby propulsion requirements for these vehicles are established and technologies critical to meeting those requirements are defined, is crucial to the success of any new launch-to-space or in-space vehicle program. Incorporating “mission success” as the primary selection criterion for this systems engineering process provides a powerful objective and quantitative tool for designing low-risk, cost-effective ORS concepts for Air Force future needs.
Integral to the total systems engineering process is verifying that the design criteria for all proposed critical technologies have been validated. This is the main thing that allows objective evaluations of development engineering schedule and cost risks and of propulsion systems’ operational and life-cycle cost risks. It also permits objective, quantitative, and consistent analysis of the trade-offs among concepts across diverse propulsion systems. In selecting a propellant, for example, one would need to look at the trade-offs between pumps and pressure-fed; pressurization subsystems for either net positive suction head or propellant feed; optimization of chamber pressure and nozzle expansion ratios for first or second stages; expendable vs. reusable first and/or second stages; metals vs. composites for tanks and motor cases; ablative vs. cooled combustion chambers; and so on.
Most important, when rigorously applied, such a verification program would force identification of unvalidated design criteria associated with technologies for critical elements of unproven propulsion systems or for upgrades of existing propulsion subsystems. These unvalidated system element design criteria, which include criteria for the element’s total operating environment, are the primary drivers of a development program’s engineering, operational, and cost risks. In the past, designs accepted without a sufficient range of criteria validation were found to have been the first cause of catastrophic failures.
Affordable Responsive Spacelift Vehicle
In 2005, the Air Force embarked on a subscale vehicle demonstration and system concept validation program, Affordable Responsive Spacelift (ARES), which it plans to evolve into the ORS family of vehicles. The ARES program was planned to start in 2005. The Air Force has been working on conceptual systems engineering for ARES and has completed an initial group of concept systems engineering analyses. This has led to a basic architecture concept for having a reusable fly-back-to-launch-site, rocket-engine-powered first stage and an expendable rocket-engine-powered second stage. Air Force documents say the ARES Hybrid is the Vector 1 roadmap medium-term solution for a revolutionary spacelift capability. They also say that an ARES flight demo in 2010 will provide confidence in full-scale system costs and operability and allow fielding a system in an affordable fashion (James, 2005).
If the ARES system design has been selected via a total systems engineering process to provide confidence in the development of a full-scale vehicle, most of the critical technologies for the full-scale configurations are locked in by default. To proceed confidently with competitive conceptual designs for the vehicle prototype starting in 2005 and then implement a selected configuration development program starting in 2006, the selection of propulsion technologies might have benefited, from a total systems engineering perspective, from incorporating those technologies in existing qualified propulsion elements or those with extensive validating data.
The committee found no sign of any transformational or revolutionary technologies that were mature enough to be considered for ARES, so they would presumably not be used in the full-scale vehicle that is expected to emerge from the subscale demonstrator. Also, the committee could identify only two existing rocket engines that might meet the propulsion system
requirements for the ARES hybrid: the Aerojet AJ26-58/59, which would constrain the first stage to LOx/RP-1, and the RL-10 family, which would constrain the second stage to LOx/LH2.
Another Air Force presentation recognizes this real situation and states as follows (Hampsten et al., 2005):
Remember … ARES-SD is the first phase of an acquisition program. [The] Air Force wants to achieve its goals using the lowest risk approach practical. ARES management team uses the term technologies in the generic sense of describing the technological means to an end. It is not intended to indicate specifically immature, “high-tech,” stretch design goals or high risk technologies. This is not a tech-push effort.
The committee concludes that ORS missions in the early part of the far term (FY18-FY30) will not have (and, in its opinion, need not have) revolutionary propulsion technologies. In fact, the various risks of committing to unvalidated technologies at this point are much greater than any potential gain in rocket propulsion system performance. If there is to be a revolutionary ORS capability in the medium term, it will come from very innovative total systems architecture and operations processes and from high margins against retained failure modes, not from revolutionary rocket propulsion systems.
Initiatives for Developing New Aerospace Propulsion Technology
National Aerospace Initiative
The NAI began in 2001 as a joint technology program by DoD and NASA. It is not to be thought of as a system development or acquisition program (NRC, 2004). “The goals of NAI are to renew American aerospace leadership; push the space frontier with breakthrough aerospace technologies; revitalize the U.S. aerospace industry; stimulate science and engineering education; and enhance U.S. security, economy, and quality of life” (NRC, 2004, p. 3). The initiative rests on three pillars: high-speed/ hypersonic flight, access to space, and space technology. An NAI executive office was created to foster collaboration between NASA and DoD and develop goals, plans, and roadmaps for the three pillars. The goal was for NAI to start by identifying the capabilities needed for future systems; to use the goals, objectives, technical challenges, and approaches (GOTChA)
process to analyze the technology development challenges and options; to establish investment plans; and, finally, to coordinate the efforts of the involved parties to execute the technology plans (NRC, 2004).
