I saw something saying that the circuits on a Venus bound space craft would overheat and basically ruin the the spacecraft.
Is that true?
If so, could they install fans to keep the circuits cool?
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6$\begingroup$ Spacecraft have landed on Venus and sent telemetry back to Earth in the past, so don't assume that the problem is insurmountable. $\endgroup$– Solomon SlowFeb 3 '20 at 18:16
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8$\begingroup$ The maximum survival time so far of a landed Venus probe is ~ 2 hours. $\endgroup$– Organic MarbleFeb 4 '20 at 0:40
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$\begingroup$ This is actually a great question! Just fyi it inspired me to ask Are fans ever used in non-crewed spacecraft? $\endgroup$– uhohFeb 4 '20 at 1:03
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1$\begingroup$ If you get too hot, just roll down the window. ;^P $\endgroup$– DrSheldonFeb 4 '20 at 15:00
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8$\begingroup$ @DrSheldon Reminds me of the joke about a guy who finds a genie lamp in the blistering heat of the desert, and wishes for a car door so he can roll down the window. $\endgroup$– Nuclear HoagieFeb 4 '20 at 15:26
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Fans work by moving cool air (or other fluid) over a warm surface. If there is no air, like in space, a fan will serve no purpose. Cooling things in space is actually a bit tricky because of this - objects on earth tend to lose most of their heat through conduction or convection, but in the vacuum of space, all you can do is radiate heat, which a fan will not help with.
Venus does have an atmosphere, so a fan wouldn't be inherently nonfunctional for space probe on the surface of the planet. But since the Venusian air is several hundred degrees Celsius at all times, you're not going to have much luck cooling electronics with airflow - at that temperature extreme, your electronics will probably be cooling the air, rather than the other way around. If your electronics normally run hotter than the surface of Venus, though, then a fan would help to cool them, but that would be an absurdly high normal operating temperature - if your electronics can already survive that, you probably don't need to cool them in the first place.
Overall, fans are not a useful way of cooling the electronics of a space probe, unless you are somewhere with a cold atmosphere (like Mars), or bring your own heat-exchanging gas into the vacuum of space (see Are fans ever used in un-crewed spacecraft?).
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5$\begingroup$ You could stretch the definition of a fan a bit, and include things like compressors and turboexpanders as part of a heat pump. In theory you could make such a thing work on Venus, though the engineering might be a bit tricky... $\endgroup$ Feb 3 '20 at 17:36
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$\begingroup$ A fan could also remove excess heat, assuming the probe can already withstand venusian temperatures. Why a probe would produce so much excess heat, I haven't got a clue, but there's that. $\endgroup$– BMFFeb 3 '20 at 18:31
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6$\begingroup$ For reference, the surface temperature of Venus is between 600 and 800 Kelvin. Making electronics that can both do something actually useful and survive that kind of conditions for more than a couple of hours is hard. $\endgroup$– MastFeb 4 '20 at 8:41
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3$\begingroup$ @Mast just fyi Would it be possible to build a probe that could operate at about 480 °C (900F degrees) without insulation? $\endgroup$– uhohFeb 4 '20 at 10:34
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$\begingroup$ Silicon carbide electronics can function at around the surface temperature of Venus, but you don't want them much higher. Fans might actually be useful for keeping near-ambient power electronics "cool" on Venus. Then again, the atmosphere's so dense that you're probably not going to need forced convection...something immersed in that atmosphere will have to be producing quite a bit of heat to get significantly warmer than the surroundings. $\endgroup$ Feb 7 '20 at 1:51
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The effect of a fan in an electronic device is to accelerate the temperature exchange between circuits and atmosphere.
But that temperature exchange works in both directions. When the atmosphere is even hotter than the electronics you expose to it, then improving the flow with a fan will make the part even hotter. That's the principle behind a convection oven or a hot-air hair dryer, by the way.
So adding fans to a probe which is supposed to stay below 100°C within the 450°C atmosphere of Venus would be rather counter-productive.
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As pointed out, a cooling fan cannot reduce the temperature below the ambient. That doesn't mean it's completely useless in this case.
First, OP said "Venus-bound." In this case a fan could be used to transfer heat from sunward sections to shadowed sections, or even better to radiating heat sinks on the shadowed side.
Even when on the planet, it could be used to distribute air cooled by a centralized cooling system (which would likely need to be active, since most electronics are silicon-based and won't do well over 150C).
If high temperature circuitry is used that can operate at about 500C, a fan would still help distribute the heat generated by high-power devices.
