Thruster exhaust effects upon spacecraft

Abstract

A study has been conducted of the effect of thruster exhaust effluent upon spacecraft. Small thrusters (micropounds to several millipounds) utilizing cesium, Teflon, ammonia, and hydrazine as fuel have been considered. A brief summary of mercury thruster effects reported by others is included. We found few problen:us with ammonia and hydrazine. The major potential interactions are the effect upon the environment density and exhaust from these thrusters can coat radiators which operate at 70°K. Cesium and mercury will coat surfaces, particularly radiators which operate in the vicinity of 100°K. Charge exchange effects are quite important in predicting some of these effects. Contaminants in the exhaust beam can be particularly important because these components do not evaporate once condensed upon surfaces. The Teflon thruster can coat any surface placed in the vicinity of the exhaust beam. This coating will be quite inert, and would require high temperatures for its removal. It would consist, in part, of the basic Teflon polymer. A preliminary study of potential cesium. thruster-Polaris star tracker interactions indicates that reflected sunlight will be a problem. Similar behavior may occur with other thruster exhausts. Further work is indicated in many areas. v *Based upon a suggestion by an Associate Editor of the Journal of Spacecraft and Rockets. 4 The evaporation rate for a surface at 2 OO'K is 7.1 x 10' atoms/cm 2 sec. Hence, the accumulation rate is roughly the arrival rate. One monolayer will accumulate on the upper 200°K surface in a little leis than one hour and on the other 200°K surfaces in about five hours. In a few days a 200°K surface will behave as though it were composed of cesium. Since the cold patch ";sees" only space and the 2000 K surfaces, the evaporation rate from these surfaces determines the rate at which cesium arrives at the cold, patch. (Evaporation is nil at 100°K.) The cold patch accumulation rate is about 5 x 10 7 ;atom/cm 2-s e c o r o n e m o n c l a y e r e a c h 2 4 0 0 h o u r s. T h e c o l d p a t c h a c c u m u l a t i o n r a t e i s n o t s e r i o u s for radiator lifetimes of a few thousand hours. A preliminary check of the 200'K wall accumulation effect shows the ,relative emissivity and absorptivity of cesium and of aluminum to be similar. As a rough guess, cesium accumulation within the radiator is not serious. We did not investigate the specular behavior of uncoated and coated aluminum, and this could introduce 'trouble, A better approach would be to eliminate the neutral cesium completely. This can easily be done with the studied geometry by either recessing the 'thruster, providing a small, shield, or extending the radiator beyond the plane of the thruster exhaust. All three approaches block the direct view of the thruster exhaust opening from the radiator, thus eliminating a direct path. Since there are no indirect paths other than interaction of the exhaust beam with the environment (an area which should be investigated), one might presume that with the modified...