Effects of thruster firings on the shuttle environment

The changes in the neutral gas composition surrounding the Space Shuttle caused by the Shuttle's Vernier Reaction Control System (VRCS) and Orbital Maneuvering System (OMS) rocket engines were measured with a quadrupole mass spectrometer aboard STS-4. There are substantial differences between the measured composition changes in the payload bay and the calculated composition of the thruster exhaust plumes. These differences can be explained by kinematic effects that occur as the exhaust products collide with surfaces and other gas phase species in the Shuttle environment. Hydrogen, because of its light mass, is enriched in the return flux to the spacecraft, and tends to permeate the Shuttle environment during thruster firings more easily than heavier species. The effect of the thruster firings on the mass spectrometer also depended on the attitude of the instrument with respect to the velocity vector. When the mass spectrometer was pointed into the velocity vector, decreases in atomic oxygen concentration were detected during the engine firings.