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From Earth to Orbit: An Assessment of Transportation Options 4 Launch Vehicle Options The Committee examined existing, planned, and potential launch capabilities, including expendable and manned U.S. and international launch vehicles as well a large number of conceptual designs. Only the vehicles that the Committee judged most important are discussed in detail. CURRENT U.S. EXPENDABLE LAUNCH SYSTEMS Current U.S. expendable launch vehicles (ELVs) can be categorized as small-, medium-, or heavy lift vehicles, as shown in Table 2. Although this assortment of expendable vehicles can satisfy most near-term U.S. national and commercial needs, with small modification as necessary, most of the small and all of the medium and heavy launch vehicle systems date back to the 1950s and early 1960s. Their design philosophy dates from an era of maximizing performance capability at the expense of operability. As a consequence, the current vehicles lack flexibility, operability, modularity, and robustness. Further, they do not incorporate payload encapsulation and are not cost-effective. For this reason, the current U.S. space launch program is constrained and limited in today's highly competitive space market. Evidence of the declining U.S. capabilities in commercial space launches is readily apparent by the trend of commercial satellites toward launches on foreign boosters, brought about by increased capability, better scheduling, and lower cost. PROPOSED U.S. LAUNCH SYSTEMS In accordance with National Space Policy, the Committee believes that there is both an opportunity and a necessity for the United States to embark on the creation of a new family of
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From Earth to Orbit: An Assessment of Transportation Options TABLE 2 Current and Proposed U.S. Launch Vehicles Performance Range Launch Vehicle Payload Lift Capability to 100 n. mi. at 28.5° (lb)1 Gross Lift-Off (lb) First Launch Year (actual or planned) Small (<4,000 lb to LEO) Scout I 560 at 38° 48,000 1979 Scout II 1,000 at 38° 110,000 Exact date uncertain Pegasus 1,000 42,000 1990 Taurus 3,200 180,000 1992 Medium (4,000–30,000 lb) Delta II (7920) 11,100 506,000 1990 Atlas II 14,100 413,500 1991 Atlas IIA 14,900 414,000 1991 Atlas II AS 18,500 516,000 1993 Titan II SLV (no strap-ons) 4,200 at 90° 340,000 1988 Titan II SLV (up to 10 strap-ons) 8,200 at 90° Not available Exact date uncertain Heavy (30,000–60,000 lb) Titan III 32,000 1,500,000 1989 (commercial) Titan IV (SRM) 39,000 1,900,000 1989 Titan IV (SRMU) 48,000 Not available 1993 Shuttle 51,800 4,500,000 1981 SOURCES: Isakowitz, Steven J. 1991. International Reference Guide to Space Launch Systems. American Institute of Aeronautics and Astronautics (AIAA); and manufacturers data sheets. 1 In general, the inclination represents the latitude of the launch facilities with the exception of launches out of Vandenberg AFB that are intended for polar orbit. However, if a higher orbital inclination is required, then the payload lift capability is decreased since more propellant is necessary to achieve the desired orbit due to the loss of the velocity component from the Earth's rotation.
