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The question For a Venus bound space probe could could a fan be used to prevent overheating? got me thinking.

Most if not all crewed spacecraft have had fans to move air for one reason or another.

However most spacecraft are un-crewed and some have substantial internal volume and lots of electronics. When my imagination runs wild I imagine a small server farm with racks full of electronics, blinking LEDs and the roar of cooling fans.

Of course liquid circulation can cool things, so an atmosphere and fans are probably not necessary.

Question: But have fans ever been used in un-crewed spacecraft?

note: By "un-crewed" I mean spacecraft that were not designed for humans.

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    $\begingroup$ Note that in a perfectly closed system, fans increase heat rather than decrease it. They don't cool the air (they just mix it), and the friction losses from the engine will generate additional heat. Fans really only work in cases where you want a certain region to be provided (efficiently) with air from another nearby (colder) region. $\endgroup$
    – Flater
    Feb 4 '20 at 10:42
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    $\begingroup$ @Flater yep, but mixing can be very helpful if there are point sources of heat, or if there are reasons to mix or move gas unrelated to heat. $\endgroup$
    – uhoh
    Feb 4 '20 at 10:52
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    $\begingroup$ @Flater fans are used in spacecraft air cooling loops to circulate the air from a heat source over a heat exchanger and back. Heat added by the fan is an unavoidable "tax'". Shuttle used air loops dumping heat into water loops dumping heat into Freon loops and then overboard. $\endgroup$
    – Organic Marble
    Feb 4 '20 at 11:50
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    $\begingroup$ @OrganicMarble: Having a heat exchanger precludes the closed system I mentioned. I merely wanted to dispel the possible implication of the question that fans by themselves cool things down, as they don't. It was unclear if this was part of the reasoning of the question, which is why I disambiguated the point. $\endgroup$
    – Flater
    Feb 4 '20 at 11:53
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    $\begingroup$ Am I correct in my impression that most unmanned spacecraft are unpressurised? $\endgroup$
    – ikrase
    Feb 4 '20 at 20:11
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Soviet planetary probes sometimes had pressurized compartments, so I suspected that they might contain fans.

This answer confirms that Venera-8 had a fan. The illustration in the answer has the inner parts mislabeled, though. The fan is really #15 in the drawing.

Reading through the source linked in that answer also confirms that Venera-5 had a similar setup, with fan. Since the other answer didn't show its diagram, I will. (source)

enter image description here

Also, according to Soviet Robots in the Solar System by Huntress and Marov, the early Soviet lunar impactor spacecraft Ye-1 (aka Luna) had a pressurized compartment with a fan.

The interior held 1.3 bar of nitrogen that was circulated by a fan between the cold outer shell and the warm electronics for thermal control...

(p. 73)

So, yes, those are three examples. I suspect all Soviet craft with pressurized compartments had circulation fans.

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    $\begingroup$ Excellent. I think this also answers For a Venus bound space probe could could a fan be used to prevent overheating?. In fact, maybe this should be moved there instead? I wouldn't want to steal a new user's first question! Then I can change this to Other than spacecraft to Venus... $\endgroup$
    – uhoh
    Feb 4 '20 at 2:16
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    $\begingroup$ @uhoh it looks like Soviet Mars probes used pressurized compartments too, so they probably had fans. I'm not sure that other question is really asking about an internal cooling fan. $\endgroup$
    – Organic Marble
    Feb 4 '20 at 2:23
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    $\begingroup$ @uhoh ? Nothing I listed was meant to be crewed. $\endgroup$
    – Organic Marble
    Feb 4 '20 at 3:35
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    $\begingroup$ I'm at a loss as to what any of this has to do with my answer. If @uhoh is being hinty about Laika's craft or similar using fans, note that the question doesn't ask for the earliest craft with fans, just if any had them. $\endgroup$
    – Organic Marble
    Feb 4 '20 at 12:56
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    $\begingroup$ @OrganicMarble Early Soviet Mars probes were internally identical to their Venus probes - they built two at a time and sent one to Venus and one to Mars. However, by the time Venera 8 was made, they knew more about the surface conditions on Venus and the probes were no longer as similar. $\endgroup$
    – Skyler
    Feb 4 '20 at 14:54
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Sputnik-1 was filled with dry nitrogen pressurized to about 1.3 bar and had a fan to control gas temperature between 20° and 30° C.

See Wikipedia in german or english.

A picture of Sputnik-1 design with the cooling fan can be found here.

Korabl-Sputnik 1, an unmanned early prototype of the later manned Vostok spacecraft, had a biological cabin with a dummy of a man. It was intended to test the life support system, so it had an atmosphere and a fan. See [Wikipedia] (https://en.wikipedia.org/wiki/Korabl-Sputnik_1).

Early russian lunar probes were also filled with nitrogen and contained a fan, Lunik and Luna.

Space probes using photographic film developed in space needed an atmosphere inside to use a wet or semidry developing process. Not only russian probes but also the NASA Lunar Orbiters. Some information about film development here.

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