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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals 5 Space Transportation Technology INTRODUCTION NASA's third pillar for success in aeronautics and space transportation technology addresses access to space and recognizes that low-cost access is the key to exploiting the commercial potential of space, as well as the key to expanding space research and exploration. Two technology goals are listed under Pillar Three that would extend the spacefaring capability of the United States and enable activities in space that are only talked about today. The two goals are as follows: Goal 9: Reduce the payload cost to low-Earth orbit by an order of magnitude, from $10,000 to $1,000 per pound, within 10 years. Goal 10: Reduce the payload cost to low-Earth orbit by an additional order of magnitude, from $1,000's to $100's per pound by 2020. While attempting to identify potential breakthrough technologies that could achieve these NASA goals, the committee noted that both goals focus only on achieving low-Earth orbit (LEO). However, this is only one aspect of the space transportation problem. Most satellites that are launched into Earth orbit, even if it is LEO, require some form of upper stage propulsion or orbital transfer vehicle to boost the satellite into an operational orbit.1 In addition, space vehicles used for scientific exploration must often travel beyond Earth's orbit into deep space. Providing this additional transport will be expensive and will add considerably to the costs of space missions. Thus, the committee suggests that NASA consider modifying the existing goals or adding additional goals to provide "stretch challenges" for: reducing the overall cost of space transportation, including the launch stage and the final propulsive stage used in orbital transfer minimizing the cost of developing far-reaching space transportation technologies that enable new deep-space missions 1 From January 1, 1988, to January 1, 1998, launched operational payloads (not including rocket stages or debris) reached the following operational orbits: low-Earth orbit (less than 2,000 km): 793 satellites; high-Earth orbit (more than 2,000 km): 422 satellites; deep-space probes beyond Earth orbit: 16; military satellites with no unclassified orbital data: 65; total operational spacecraft launched: 1,296. (Devere, 1998)
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals NASA's current space transportation goals reflect the belief that customers will require much less expensive, more reliable, and more flexible launch services than are available today. The major cost drivers for today's expendable or reusable launch systems are listed below: amortization of large development costs complex operations: vehicle assembly, checkout of numerous complex interfaces, and launch command and control maintenance, monitoring, and perpetual improvements in hardware designed for performance, not robustness limited reuse of hardware low launch rates Estimates of the approximate cost per pound to orbit for several U.S. expendable launch vehicles are shown in Table 5-1. Table 5-1 Approximate Cost per Pound for Major U.S. Launch Vehicles Vehicle Cost Per Pound Low Earth Orbit Geosynchronous Earth Orbit Delta II $4,500 $25,000 Atlas IIA $5,800 $29,000 Titan III $5,000 $28,000 Titan IV-SRMU (no upper stage) $4,600 — Centaur (with upper stage) — $26,000 Source: Dawson, 1994. The committee believes that the low-cost attributes of future launch systems will be simplified launch operations, robust design and operating margins, and near complete reuse of hardware. Large design and operating margins will insure long life and minimum checkout and maintenance costs. Complete or near-complete reuse of hardware will keep replacement costs low. Thus, the most viable way to achieve the NASA goals for low-cost access to space is to develop robust, highly reusable launch vehicles (RLVs) with aircraft-like maintenance and frequency of operation. The potential enabling technologies identified by the committee are discussed in the remainder of this chapter.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals POTENTIAL ENABLING TECHNOLOGIES Advanced Air-Breathing Engines A major constraint on improving the overall performance and lowering the cost of today's launch systems is the relatively low specific impulse of conventional rocket propulsion.2 Air-breathing engines with their vastly superior specific impulse at lower flight speeds (M ≤ 12) offer much improved mass ratios and more robustness in vehicle design. Unfortunately, air-breathing engines are usually more complex than rocket engines and have significantly lower thrust-to-weight ratios because of their heavier engines. In addition, the higher drag associated with atmospheric flight reduces the effective specific impulse. Nevertheless, air-breathing engines, in combination with chemical rockets, have the potential to improve payload fraction, be more robust, and would be more reusable than existing expendable launch vehicles, which would result in lower costs per pound of payload to orbit. One possible way to reduce the weight of the air-breathing portion of combined air-breathing/rocket engines is to eliminate heavy turbo-machinery by condensing the incoming airflow and pumping the air in the liquid phase using a much smaller pump. In this scenario, the airflow would be liquefied by using the onboard cryogenic propellants in a heat-exchange process. This class of engine is called a liquid air cycle engine (LACE). In the basic cycle, air is liquefied and pumped to a rocket-thrust chamber. The liquefying