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1$\begingroup$ I am assuming you mean for this fan to be used in conjunction with a sealed enclosure full of gas or something? I ask because otherwise, I cannot see how these fans could work, given the near-vacuum of outer space. $\endgroup$ Feb 4 '20 at 21:19
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$\begingroup$ Yes, I'm talking about circulation of contained gases...a fan in vacuum is a self-powered pinwheel. $\endgroup$ Feb 5 '20 at 13:49
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$\begingroup$ +1 Technically true, but if your need for heat transport is big enough to consider active solutions, you might rather want to use some more efficient fluid for the job. So technically, you would use a pump. Also I don't envy whoever has to program the attitude control for a spacecraft that has large fans periodically turning on and off... $\endgroup$– mlkFeb 5 '20 at 15:57
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$\begingroup$ Granted, but the OP's question was whether fans could be used, not whether they were the best option. Just thinking outside the box a bit. $\endgroup$ Feb 5 '20 at 18:35
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Heat is exchanged via three mechanisms:
- convection (transfer of heat in gases and fluids)
- conduction (transfer of heat between touching solid objects)
- radiation (transfer of heat via thermal radiation)
In the presence of an atmosphere, transfer via convection is usually the most efficient, followed by conduction. Radiation is -- by far -- the weakest form of heat transfer.
On the way to Venus, the probe is in space and thus the strongest form of heat transfer is unavailable. There is no atmosphere one could use to transfer heat from or to. This leaves conduction and radiation. Heat travels through the probe via conduction. The electronics heat the probe. But usually, conduction is not sufficient to heat all of the probe equally, it will still have hotter parts and colder parts. The excess heat is shed via radiation, usually with special devices aptly called radiators (which look a lot like solar panels).
But as you move closer to the sun, more heat is absorbed by the probe, while radiating it gets more difficult. The equilibrium temperature rises and the probe becomes hotter.
As soon as you enter the Venusian atmosphere, convection becomes available as a method of heat transfer and a fan becomes applicable. However, the atmospheric temperature at the surface of Venus is up to 470° C. Heat transfer works in both directions, if you blow hot air (or gases, or fluids) over a cold surface the cold surface becomes warmer (blow hot air over ice, it melts quicker).
So basically, a fan would just expedite the demise of the probe since the hot, acidic venusian air is allowed to efficiently transfer it heat to the electronics.
In order to remove heat, you need something colder you can transfer heat to. In space, there isn't any matter around you could transfer heat to, so radiation is the only option. That is why the ISS has such large radiator - it is to shed excess heat.
In the venusian atmosphere, you have matter for heat transfer. But that matter is already much hotter then the operating temperature of the probe, so you would typically try to minimize heat transfer so that the probe stays cool longer and have a longer working time.
Note that fans only feel cold to a human. A fan on your desk doesn't actually cool anything. A fan on your desk blows dry air that is the same temperature as room temperature over your skin. The sweat on your skin evaporates better since humidity is carried away in the air stream. It is the process of evaporation that locally lowers temperature since water needs energy to go from fluid to gaseous - this energy comes in the form of heat that is removed from your skin, making it feel colder.
You can, however, use a fan internally, e.g. to circulate gases or fluids inside the spacecraft to distribute heat inside the spacecraft, e.g. from a cold storage to warmer sections like the hull. In that case, the fan isn't used for cooling, though, but for circulation (cf. Are fans ever used in un-crewed spacecraft?). But you can not make the spacecraft colder per se with this technique, except when transferring heat that way to a radiator where it is then removed by thermal radiation.
Note that in any closed system, a fan necessarily only adds heat because it does work - the motor has to turn, with all that entails.
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1$\begingroup$ Small correction: The surface temperature of Venus is hot, but not that hot. It only gets up to 470°C, not 600°C-900°C. en.wikipedia.org/wiki/Atmosphere_of_Venus $\endgroup$– PhilippFeb 4 '20 at 11:29
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$\begingroup$ @Philipp 470°C ~ 900°F, but i'll make that more clear ;) $\endgroup$ Feb 4 '20 at 11:46
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$\begingroup$ There is a fourth mechanism to remove heat: evaporation of water as used by the Apollo Lunar Module and the space suits worn on lunar surface. $\endgroup$– UweFeb 5 '20 at 18:34
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$\begingroup$ @Uwe That is not one of the three ways to exchange heat. But if you have read my answer, I do mention evaporative cooling. $\endgroup$ Feb 5 '20 at 23:54
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It is possible to cool with a fan below ambient temperature with the help of the thermoelectric cooler like these.
From the other side, devices under the link only support about 70 degree temperature difference so five must be cascaded for Venus, using 150 W in total that is very demanding for a battery operated spacecraft. The outer devices are also not protected from the Venus heat and must be able to withstand it.