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From Earth to Orbit: An Assessment of Transportation Options launch vehicles together with a fully integrated complex of associated ground facilities. Given the advances in engineering science and technology (especially in avionics) during the last quarter-century, systems having significantly enhanced economy, reliability, efficiency of operation, and performance can be designed and built. Development of the new launch vehicles could help revitalize the private sector and allow it to meet the severe challenge of foreign competition for commercial space launches. Equally important, this program could stimulate the basic and applied space technology research that is crucial for the continued vitality of a U.S. role in space. National Launch System (NLS) The National Aeronautics and Space Administration (NASA) and the Department of Defense (DoD) have begun work on a new family of launch vehicles, the National Launch System, which is sometimes referred to as the New Launch System. The proposed National Launch System was conceived to implement national space policy,1 which is discussed in Chapter 1. The NLS concept consists of a family of vehicles with various payload capabilities. NASA and DoD plan to construct the supporting infrastructure and the vehicles in a coordinated fashion, while incorporating a design philosophy that emphasizes reliability and operational efficiency rather than maximum performance. Common components and subsystems will reduce development and manufacturing costs, and streamlined operating procedures will reduce the launch turnaround time. The Committee supports the new system and encourages this approach while discouraging the nation from continuing the old way of doing business. The NLS Joint Program estimates development costs of $11.5 billion for a first launch in 2002. This estimate includes system design and integration, the Space Transportation Main Engine, development of three launch vehicles, a cargo transfer vehicle, an upper stage, and manufacturing and launch facilities. The current NLS concept consists of three vehicles: (1) the NLS-1, a 135,000-pound payload class vehicle that will utilize a new core design, four STMEs, and strap-on boosters; (2) the NLS-2, a 50,000-pound payload class vehicle that will use six STMEs, but no strap-on boosters; and (3) the NLS-3, a 20,000-pound payload class vehicle that uses the STME, no strap-on boosters, and other equipment such as the guidance and control systems common to the NLS-1 and the NLS-2. While the specific vehicle configurations are still evolving, NASA and DoD currently plan to develop the core vehicle and launch the 135,000-pound payload class NLS-1 in 2002, to launch the 50,000 pound payload class NLS-2 vehicle next, and to bring the 1 National Space Council. July 10, 1991. National Space Policy Directive 4.
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From Earth to Orbit: An Assessment of Transportation Options 20,000-pound payload class NLS-3 vehicle to initial operation within two years after NLS-2. Redesigned Solid Rocket Motors (RSRMs) or the Advanced Solid Rocket Motors (ASRMs) will be used to augment thrust on the 135,000-pound payload class NLS-1 core vehicle. It is anticipated that the NLS-1 could be used to deliver payload to Space Station Freedom (SSF). However, firm requirements for this largest vehicle were not apparent to the Committee. In addition, it is the most complex and expensive member of the NLS family. The Committee believes that the 20,000-pound payload class vehicle (NLS-3) should be the first of the proposed NLS family to be designed and built in coordination with new launch facilities. Plans for NLS-2 and NLS-1 should be pursued as funding becomes available. Lessons learned from the NLS-3 system design may influence the final design approach taken on the more ambitious, larger-scale NLS-2 and NLS-1. In addition, the NLS-3 is the least complex and least expensive member of the NLS family, and the one most likely to have potential commercial, as well as national security applications. An analysis of mission requirements presented by NASA, DoD, and the commercial sector (Table 1) shows that there appear to be requirements for 20–30 launches per year, with the greatest potential growth of unmanned launch vehicle traffic suited for the 20,000-pound class vehicle. Since the Air Force has recently committed to using the Titan IV (40,000-pound payload category) by procurement of 41 new vehicles with a purchase of 10 additional vehicles (with an option for another 22 vehicles planned), there is no immediate need for a new vehicle in the 50,000-pound payload class. Thus, the Committee believes that building the NLS-3 vehicle first is of highest priority and would better suit national needs. The 20,000-pound payload class vehicle (NLS-3) would utilize many of the new technologies now contemplated for introduction to the NLS family. Starting the smaller NLS vehicle first will allow more time to refine the requirements for the larger NLS vehicles. Figure 1 shows a conceptual National Launch System schedule as compiled by the Committee. The schedule shows the priorities of the Committee and is based on plans and projections from several government agencies that were presented to the Committee during the study. The Committee explored possible sequencing of events to avoid unacceptable major funding aggregation in any one year. In examining possible timing for the NLS vehicles, a question arises concerning the feasibility of designing the first stage to be partially or wholly recoverable. The Committee did not have the time or means to evaluate the potential for this design direction. There will be time, however, in the NLS program to permit concepts of this nature to be explored prior to committing the configurations of NLS-2 or NLS-1 to detailed design and construction. The Committee believes that there will be future requirements for heavy lift capability to serve the Space Exploration Initiative, but that it is too early to determine the specific performance criteria that will dictate heavy lift design approaches. Early reconnaissance science programs should be planned with the currently available ELVs augmented by NLS-1 to NLS-3, as they become available.
