Propulsion for the space station

Abstract

Propulsion for the Space Station requires a unique, challenging combination of features. The propulsion requirements demand responsive capabilities for orbit control and countering on-board or externally induced forces, and precision control for station-keeping with minimum disturbances. The propulsion design must provide high performance to minimize propellant requirements, have extremely high reliability, provide an extended long-life capability, control emissions and outgasses that could impact in-space operations, and provide maximum safety. Comprehensive evaluations have been performed of numerous design options; these included monopropellants, earth-storable bipropellants, subcritically and supercritically stored cryogenic propellants, use of residual and/or waste fluids, pump-fed and pressure-fed systems, systems with and without gas accumulators, various pressurization methods, and several types of thrusters. The NASA Space Station program office has defined a baseline propulsion system based on hydrogen/oxygen thrusters, using an on-board water electrolysis system to provide the propellants. This system is combined with a low thrust (100 millipound) resistojet propulsion module that can operate using excess or waste fluids. With this combined on-board propulsion system the need to transport propellants to orbit is basically eliminated, as is the need to collect, store, and return waste fluids to earth. Prototype thrusters for the on-board propulsion have been fabricated and tested, with excellent performance and durability demonstrated. Features that enhance long life, maintainability, servicing, and health monitoring have been studied and are being incorporated. A versatile propulsion test bed has been designed and an extensive demonstration program initiated.