Temperature control of a liquid helium propulsion system

Two spacecraft, gravity probe B (GP-B) and the satellite test of the equivalence principle (STEP), incorporating onboard 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, the ultra low flow rate helium thrusters for such a propulsion system are now considered proven technology. This article is concerned with implementing these thrusters into an effective overall propulsion system. A thermodynamic model relating the temperature, pressure, and flow rate of the propulsion system is derived. Based on this model a controller is developed which regulates the liquid helium supply temperature and pressure by varying the net heliqm mass flow rate through the thrusters. We show how the net mass flow rate can be controlled independently from the desired output thrust. The manifold pressure upstream of the thrusters is shown to remain remarkably stable even with fairly large flow rate variations. Using the GP-B spacecraft as an example we conclude that it is feasible to build a liquid helium based propulsion system with a very stable supply temperature and pressure.