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From Earth to Orbit: An Assessment of Transportation Options Figure 1 Conceptual priority schedule for the National Launch System (NLS). Although the concept of a family of vehicles with common cores and engines is frequently mentioned, the Committee believes that actual construction and operation are more difficult. The various vehicles should be designed for optimal operability and reliability, and the family relationship could and should be maintained more at the subsystem and component levels. The Committee recognizes that such a course of action would represent a departure from the postulated family resemblance of the NLS series and believes that the reality of producing vehicles could be considerably different from the design concepts presented.
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From Earth to Orbit: An Assessment of Transportation Options Taurus Taurus is a new expendable launch vehicle under development and with a firm order for 1992 delivery to the Defense Advanced Research Projects Agency (DARPA). It is a derivative of the novel Orbital Sciences Corporation air-launched space vehicle called Pegasus. It incorporates the three Pegasus stages slightly modified for a heavier payload and ground launch. The vehicle is boosted from the Earth by a first stage Peacekeeper booster. The performance goals of this system are shown in Table 2. Medium Launch Vehicle III (MLV III) The Committee is aware of a current Air Force competition, the Medium Launch Vehicle III (MLV III), which is aimed primarily at the 12,000-to 15,000-pound payload class for launch of the second block of the Global Positioning Satellite System (GPS). This competition has limited application, and the Committee did not review the specific vehicle proposals, which are proprietary at this stage. CURRENT U.S. MANNED SYSTEMS The Space Shuttle The Space Shuttle is the only U.S. system that provides human access to orbit. It currently consists of the Orbiter, external tank, and two RSRMs. The Space Shuttle, in spite of its need for continued upgrading, remains a unique capability for round-trip transportation to Earth orbit and is likely to remain so well into the first decade of the next century. Therefore, the viability of our manned space program depends critically on the maintenance, refurbishment, and upgrading of the Shuttle system over the coming years. It is important to recognize that obsolescence or decay of any element of the Shuttle system will lead to safety problems, schedule slips, and unexpected cost increases. As emphasized elsewhere in this report, the launch infrastructure exerts a controlling influence on overall launch costs, and careful consideration should be given to systematic infrastructure improvements. The Orbiters are complex and sophisticated vehicles and are the heart of the Shuttle system, and as such, critical to human access to space. At present, there are no plans for increasing the size of the four-Orbiter fleet, and the fleet is fully scheduled for many years, so all Orbiters must be maintained in as effective an operating condition as possible. In addition to maintenance and refurbishment, several upgrades of the Orbiters have been proposed, and
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From Earth to Orbit: An Assessment of Transportation Options some have been initiated. Among the most critical of these are improved turbopumps for the Space Shuttle Main Engine (SSME). The Committee endorses these changes and recommends that they be incorporated into the entire fleet of Orbiters, as practical. As other opportunities for improvements develop, they too should be implemented if they promise significant benefits in operability or reliability. Although the Committee understands that the changes will be expensive, it believes that the cost will be insignificant compared to that of another Shuttle loss. Investment in improvements for the Space Shuttle Orbiter and its subsystems should be continued. New, enabling technology is needed for Orbiter replacement or a new personnel carrier. The oldest Orbiter will be over 20 years old in the year 2000 and long lead times are necessary for a new, human-rated space vehicle. Therefore, research and technology development with the goal of developing a new personnel carrier should be continued. PROPOSED U.S. MANNED SYSTEMS Currently, the United States has no approved programs for producing new operational manned launch capabilities. However, there are two programs in advanced development to explore the technical feasibility of reusable Single-stage-to-orbit (SSTO) vehicles. Each program