agent is liquid cryogenic hydrogen, which is pumped through a heat exchanger as it, in turn, flows to the rocket thrust chambers. Although the basic LACE engine was once limited to low specific impulses (~ 1,000 sec), more sophisticated cycles were identified in the 1960s during the first U.S. Air Force Aerospace Plane Program. Recently, research on LACE engines has been revived in Japan and Russia, where test engines have been successfully demonstrated. Significant advances have been made in heat-exchanger technologies, and problems with icing have been resolved. The Russian Space Agency's program to evaluate reusable space transportation system technologies, which began in 1993, includes substantial work on LACE derivatives and air liquefaction systems. A separate and important propulsion development related to LACE is the invention of the "deeply cooled" engine. This engine cycle is similar to LACE, but the air cooling stops short of actual liquefaction. The deep-cooling cycle has been confirmed as the basis of the British RBSUS engine, which was under development for the horizontal takeoff and landing RLV project. Analytical studies, principally in Russia, have demonstrated promising performance levels for this class of engines. Another "cryogenic" class of engine that should be mentioned is the Japanese Air Turbo-Rocket Expander Cycle (ATREX). In this air turbo-rocket, the fan is driven by a turbine 2 Current levels of specific impulse for rocket engine ranges from approximately 200 to 400 seconds, depending on the type of propellant.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals powered by heated hydrogen. The hydrogen is heated by means of a heat exchanger located in the afterburner. Interestingly, a hydrogen precooler is used in the engine intake. A version of this engine has been extensively ground tested, and tests under simulated flight conditions are planned. Unlike rocket engines, different types of conventional air-breathing engines perform best at different flight speeds. For example, turbine engines perform best from takeoff to speeds of about Mach 3.5. Subsonic combustion ramjets perform best at speeds from about Mach 2.5 to Mach 6.0; and supersonic combustion ramjets from about Mach 6.0 to Mach 12 to 15. In general, rocket propulsion is required for speeds of more than Mach 12 to 15. Consequently, propulsion from takeoff to orbit will require a multimode engine that combines air-breathing and rocket elements. The propulsion design problem can be greatly simplified by using a two-stage vehicle with separate air-breathing and rocket stages. Unfortunately, realistic evaluations of the potential benefits of air-breathing systems for single-stage or two-stage RLVs cannot be made at this time because the basic parameters of combined-cycle engines are not known. Cost issues associated with their potential use in RLVs are also poorly understood. Therefore, a thorough technical assessment of state-of-the art systems must be made as a starting point for evaluating their promise and potential. A system design approach to this assessment should be considered, with cost as the independent variable. This assessment could then be used as a basis for an aggressive R&D program. The technical assessment should encompass the following engine concepts: rocket-based combined-cycle engines, including rocket-scramjet systems and rocket-ramjet systems interturbo-rocket engines, including the U.S. air-core enhanced turbo-ramjet engine and the innovative Japanese ATREX cryogenic fuel engine cycles, including LACE-derived engines and the deeply-cooled engines explored by the United Kingdom and Russia gas turbine engines incorporating advanced materials and better aerodynamics that could improve thrust-to-weight ratio Finding. A system study is required to select the most cost effective combined air-breathing/rocket engine for RLVs. The study must be detailed enough to identify promising technologies and should assess the benefits of engines relative to pure rocket-based propulsion systems incorporating advanced technologies. Pulse Detonation Wave Engine The detonation process has been extensively studied, and over the years, sporadic attempts have been made to apply the detonation process to rocket engines. Although these efforts have met with limited success, the concept is worth another look. The benefits of a detonation wave engine include improvements in thermal and volumetric efficiency. The
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals effective pressure ratio across the detonation wave increases chamber pressures to at least six times those of the unburned fill mixture upstream of the wave front. This means that the pulsed detonation wave engine could provide the equivalent performance of a high chamber pressure conventional rocket engine while operating at one-sixth the pressure. This represents an increase of 10 to 15 percent in potential specific impulse. Because the propellant feed pressure is so low, simpler, lower pressure discharge pumps can be used. Finally, the concept can combine both air-breathing cycle operation, using air as the oxidizer, and rocket cycle operation, using onboard propellant, which makes a detonation wave engine an attractive propulsion system for transatmospheric and reusable vehicles. Several NASA and DOD projects are in progress under the Small Business Independent Research Program that are focused on rocket and air-breathing pulse detonation wave engine concepts to demonstrate feasibility and proof of concept. Scaling limits, process controllability, and fast acting valves for booster-sized engine concepts are