Entrapment of heat

[DerekSmith]

Why you need to keep it warm?
We need to use only dry elements because all water will evaporate in the vacuum.
Dry batteries and capacitors are cold-proof and do not require any heating.
OVERHEATING is the issue there.
All electronics must be tested in vacuum and heat before sending anywhere in space.
Chandrayaan-1 failure is a lesson for all of us to learn.

You say that OVERHEATING is the issue as if it is the ONLY issue. You also suggest that a 'dry' system will coldproof our project. What cells are you proposing we use that do not contain a liquid water phase internally?

You are also ignoring the issue that all components must be connected to a thermal 'bus' in order to ship their heat to some external radiator, and that the temperature of that bus will depend exactly where on it a component is stationed and the heat flux emanating from each component.

Granted, units equivalent to thermal 'zenner diodes' are available - variants of the 'heat pipes', but I have not read that use of these clever devices is planned, so some components are going to be hot and some very cold - until something causes us to need to turn round, or move into the glare of a heated hill of rock.

I genuinely do not believe that it is within the capability of a small machine to manage its thermal budget for several days on the surface of the moon, but I am keen to be proven wrong by model calculations and trial results.

I have to conclude that the only way to collect the Lunar X prize is to make the 500m trip really fast - in seconds or minutes, then deploy a directional PV panel to 'manage' our shade control and utilise passive heat tube thermal diode technology as the heart of our thermal bus.

Derek

[tristancho]

Hey Derek,
I do like your thermal view. I am agree that 500 meters has to be run very fast.
Best,
Joshua

[spinnakr]

I realize this is a little late coming to the discussion, but whatever. I have a few tidbits of info that should be helpful.

First, forget convection and conduction. Forget aerogels and other insulating materials. As a few others have pointed out, the ONLY thing we need to worry about is radiation. Radiation can very easily be managed via thin metal foil - IE, gold leaf or very thin aluminum. Gold vs aluminum is a very complicated question: aluminum, although less dense, cannot be rolled as thinly as gold, meaning that the actual mass of the foil per square meter is pretty close (close enough to ignore). Although gold offers superior reflectance with lower absorption above about 750 nm, its performance isn't great at any smaller wavelengths. Judging simply off http://en.wikipedia.org/wiki/Reflectivity it would be best to go with aluminum.

Thermal conduction of regolith is dismal. And I do mean dismal. As in:

1.5 x 10^-5 W/cm K in upper 1 to 2 cm
1.5 x 10^-4 W/cm K at a depth of ~1m
Heat flow estimated at 2.2 x 10^-6 W/cm^2 +/- 20%

That means a few things. First, it means that about a meter below the surface, the temperature of regolith is much steadier - possibly within operating temperatures of electronics. I saw a number for this, and if I remember correctly it was around 200K. According to here it's about 250K, but I'm not sure I trust that site. This in turn means that there is a possibility of powering a generator of sorts at a base station (aka the lander) purely by the temperature gradient between the top and bottom. A stirling engine would be ideal for this, and has actually been proposed by NASA for a moon base. This would be much more practical than a self-burying rover; however, due to mass concerns on the lander, this still probably isn't practical. HOWEVER, this also means that - assuming a decent mechanical separation between the contact surface with the ground (wheels, tracks, sphere, whatever), heat conductance should, for all intents and purposes, be negligible. More exactly: given the above estimated conductance, and assuming PERFECT conductibility within the rover, with one square meter of contact with the lunar surface (orders of magnitude higher than we will see), we would need to dissipate 0.022 W of conducted heat. That is several orders of magnitude below any electrics we put on the rover. In other words, ignore conduction as well.

So, since radiation is the only method of heat transfer, then a working fluid going through heat pipe (a lunar radiator, basically) remains by FAR the best way to control heating issues. It also offers the ability to store heat to release during the lunar night, which could conceivably let us survive the night. Note that there ARE heat pipes available with very good emissivity and very poor absorption, and that some are very lightweight (5 kg / kw). I am unsure how scalable they are, though, so I don't know if we could find ones small enough for our purposes.

EDIT: Page 34 of the Lunar Sourcebook says: "At the Apollo 15 site, the mean temperature at a depth of 35 cm is 45 K higher than that of the surface; at the Apollo 17 site, the difference is 40 K."

[tristancho]

DELETED

[spinnakr]

This is about -24 ºC. The kinetic of temperature in the Moon's surface is very fast from day temperature to night temperature; also change very fast from light to shadow areas because no atmosphere in there. For the inner temperature inside the regolith you may look at the IR radiation of the Moon that not depends neither Sun radiation nor albedo radiation.

I realize the lunar surface is highly dynamic, but that is irrelevant to what I said regarding the difference in temperature between the lunar surface. I wasn't trying to provide estimations for anything, or do any calculations; I was merely saying (as others here have) that the lunar "underground" stays at a far more stable temperature than the lunar surface. Theoretically this could be used to generate power, but unfortunately, due to mass considerations this is largely infeasible. Also, the low conductivity of the regolith certainly doesn't help - you'd have to have a large heat exchanger to extract even a little energy, or work in milliwatts. This was also pointed out on the forums here. Ultimately, my point was that (as others have said) rover heat loss/gain from conductance is negligible.

I'm not really sure what you are suggesting as far as the inner temperature of regolith. Are you saying that its temperature can be found by measuring IR black body radiation? Okay, but how is that relevant? My whole point was that from a thermal management perspective, the lunar soil can be ignored - except, of course, when talking about black body radiation.

[scasey]

Great set of comments Spinnakr. Below the lunar surface you would clearly measure temperature directly. An on going question is the generation of power through a heat sink anchored to the lunar regolith (beneath the surface) and a radiator coupled to space (e.g. a black antenna radiating out into space). There must be some papers on this in the literature saying 'yes it's possible' or 'no it's not'.

Providing some references would be very helpful.

SC