Hubble Space Telescope Low Pressure Venting Analysis, Thermal Vacuum Test Solution, Validation And Flight Prediction

Abstract

There are three primary factors which drove the low pressure venting performance of the Hubble Space Telescope (HST) aft shroud. The first two arise from design requirements. An important portion of the HST mission is to image very dim objects in deep space. Thus, stray light from the exterior environment into the optical path must be held to an extremely low level. Light leakage into the aft shroud must be constrained to approxi-mately 7.0 X 10-09 of the incident solar flux. In order to achieve this level, light rays incident on the external apertures of the aft shroud vents must be made to reflect many times off of the blackest possible surfaces (baffles) before they reach the interior of the aft shroud. Unfortunately, light rays (photons) and molecules of gas in the free molecular flow regime (collisionless) behave somewhat similarly - a 'random walk' through the vent with diffuse reflections from the walls of the vent passage and its baffles. Thus baffled vent configurations which effectively limit light leakage also tend to severely restrict the efficiency of free molecular flow through the vent.