Thermal Design and Flight Experience of the Mars Exploration Rover Spacecraft Computer-Controlled, Propulsion Line Heaters 2004-01-2412

As part of the Mars Exploration Rover (MER) project, the National Aeronautics and Space Administration (NASA) launched two rovers in June and July of 2003 and successfully landed both of them on Mars in January of 2004. The cruise stage of each spacecraft (S/C) housed most of the hardware needed to complete the cruise from Earth to Mars, including the propulsion system. Propulsion lines brought hydrazine propellant from tanks under the cruise stage to attitude-control thrusters located on the periphery of the cruise stage. Hydrazine will freeze in the propellant lines if it reaches temperatures below 1.7°C. Thermal control of the propulsion lines was a mission critical function of the thermal subsystem; a frozen propellant line could have resulted in loss of attitude control and complete loss of the S/C.

The MER cruise stage thermal design employed a computer-controlled thermostatic heater system to keep the propellant lines within their allowable flight temperature limits (17°C to 50°C). The MER propellant line thermal design differed from previous propellant line heater designs in that the line heaters were placed only in areas of highest potential heat loss (not along the entire length of the lines) and that computer-controlled thermostats were used instead of mechanical thermostats. Computer-controlled thermostats enabled setpoint flexibility; adjustments to setpoints were made after solar thermal vacuum testing and during flight.

This paper covers the design, thermal testing and flight experiences with the computer-controlled thermostats on the propulsion line heaters. Flight experience revealed heater control behavior with propellant loaded into the system and during thruster firings that was not observable during system level testing. Explanations of flight behavior, lessons learned and suggestions for improvement of the propellant line heater design are presented in this paper.