Item — Large Momentum Wheels Criticality — Enhances probability of program success SPS systems analyzed to date have provided attitude control torques with high performance electric thrusters. Success of this approach depends on development of high performance, high thrust, long life electric thrusters. Delivery of large quantities of propellant to GEO and effects of thruster exhaust on the vehicle post severe problems. Large momentum storage devices alleviate these problems by elimination of consumables from the attitude control system. Present Capability: The largest momentum storage device identified is a 2-degree~of-freedom (DOF) control moment gyro producing 6100 n-m-sec of angular momentum at a unit mass of 300 kg. Maximum torque per axis is 270 n-m per axis. Use of these devices for attitude control of SPS requires 25,000-250,000 units with a total mass of 15 x 10$ - 150 x 10$ kg for control of the axis perpendicular to the orbit plane. Such a system is prohibitively heavy and complex for SPS. Analysis and design efforts to date indicate that momentum wheels of large diameter can provide realistic torques and angular momentum. A 400-m diameter aluminum wheel can produce 10° n-m-sec at a unit mass of 2500 kg. Complete control of an SPS vehicle is possible with a small number of such units. Required Capability: SPS designs analyzed to date require 10®-3 x 10$ n-m-sec of angular momentum and torques up to 2 x 10^ n-m. The torque and angular momentum should be provided by a small number of units which may be 10's or 100's of meters in diameter. Metal structures can provide satisfactory performance; however, advanced composites may further reduce mass or size because of their improved strength to weight ratios. Development Plan: The first step is to develop design and operating criteria for such large wheels and use them for preliminary definition of feasible concepts. The second step is to design in detail a prototype model of a wheel. Third step is to fabricate prototype models and subject them to ground and space environment testing to verify their performance for SPS applications. Item — Electric Thrusters Criticality ~ Critical to program success SPS concepts variously employ electric thrusters for inter-orbit transportation, attitude control, and stationkeeping. Very high specific impulse performance is required to keep total propellant requirements from becoming excessive. Propellant requirements for lower performance chemical propulsion systems used for attitude control and stationkeeping may exceed 10$ kilograms per year. Present Capability: Thirty-cm thrusters have been demonstrated and tested with cesium, mercury, xenon, and argon propellants. An argon 60-cm system has been tested with reported ISP ='8070 sec at 200 kw and 13,000 sec at 400 kw. The larger, 30 cm and greater, are somewhat more efficient than
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