The goals and direction of NAI changed on January 14, 2004, when President Bush announced a plan to develop and test a new crew exploration vehicle (CEV) by 2008 and to carry out human missions to the moon (circa 2014) and, later, to Mars (NRC, 2004). This vision for space exploration was announced after the committee had submitted its draft evaluation of NAI to external peer review.
NAI represents cooperation, better utilization of resources, and maximization of synergies. However, it is hard to identify how the money allocated for the NAI is being spent beyond the first layer of general funding. The recommendations and observations of the NAI evaluation committee are still valid. A continuous update of Air Force needs and their inclusion in future revisions of NAI plans or strategy would strengthen the initiative.
Although funding for the program has been severely limited, contractors have had considerable freedom to develop new technologies that could improve the performance and life of both solid and liquid rocket engines. The main difficulty voiced by several of the contractors is that there is no clear definition of Air Force needs.
Integrated High-Payoff Rocket Propulsion Technology
The IHPRPT program was initiated in 1994 and has been in place for a dozen years. It is a joint government-industry effort focused on affordable technologies for revolutionary, reusable, and/or rapid-response, global-reach military capabilities. It addresses sustainable strategic missiles, long life or increased maneuverability, spacecraft capability, launch vehicle propulsion, and high-performance tactical missile capability. It attempts to emulate the IHPTET program, which was successful in developing and testing new turbine engine technologies.
One significant limitation of the program is severe underfunding for component testing and validation. The uncertain and disappointing future for commercial launch opportunities and a lack of real requirements from DoD has discouraged the major rocket propulsion companies from investing their decreasing independent resources in new propulsion subsystems (NSPD, 2005). A clear expression of DoD needs would give the IHPRPT program more focus and streamline its efforts.
Finding 4-8. The AFRL Space and Missile Systems Division is undertaking a variety of interesting and potentially valuable in-house programs. It appears to be developing technology that will be very useful, such as predicting the existence of certain energetic compounds and their synthesis, determining the coking properties of hydrocarbon propellants, and developing combustion instability models. Unfortunately, there does not appear to be much in-house work in the liquid engines and solid motors areas. The more basic work seems to be of high quality, but its basic nature and not knowing where the Air Force wants to be in the future make it very difficult to set the priorities for these efforts or even determine if they are the best ones to undertake. A thorough review by outside experts might help in prioritizing the efforts.
Recommendation 4-8. The Air Force should develop in-house test beds for liquid, solid, and hybrid rocket motors. Because limited funding seems to be at least part of the reason this is not being done, the Air Force should seek to increase the funding for both liquid and solid rocket test beds at AFRL.
The U.S. Rocket Propulsion Industry
The U.S. rocket propulsion industry and associated space transportation business have been in a steady decline since the end of the Apollo program (~1972) and the cold war arms race. A turnaround in the propulsion and space transportation industry was expected in the wake of the space shuttle and International Space Station programs. The space shuttle program (or National Space Transportation System), which had to develop three new liquid rocket engines—the space shuttle main engine (SSME), the orbital maneuvering engine, and the reaction control engine—and the world’s first large, segmented, and reusable-case solid rocket motor, did not reverse the decline after the Apollo era; it only slowed the rate of decline until the late 1970s.
In general, the development of rocket propulsion technology by the United States for all spaceflight applications has significantly lagged development by the rest of the world since the initial certification of the space shuttle. This lack of progress in rocket propulsion technologies over such a long period has resulted in several deficiencies in the nation’s space program. Most notable is the reduced reliability of U.S. launch and space vehicles, as evidenced by the increased number of flight failures during the late 1990s and into this new decade, as well as the large loss of U.S. share of the world
market in both the space launch and spacecraft industries, with the U.S. share having fallen from about 80 percent in the late 1970s to less than 20 percent in 2002.
In the last three decades, only one new U.S. government-sponsored booster engine, the SSME, has been developed and gone through flight certification. Some significant upgrades have been incorporated into the SSME since the original certification for flight in the 1970s. These upgrades increased reliability and safety and somewhat increased mean time between engine refurbishments. They did not, however, appreciably advance rocket engine technology. Since the 1970s, the number of firms capable of major engine development has shrunk significantly. This industry downsizing, combined with consolidation, points to the diminution of U.S. ability to meet DoD’s propulsion needs for a new ORS family of vehicles starting with ARES in about 2015. Basically, the nation’s current capabilities in space propulsion and space transportation are but a fraction of the capabilities that were evolved starting in 1954 for the ICBM program and culminating in the early 1970s with the end of the Apollo program. Those programs helped the United States respond to international crises and eventually win the cold war.
Since 1980, only one new first-stage rocket engine has been developed in the United States. This engine, the RS-68, was funded primarily by Boeing Rocketdyne. It was developed as a low-cost, expendable booster engine for the Delta IV EELV. Engine performance of the RS-68 is poorer than that of the 1960s-era Saturn V second- and third-stage J2 engines, both of which were simple open-cycle, gas-generator-powered designs. However, by incorporating comprehensive modeling, computer-aided design/manufacturing, and advanced manufacturing technologies of the 21st century, the developer realized important advances in engineering methodology and capabilities. The latest manufacturing technologies would be very beneficial for production runs of, say, 30 to 50 engines per year. However, it turns out that the EELV program will require no more than 5 to 8 engines a year. Moreover, no commercial market for very large boosters ever materialized. So the RS-68 offers almost no unit cost advantage over older engines that are available from several countries.
