Ref. All states that current engine development at LeRC has concentrated on a 12 inch (30 cm) thruster, and that extension to 40 inches (1 meter) seems within technical feasibility. Ref. A3, an early contractor document, mentions the need for 5 foot (1.5 m) diameter ion thrusters. Ref. All states that the grid material will be the limiting factor in the size, as the grids distort thermally during thruster operation. The choice of propellant for ion devices, also a major development concern, has received some attention. Ref. All states that current emphasis is on mercury, but that use of this material on the scale needed for the SPS may result in contamination of SPS components. Argon is also a potential propellant candidate, and Ref. Al2 describes a transfer system based on MPD (magnetoplasma-dynamic) electric thruster modules utilizing argon propellant. An LO^/LH^ rocket system is under consideration for propulsion during solar eclipse and for the return from GEO to LEO. With respect to the third vehicle category, i.e., personnel and equipment transport, Ref. A9 postulates the use first, for transport to LEO, of a baseline shuttle, followed in development by a growth shuttle, then an advanced system. Characteristics of these systems can be summarized as follows: Baseline Shuttle 10 persons and 30 tons (27,000 kg) payload Growth Shuttle 80 persons or 50 tons (45,000 kg) payload Advanced Systems ~50 persons or 20 tons (18,000 kg) payload For LEO to GEO transport, Ref. A9 postulates a 10 ton payload vehicle for final assembly and maintenance tasks, and a 40 ton payload vehicle for fabrication and assembly tasks. In addition to transport requirements such as these, vehicles for handling materials at the assembly site would be required. Ref. All cites the need for "maneuver tugs" to maneuver and transport large equipments, materials, propellants, etc., in the vicinity of the assembly site and the "propellant farms." The system outlined in Ref. Al3 would minimize "free flying" by attaching support equipment to the construction site. With respect to the fourth vehicle category, i.e., interim types of
RkJQdWJsaXNoZXIy MTU5NjU0Mg==