I found the Cody's Lab videos from Scott Manley's video as described in this answer. Browsing through I learned from the first 30 seconds of Hot Wax Bottle? that the phase change of wax was used on the Apollo Lunar Roving Vehicles.

Power was provided by two 36-volt silver-zinc potassium hydroxide non-rechargeable batteries with a capacity of 121 A·h each (a total of 242 A·h), yielding a range of 57 miles (92 km).[16] These were used to power the drive and steering motors and also a 36-volt utility outlet mounted on the front of the LRV to power the communications relay unit or the TV camera. LRV batteries and electronics were passively cooled, using change-of-phase wax thermal capacitor packages and reflective, upward-facing radiating surfaces. While driving, radiators were covered with mylar blankets to minimize dust accumulation. When stopped, the astronauts would open the blankets, and manually remove excess dust from the cooling surfaces with hand brushes.

How much wax was carried to the Moon for this reason? Was there any way to judge how much wax had melted, and how much remained solid, or some kind of "your wax is toast" warning when no more solid wax available for phase change?


Looking here the mass is shown as 2.25lbs. They list motor and battery temperature sensors, backed up by information elsewhere on what sensors made it onto the rover instrument cluster but that would only provide limited information on the wax state.

Would be interesting if the full transcript that went with the presentation was available but the fact they describe several different temperature prediction models tried during the program is suggestive that they had no way of directly reading what state the wax was at once it reached the phase change temperature and depended on ground based live modeling to plan EVAs rather than anything available on the moon.

An interesting candidate to give a state indication should there have been time/mass/need are these.

  • $\begingroup$ The NASA link is fascinating! Since links break over time, it would be great if you could choose a few screenshots of key images or points, and include them here? It's not necessary but future readers may thank you. $\endgroup$
    – uhoh
    Oct 21 '18 at 6:59

The LRV Manual states that 3.5 lb of octadecane wax was used for the Drive Controller Electronics (DCE) and 2.5 lb of eicosane wax was used for the Signal Processing Unit (SPU) .

During operation, the 60 pound batteries were a great heat sink for their own internally generated heat, and an additional heat sink for some of the other electronic components. Flexible thermal straps were designed and tested to enable heat conduction from the Signal Processing Unit (SPU) to Battery 1 and from the Directional Gyro Unit (DGU) to Battery 2. The Drive Controller Electronics (DCE) had to be positioned too far from the batteries for effective thermal strapping. Passive thermal “heatpipes”, which are used extensively on present day spacecraft, were not mature enough designs at the time of LRV development, so another heat storage and transfer method was needed. Therefore, fusible mass “wax tanks” were used to store excess heat generated in the DCE (3.5 lbs. of Octadecane wax) and also for the SPU (2.25 lbs. of Eicosane wax) during operation.

The added advantage of these wax tanks was that they acted as “thermal dampers”, that maintained the DCE and SPU at constant temperatures while the wax was being melted. Then, the wax would be solidified for re-use when the dust covers were opened at the end of driving on each EVA and thermal radiators on top of the batteries and wax tanks were exposed. It was planned to have the dust covers automatically close using bi-metallic spring actuators when battery temperatures reached a safe lower temperature.


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