While the United States developed almost no new booster rocket technology during the last 30-plus years, the new spacefaring nations of Europe, Asia (including India), and the Middle East have been developing their own new vehicle and propulsion systems to catch up. Along with the former Soviet Union, they are believed to have developed 40 to 50 new engines
using several propellant combinations in addition to LOx/LH2. Many of these engines can now be considered to be today’s state of the art.
Based on these observations, it is probably no coincidence that the U.S. share of the space launch market has eroded badly in the last 40 years and along with it, U.S.-built launch vehicle reliability. Of the potential worldwide market for commercial launch systems of $8 billion to $10 billion per year, the United States now captures only about $1 billion to $2 billion.
A similar trend has been observed for the development of propulsion technology for upper-stage and in-space products. Advancements will be needed in both areas to enable the Air Force’s future total capability missions. Most of the U.S. in-space propulsion developments in recent times have been privately funded, with some support from the government. However, most of the government-sponsored projects were stopped for one reason or another before any significant advances in technology readiness could be achieved.
Finding 4-15. The severe industry downsizing and consolidation causes concern about U.S. ability to meet the propulsion needs set forth in the SMP FY06 (AFSPC, 2003) for a new operationally responsive family of spacelift vehicles, starting with ARES in 2010 and ORS in 2015. DoD and Air Force commitment to fully develop these new robust launch vehicles might help rejuvenate the U.S. aerospace industry, provide more employment opportunities for young aerospace engineers, and reverse the current decline in rocket propulsion design, development, testing, and production capabilities. This in turn could create synergies and capabilities that would present future leveraging opportunities for the Air Force.
Recommendation 4-15. The Air Force and DoD should devote more of the annual S&T rocket propulsion budget resources over the next few years to rocket propulsion; to technologies that would enable the successful introduction of mission-based ORS; and to other flexible, small-satellite launch capabilities in the medium term. The committee’s estimate of the additional focused investments needed is $50 million to $75 million annually.
ROCKET PROPULSION SYSTEMS FOR IN-SPACE OPERATIONS
All the U.S. military and civil strategic satellites and technology platforms require propulsion subsystems operating in space to provide the
impulse for adjusting velocity, changing orbit altitude, controlling attitude, station keeping, and deorbiting at the end of life. These propulsion needs are being satisfied currently by state-of-the-art chemical propulsion and, increasingly, by electric propulsion subsystems. Of note here is that the state of the art has been undergoing significant changes over the past 15 years and is therefore very advanced in many areas. For kilogram-class developmental satellites with unique mission capabilities, micropropulsion systems may be required for all maneuvers other than rapid inclination change.
However, in contrast to rocket propulsion for access to space and in near space, the potential for improving the performance of in-space thrusters and electric power generation and energy storage in space is still very great. Some of these technologies, such as various types of electrically powered thrusters or high-energy monopropellants, have the potential to directly enable transformational in-space capabilities for military systems. Air Force/DoD long-range plans have identified, and are working on, needs for many types of operational maneuvers in near space and in space. A systems architecture for seamless air-space operations will enable ORS.
Recommendation 5-1. DoD should support extensive basic research and technology projects for various in-space propulsion thruster concepts and for in-space electric power generation and energy storage. This fundamental long-range support need not be tied to any specific mission or platform requirement. The current range of technical opportunities is so great that progress will be directly proportional to annual resource allocations over the next 10 years. The committee estimates that at least $20 million should be considered as a yearly allocation in these areas.
Current Propulsion Technologies
The conventional chemical liquid propellant propulsion systems now in use are either monopropellants or bipropellants. Liquid bipropellant systems are better performers but are more complex and deliver a fuel and oxidizer mixture that reacts chemically in the combustion chamber. Monopropellant systems provide a single propellant that decomposes at the catalyst bed of the combustion chamber. Widely used, highly reliable, state-of-the-art chemical systems are the monopropellant hydrazine (N2H4) and
bipropellant propulsion systems such as mixed oxides of nitrogen (MON) and monomethylhydrazine (MON/MMH). For orbit circularization and station acquisition, bipropellant engines containing MON/N2H4 are also in use.
The expanding range of spacecraft size and the changes in the commercial spacecraft industry environment have presented new challenges to the chemical propulsion community. There has been a clear need to come up with better performing propellants and/or thrusters. The advent of power-rich spacecraft architectures opens up propulsion options that can provide both high power and high specific impulse (Isp). One option, reducing the onboard propulsion system’s wet mass requirement, could allow decreasing spacecraft mass or increasing payload. It could also allow placing greater demands on the propulsion system, including increased reposition or longer duration orbit maintenance, thereby increasing useful life. Another outcome of reducing the propulsion system’s wet mass might be its enablement of a stepdown to a lower-weight-class launch vehicle. These performance enhancements targeted by commercial satellite owners are also desirable for military satellites. The propulsion industry has accepted these challenges and is transitioning to electric propulsion.