critical technologies that will have to be demonstrated. Finding. Pulse detonation wave engines could provide the equivalent performance of high chamber pressure conventional rocket engines while operating at one-sixth the pressure, representing an increase of 10 to 15 percent in potential specific impulse. Critical technologies for pulse detonation wave engines include scaling limits, process controllability, and fast acting valves for booster-sized engines. High Thrust-to-Weight Rocket Engines A 1995 NRC study on NASA's Reusable Launch Vehicle Technology and Test Program found that prime contractors involved in the program believed that a very high (greater than 75) sea level thrust-to-weight ratio (T/W) would be required for RLVs with rocket engine propulsion (NRC, 1995). Currently, the T/W goal for the Lockheed Martin Venture Star RLV is 83. This T/W target represents increases of approximately 30 percent over both the space shuttle main engine (and the Russian Energia RD-0120, both of which are operational, high performance engines. Achieving this very difficult target will require the technical evolution of current materials and component designs that will not be proven until early in the next century. Technology advancements beyond those that have already been identified will be necessary for NASA to realize its low cost per pound to LEO goals. Sea level T/W goals should be higher (≥ 100) for reusable lightweight vehicles with appropriate lifespans and design margins. Advanced materials and fabrication methods will have to be developed to reduce weight, raise allowable operating temperatures and pressures, dampen vibrations, increase strength, and enable revolutionary system designs. Every engine component, including ducts, valves, manifolds, and casings should be considered an opportunity for innovative improvements in technology.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals Finding. The thrust-to-weight ratio necessary to enable rocket propulsion-based RLVs to meet the NASA launch cost goals will require significant reductions in the weight of engine components. Advanced materials and fabrication methods will have to be developed to reduce component weight without compromising performance. Variable Expansion-Ratio Nozzles The optimum thrust coefficient for a rocket nozzle is achieved at an expansion ratio where the exit pressure of the exhausting gases matches the ambient pressure. Launch vehicle booster rockets used today all have fixed-area ratio nozzles. A booster rocket flying through the atmosphere can only match the pressures at one point in altitude. At all other altitudes, the nozzle is either under-expanded, or over-expanded, which causes a slight reduction in the exhaust velocity and, thus, a loss of energy. Nozzles for booster rockets are designed for an area ratio that minimizes the loss of specific impulse over the entire flight path. Increasing the specific impulse is important for all classes of launch vehicles. However, because of the premium on performance, advantages associated with variable expansion-ratio nozzles will especially benefit RLVs, especially single-stage-to-orbit RLVs. Although the exact thrust specific impulse benefit will depend on the engine design and flight profile of a given launch vehicle, variable expansion-ratio nozzles enable efficient altitude compensation that increases the average nozzle thrust coefficient over that of a conventional fixed area-ratio nozzle. The concept has existed for more than 30 years, and a variety of shapes and configurations have been studied in wind tunnels and experimental rocket engine firings have been tested at small and moderate scales. Plug nozzles and expansion-deflection nozzles have been studied the longest. Aspirating slot nozzles have recently been investigated as well. Lockheed Martin is incorporating a linear aerospike (plug-like) nozzle version into the X-33, but the annular aerospike should be investigated for other RLV concepts. A variety of other concepts have also been investigated, such as dual-throat, dual-bell, extendible cones. Although they do not provide continuous altitude compensation, they do incorporate step changes in area ratio. These concepts are not optimum, but they do provide better trajectory averaged performance than fixed area-ratio nozzles. The dual-bell cone has an added advantage in that it induces separation in the near sea-level mode, which increases sea-level thrust, an attractive feature for some RLV designs. All of these variable expansion-ratio nozzle concepts should increase overall engine T/W and should affect vehicle design in a way that reduces overall RLV structural weight requirements. Finding. Variable expansion-ratio nozzle configurations provide altitude compensation to improve trajectory averaged performance. To be most beneficial to RLVs, these nozzle
