The new operational strategy for XMM-Newton's tank thermal control, after Low Temperature Protection thermostat failure

摘要

During the Eclipse season in July 2009, XMM-Newton had a failure on a Low Temperature Protection (LTP) thermostat in the heater circuit for the thermal control of the Tank 1 of the Spacecraft, and its temperature dropped down to 21.25 degC, below the nominal threshold of 24.6 degC. The Reaction Control System of XMM-Newton is based on 4 tanks connected in series to feed the main one (Tank 1) with pressurized gas, and a temperature gradient among the tanks could cause fuel migration between the tanks and in the worst case, if the main tank is emptied, this in turn can lead to gas bubble ingestion in the pipes downstream feeding the thrusters. The main tank temperature is now controlled by means of uplinking a duty cycle to the Transistor Switch (TSW), connected in parallel to the failed LTP thermostat, and manually reacting to any temperature deviation outside the nominal range. This duty cycle is implemented via Time Tagged Commands which are generated on ground following an operational strategy that has to take into account safety issues and contingency situations. The thermal cycle for the Tank 1 is variable because it depends on the attitude, the position in the orbit and the season, therefore it can not be easily predicted. The strategy adopted is an adjustment of the thermal cycle to be done every revolution, based on the previous one, changing the time of activation and deactivation of the TSW. The operation is implemented in three different layers: ⅰ) an automated script to calculate the time of the Time Tags in order to correct the thermal cycle; ⅱ) a procedure to calculate manually the Time Tags in case automation fails, and ⅲ) an Operational Instruction for the Spacecraft Controller to operate the TSW in case the temperature goes out of the nominal range. This paper describes the failure suffered by XMM-Newton and the following operational strategy adopted to cope with the failure.