Crew transportation from Earth to LEO will be accomplished by a personnel launch vehicle (PLV), which may be a Space Shuttle derivation, and from LEO to GEO by a chemically powered personnel OTV (POTV). The POTV is expected to incorporate a conventional chemical rocket, probably O2/H2, to provide a short transit time system (1 day or less). The thrust level necessary to achieve the short time of flight precludes high-specific-impulse, low-thrust electrical systems. B. Heavy Lift Launch Vehicle The HLLV is designed for transporting all SPS freight, except crews and high-priority cargo, from Earth to LEO. The launch site is assumed to be the NASA John F. Kennedy Space Center (KSC), and payloads are launched into an approximately 90 by 500 km, 28.5° inclination nominal insertion orbit. Payload rendezvous capability is provided by the orbital maneuvering system (OMS) to decrease second-stage velocity requirements. This imposes a weight penalty of approximately 3 percent on the payload for the OMS, including propellant, and requires a subsequent return to Earth for the OMS engines and avionics. The cost of OMS recovery has not been investigated. The HLLV will provide a payload environment, such as acceleration, shock, vibration, temperature, etc., similar to that provided STS payloads, but will provide no additional services. The key figure of merit for the HLLV is the cost per pound of payload to LEO. Minimizing this cost requires attaining as much reusability as possible with as little refurbishment and parts replacement as can be achieved. Reuse goals of 300 and 500 flights were considered from a structural design (fracture mechanics) standpoint and are suggested as the range for launch vehicle replacement calculations and costing purposes. No particular requirements for advanced technology were assumed in the HLLV studies. Hydrocarbon fuel engines are considered best for launch vehicle first stages because the greater fuel density relative to hydrogen allows enough decrease in structure with related cost advantages to outweigh the higher specific impulse of hydrogen fueled engines. The engines that were considered are presented in table VI-1. The HLLV candidate configurations are presented in table VI-2. These candidates represent the range of launch vehicle concepts suggested by the section on Technology Forecast of the Outlook for Space report and by NASA and industry experts. Other candidates, such as mixed ballistic and winged systems and very large (450-ton payload) single stage to orbit (SSTO) vehicles were considered in a study contracted by Boeing (NAS 9-14710). Study analyses conducted to date did not identify the mixed systems as leading candidates, although the large SSTO was considered a close competitor to the two-stage ballistic vehicle. The modified SSTO is an SSTO launch vehicle with a 100- to 175-metric ton payload capability. It features an expendable external hydrogen tank (hence "modified" SSTO) and uses 15 uprated (4000-psia chamber pressure)
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