To ensure the broader application of ion or Hall thrusters, greater emphasis needs to be put on developing the components of the entire electric propulsion subsystem, which includes not only the thruster but also the propellant feed system and the power processing unit (PPU). Historically the PPU has been the dominant cost driver for electric propulsion systems because it calls for heavy power converters and thermal management systems. Aerojet Redmond designs and builds high-power converters to support the electric propulsion subsystems it manufactures. It is also working on a solar electric direct drive that uses a high-voltage solar array to provide power directly to a Hall thruster at voltages needed to drive thruster discharge. Qualification of a solar electric direct drive would greatly reduce the cost and weight of a Hall electric propulsion system while also reducing array size. Reduction in array size brings added savings in spacecraft weight. The potential payoff for direct drive makes this goal extremely worthy of pursuit.
Current Propulsion Research
In 1994, DoD, the Air Force, and NASA established the IHPRPT program. This joint government and industry effort includes developing technologies for extending the life of spacecraft and for in-space maneuvering of various assets. Performance metric goals for spacecraft propulsion under IHPRPT were defined. Many basic concepts already in use on commercial communication satellites were leveraged for the benefit of DoD and the Air Force and will continue to be exploited for IHPRPT.
Projects under IHPRPT at AFRL, NASA, and contracted to industry include alternative propellants for liquid propellant engines and combination thrusters having dual-mode capability.
Energetic Monopropellants. Research and development on several energetic monopropellants is under way in-house at Edwards Air Force Base and at Aerojet Redmond. Two of the propellants under study are hydroxylammonium nitrate and AF-315E. Their main theoretical advantages are higher density and higher Isp (260-270 sec) than state-of-the-art hydrazine. They are claimed to be less toxic, but the relevance of this characteristic for military missions is unclear.
Combination Thrusters, Dual-Mode Capability. The first thruster designed to operate in either a bipropellant or monopropellant mode has been designated the secondary combustion augmented thruster (SCAT). In its monopropellant mode, it decomposes hydrazine in a catalyst-bed chamber. The decomposition products flow out through a second small chamber and exit through a conventional nozzle with an expansion ratio of about 100:1. In this mode the thruster has an Isp of about 230 sec and can provide thrusts from 0.8 to 4.5 lb. In its bipropellant mode, N2O4 is turned on. It cools the second chamber, vaporizes, and then combusts the NO2 vapor with the N2H4 decomposition products in the second chamber to produce an Isp of about 325 sec up to 14 lb thrust. Because the second chamber is regeneratively cooled it can be made of nonrefractory metals such as nickel. This provides essentially unlimited operating life. Dual-mode capability provides opportunities for optimizing in-space operations.
On-Orbit Refueling: Orbital Express. Northrop Grumman Corporation and Boeing are finishing up the DARPA Orbital Express spacecraft contract to demonstrate the practicality of on-orbit refueling of spacecraft hydrazine propulsion systems. DARPA is interested in in-space refueling because some fully functional satellites had to be retired when the original propellant load carried into orbit with the satellite became exhausted. If spacecraft could be designed to be refueled in space, many could continue to operate for much longer times. The next important step would be to extend the technology to the transfer of MON and subsequently to LOx. A refuelable LOx/N2H4 system could be a transformational capability for in-space, high ΔV maneuvering of large platforms, space tugs, and fly-out weapons.
Recommendation 5-2. DoD should fund total architectures and operations studies for various future DoD/Air Force missions to determine the advantages of on-orbit refueling capability. Future funded technology work should complete the validation of full operational design criteria for the transfer of hydrazine. Those basic design criteria should be expected to be applicable to other storable low-vapor-pressure fuels like MMH. A subsequent program should be instituted to extend the technologies to storable oxidizers such as MON and, finally, to LOx. The committee believes a funding level of $10 million per year, in addition to that discussed in Recommendation 5-1, over the next 10 years would permit finalizing an IOC module for N2H4 and pursuing subsequent technology demonstrations with MON and LOx.
Electric propulsion projects already carried out or in progress under IHPRPT include the following:
Orbit insertion: 4.5-kW Hall-effect thrusters, 25-cm xenon ion thruster, 20-kW Hall-effect thrusters, and XOCOT (type of pulsed plasma thruster).
Orbit attitude/position changes: 200-W Hall-effect thrusters and 600-W Hall-effect thrusters.
Propulsion systems used to propel fly-out and maneuvering of items: relatively high-thrust dual-mode thrusters.
Altitude control: using micropulsed plasma and colloid thrusters.
Critical Needs for Meeting In-Space Propulsion Goals
At the present time, there is very little national effort in advanced in-space propulsion. There is almost no work on very high power (greater than 50 kWe) electric thrusters; in particular, there is almost no technology development under way on Lorentz force accelerators (electromagnetics), which offer higher thrust (1-5 lbf) as well as high Isp (>5,000 sec). In addition, there is little investment in the companion areas of very high power sources and highly efficient energy storage systems to enable high-power, high-performance electric propulsion to function unimpeded by system power and energy constraints.
