Applying Contamination Modeling to Spacecraft Conventional Propulsion System Designs and Operations

Molecular and particulate contaminants generated from the operations of a propulsion system can impinge on spacecraft critical surfaces. Plume depositions or clouds can hinder the spacecraft and instruments from per- forming normal operations. The interconnection between the functions of spacecraft contamination modeling and propulsion system implementation is presented. An innovative contamination engineering approach is addressed during a spacecraft mission, which includes concept design, manufacturing, integration and test, launch, and on-orbit operations. A summary of the implementation on several successful missions is also presented. NE potential source of concern facing the instruments of or- biting spacecraft is the effect of molecular contaminant inter- action with sensitive thermal control and optics surfaces. Typically, the sources of these on-orbit contaminants can be categorized into e ve general areas: 1 ) material outgassing (water, hydrocarbons, sil- icones) from materials of construction; 2 ) spacecraft and multiple- layer insulation venting; 3 ) e uid leakage from pressurized vessels (e.g., cryogen tanks ), dumps, and lubricant loss; 4 )exhaust material generated through thruster e rings; and 5 ) extravehicular activity. 1 Once released, contaminants can propagate to the receiving sur- faces through direct line-of-sight transport (direct e ux), ree ections with spacecraft surfaces, and scattering through self-scattering or with the local ambient atmosphere (return e ux). The efe ciency of these transport mechanisms is a complicated function of spacecraft geometry, mission /e ight operations, and environmental effects. In the past the purpose of computer modeling was concentrated in the assessment of contamination damage during the late design phase, integration and test, and on-orbit operation. The impact of modeling on the mission was limited to minor design changes (such as vent locations ), verie cation (for meeting contamination require- ments), and on-orbit operation (such as operational constraints im- posed to avoid contamination ). Becauseofincreasedsensitivityofspacecraftcomponentstocon- tamination effects, contamination engineering has begun to play a more notable role in overall spacecraft development. Early involve- mentrepresents themosteffectivedirectionoffuture contamination modeling efforts. By ine uencing the early design, cost savings can be very signie cant because many inefe cient contamination avoid- ance remediesestablishedlate in thedesign cycle canbeeliminated. In recent years improved contamination modeling techniques have been used extensively by contamination sensitive projects to improve spacecraft and instrument performance during the early design stage. One good example is the detailed modeling effort for the Tropical Rainfall Measuring Mission (TRMM). Contami- nation modeling efforts for this mission resulted in several design changesespeciallyinthepropulsionsystem.Thepaperdescribesthe