has the potential to include manned operation. They are the Single-Stage Rocket Technology Program (Delta Clipper, DC-Y) supported by the Strategic Defense Initiative Office (SDIO) and the National Aero-Space Plane Program (NASP), jointly supported by DoD and NASA. Studies are also underway to explore the vehicle options for assured crew return from Space Station Freedom. Although assessment of the specific crew return vehicle designs is not within the scope of this study, it is of concern to the Committee because of the potential impact on the Earth-to-orbit requirements necessary to place the vehicle at Space Station Freedom. The Single-Stage Rocket Technology Program Delta Clipper (DC-Y) The McDonnell Douglas Delta Clipper (DC-Y) was recently selected in the SDIO Single-Stage Rocket Technology Program (SSRT) as the most promising concept of a reusable single-stage-to-orbit vehicle. The proposed vehicle is envisioned to provide 20,000 pounds of payload into a low-Earth orbit with low recurring costs, operational flexibility, and a rapid turnaround time. Unfortunately, all of these desirable attributes require changes that increase inert and propellant weights—weights that, in single-stage vehicles, directly (i.e., pound for pound) reduce the payload. Therefore, to be useful, a reusable, single-stage vehicle must represent a compromise between the desirable properties and the size of the payload. In the Committee's
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From Earth to Orbit: An Assessment of Transportation Options opinion, the DC-Y represents a promising and logical approach to the realization of such an objective. The design makes maximum use of existing materials. Only when absolutely necessary, will they be replaced with high-performance, light-weight composites, and advanced metallic or efficient metallic/nonmetallic insulation systems. Some of these materials have been, or are being, developed by the NASP program and promise higher strength and longer life at elevated operating temperatures. However, it should be noted that only limited data are available on the materials properties and that the structures and materials represent a crucial area of uncertainty that may well determine whether a single-stage-to-orbit vehicle with sufficiently low empty weight could be built in the next few years. Thus, the Committee recommends that, as a first priority, the SDIO concentrate on developing information on properties, joining, and fabrication of new materials to allow a confident (e.g., using aircraft safety factors) design of the DC-Y. As originally conceived, the DC-Y was to use a novel modular plug engine2 that appeared especially well suited to a single-stage-to-orbit vehicle. Unfortunately, technical immaturity, as well as the high risk associated with the development of such an unconventional device, prompted a change to a still-undeveloped but more conventional modular bell engine.3 Although development of the bell engine is likely to take less time and cost much less than the development of the modular plug engine, it is not as neatly congruent with the DC-Y and may create some drag and heat transfer problems. The Committee also notes that instead of a new modular bell engine, it may be possible to use other engines. Consequently, the Committee recommends that the SDIO consider reducing the projected engine development costs by seriously examining the possible use of other engines. In 1993, the SDIO plans to conduct suborbital test flights of a one-third scale model (DC-X) propelled by modified RL1O engines. The purpose of these suborbital flights is to demonstrate safe return to the launch site and to provide information on the aerodynamic interactions between the engines and the vehicle. The model may also indicate some of the control problems that are likely to be caused by crosswinds during transition from lifting reentry to, and during, thrust braking. In addition, data on time involved in turnaround and maintenance will also be sought. However, because of scaling and lower gross weight, these flights are not likely to simulate the conditions that the full-size DC-Y will experience upon reentry. Thus, because the reentry conditions will not be simulated properly, these tests cannot be expected to yield a good indication of the adequacy of the structure and its heat protection system. In addition, since the RL1O engines are different from the modular bell engine that is still to be developed, even the information on operation and maintenance problems may be distorted. It 2 This engine is described more fully in Chapter 6. 3 This engine is described more fully in Chapter 6.
