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One of the biggest problems with stealth in space is heat.

My idea was to have a gap between hulls filled with a liquid similar to liquid nitrogen which would chill the outer hull and electric circuits to low temperatures.

Would doing this make a drifting ship disguised as an asteroid harder to detect?

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    $\begingroup$ Generating gasses from your ship might make it more visible than the heat you're covering. People out there will be looking for cold things generating vapor (like comets) as navigational hazards, if nothing else. $\endgroup$ – DWKraus Jan 19 at 22:24
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    $\begingroup$ Liquid nitrogen is only cold because at Atmospheric pressure, it will phase change to a gas, requiring latent energy to boil. Once all of the LN2 has boiled away, the N2 can continue to heat up. It would therefore be possible to build a 'heat capacitor' like in Mass Effect using this principle. You can even vary the temperature of the heat capacitor by varying the pressure. However, this would not be very 'space efficient'... $\endgroup$ – Aron Jan 20 at 10:24
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    $\begingroup$ Yet another "I don't understand the basics of physics" questions. $\endgroup$ – Ian Kemp Jan 20 at 12:15
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    $\begingroup$ You do not ask for alterantive ways to remove heat from your ship but maybe some kind of advanced heat to electricity generator such as en.wikipedia.org/wiki/Thermoelectric_generator would allow a plausable way for your to do something to transform the heat into something less detectable. $\endgroup$ – TafT Jan 20 at 14:10
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    $\begingroup$ @TafT The hot thing cools down more than the cool thing warms up, according to Wikipedia. $\endgroup$ – wizzwizz4 Jan 21 at 10:21
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If your aim is to use liquid nitrogen to mask heat, it will be just a temporary mitigation.

The heat from the ship will cause the liquid nitrogen to heat up and evaporate in the vacuum of space.

If you want to keep the liquid nitrogen in place, you need to cool it down and dump that heat away. But dumping the heat away is what your ship was doing already before you put the liquid nitrogen in place.

Basically this system will attenuate your thermal signature as long as your reservoir of liquid nitrogen lasts.

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    $\begingroup$ space is about 3K, liquid nitrogen is about 77K. Liquid nitrogen won't hide your thermal signature. It may temporarily reduce it, but you'll still be there, visible against the background. Liquid helium is at 4K though so it would work much more like how you describe $\endgroup$ – Tristan Jan 20 at 10:10
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    $\begingroup$ It'll evaporate, not sublime as it's not starting out as a solid $\endgroup$ – SiHa Jan 20 at 10:49
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    $\begingroup$ it can as well sublime - depending on the pressure it is kept at. $\endgroup$ – fraxinus Jan 20 at 19:40
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    $\begingroup$ @GarrettMotzner, The difference between the liquid nitrogen trick and "freezing the ship's hull in a spaceport" is that there's a phase change when the nitrogen boils. The temperature of your entirely-solid hull will start to rise from the very moment that you start to generate heat inside of it. But the temperature of a hull that is cooled by an evaporating liquid will not even begin to rise until all of the liquid has boiled away.* But like L.Dutch says, the effect is temporary. Eventually, all of the liquid boils away. $\endgroup$ – Solomon Slow Jan 20 at 22:19
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    $\begingroup$ @Tristan: Thermal emissions go up with the 4th power of the temperature. That's one of the highest exponents you're likely to see in ordinary physics. Cooling down a ship to 1/4 (300K to 75K) reduces its thermal emissions to 1/256, so 99.6% less. In the asteroid belt, that is effectively invisible. $\endgroup$ – MSalters Jan 21 at 12:03
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Depends where your ship is.

If you're in, say, Earth's orbit, or anywhere inside the Main Belt, anyone who happens to detect the ship with passive sensors might wonder why an asteroid so close to the sun is so cold (its surface ought to approximate black body temperature for its distance from the star) -- and if you're in interstellar space (or near it, like in the Oort Cloud) it'll still shine like a beacon because liquid nitrogen is too hot.

