Low Thermal Loading and Operational Voltage Limits as Methods for Enabling MMRTG as a Power Source for Lunar Missions

The Multi-Mission Radioisotope Thermoelectric Generator (MMRTG) is a readily available, long-lived, small scale, versatile power supply that can generate uninterrupted power on the moon during the long lunar nights. Lunar surface conditions, however, can be very harsh. Lunar daytime highs could cause the critical hot junction temperature (T
hj
) to exceed its operating limits, while the cold lunar nights can generate deep thermal cycles that could induce significant stress. An energy balance for objects on the lunar surface was performed, and an approximate lunar thermal sink for the MMRTG was obtained. This approximate sink does not incorporate potential self-shading from incident solar radation created by the MMRTG geometry. It is hypothesized, however, that the effect of this self-shading on the lunar surface will result in a decrease in operational temperatures. Recently developed thermal models have been improved so that they can calculate critical temperatures for an MMRTG on the lunar surface. Using the approximate lunar thermal sink, results from the model indicate that under the nominal operating conditions of 28 V and 2000 W
th
of thermal inventory, the MMRTG can operate on the lunar surface while remaining under the maximum T
hj
by a margin of greater than 25 °C. Operating under worst-case nominal conditions, 28 V and 2048 W
th
of thermal inventory, still allows operations on the lunar surface with over 15 °C of margin. Operating at higher voltages will increase temperatures within the MMRTG, while lower voltages are expected to cause T
hj
to decrease. An off-nominal 32 V system will still have at least 15 °C of margin if the system can be held at 2000 W
th
or less of thermal inventory, but that margin shrinks to almost zero if the maximum specification of 2048 W
th
happens to be implemented. Lowering the thermal inventory can lower T
hj
, but it will also decrease beginning-of-life power. The relationship between these three parameters appears to be linear, with a 48 W
th
decrease in thermal inventory causing T
hj
and beginning-of-life power to decrease by 11.2 °C and 4.8 We, respectively. Missions that want to use MMRTG on the lunar surface should carefully consider the voltage they want to pull from the generator. If the operating voltage chosen is high, then a decrease in thermal inventory is one potential option for keeping the T
hj
below the operational maximum. Results also indicate that the MMRTG will experience a thermal cycle of
$\Delta\mathrm{T}=47\ {}^{\circ}\mathrm{C}$
during the lunar diurnal cycle. Empirical testing of a prototypic system or subsystem is anticipated to be the best method for determining the effect of lunar induced thermal cycles on the MMRTG. Currently, the Engineering Unit is available and is not a critical testing resource for NASA. As such, potential damage to the unit from thermal cycle testing would not deprive NASA of valuable life test data on the MMRTG. Other thermal cycle testing options include an MMRTG thermoelectric module mounted in a suitable testing frame.