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39 votes

Could a fan be used to prevent overheating on a Venus bound space probe?

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 - ...
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36 votes

Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed?

At least for the Space Shuttle, freezing was OK, but thawing out was bad for piping. Hydrazine contracts when it freezes, so it can 'superpack' (more fluid flows in, then freezes, etc.)...then when it ...
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30 votes
Accepted

Are fans ever used in un-crewed spacecraft?

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 ...
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30 votes

Why exactly does Curiosity's RTG make more electricity on cold days?

There is Carnot's theorem for the theoretical maximum efficiency of heat engines. It is valid not only for mechanical engines like steam engines or Stirling engines but also for solid state devices ...
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30 votes

Could sheets of stacked graphene be used as part of a heat shield, since its melting point is 3000k to 5000 K

For a non ablative heat shield you need a material with a very high melting point and a very low thermal conductivity. It should not burn in hot air. Unfortunately graphene seems to have a high ...
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27 votes
Accepted

Why are RTGs different colors?

Answer: Thermal radiating coating technology has improved, so they are no longer forced to be sub-optimally black in visible light. They can now be white and reflect incident sunlight to improve ...
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27 votes

Why are RTGs different colors?

The big difference between the two darker RTG fins (Black and Grey) and the white RTG fins, is that the white fins were destined for use in an atmosphere (Mars). The presence of an atmosphere, even ...
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26 votes

Where exactly are the Apollo space suit sublimators venting steam into space? is there a photo of an astronaut in space that shows the opening?

It can be hard to see because it's on the front of the PLSS behind the astronaut's helmet. (NASA photo of Irwin on Apollo 15, I cropped it and added the arrow) Here's an exploded view showing the ...
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25 votes
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How are the electronic circuits in interplanetary missions protected from extremely low temperatures?

Space in the vicinity of the Sun is not that cold, perhaps even out to Jupiter. One side of the spacecraft will be receiving sunlight while the side shielded from sunlight radiates into empty space. ...
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24 votes
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Why do many RTGs have finned heatsinks, when this is considered bad for radiation heat transfer?

With typical active radiators on spacecraft, heat is transferred away from the sources into the radiators through forced convection - as heated coolant. At that point the only concern remaining is to ...
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21 votes
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Why is the sunny side of JWST's sunshield purple and covered with silicon?

From this presentation James Webb Space Telescope (JWST) Technology Discussion For APPEL Forum, dated August 7, 2008, I see this: Low $\alpha_S$/$\epsilon_H$ to minimize absorbed solar heat load ...
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20 votes

Why do many RTGs have finned heatsinks, when this is considered bad for radiation heat transfer?

Multi-fin radiators are worse per unit mass. But for an RTG, it is absolutely vital to provide a very large thermal gradient between the (very small) core and the outer layers. Adding more fins still ...
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18 votes
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Now that Perseverance is "hot" (RTG in place) and before it gets to deep space, how will it stay cool?

2kW is not that much on Earth You've mentioned radiation and convection in your answer (you forgot conduction). Turns out the properties of Earth's atmosphere make conduction and convection way ...
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  • 975
16 votes

temperatures of metal facing the sun in space

A single number is impossible to give, unless you exactly specify the kind of metal. So I'll answer in the general sense. The first question to answer is "How much energy does the sheet absorb?&...
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15 votes
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Why the thermal imaging of Mercury's surface requires a telescope on a jet flying through an eclipse?

Why not a satellite-based telescope to observe Mercury in the thermal infrared? Space-borne satellites that are designed to look at the Sun (e.g., SOHO) aren't instrumented to look in the thermal ...
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15 votes
Accepted

Is it feasible to paint SpaceX Starlink satellites black so as not to frequently saturate the CCDs of the Large Synoptic Survey Telescope?

In general satellites are not "painted". They are covered in a variety of Multi-Layer Insulation (MLI) blankets with varying optical qualities. I have seen MLI in silver, black, and gold - sometimes ...
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  • 4,159
15 votes

Are fans ever used in un-crewed spacecraft?

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 ...
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14 votes

Is it bad if hydrazine freezes on a spacecraft? Is it always kept as liquid, or can it be safely allowed to freeze and then thawed when needed?

The Olympus satellite (1989-053A, 20122) lost pointing and power for long enough that all the fuel froze. It was recovered after a couple months and the fuel defrosted. I couldn't easily find what ...
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13 votes
Accepted

Why does the Alpha Magnetic Spectrometer (AMS) on the ISS use carbon dioxide for coolant?

1) There is no superconducting magnet in AMS-2. This would have required cooling with liquid helium resulting in a limited life time of only 3 years because of helium evaporating. Instead, they used a ...
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13 votes

Why exactly does Curiosity's RTG make more electricity on cold days?

All heat engines, whether mechanical or solid state, produce work based on heat flow across a temperature difference. The maximum efficiency of a heat engine depends on how large that difference is.
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  • 8,421
13 votes

Is a Raptor 2's thermal output really comparable to that of a nuclear power plant?

On average it isn't wrong as a "close-enough" analogy. All but one of Britain's 10 operating nuclear power stations are scheduled to close by 2023 and two of these are planned to close at ...
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12 votes
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Are there any promising next-generation space station cooling technologies?

There are multiple possible improvements for moving the coolant, in pumping systems. There are piezoelectric pump and electrohydrodynamic pumping systems being worked on, with main advantage being ...
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12 votes
Accepted

How cold is the Martian sky at night? Or the day for that matter?

You've asked a question that is very difficult to answer accurately without in situ measurements, which apparently we don't have. The short answer: We don't know closer than ~100K! There was an ...
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12 votes

Insulation on rockets--why take it to space?

The insulation's job is also to prevent ice from forming on the rocket's skin (and breaking off during ascent). Shards of ice are more dangerous than chunks of foam. Even if ice were no problem, for ...
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12 votes
Accepted

Is a Raptor 2's thermal output really comparable to that of a nuclear power plant?

Power is in units of "energy rate," joules per second, and thus it is relatively easy to estimate for a rocket engine: take the "energy rate" of the exhaust gases at the nozzle ...
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11 votes
Accepted

How have space suits dissipated the heat removed from astronauts?

For Apollo, space suits used the Primary/Portable/Personal Life Support System during EVAs. This is a schematic of the PLSS: The PLSS used sublimation cooling. Water was run over a heat exchanger, ...
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  • 122k
11 votes

Is it normal for thrusters to "ice up"?

When this was captured, it was at the end of the rocket flight. Looking carefully at this, I don't see any other similar nozzles. Furthermore, I didn't see any evidence of this being used in flight. I'...
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  • 119k
11 votes

How can I make robust guesses about waste heat from satellites?

As a first approach, you can assume all electric power will be turned into heat. Some of the power will be used to do something first, but electrical and mechanical resistance will eventually turn all ...
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  • 122k
11 votes
Accepted

Why is the Heat Shield of ESA's Solar Orbiter Black in colour?

Apparently, black was chosen despite its thermal properties. This 2005 paper was written at the end of the assessment phase where technical feasibility of the mission has been demonstrated. It shows ...
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  • 122k
10 votes
Accepted

Mathematically, what is the argument in favor of skipping reentries?

The two systems that I know of that were qualified for skip entry, Apollo and Shuttle, did so purely for cross-range capability. The skip entry would have been used for an emergency return that could ...
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