Now, the general idea seems sound, until you wonder where you're going to dump waste heat from inside the ship to avoid evaporating the liquid (something-or-other) in the tween-hulls space. In the end, the ship still has to radiate all the heat that's produced inside to keep from cooking the occupants and/or electronics etc. -- all your chilled outer hull can do is delay the inevitable, and for that, it's really no better than filling that same space with a vacuum (like in a Dewar flask) to limit heat transfer to the outer hull.

Any of these methods require a way to store heat inside the inner hull for some period of time, and can only last as long as that heat storage can be maintained.

The one exception here is if you're trying to hide in a region of space where objects run around 50-100K at the surface; in that case, as suggested in comments, you could use liquid nitrogen mixed with small (or trace) amounts of contaminants like carbon monoxide, ethane, and so forth with similar boiling points (they'll tend to have similar molecular weights, so you at least know which boiling points to look up), and allow it to evaporate to look like a small, prematurely active comet (even to a spectrgraph, if you get the gas mix right). This would prolong the life of your cloak, more so if you can deeply minimize heat production inside the spaceship.

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    $\begingroup$ If the ship was large enough it could contain a vast inner vacuum area that could be slowly filled with evaporating nitrogen. It should work for a while until the pressure inside reached dangerous levels. $\endgroup$ – Slarty Jan 19 at 23:05
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    $\begingroup$ I'd upvote just for "liquid nitrogen is too hot" (you'd need liquid helium at ~4K, still a degree hotter than the background at 2.7K and likely causing all kinds of issues with the containing materials). But the rest of the answer is correct as well. $\endgroup$ – Peter - Reinstate Monica Jan 20 at 10:08
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    $\begingroup$ came here to post pretty much exactly this answer, but glad to see it was already here, and the phrase "liquid nitrogen is too hot" is solid gold $\endgroup$ – Tristan Jan 20 at 10:11
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    $\begingroup$ @Aron Looks as if it's too hot as a liquid even at zero pressure, see engineeringtoolbox.com/nitrogen-d_1421.html. $\endgroup$ – Peter - Reinstate Monica Jan 20 at 10:40
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    $\begingroup$ @VaradMahashabde Latent heat of vaporization isn't generally huge unless the liquid is water. And keeping the vapor is just a way of storing the heat inside the ship -- still has the same limitations. Further, you wouldn't run an ion drive when you're hiding; the drive plume emits light as the electrons recombine with the ions. $\endgroup$ – Zeiss Ikon Jan 20 at 18:52
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Nitrogen isn't inherently cold. It has some properties that make it useful for refrigeration/cryonic applications: it's easily available, not very reactive, and because it stays liquid at very cold temperatures, we can pipe it around or dunk things in it. You can think of it as something that transports or stores "cold", but it doesn't make "cold".

So it doesn't really answer the question of "where does the cold come from?" aka "where are you moving the heat to?"

Spacecraft contain components that generate heat: electronics, engines, people. Unless you want your spacecraft to melt, you have to get rid of that heat somehow, and in the long run you have to get rid of it as fast as you're generating it.

On Earth, the three ways of shedding heat are conduction, convection, and radiation.

In space, conduction isn't a thing (your ship isn't touching any other objects), so you're left with the other two.

  • Radiation is the usual method, but that's how you get detected - effectively you're shining red-hot, for some values of "red".
  • Convection means carrying around some kind of thermal mass, transferring heat from the rest of your ship into that mass, and then jettisoning the mass. But then even if your ship is cold, the ejecta will be hot and detectable. If you only heat it up a little bit above ship temperatures, it's not going to be weight-efficient - you'll be carrying around a lot of mass and you'll run out quickly. (Nitrogen is not a great choice for this, BTW - you want something with a high heat capacity per kilogram.)