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From Earth to Orbit: An Assessment of Transportation Options is, thus, doubtful that the suborbital flight program will appreciably reduce the risks and shorten the time of the DC-Y development. Therefore, the Committee believes that the SDIO should reconsider the value of the DC-X flight tests, relative to the more critical need for demonstration of the adequacy of the proposed low-weight structures and heat protection systems. The Committee believes that the use of a lifting body for a reusable single-stage-to-orbit vehicle holds promise and may eventually lead to reduced launch costs, short turnaround time, and operational flexibility. However, the DC-Y has not yet reached the stage at which it can be considered for Earth-to-orbit transportation. National Aero-Space Plane The National Aero-Space Plane (NASP) is a research program that focuses on the technologies needed for a manned, experimental vehicle designated the X-30. The NASP is envisioned to be fully reusable, designed for aircraft-like takeoff and landing, and capable of reaching orbital speeds. Its propulsive systems are to be largely air-breathing, consisting of combined ramjet-scramjet engines fueled by slush hydrogen (a denser form of liquid hydrogen). Should the development proceed on schedule, the decision whether to start building the X-30 experimental vehicle will be made in 1993. The purpose of the program was defined in a July 1989 National Space Council memorandum4: (1) to explore the limits of air-breathing propulsion; (2) to obtain aerodynamic and other data within the hypersonic flight regime; (3) to develop technologies that would enable new hypersonic vehicles; and (4) to demonstrate Earth-to-orbit ascent and rapid turnaround. To meet its objectives, the NASP program is following a flexible philosophy of design and manufacturing that embodies current engineering materials and their applications while permitting, when necessary, the incremental introduction of new materials and techniques. In particular, the program is proceeding with the development of composites, ceramics, and intermetallic matrices reinforced by ceramic fibers, which should be useful for elevated-temperature applications and may reduce weight. However, the new materials require rigorous testing to determine their physical properties before they can be used in the analysis, design, and fabrication of the X-30 vehicle. The NASP program has also been developing and making extensive use of new computational techniques as well as experiments to assist in the design analyses. The use of computers as a tool depends on the ability to confirm the validity of the models, which can be done only by extensive experimental testing prior to flight. However, complete engine testing 4 National Space Council. July 20, 1989. Memorandum.
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From Earth to Orbit: An Assessment of Transportation Options is impossible, because the facilities needed for such a task would require impractically high amounts of energy. Consequently, the X-30 will have to begin its flight testing with the scramjet engine not fully proven. This puts very special demands on the X-30 because, during testing of the air-breathing engines, the vehicle will need an additional propulsive system (probably a rocket) to place the engines in proper test conditions and also serve as a backup should the air-breathing engines fail. The Committee recognizes that the scramjet engines cannot be fully developed on the ground and must be tested in flight. It endorses such flight research as soon as the basic technology development is at a stage to make it most worthwhile. It should be apparent from the above discussion that the X-30 will not be easily convertible into an operational NASP vehicle capable of providing low-cost aircraft-like transportation from Earth to orbit. Even if the X-30 were to meet its current schedule and be completely successful, a new, separate development program would be needed for an X-30-derived NASP vehicle that could become a part of our Earth-to-orbit transportation system. In view of the very high risks inherent in this concept and the long time that is likely to be required to overcome them, the Committee decided to exclude the NASP from consideration as a viable option for currently foreseeable Earth-to-orbit transportation. Nevertheless, the Committee considers the NASP a stimulating and productive research and development program. The development of materials technology and of air-breathing hypersonic propulsion deserves continuing and vigorous support. In addition, applications of the NASP-derived technology should be considered in cruise vehicles, as well as SSTO vehicles. Assured Crew Return Capability Vehicle options for crew return capabilities during the permanently manned phase of the Space Station are currently being investigated. During extended stays on Space Station Freedom, it will be necessary to provide assured crew return capability in case of medical emergencies or other difficulties that require evacuation of the Station. As stated earlier, although the assessment of the crew return vehicle is not within the scope of the current study, it is of concern to the Committee because of the potential impact on the Earth-to-orbit transportation requirements to place the return vehicle at Space Station Freedom. Currently, studies are underway to examine various crew return options. Three types of vehicles are being considered. One is a reentry-type vehicle, estimated to weigh 12,500 pounds, which can be launched on the Shuttle or by an expendable launch vehicle. Preliminary studies of this concept are underway by two teams, one led by Rockwell, the other by Lockheed. These studies should be completed in one year, at which time a selection will be made. The program is expected to cost between $1 billion and $3 billion, and should be completed by the year 2000.
