Figure 8.19 Space Vehicles of a Lunar Transportation System 8.7.2 Electric Propulsion NASA has studied the manned exploration of the lunar surface and its use for a large scale exploration [Bryan PALASZEWSKI, 1988]. We attempt to use these studies as a basis for describing the efficiency of electric propulsion for the round trip transportation to the Moon. Electric propulsion is characterized by its high Isp (1000 s to 10000 s) and low thrust. The ratio of thrust to weight of this type of propulsion is less than 0.01 see Figure 8.20. Of course, the performance strongly depends on the technology and also the power of the generator. The low thrusts do not allow liftoff capability from the Earth or from the Moon. Thus this type of propulsion will be used for an OTV from LEO to LLO. In the transfer, the OTV departs from LEO at an 500 km altitude orbit; the LLO is at an 100 km altitude 0.0 degree inclination orbit. Transfer to the Moon will require a combination of chemical propulsion and electric propulsion. In part, the step from Earth to LEO and from LLO to the Moon surface require chemical propulsion. A major difference between chemical and electric propulsion concerns the AV required for orbit transfers. High thrust orbit transfers from LEO to LLO require AV of 4000 m/s, for example. This includes LEO departure, trajectory correction and LLO insertion. In the low thrust case, the AV is 8000 m/s. Thus, electric propulsion systems need twice as much AV than chemical propulsion for a lunar mission. This is mainly due to the gravity losses and the attitude control losses. Among electric propulsion two main categories exist: the solar electric propulsion and the nuclear electric propulsion. In order to illustrate both of them we address the following examples: -nuclear electric OTV: Xe Ion OTV( 1MW ) H2 Arcjet OTV (1MW) Magneto Plasma Dynamic OTV (1MW) -solar electric OTV: Xe Ion OTV ( 300 kW) Figure 8.20 shows a nuclear electric lunar transfer vehicle design. The vehicle uses a dynamic conversion nuclear reactor to power the electric propulsion thrusters. The payload and the reactor are separated by a boom. This boom minimizes the radiation exposure of the payload to the reactor. The main propulsion system is near the center of mass of the transfer vehicle. This design is a side thrust configuration.
RkJQdWJsaXNoZXIy MTU5NjU0Mg==