Air Force long-range planning will continue to evolve new needs for both strategic satellites and the responsive introduction and repositioning of tactical military satellites or space vehicles of various types. For repositioning large strategic capital assets, one could utilize an onboard, low-thrust, very high fuel efficiency electric propulsion system such as Hall-effect thrusters that would fire continuously to complete a large station change in weeks or months. Alternatively, one might use a modest-performance (330-360 sec) chemical propulsion thruster at 100-200 lb thrust. Velocity changes of hundreds of feet per second could be achieved in minutes to hours, permitting position changes of thousands of miles per day. A third way to implement large, rapid station changes would be to have a space tug with either high-performance electric propulsion for slow strategic moves or high-thrust, modest-performance chemical propulsion for responsive maneuvers.
Another important technology that would permit rapid multiple maneuvers of critical assets would be an on-orbit refueling system, such as was discussed under chemical propulsion, to resupply the propellants during or after changing station. The on-orbit refueling capability would enable a space asset to stay alive for as long as everything kept working functionally and to make as many rapid station changes as required. The committee recommended that DoD fund a significant program in this area (see Recommendation 5-2).
A near-term need is to characterize spacecraft/plume interactions. There is a study under IHPRPT for in-space validation of modeling and simulation predictions of Hall thruster electromagnetic fields and plume interaction with spacecraft. A Northrop Grumman/Busek 300-W Hall electric thruster, which might provide some data to anchor the models, was planned for flight in 2006.
All electric propulsion thrusters at any power level need PPUs having greatly reduced mass per kilowatt. One promising approach to this is a solar-electric drive that uses a high-voltage solar array to provide power directly to a Hall thruster.
A larger industrial base than presently exists is required for assured production of complete electric propulsion systems (thrusters, feed, and PPUs). For Hall electric thrusters there appear to be two sources: Aerojet Redmond and Northrop Grumman/Busek. For ion thrusters the future is uncertain, because Boeing has sold its Torrance, California, electric propulsion facility to L-3 Communications.
PROPULSION SYSTEMS FOR STRIKE AND TACTICAL MISSILES
This section addresses propulsion technologies applicable to military airborne, extended-range strike and tactical missiles. Such missiles include surface-to-surface; surface-to-air; air-to-air; air-to-ground, extended-range global strike; and air-to-near space. Currently, almost all these missile types use standard Class 1.3 hydroxyl-terminated polybutadiene/ammonium perchlorate (HTPB/AP) propellant solid rocket motors. Very little new work has been conducted over the last 10-15 years to improve solid rocket motor propulsion systems for missiles by the S&T elements of the Services or by the OSD. Most of the IHPRPT funding for solid rocket technology has been for strategic sustainment to maintain some level of capability and industrial base. For one reason or another, none of the few advanced propulsion technologies that have demonstrated significant improvements has been transitioned into an operational system to date.
IHPRPT Goals for Improving Missile Propulsion
Most of the IHPRPT goals for improvements in propulsion systems for tactical missiles are proving to be somewhat unrealistic for the medium term. People talk about higher combustor operating pressures (>2,000 psia) and high-energy propellant formulations, but they cannot demonstrate them with acceptable margins. Solid propulsion technology has run into a ceiling dominated by throat erosion for the best materials that experts have devised. This same limit has prevented using a number of new high-energy propellants. By their very nature, tactical missiles have higher chamber tem-
peratures and higher throat velocities, and in many cases their products of combustion are chemically incompatible with nozzle materials, all of which results in unacceptable throat erosion.
The Army has done considerable work to demonstrate missile trajectory and energy management using storable gelled liquid propellants and pintle-in-the-throat throttleable solids. This work has achieved reasonable success in increased missile range, accuracy, and real-time retargeting. For example, using gelled propellants for energy-managed propulsion systems, the Army was able to flight demonstrate doubling the range of a tube-launched, optically tracked, wire-guided antitank missile from 4 to 8 km and hit the intended target. However, as already stated, these technologies have not yet been transitioned into any operational system.
Current IHPRPT Research
Missile propulsion technology work under IHPRPT is divided into three categories: solid propellant motors, hybrid rocket motors, and gelled propellant motors.
Solid Propellant Motors
Solid propellant technology work in-house at AFRL in solid motor design and hardware demonstrations appears to be minimal. Basic research in a number of very advanced areas is of high quality, however. An important IHPRPT project contracted to both Aerojet and Alliant TechSystems is to develop and validate advanced computational tools. The intent is to replace empirical models with more physics-based models. AFRL is also working on a demonstration solid propellant motor that incorporates a desubmerged lightweight nozzle, reduced part counts and fewer interfaces, a carbon-carbon exit cone, a wet-wound graphite/resin system, the elimination of dome reinforcements, strip-wound Kevlar ethylene propylene diene monomer (EPDM) rubber insulation, a Class 1.3, 90 percent solids HTPB/cyclotrimethylene trinitramine/royal demolition explosive (RDX) propellant, a consumable igniter, and a smaller electromechanical thrust vector assembly (EMTVA).