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From Earth to Orbit: An Assessment of Transportation Options A second concept involves the use of the Russian Soyuz vehicle. Studies are being made by NASA to investigate the possibility and feasibility of this option. A third, more ambitious concept, is a mini-shuttle or personnel launch system with two-way capability, configured to ensure a runway landing at a selected site. This is now only in the conceptual stages as an in-house NASA study. In the Committee's view, the requirement for assured crew return from the Space Station poses no launch vehicle requirements that are outside the capabilities of systems planned for other purposes. EXISTING INTERNATIONAL LAUNCH SYSTEMS International launch systems are shown in Table 3. Most of the international vehicles possess payload capabilities that are in the small to medium payload class range (up to 40,000 pounds). Until the 1980s, the United States had negligible international competition for commercial space transportation and considered its commercial capabilities a spin-off of national space requirements. In recent years, foreign space activities have increased to the point where the majority of worldwide commercial traffic has gone overseas. Although the largest segment of commercial traffic has currently gone to the Ariane, a number of additional foreign competitors are now or soon will be entering in this market. From Table 3, which summarizes some of the most important competition, it is apparent that the primary contender for commercial traffic (in the medium 8,000-to 25,000-pound payload range) is the Ariane-4. Japan and China promise to be stronger contenders in the near future. For the heavy lift market (40,000 pounds and up), Europe and even the newly formed Commonwealth of Independent States are primary contenders. If lunar and Mars exploration missions are planned, then the former Soviet Union's Energia may be capable of supporting such missions. Table 3 provides an indication of how the international competition for commercial space traffic in the medium and possibly heavier payload ranges is growing. If the United States is to remain a player in the commercial arena, it is clear that it must enhance its space transportation capabilities. Energia The largest Soviet-developed launch vehicle, Energia, was first launched in 1987. It has a core structure with liquid-oxygen/liquid-hydrogen engines and four liquid-oxygen/hydrocarbon strap-ons, resulting in 7.8 million pounds of takeoff thrust, capable of placing 194,000 pounds of cargo payload in low-Earth orbit, or 66,000 pounds when the reusable Buran (similar to the
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From Earth to Orbit: An Assessment of Transportation Options TABLE 3 International Launch Systems Origin Launch Vehicle Payload Lift Capability Gross liftoff (lb) First Launch Year (actual or planned) (lb) n. mi. at inclination Europe Ariane-4 (40) 10,800 100 at 5.2° 529,000 1988 Ariane-4 (42P) 13,400 100 at 5.2° 747,000 1988 Ariane-4 (44L) 21,100 100 at 5.2° 1,040,000 1989 Ariane-5 39,600 300 at 5.2° 1,570,000 1995 Ariane-5/Hermes 48,500 50 × 250 at 5.2° Not available 2002 China Long March (CZ-2C) 7,040 108 at 28.2° 421,000 1975 Long March (CZ-2E) 20,300 108 at 28.2° 1,023,000 1990 India ASLV 330 216 at 43° 86,000 1987 PSLV 6,600 216 at 43° 606,000 1992 Israel Shavit 350 100 at 43° Not available 1988 Japan H-1 7,000 100 at 30.2° 308,000 1986 H-2 23,000 100 at 30.2° 582,000 1993 USSR Zenit-2 30,300 108 at 51.6° 1,012,000 1985 Proton 44,100 100 at 51.6° 1,550,000 1967 Energia 194,000 108 at 51.6° 5,300,000 1987 SOURCES: Isakowitz, Steven J. 1991. International Reference Guide to Space Launch Systems. AIAA; and manufacturers data sheets.
