Figure 8.18 Ratio of Thrust to Weight for Different Propulsion Systems Due to the fact that almost all technology is available a cislunar transportation system could be realized within less than a decade. Lunar Bus (LB) Payload transport between the lunar surface and LLO is carried out by a Lunar Bus Figure 8.19 which is designed to accommodate the special requirements of lunar ascent/descent. The AV- requirement for lunar ascent/descent is assumed to be 1900 m/s respectively 2000 m/s (higher reserves for landing maneuver) These Av-requirements include losses due to gravity and thrust vector control and reserve propellant. The flight time for ascent or descent is less than one hour. Orbital Transfer Vehicle (OTV) Payload transfer between the LLO and GEO is carried out by an Orbital Transport Vehicle. The Av- requirement between LLO and GEO for one interorbital flight amounts to 2000 m/s and the transfer time is about 3-4 days. Aeroassisted Orbital Transfer Vehicle (AOTV) Payload transfer between LLO and LEO is carried out by an AOTV. After leaving LLO the AOTV reaches Earth where it dives into the upper layer of the Earth's atmosphere to reduce its velocity using an aerobrake maneuver from about 11 km/s to the velocity of LEO to about 8 km/s and then insert into the space station orbit. The Av-requirement from LLO to LEO is about 1000 m/s (800 m/s are required for lunar escape acceleration and midcourse maneuver and 200 m/s are required for Earth's atmosphere entry and leaving to reach the 400 km space station orbit in LEO). From LEO to LLO the AOTV accelerates to Earth's escape velocity (AV=32OO m/s) and when reaching the moon the AOTV decelerates in order to go into LLO (AV=800 m/s including midcourse correction). The flight time for each interorbital transfer is about 3 days. The technology for an aerobrake maneuver of an AOTV in the Earth's atmosphere will need to be developed.
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