At the U.S. Army Missile Command, Huntsville, Alabama, solid propellant missile propulsion technology is focused on three high-leverage areas: controllable thrust propulsion (CTP), insensitive munitions (IM),
and new materials. Controllable thrust has the potential for significant system benefits. It can provide extended range and shorter time to target at mid-ranges in a single system. Controllable thrust systems also can reserve propellant energy for end-game performance. Thrust profiling for either ground- or airborne-launched missiles can be provided by using solid propellant motors with a variable area nozzle, hybrid solid fuel with gelled storable oxidizer, or gelled storable liquid propellants.
Hybrid Rocket Motors
Lockheed Martin Michoud Operations has worked on hybrid propulsion technologies since 1989. Because the fuel is inert, launch vehicles or missiles that use these propellant combinations can achieve good performance and gain the benefits of having a nonexplosive propellant combination. Controllable thrust hybrid rocket motors are being investigated by the Army. Two types of hybrid rockets have been considered: (1) a conventional hybrid rocket in which liquid or gelled oxidizer is injected into the port(s) of the solid-fuel grain or the fuel-rich propellant grain for combustion and (2) a gas-generator type of hybrid rocket in which the fuel-rich solid propellant grain burns in its own combustor and the discharged products are further burned with the oxidizer-rich gases in a postcombustor. In some special but rare cases, an inverse hybrid can be considered in which the solid grain is made of oxidizer-rich material and the injected liquid is a fuel-rich material. In the past, hybrids suffered from significant instability problems because of low regression rates. Recently, there have been several significant breakthroughs in hybrid technology at Lockheed Martin Michoud, Stanford University, and Orbitec. These hybrid technology advances with their inherent safety should greatly advance hybrid propulsion applications for space and missile systems.
Gelled Propellant Motors
Gelled propellants can meet field and aircraft operational and handling requirements. They have the potential of being inherently insensitive to IM threats because the fuel and oxidizer are stored in separate tanks. A throttling gel engine using a passive pintle demonstrated a turndown ratio of 12:1 while maintaining greater than 98 percent Isp efficiency. Design criteria for gelled propellant propulsion systems for missiles of almost any size have been validated at Northrop Grumman Corporation. The critical
design element is a central injector with a single sleeve that permits throttling, no dribble face shutoff, and restart.
Recommendation 5-4. DoD should ensure that the development of advanced tactical missiles, responsive global-reach missiles, and ABMs satisfies four key requirements: effective energy/trajectory management, higher-energy-density performance, minimum smoke exhaust, and insensitive propellants. The S&T part of the DoD/Air Force strategic plan for missiles should focus on the technologies and design criteria necessary to meet these goals. The committee’s estimate of annual funding that would be required to make reasonable progress in establishing advanced capabilities in these areas is $20 million to $30 million.
Two Potentially Transformative Concepts
There are a couple of potential opportunities for transforming how certain tactical, responsive global reach, and ABM missions could be achieved. One system concept utilizes self-contained ABVL modules. Another would make use of a multimission modular vehicle (MMMV). Both airborne launch concepts could transport rocket-powered missiles to high-altitude launch points at optimum geographic locations, enabling broad flexibility with respect to launch time, azimuth, orbital inclination, and time to target for missiles used for ABM missions, tactical support, or long-range global strike. Such concepts could provide a rapid response capability beyond what is available today to counter emerging threats. Launching missiles from a flying aircraft platform can dramatically improve both missile performance and time to target. High-altitude air launch allows the rocket to bypass the initial parts of a ground-launch trajectory, where combined negative effects can result in a velocity loss of around 3,500 feet per second.
Recommendation 5.6. The Air Force and DoD should sponsor a detailed system engineering study of using the MMMV air-based launch system for medium-sized missiles in combination with the air-based vertical launch study for various types and sizes of missiles called for in Recommendation 5.5, thereby ensuring that both studies are focused on the Air Force/DoD optimization criterion “mission success.” The studies would identify the propulsion technologies (modifications or new concepts) that should be evolved in order to take full advantage of such air-based launch platforms for operationally responsive missions.
OUTLOOK FOR ALL ROCKET PROPULSION SYSTEMS: ACCESS TO SPACE, IN-SPACE OPERATIONS, AND AIRBORNE MISSILES
As shown in Tables 4-10 to 4-14, funding for technology programs such as IHPRPT and for sustaining improvements in propulsion engineering on various weapon systems now accounts for a much smaller fraction of the overall research and engineering (R&E) funding line. This limits the accomplishments of propulsion improvement efforts and minimizes the opportunity to train the next generation of designers and production specialists. Personnel demographics indicates that many individuals with critical skills in the development and production of large missiles and launch vehicles will retire in the same time frame. One outcome will be to limit the capabilities and flexibility of U.S. space assets that are crucial to support the warfighter and without which the national defense will be compromised. The consequences of this funding situation have been eroding U.S. aerospace capability for many years. Unless there is a serious commitment to reversing this trend, the ability of industry to provide the high-quality engineering and production capability for realizing the Air Force’s medium- and far-term transformational in-space and missile goals will be at risk.