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals configurations should be lightweight, should contribute to increases in overall engine T/W, and should reduce overall structural weight requirements. Advanced Propellants and Storage Methods The rocket propellants in use today were developed more than 30 years ago, and the potential energy of these propellants is close to the maximum. Nevertheless, the launch community, both government and commercial, will continue to rely on chemical rocket propulsion for the foreseeable future. Therefore, technology breakthroughs in propellant performance, density, and affordability will be crucial to satisfying NASA's space transportation goals. Current research on rocket propellant capability is being led by the Air Force, which has maintained a small research program in this technology area for the past 10 years. However, this program is not sufficient to bring new concepts and technologies to fruition and would benefit from the aggressive participation of NASA. The technologies that should be investigated in a more robust program are discussed below. Recombination of Highly Energetic Atomic Ingredients Several schemes for achieving significant increases in specific impulse performance and density are possible based on the energy and low molecular weight of hydrogen. Proposed concepts for study include cryogenic solid hydrogen, metallic hydrogen, the carbon and carbon-boron absorptivity of hydrogen, and cryogenic solid oxygen. These propellants could provide specific impulse increases as high as 200 seconds over today's state of the art, and could be the basis for achieving launch costs of less than $100 per pound of payload to LEO.3 Improvements in performance with solid hydrogen are achieved by exploiting the recombination energy of energetic atomic ingredients, such as boron, carbon, hydrogen, and lithium. Approaches being pursued involve packaging these energetic and reactive ingredients in cryogenic solid matrices in order to separate them physically from one another by the hydrogen host molecules. A similar approach uses a mixture of ozone in solid oxygen with a potential increase in specific impulse of up to 50 seconds. Steady progress has been made in research on cryogenic solid propellants. Laboratory experiments have demonstrated the feasibility of ingredient storage, albeit at low concentrations, and small thruster experiments have demonstrated the stable combustion of pure cryo-solid propellants in a hybrid configuration. Much more research will be necessary before this can be called a breakthrough technology and before its readiness for transition into real launch systems can be demonstrated. Research areas are listed below: 3 Current levels of specific impulse for rocket engines range from approximately 200 to 400 seconds, depending on the type of propellant.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals computational studies of the dynamics, thermodynamics, and spectroscopy of energetic additives to cryogenic solids spectroscopic characterization of highly energetic species trapped in energetic matrices at concentrations of at least 1 percent production methods for energetic species scale-up production of cryogenic solid propellants with energetic species concentrations of at least 1 percent methods of transporting and combusting doped cryogenic solids Hydrogen Storage at High Effective Densities In many ways, hydrogen is the ideal chemical rocket propellant. Its one great disadvantage is its extremely low storage density as a cryogenic liquid. If a way can be found to store hydrogen at ambient temperature, in an absorbed or adsorbed state and at an effective density much higher than liquid hydrogen (.07 gm/cc), this could be a real breakthrough for the transportation industry as a whole (automotive, aircraft, and space). Recently, a number of researchers have reported the development of carbon nanotubes that are capable of absorbing/adsorbing large quantities of gas, such as hydrogen (Dillon et al., 1997). The effective density of the adsorbed material may be substantially increased as a consequence of the attractive potential of the pore walls, where pores are of molecular dimensions. These results suggest that nanofibrous carbon material may have the capability to store hydrogen at very high effective densities, which would mean that much smaller and lighter tanks and associated structures could be used in a hydrogen-fueled launch vehicle. Although the properties of these carbon materials are still being explored in the laboratory, and there will probably be some challenges with respect to the large-scale production of carbon nanotubes, this technology could contribute to the development of a truly low-cost single-stage-to-orbit vehicle. Metallic Hydrogen Metallic hydrogen is the optimum form of solid hydrogen and consists of all hydrogen atoms in a disassociated metallic state. There have only been glimmers of feasibility in the synthesis of metallic hydrogen, but because of its high payoff (up to 1,200 sec specific impulse) some investment by NASA in research on metallic hydrogen would be warranted. Finding. Notable improvements in chemical propellants, which could be important to the achievement of NASA's space transportation goals, are possible. Potential advances include the recombination of highly energetic atomic ingredients, hydrogen storage at high effective densities, and the development of metallic hydrogen. However, the potential of these advances may not be realized unless NASA increases its research support.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals Integrated Aerothermal Structures In order to reduce launch costs, tremendous advances will have to be made in most, if not all, parts of the space transportation system. The committee believes that an area of equal importance to advanced propulsion technologies and advanced propellant technologies is the airframe, or integrated aerothermal structure, of the reusable launch system. This highly integrated structure must efficiently incorporate the entry thermal protection system, as well as the propulsion system and propellants. Integration will be particularly complex if some form of air-breathing propulsion system must also be integrated into the airframe. Systems studies will be necessary to select the most cost-effective integration strategies for launch vehicles. Improvements in the following