Thermal Engineering of Mars Pathfinder MOx Chemical Cell

A thermal engineering analysis of the Mars Pathfinder MOx chemical cell was performed to determine the feasibility of raising the temperature of the soil and air cells to operating conditions using passive heating in the Mars environment. A critical 10° C rise in the soil sensor above the ground temperature is needed to activate the MOx chemical cell. Little relative spacecraft power is available to heat the instrument to its desired surface operating temperature. Realistic analytical bounds of thermal performance of the MOx chemical cell were predicted using a multi-discipline approach that consisted of materials, thermal, and structural analyses. The models accounted for solar heating, conduction to the ground, radiation to space, convection to the Mars atmosphere, and spacecraft power. Parametric thermal control methods were evaluated that consisted of evaluation of coating properties, thermal isolation of the sensor head from the Mars surface, sensor head temperature, and contact coefficient of the head to the ground.

Predicted analytical thermal performance shows that a 10° C gradient between the MOx sensor head and the Martian soil is feasible using 0.5 Watts of spacecraft heater power or can be achieved using passive thermal control by closely coupling the soil sensor to the head. The air sensor can be as much as 20° C above the atmospheric temperature by closely coupling this sensor to the head. Conduction to the ground is the dominant mechanism for cooling of the sensor head on Mars and is a strong function of the contact coefficient of the head to the ground. Passive thermal control of the sensor is possible using a combination of thermal control coatings on the head and thermal isolation of the head from the surface. This paper discusses the multi-discipline methodology used to analyze and predict thermo-mechanical performance of the MOx chemical cell. The analytical performance envelope determined in this study was used to help refine the design of the MOx instrument.