Jet fuel costs have risen by a factor of 2.2 since 2004, and this rising cost of jet fuel is a major expenditure for warfighter support. Estimates are that the DoD fuel bill was $6.8 billion to $9.4 billion higher in 2005 than in 2004 due to fuel price hikes and the additional cost incurred in transporting fuel to the battlefield. Further, DoD is increasingly dependent on foreign sources of refined fuels and relies on supplies from refineries that are vulnerable to terrorist attacks. Finally, DoD needs to use fewer varieties of fuels to simplify logistics, and it needs high-thermal-stability fuels to facilitate the thermal management of aerospace vehicles.
The propulsion industry today, air-breathing and rocket, is taking advantage of materials and processing investments spurred by the ManTech
program of the past. In retrospect, the Services and DoD/DARPA funding spawned the entire aerospace materials supply chain that is in place today. As we look to the future with very limited and restricted ManTech, such investments are being driven by commercial engine needs and DoD is just tagging along. More of the advanced work will migrate offshore. For instance, SiC fiber used in highest temperature ceramic matrix composites (CMCs) is supplied from Japan, and TiAl processing is rapidly advancing in Europe. Since the DoD production base is not large, these new material technologies only become economical if there are commercial applications. This presents an opportunity for realistically planned ManTech programs that will provide baseline manufacturing technology for high-performance defense systems and also leverage requirements of the nondefense aerospace sector (DSB, 2006).
Recommendation 6-2. The Air Force should fund ManTech at a level sufficient to enable future advances in materials for propulsion technology.
Investment Strategy Options
The range of challenges facing the U.S. military over the next 15 years requires that the United States field air-breathing propulsion systems that cover the Mach number range from 0 to Mach 16 and rockets that provide cheap, reliable, and ready access to space. These requirements, which are greater than the United States has faced since World War II, come at a time when existing equipment is demanding an ever-increasing share of the DoD budget for fuel and sustainment and when S&T is receiving less than 1.5 percent of the DoD budget. Clearly, this situation calls for DoD to define and then implement an optimal investment strategy for its limited resources. The committee examined a number of strategies and found that while no single strategy was right for all programs, several did allow DoD to optimize the impact of its investment.
VAATE, like its predecessor IHPTET, utilizes and focuses all government and contractor independent R&D (IR&D) money and resources on the achievement of a common goal. No other DoD technology program has the reach or leverage of the VAATE program, whereby contractor IR&D funds are applied to extend government technology funding. An aggressive VAATE program is needed to meet the warfighter’s capability requirements.
The focus and leverage of the IHPTET program allowed the United States to field the most advanced fighter engines (F135 and F136) in the world. The IHPTET investment strategy had three main characteristics: (1) it gave all government and contractor technology funds and resources a common set of goals, (2) it had a relatively stable funding profile consistent with the time frame for developing and demonstrating new technologies, and (3) it tracked investment versus achievement of the technical goal. VAATE improves on the IHPTET investment model: Not only does it maintain the three IHPTET characteristics, but it also focuses on system solutions (high-impact, integrated solutions) as opposed to specific gas turbine improvements and makes affordability a key metric.
A small share of the savings in sustainment and fuel costs of existing aircraft generated by IHPTET could be allocated to properly fund VAATE, which requires approximately $300 million per year focused on warfighter needs if the United States is to maintain its lead in gas turbine propulsion.
The legacy fleet, which will make up over 80 percent of the 2018 warfighting capability (if the Joint Strike Fighter is included), needs drastic attention. In FY04, the cost to sustain the legacy fleet was $4.2 billion, and assuming a fuel cost of $1 per gallon, fuel amounted to an additional $4.7 billion. In other words, in FY04 the cost of operating the legacy fleet was approximately two-thirds of the total DoD propulsion budget. These costs will continue to increase, because the DoD fleet will grow older and the true cost of fuel will be much, much higher than the assumed $1 per gallon in FY04. Unless strong action is taken, the growing proportion of the DoD propulsion budget allocated for sustainment of the existing fleet and fuel for it will become a death spiral, with the portions of the budget allocated for procurement, development, and technology always decreasing. See Figure 7.1 for additional detail.
The committee found that the breakdown of the DoD budget for fuel, maintenance, and product improvement for different commands and Services is wasting potential opportunities to decrease the cost of sustainment and fuel (see Chapter 3 for examples). To illustrate: An engine program office that has money to improve an engine is disconnected from the budget for maintenance cost of the engine and also from the budget that buys fuel to operate the engine. The committee found many cases where this lack of connecting authority and responsibility was causing poor decisions on trade-offs that were meant to reduce the costs of sustainment and of fuel burned.
Connecting the authority to make small investments to improve legacy propulsion systems with the responsibility for reducing sustainment and fuel costs for the same systems would allow good investment decisions to be made. Today, since no one person or command is responsible for the total cost and investment, no such trades are made, or they are very infrequent. Budgets spent on sustainment and fueling of the legacy fleet leave no room for the acquisition of new aircraft or the development of technology.