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From Earth to Orbit: An Assessment of Transportation Options U.S. Orbiter) is carried. The strap-on boosters are powered by the RD-170 engines discussed in Chapter 5. Zenit The Zenit intermediate-size Soviet launch vehicle was first launched in 1985. It can be used as a two-or three-stage vehicle and is expected to supplant earlier launch vehicles such as Soyuz for both unmanned and manned missions. The first stage is essentially identical to the Energia strap-on boosters; it is powered with the same RD-170 engine. With an approximate capability of 30,000 pounds into low-Earth orbit, it has some potential in the commercial market. Ariane The Ariane launch system is a family of vehicles that was designed primarily to launch European satellites. Development work for Ariane-4 began in 1982 and the first launch occurred in 1988. There have been more than 15 launches of the Ariane-4 and it is rapidly taking over the commercial market. PROPOSED INTERNATIONAL LAUNCH SYSTEMS Ariane-5/Hermes The space transportation system incorporating the Ariane-5 launch vehicle and the Hermes space vehicle is a European Space Agency (ESA) project. In addition to launching the Hermes manned spaceplane, the Ariane-5 launch vehicle will be used to place heavy payloads in low-Earth orbit (LEO) or geosynchronous Earth orbit (GEO). Ariane-5 is a two-stage launch vehicle comprised of a core stage with a single liquid-oxygen/liquid-hydrogen rocket engine and two reusable solid rocket (HTPB propellant) boosters. Thrust at liftoff is 3.5 million pounds and payload to LEO is 48,500 pounds. Flight qualification of the Ariane-5 launch vehicle is targeted for 1995. The Hermes space vehicle is recoverable and will be deployed by Ariane-5 only when a mission requires a crew. As presently proposed, Hermes is a delta-wing vehicle that can carry a three-member crew, cargo, and an expendable resource module. The primary missions are to service the Columbus Free-Flying Laboratory and to visit Space Station Freedom. On completing its mission, Hermes will return to Earth and land at a predefined terrestrial landing
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From Earth to Orbit: An Assessment of Transportation Options site. The Hermes program is currently funded, but cost increases due to technical changes and program extensions make its future uncertain. Hotol Hotol originated at British Aerospace (UK) in 1983 from studies on fully reusable, low-cost launch vehicles for space operations. Hotol was originally conceived as an air-breathing rocket vehicle, which would have necessitated developing a novel propulsion system in parallel with an advanced airframe. However, the 1989 availability of the Antonov-225 (AN-225) heavy lift aircraft led to consideration of a rocket-propelled concept in which the vehicle is launched from the AN-225 at an altitude of 5 nautical miles before separating and accelerating to orbit using its own liquid-oxygen/liquid-hydrogen rocket engine. Use of a Russian engine from Energia is under consideration. Use of the AN-225 avoids the need for a launch site or a launching trolley for Hotol and improves the vehicle performance. Since 1990, a joint British/Russian effort has been undertaken, with the British responsible for the aerospace plane and support systems, and the Russians responsible for the rocket engine and problems of vehicle separation. The Hotol design allows for either crew members or a cargo capsule to be carried. Sänger In 1988, the Federal Republic of Germany initiated a national program to develop technologies required for Sänger, an advanced space transportation system. Sänger is presently in the concept definition stage, and funding is believed to be at a very low level. It is being designed as a space transportation system consisting of two stages. Sänger's first stage is a hypersonic transport aircraft. Two different second stages are under consideration: (1) the winged reusable version designated HORUS (Hypersonic Orbiting Reusable Upper Stage), capable of carrying two to six crew members and 4,400 to 8,820 pounds of payload into a near-Earth orbit, or (2) the expendable payload rocket CARGUS (Cargo Upper Stage), which can lift up to 33,075 pounds into near-Earth orbit. The Sänger first stage carries either HORUS or CARGUS to an altitude of 20 nautical miles where separation takes place at a Mach number of approximately 7.0.
Representative terms from entire chapter: