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What methods keep the internal liquids on lunar rovers from freezing? I understand that temperatures on the Moon are extremely low, way past freezing, especially in places where the Sun does not shine, yet fuels, pneumatic fluids and other liquids essential to rover operations stay functional. How is this done, besides the obvious heating coils and thermal wrappings that may be used? Are these liquids chemically endowed with certain "anti-freeze" components?

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    $\begingroup$ pneumatic fluids? Do you think of hydraulic fluids? $\endgroup$
    – Uwe
    Jun 6, 2022 at 17:59

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If by rovers you mean the Lunokhod and Yutu rovers, they both use (used) nuclear radioisotope sources to assist in thermal control through the lunar nights. Applicable fluids would be battery electrolytes and gear lubricants. Batteries cannot be allowed to freeze, and so will depend on the radioisotope sources for survival. Gear lubricants can be allowed to freeze, so long as they are warmed up before use. That can be done with electrical heaters once the Sun comes up.

As noted in the comments, Yutu uses Li-ion batteries. They probably need to be kept above -20 °C.

Lunokhod had a big turtle-like lid that had the solar panels on the inside. During the lunar day, the lid was open for the solar panels to collect energy. During the lunar night, the lid was closed, providing thermal insulation to help retain the heat from the radioisotope source.

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I'm not aware of any liquids essential to rover operations. Motion can be performed by electric stepper motors and gears. Instead of fuel, electricity from solar panels or RTGs (radionuclide thermic generators) is used, stored in lithium ion batteries.

If liquids are needed for scientific purposes, they have to be kept at the proper temperature by heating/cooling and thermic isolation, as you state.

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    $\begingroup$ One minor point: Li-Ion aren't necessarily the first choice, precisely because of their temperature dependence. $\endgroup$ Jan 23, 2014 at 22:48
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    $\begingroup$ There aren't many rovers on the moon, so here two cites: "Yutu's power supply subsystem consists of two solar panels: a set of lithium-ion batteries, hibernation awaking modules and a power control unit" (wiki.china.org.cn/wiki/index.php/Yutu_(Jade_Rabbit)); "Powered by Lithium-Ion Batteries, NASA Spacecraft Explore Mars and the Moon" (techbriefs.com/component/content/article/27-ntb/features/…). For what-ever reason, Li-ion batteries are taken at least in several cases. $\endgroup$
    – Gerald
    Jan 23, 2014 at 23:38
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We often think of the freezing point as being the temperature where water freezes (273.15 K). There are several scales for measuring temperature, for simplicity let's use Kelvin here. If you have watched a pot of water boil away you are aware the water has three states: vapor (steam), liquid (water), and solid (ice). The point where a solid turns to a liquid is often called the melting point.

According to Wikipedia, temperatures on the Moon range from 26 K to 390 K. If we look at a list of melting points we see that element like hydrogen and neon have melting points below 26 K but they also boil around 20 - 27 K, so they would not be a good liquid for using on the Moon, as they would vaporize in all but the coldest lunar winter day.

The lunar equator has temperatures ranging from 100 k to 390 K. As we look at the list of melting points there are some that are liquid around 100 K but they all have fairly short liquid ranges oxygen has one of the bigger ranges of about 45 degrees, but that is between 54 K and 90 K. It seems our biggest risk then is not in finding an element that will flow during lunar cold spells, but rather one that will not boil during the warm spells. Even water, which boils at 373.16 K, can boil on the Moon.

There are also considerations for pressure, as it can modify the melting and boiling points. In short, there is no single element that will work as liquid on the Moon, so any fluid would either need to be a compound or a solution (antifreeze) to keep it from freezing, some heat would likely be required for the coldest periods, and pressures maintained to prevent boiling from extremes of temperature or exposure to vacuum.

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  • $\begingroup$ Thank you, James, Gerald, and Mark. Your answers are extremely valuable to me, as my endeavors are in researching viable fluids for a lunar vehicle. The major difficulty right now is indeed the "freezing" risks of certain liquids, and so your input is critical. Again, thank you all for your generosity! $\endgroup$
    – user32016
    Jan 25, 2014 at 18:48
  • $\begingroup$ I am intrigued by the idea of a lunar winter. For sure the Moon's axis has an inclination (though it is tiny in comparison to Earth's). But as there is no atmosphere or bodies of water I doubt there are any seasons of any sort. Or am I wrong? $\endgroup$ Jan 28, 2014 at 13:19
  • $\begingroup$ @adrianmcmenamin that sounds like a question you should formally ask. $\endgroup$ Jan 28, 2014 at 13:42
  • $\begingroup$ @JamesJenkins Are there seasons on Luna? $\endgroup$
    – user
    Jan 30, 2014 at 20:54
  • $\begingroup$ Water boils at 373.16 K if the pressure is 1.013 bar, but under 1.79 K it will boil at 390 K. $\endgroup$
    – Uwe
    Jun 6, 2022 at 21:30
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Supplementary answer:

Vehicle liquids freeze on the Moon -- how has this problem been solved?

There's an example where liquid/solid transitions really are a feature, not a bug.

How much wax is on the Moon? (Lunar Roving Vehicles) and answers there describe how the Apollo era lunar rovers used "hot wax capacitors" as a place to absorb generated thermal energy (a heat sink), primarily by the wax's respectable enthalpy of fusion of about 200 Joules per gram.

They were allowed to cool by radiation to space later, and and provide warmth back if something, the batter especially got too cold during periods of no use. They would be allowed to solidify and be prepared for the next day's melting.

What is the "space grease" used to lubricate the ISS robotic arm? What are the material considerations for it? and answers therein describe vacuum compatible lubrication and sealing materials that remain... grease-like even at zero pressure and respectable temperature swings.

However I don't know which if any of these were used to "grease the wheels" of lunar exploration.

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    $\begingroup$ Fun to read about 'wax on the moon' here. Years ago a friend used PCM (phase change material... wax) as a heat dump on a classified project. We didn't really care about the wax refreezing. If the wax began to melt it meant something was very close to getting blown up. There was quite a family of PCMs to select from, tailored by molecular weight and distribution about mean weight in order to get just the right combination of properties. $\endgroup$
    – BradV
    Jun 6, 2022 at 21:55
  • $\begingroup$ @BradV the shuttle fuel cells (totally non computerized marvels of electromechanical engineering) used tailored wax capsules in actuators to move thermal control valves. $\endgroup$ Jun 7, 2022 at 2:49
  • $\begingroup$ @Organic Marble interesting... can you point me to a diagram of the valve mechanism? $\endgroup$
    – BradV
    Jun 7, 2022 at 12:35
  • $\begingroup$ @BradV I don't know of one in open literature. There is a discussion on their function on 2.8.17 here: nasa.gov/centers/johnson/pdf/… $\endgroup$ Jun 7, 2022 at 13:07
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    $\begingroup$ @Organic Marble THX! $\endgroup$
    – BradV
    Jun 7, 2022 at 21:55

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