The design of a propulsion system using vent gas from a liquid helium cryogenic system

Two spacecraft, Gravity Probe B (GP-B) and the Satellite Test of the Equivalence Principle (STEP), incorporating on-board liquid helium cryogenic systems are scheduled to fly around the turn of the century. Effective propulsion systems can be implemented for these spacecraft by directing the helium gas which boils off from the cryogenic systems in specific directions through a set of thrusters. Due to extensive development and testing work at Stanford and elsewhere, the ultra low flow rate helium thrusters for such a propulsion system are now considered proven technology. This thesis is concerned with implementing these thrusters into an effective overall propulsion system. Our primary concern is maximizing the forcing capability or control authority of the propulsion system. A worst case analysis of the thruster system is performed. Various techniques are derived to calculate the worst case output or minimum control authority of the thruster system. The techniques are general and apply to any parallel configuration of actuators. They are used in this thesis to design a propulsion system for GP-B which maximizes the minimum control authority and to study redundancy of the GP-B thruster system under a worst case thruster failure. A general thruster controller is derived for a liquid helium propulsion system which can generate desired output forces and at the same time vent the required helium gas needed to cool the cryogenic system. We show how the helium gas vent rate can be controlled to maintain a stable liquid helium supply temperature and pressure. An on-orbit thruster calibration scheme is developed to make sure the correct output force is generated. Using a nonlinear simulation of the spacecraft dynamics, the calibration scheme is shown to calibrate the 108 parameters needed to describe the outputs of a set of 18 thrusters to better than 1% accuracy. Finally an on-orbit mass trim control system is derived to reduce the centrifugal force and moment disturbances on the propulsion system due to mass property offsets.