In the long run, there is no physics-compatible solution that will stop you from emitting heat, which then makes you detectable. About the only thing you can do here is bank that heat for short periods - you might be able to chill your hull while heating up an insulated reservoir hidden inside the ship, which might make you harder to detect for a while, but eventually you'll need to dump that heat one way or another and then you'll be extra-visible. This could be tactically useful if you know exactly when you need concealment, but not for ongoing concealment.

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    $\begingroup$ Yes! Good dissection of the logical error: 'It doesn't make "cold".' $\endgroup$ – Peter - Reinstate Monica Jan 20 at 10:43
  • $\begingroup$ What about radiating the heat away directionally so that, even though you're detectable, it'd be only within a narrow cone (that likely is pointing away from whatever's trying to detect you)? $\endgroup$ – jamesdlin Jan 21 at 22:14
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    $\begingroup$ This is a great answer both in that it explains the physics, and explaining why thermal masking might be possible for short periods, but not indefinitely. I'm imagining that prolonged use of "thermal stealth" would cause the inside of the ship to get hotter and hotter until the crew begin suffering heat stroke. If you're not venting heat away, then it's gotta be staying in the ship! $\endgroup$ – Mike Ounsworth Jan 21 at 23:13
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    $\begingroup$ @jamesdlin Atomic Rockets covers this. If the cone is too narrow, you need an excessive amount of power just to pipe heat into the radiators properly (and that means turning up your reactors, which make even more heat...), and if it's even decently wide, all your adversary needs is a smattering of drones around the system for one to pick up the signal and relay the detection down the line. $\endgroup$ – parasoup Jan 22 at 5:55
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    $\begingroup$ @MikeOunsworth Kinda like old-fashioned submarine warfare, where staying submerged meant running down your batteries and building up CO2. $\endgroup$ – Geoffrey Brent Jan 22 at 22:33
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The real problem is, where is all that heat coming from? You see, if you stop all systems on board, your ship will cool down to the same temperature of any asteroid around - so is, depending exclusively on how much radiation is exposed to - but, of course, that is probably too cold for the critters inside it.

You can, of course, cool down your external hull trying to disguise your heat signature, but not for too long. Your liquid similar to liquid nitrogen will soak up heat very quickly, and thus will stop working. You'll either run out of liquid, or will have to cool it down in a termal cycle, but you can't do that without getting rid of the excess heat somewhere else... thermodynamics is a hard mistress.

But yes, for a short period of time you could reduce your heat signature, for example before making an infiltration attempt on enemy territory. It won't last very long, so you pray to get through quickly.

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Geoffrey has it right but let me rephrase it.

Your actual problem is to cool down your hull.

Phrased this way it is immediately clear that flooding a whole space between two hull layers is wasteful and unnecessary. For example, a proven way to cool down surfaces (you'll need to get down to 3K or so to match the microwave background) are pipes with coolant, much like in your freezer. Since nitrogen becomes solid at around 60K even at zero pressure you'll need something like liquid helium in those pipes. That will bring your temperature down to at least 4K. The technology exists today — it is used to cool superconductors.

Other posts have pointed out that keeping the hull cold is unsustainable because it prevents radiating the heat away which is generated inside by electronics, machines and organisms. The ship will heat up over time; darned entropy. But for a while it can work, principally.

The main goal of such a ship design would be to minimize the amount of heat which reaches the surface from the interior. The reason is two-fold: First of all, less heat is obviously easier to deal with. But secondly, and perhaps most importantly, what we have here is basically an inside-out fridge, where the cold surface faces outside. Like all mechanisms, the cooling creates additional heat. That's well-known for laptops: If you can get the heat production down to a level that can be cooled passively you have an additional jump in battery time because the active cooling itself needs energy which is now available for computing. The following strategies can be employed to increase mission time:

  • Minimize the heat flux to the outer surface. This will reduce the necessary cooling effort and, as said above, reduce heat production from cooling. The heat flux can be tackled from two sides:
    • Minimize the overall energy consumption of the ship. Eventually all energy ends up as heat and must be dealt with. A good design starts at the root of the problem. Minimal life support, no creature comforts.
    • Interrupt the heat flux through insulation. The most effective insulation will be much like a thermos: A multi-layer hull with the layers separated by vacuum, with mirror surfaces.
  • Have a dedicated heat sink inside the ship which in turn is insulated and can be heated up, thus storing the produced heat. The heat sink could be heated to temperatures above the rest of the ship's interior with heat pumps, but there is probably a sweet spot above which the heat pumps themselves create so much additional heat that it outweighs a higher heat sink capacity.

That reservoir, as well as any other possible part of the ship, should be cooled down as far as possible before a mission. If the vessel were unmanned it could be cooled down to 3K throughout (which probably means that it must be away from any warm planet, let alone star, when the mission starts).

So what we have is a flying thermos that looks like the cooling surfaces in your freezer ;-).

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  • $\begingroup$ By the way, if someone played Mass Effect, the protagonist's ship had a "silent running" feature, where she was not dumping excess heat into the space, but was conserving it within the hull for a short period of time. And yes, being a good thermos, she was getting heated to inhumane temperatures on the inside while being as cool as the ambient space. $\endgroup$ – Xobotun Jan 21 at 9:44
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Only temporarily (and only assuming the liquid nitrogen was cold as the background -- liquid helium temperatures are really the temperatures you're going for if you want to match thermal background radiation, and at that temperature nitrogen is a solid).

Let's say at t=0, you have a warm spacecraft with internal heat sources (people, fuel cells, equipment) and a cold jacket of ultracold material (liquid helium, solid nitrogen, whatever). At that time, the jacket is emitting blackbody radiation that looks like the thermal background of the universe, and the ship is invisible. At t=t1, sometime later, the jacket has absorbed some heat from the warm interior of the ship. Now, one of two things happens. Either the heat goes into changing the state of the jacket (say, boiling off some liquid helium), in which case the remaining jacket's temperature remains the same and the ship stays invisible (but for the venting gasses), or the jacket warms up (say, if you've just warmed the solid nitrogen coat, but haven't reached a phase transition), in which case the blackbody spectrum changes and the ship becomes more visible than before. In the first case, you're fine until you run out of jacket to boil off; then your ship is visible. In the second case, the jacket keeps warming until it has the equilibrium temperature of the interior of the ship. If it hasn't undergone a phase transition (e.g. let's say it started as ultracold iron, and now it's just lukewarm iron), it now has a blackbody spectrum matching the naked ship, and either way your ship is visible.

The next thing that you'll probably say is, "Well, what if I keep the jacket cold?" The question is, how will you do that? If you have a magic heat sink for the heat that the jacket will absorb from the interior, just pump the interior heat directly into that sink -- no need for the jacket. Unfortunately, any scheme for avoiding exterior radiation will presumably involve pumping heat into an interior reservoir, which will get hotter and hotter until the heat is no longer containable, and the ship will be (briefly) VERY visible.

So the short answer is, no: your interior heat has to go somewhere, and while you can delay its emission, you can't prevent it. (Now, if the goal is to temporarily cloak a ship, that's doable, at the expense of having to dump more heat later.)

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Yes! It will mask its thermal signature and that could make you invisible to a thermal scanner but only if the spaceship and the surroundings are at the same temperature and this will not be the case in most scenarios in outer space, the temperature in outer space is −270 °C, nitrogen is to hot for it at around −160 °C and cause of the vacuum this would turn solid , helium on the other hand can be in a liquid state at −269 °C , this could work but you still have yo deal with the radiation energy coming from within the ship, maybe a super reflective surface in the inside that concentrates all that energy to a point were its stored until its emitted in a laser beam once it reaches maximun capacity (this gives room for a lot of tense story writing when the bad guys are looking for your ship and the heath storage system its about to mealt down), but some how this doesn't convince me... Another idea would be to have a sphere shaped shield or spaceship designed in such a way that its made with very small 'tiles' that are paired with another tile in the opposite side of the sphere, both receiving the spectrum of the other and emitting it in their respective side, dont know if the black body radiation of the materials to create that simulated spectrum would interfere with the signal emitted making this idea worthless.