Finding 7-1. History has demonstrated that the introduction of new technology into existing weapon systems—i.e., spiral development—can be a very cost-effective way to upgrade warfighting capability.
Recommendation 7-3. The Air Force and DoD should apply spiral development to all weapons systems that are in service longer than it takes to develop a new generation of technology.
History shows that spiral development has been applied to the important Air Force and Navy fighter engines. For example, propulsion systems for the F-16 and F-15 aircraft underwent spiral development programs to improve their thrust and reliability, a very cost-effective way to increase their warfighting capability. The main derivative programs for these engines (e.g., F100-220 and -229 versus the F100-100) bundled technology packages from IHPTET or IR&D programs to markedly enhance performance. Currently, DoD is not leveraging the major F-22 and F-35 propulsion investments by providing spiral development programs (e.g., EMDPs) to meet the requirements for other aircraft. For example, derivates of the F119/F135 or F120/F136 engines are candidates to power future global strike aircraft. Spiral development is the most cost-effective way to maximize the effectiveness of long-lived weapon systems.
Commercial experience has demonstrated that the government is a very high-cost integrator. DoD has made progress over the past several years in adopting commercial best practices. Commercial best practices clearly set requirements and then contract with suppliers to meet those requirements.
Recommendation 7-4. DoD should adopt commercial best practices to reduce costs and exploit the technical expertise of its research laboratories to enhance the integration process in its product centers and depots.
The idea of a 1-year engine demonstrator program is a good one. The current trend is to move from IHPTET, where a major technology demonstration occurs every 1 to 2 years, to VAATE, where a major technology demonstration is planned once every 5 to 7 years. This long time between major technology demonstrations will accelerate the demise of the U.S. technology lead in propulsion systems.
Recommendation 7-5. DoD and major propulsion contractors should define the process changes needed to produce 1- to 2-year technology demonstrations. Decreasing the interval between demonstrations of technology in major propulsion systems will increase the rate of technology development.
Recommendation 7-6. To reduce the cost of fuel burn and of sustaining the portion of the existing fleet that will be in service in 2020, DoD should develop innovative contracting methods to facilitate the incorporation of evolving technologies into existing engines.
Finding 7-5. A focused effort, probably by DDR&E, to catalog and make accessible the findings of past technology programs would be highly useful.
Recommendation 7-10. DDR&E should undertake a focused effort on cataloging and making accessible the findings of past technology programs, perhaps even combining the IHPTET, IHPRPT, and VAATE databases at the lower taxonomy levels to enhance technology cross-fertilization. It should also set up a feedback process and facilitate a cross-cutting flow of S&T during the development, acquisition, and sustainment phases.
AFSPC (Air Force Space Command). 2003. Air Force Space Command Strategic Master Plan FY06 and Beyond. October 1. Available online at http://www.cdi.org/news/space-security/afspc-strategic-master-plan-06-beyond.pdf. Last accessed on October 31, 2006.
Bexfield, James, and Lisa Disbrow. 2005. MORS Workshop on Capabilities-Based Planning: The Road Ahead. Washington, D.C.: Military Operations Research Society. December. Available online at http://www.mors.org/publications/reports/2004-Capabilities_Based_Planning.pdf. Last accessed on August 9, 2006.
DoD (Department of Defense). 2004. Department of Defense Space Science and Technology Strategy. Washington, D.C.: Defense Research & Engineering. July 31.
DSB (Defense Science Board). 2006. The Manufacturing Technology Program: A Key to Affordably Equipping the Future Force. Washington, D.C.: Office of the Under Secretary of Defense for Acquisition, Technology and Logistics. February. Available online at http://www.acq.osd.mil/dsb/reports/2006-02_Mantech_Final.pdf. Last accessed on August 9, 2006.
NRC (National Research Council). 2003. Materials Research to Meet 21st Century Defense Needs. Washington, D.C.: The National Academies Press. Available online at http://newton.nap.edu/catalog/10631.html. Last accessed on August 11, 2006.
NRC. 2004. Evaluation of the National Aerospace Initiative. Washington, D.C.: The National Academies Press. Available online at http://www.nap.edu/catalog/10980.html. Last accessed on March 31, 2006.
NSPD (National Security Presidential Directive). 2005. U.S. Space Transportation Policy. NSPD-40. January.
USAF (U.S. Air Force). 2005. Master Capabilities Library.
Hampsten, Ken, Jim Ceney, and Gus Hernandez. 2005. “ARES subscale demo Phase I,” Presentation to HLV Industry Day on March 7. Space and Missile Center. El Segundo, Calif., LAAFB.
James, Larry D. 2005. “ARES Industry Day AFSPC,” Presentation to HLV Industry Day on March 7. Space and Missile Center. El Segundo, Calif., LAAFB.
Sega, Ron. 2005. “Department of Defense propulsion science and technology,” Presentation to the committee on May 24.
Smith, Scott. 2005. “Air-based vertical launch,” Presentation to committee member Gerard Elverum on December 19.