enabling technologies will be necessary for the production of durable, long-life, integrated thermostructures: integrated cryotank/insulation/thermal protection system designs attachments, joints, and seals long-life, durable thermal protection systems and cryotank material development vehicle-integrated conformal/integral designs purge, vent, and cooling provisions composite cold structures ceramic hot structures Each of these technologies will have to be life-cycle tested in representative environments for thousands of cycles to assess their robustness. Finding. For RLVs designed to achieve NASA's launch cost goals, lightweight, integrated aerothermal structures will be critical. System studies should be performed to select the most cost-effective integrated thermostructure. Technology development will also be required for a number of critical subsystems. Novel Launch System Concepts Achieving the NASA cost goals may require revolutionary concepts and somewhat unconventional approaches to launch systems. Many concepts in this category, such as electric and solar propulsion, solar sails, and most forms of nuclear propulsion, are most appropriate to orbital transfer vehicles or station keeping. Nuclear propulsion could be a high thrust device suitable for boosters, but the public is not likely to accept this technology for use in this application. Antimatter is a speculative, yet feasible, approach that has great potential if it can be stored in quantity and the energy release mechanism can be harnessed. Laser propulsion is another promising idea that is already under investigation.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals Ground-based techniques that assist the launch vehicle, including rocket sleds, ''mag lev" rails, and pulse detonation tubes, are also being investigated. However, the committee believes that these concepts, although they may be technically feasible, only address "niche markets" for access to space. The very high acceleration loads on payloads launched by these techniques would probably limit their utility to a small, but perhaps very important class, of payload, including propellants and rugged structures, which will be important when an in-orbit infrastructure for assembly and refueling is developed. However, novel launch vehicle configurations and automated launch operations based on systems approaches aimed at reducing costs and increasing reusability appear to have wider applicability and may approach NASA's first launch cost goal. Novel Launch Vehicle Configurations Compared to the accepted design configurations for expendable launch vehicles, all RLVs in use or being considered are novel or unconventional. This began with the Space Shuttle, and continued with the Boeing (formerly McDonnell Douglas) DC-X/DC-XA, with its vertical takeoff, vertical landing demonstrations. The committee was briefed on many of the RLV concepts currently under development, including the NASA-funded X-33 and X-34 projects, the Air Force-funded Space Operations Vehicle and Space Maneuvering Vehicle demonstrators, and two of the many privately funded approaches to reusability4. The committee did not analyze any of these RLV concepts in any detail. However, the committee believes that NASA's first access to space goal of $1,000/pound to LEO could potentially be achieved by 2010 using systems engineering approaches that combine demonstrated technologies and infrastructure into a launch capability optimized for minimum total cost and reusability. Automated Launch Operations Reducing the cost of labor-intensive launch operations through automation is another important component of reducing launch costs that should be pursued in parallel with novel launch vehicle concepts. Increased automation should be based on optimization of human/computer interactions during the design stage of launch vehicles and their related launch infrastructures, and from enhanced human/computer integration in launch processing and operational command and control. Finding. Leveraging novel reusable launch vehicle concepts and automated launch operations based on demonstrated technologies and systems approaches aimed at reducing costs and increasing reusability may approach NASA's 10 year launch cost goal. 4 Space Access LLC and Kelley Aerospace Corporation presented their concepts to the committee. Other reusable launch vehicle concepts are currently being designed or developed by commercial launch vehicle firms such as Pioneer Rocketplane, Inc., HMX, Incorporated, and Kistler Aerospace Corporation.
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Maintaining U.S. Leadership in Aeronautics: Breakthrough Technologies to Meet Future Air and Space Transportation Needs and Goals REFERENCES Dawson, T. 1994. Perspectives on U.S. Space Launch Systems—A Staff Background Paper. Subcommittee on Space, Committee on Science, Space and Technology, U.S. House of Representatives, Washington, D.C. Devere, T. 1998. E-mail to the Committee to Identify Potential Breakthrough Technologies and Assess Long-term R&D Goals in Aeronautics and Space Transportation Technology from Taft Devere, U.S. Space Command, June 12, 1998. Dillon, A.C., K.M. Jones, T.A. Bekkedahl, C.H. Klang, D.S. Bethune, and M.J. Heben. 1997. Storage of hydrogen in single-walled carbon nanotubes. Nature 386 (Mar 27): 377–379. NRC (National Research Council). 1995. Reusable Launch Vehicle Technology Development and Test Program. Aeronautics and Space Engineering Board, Committee on Reusable Launch Vehicle Technology and Test Program. Washington, D.C.: National Academy Press.
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