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    $\begingroup$ Welcome Alex. Thank you for contributing. I am downvoting your answer, simply because your answe isn't well grounded in science. It seems most of your experience and intuition exists in Standard Temperature and Presure. You will find neither in space. The limit of the temperature that LN2 can get down to in a vacuum is really down to how low a pressure you can get, and Adiabatic Expansion. $\endgroup$ – Aron Jan 20 at 10:20
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    $\begingroup$ @Aron I don't think that's true. LN2 will become solid below 60K or so at zero pressure -- indeed, at any pressure -- if I read the diagrams correctly. $\endgroup$ – Peter - Reinstate Monica Jan 20 at 12:03
  • $\begingroup$ Can you clarify the "sphere shaped shield" idea? $\endgroup$ – Peter - Reinstate Monica Jan 20 at 12:05
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Perhaps water could be sprayed into a cloud, which will scatter and mask the heat in the direction you choose.

Some heat radiator system proposals suggest spraying water on the radiator as a way to dump heat quickly For combat operations, perhaps? When high heat systems must be cycled faster than the radiator can handle.

This same water supply could be applied to spray a cloud towards the ship or station one wishes to hide your heat from?

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Objects give off different wavelengths and amounts of radiation based on their temperature which is called black-body radiation. The radiation is given off in a continuous spectrum in a curve with a peak at a certain frequency depending on the temperature. There's a cool interactive visualization published by the University of Colorado that lets you play with parameters. Unfortunately it only lets you go down to 200K, but fortunately they made the source code available. If you play around with their simulation:

  • the sun is at 5850K, gives off 84 million watts per square meter with a peak of 500 nanometers which is in the visible range
  • a body at 300K gives off only 459 watts per square meter with a peak at 9,659 nanometers which is in the infrared
  • at 77K (modifying source required) it only gives off 2 watts per square meter with a peak at 37,633 nanometers which is also in the infrared, need to get to about 1,000,000 nanometers to push it into microwaves

So enemies could detect you, but you're giving off 1/200th the energy of something at 300K and 1/45th the energy of something at 200K (typical asteroid), so you would be that much harder to detect. Also their equipment might need to be tuned for different frequencies or they may have to have different equipment all together to detect you compared to a normal ship. If stealth ships are a new technology, they might not be prepared and miss you all together.

But you have to think about where the energy from your goes. If you have a layer of vacuum between your actual hull and the liquid nitrogen, you'll cook inside if you're producing much energy at all. If you dump heat into the liquid nitrogen it will boil and increase the pressure until your container bursts. If you're near the sun and don't reflect it's radiation, that will warm it up too (ala 200K in the asteroid belt) It might be efficient to have blocks of regular water ice for this purpose. 1kg of ice needs to absorb about 330kJ of energy to melt, then it will take another 420kJ to raise it to the boiling temperature. So you can have a certain energy budget you can maintain your stealth mode for before you need to vent some of that heat. If enemies are only in one direction, you could have heat pipes transferring that heat to a large radiator that could emit heat away from your enemies which would be nearly impossible to detect from the opposite direction.

You might be able to rendezvous with a comet or asteroid and dump some heat into that. You would need some form of propulsion that would let you perform a rendezvous without being seen though, like some form of reactionless drive. Even if you vent the liquid nitrogen for thrust, it will expand to make a larger cloud and will be warmed by the sun which will both serve to increase your energy signature and provide a way to track your trajectory. If you start lighting off a fusion drive, you can forget about stealth.

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