Would a Submarine Work as a Spaceship?

xkcd's What If?
5 Mar 202404:23


TLDRIn this intriguing video script, the presenter explores the fascinating idea of sending a nuclear submarine into orbit. While acknowledging the initial assumption that submarines wouldn't fare well in space, the script delves into unexpected challenges, such as overheating due to the lack of cooling mechanisms and the potential for reactor meltdowns. The presenter then proposes an ingenious solution: using the submarine's ballistic missiles in reverse to de-orbit and re-enter the atmosphere, albeit with significant risks. The script concludes with a humorous reminder about disabling the detonators, leaving viewers captivated by the thought-provoking and entertaining exploration of this unconventional scenario.


  • ๐ Nuclear submarines are not designed to operate in space and would face significant challenges if placed in orbit.
  • ๐ฆ Submarine hulls can withstand high water pressure, but are not designed to hold in air over an extended period.
  • ๐ฌ While carbon dioxide can be scrubbed, submarines cannot produce oxygen indefinitely without a water source.
  • ๐ฅ The biggest issue would be overheating from the nuclear reactor, which relies on seawater for cooling.
  • ๐ Space near Earth is technically 'warm' due to the high kinetic energy of molecules, but this heat is negligible.
  • ๐ก Without a way to dissipate heat, the submarine's reactor would overheat and likely melt down within an hour.
  • ๐ Firing missiles backwards could theoretically provide enough thrust to de-orbit the submarine, but it would break apart upon re-entry.
  • ๐ฅ There is a tiny chance of survival by sheltering in the right part of the wreckage and using a parachute before impact.
  • โ ๏ธ Disabling the missiles' detonators would be crucial to attempting this highly inadvisable maneuver.
  • ๐ค Overall, putting a nuclear submarine in orbit is impractical and extremely dangerous without significant modifications.

Q & A

  • Why wouldn't a nuclear submarine burst in space?

    -The hull of a nuclear submarine is designed to withstand extreme external pressures of up to 50-80 atmospheres from water. This makes the hull strong enough to contain the internal pressure of just 1 atmosphere of air, preventing it from bursting in the vacuum of space.

  • How would a nuclear submarine run out of oxygen in space?

    -Nuclear submarines extract oxygen from water through an electrolysis process. In space, there is no water available, so they would eventually run out of the oxygen reserves they carry and be unable to manufacture more air.

  • Why would a nuclear submarine overheat in space?

    -The nuclear reactor on a submarine generates a tremendous amount of waste heat, around 100 megawatts. This heat is normally dissipated by the surrounding seawater. In space, without water to act as a coolant, the heat would rapidly build up, causing the submarine to overheat and potentially leading to a reactor meltdown.

  • How could a nuclear submarine theoretically de-orbit itself?

    -By placing the ballistic missiles carried on the submarine in the launch tubes backwards, they could be fired to provide enough thrust to slow the submarine down and cause it to re-enter the Earth's atmosphere. However, this would likely result in the submarine breaking apart during the uncontrolled re-entry.

  • What precaution would be critical if attempting to de-orbit a nuclear submarine?

    -It would be absolutely critical to remember to disable the detonators on the missiles before attempting such a maneuver, to prevent the nuclear warheads from detonating during the re-entry process.

  • Why is space considered 'warm' near Earth, according to the explanation?

    -Space near Earth is considered warm because the molecules present can have very high average kinetic energies, corresponding to thousands of degrees. However, the low density means this heat is not effectively transferred, similar to how sparks from a grinder don't burn skin.

  • How does heat transfer differ in space compared to on Earth?

    -In space, there is no air or water to facilitate heat transfer by conduction or convection. The only way for heat to be lost is through radiation, which is a relatively inefficient process, making it harder for objects like a submarine to cool down.

  • What analogy is used to explain why the 'hot' molecules in space don't make it feel warm?

    -The analogy of sparks from a grinding wheel is used. Even though the sparks are thousands of degrees hot, they are tiny and there are so few of them that they don't appreciably warm up a person's skin when they land on it.

  • How many Trident missiles would be needed to potentially de-orbit a nuclear submarine?

    -According to the explanation, the 24 Trident missiles carried by an Ohio-class nuclear submarine, if fired backwards from the launch tubes, could theoretically provide enough thrust to change the submarine's speed by around 100 m/s, which is in the range required for a de-orbiting maneuver.

  • What is the key limitation that prevents a nuclear submarine from simply using its engines to de-orbit?

    -Nuclear submarines do not have engines or thrusters designed for propulsion in space. Their only 'rockets' are the ballistic missiles they carry, which are not attached to the submarine and thus cannot provide meaningful propulsion when fired normally.



🚀 Surviving a Nuclear Submarine's Re-entry from Orbit

The paragraph discusses the hypothetical scenario of a nuclear submarine ending up in orbit and the challenges it would face in attempting to re-enter Earth's atmosphere. It explains that while the submarine's hull could withstand the pressure, the lack of water for oxygen extraction and the inability to dissipate heat from the nuclear reactor would pose significant threats. The paragraph then explores the possibility of using the submarine's ballistic missiles in reverse to slow its speed and initiate a controlled re-entry. However, it acknowledges the high risk of the submarine breaking apart during atmospheric re-entry due to its lack of heat shielding and aerodynamic instability. It suggests that with precise positioning and an acceleration couch, there is a tiny chance of surviving the rapid deceleration, provided the missile detonators are disabled beforehand.



💡Nuclear Submarine

A nuclear submarine is a submarine powered by a nuclear reactor. The video discusses the potential challenges of putting a nuclear submarine in orbit, as highlighted by the question, "How long could a nuclear submarine last in orbit?". It explains that while the submarine hull can withstand immense water pressure, it is not designed to operate in the vacuum of space.


Orbit refers to the curved path of an object revolving around another object, typically a planet or a star, due to the force of gravity. The video explores the scenario of a nuclear submarine being placed in Earth's orbit and the difficulties it would face, such as lack of water for oxygen generation, overheating due to the reactor's heat output, and the inability to de-orbit without specialized propulsion systems.


Pressure is the force applied perpendicular to the surface of an object per unit area. The video mentions that submarine hulls are designed to withstand external pressures of 50 to 80 atmospheres from water, which is why they can contain the internal pressure of one atmosphere of air without bursting in space.

💡Carbon Dioxide (CO2) Scrubber

A CO2 scrubber is a device used in submarines and spacecraft to remove carbon dioxide from the air, making it breathable. The video notes that submarines use CO2 scrubbers, which can run indefinitely as long as they have power, so dangerous carbon dioxide buildup would not be an issue for a submarine in orbit.

💡Oxygen Generation

Oxygen generation refers to the process of producing oxygen, which is essential for human respiration. The video explains that nuclear submarines use electricity to extract oxygen from water, but in the vacuum of space, there is no water, so they would not be able to manufacture more air once their oxygen reserves are depleted.


Temperature is a measure of the average kinetic energy of particles in a system. The video discusses the concept of temperature in space, explaining that while space itself is very cold, the molecules in Earth's orbit can have high kinetic energies, leading to the counterintuitive notion that space near Earth is actually warm.


Conduction is the transfer of heat through a material by the collision of particles. The video mentions that in space, without air or water around the submarine, heat loss by conduction or convection is limited, making it difficult for the submarine to dissipate the heat generated by its nuclear reactor.


Radiation is the emission or transmission of energy in the form of waves or particles. The video explains that in the absence of conduction or convection, the primary mode of heat transfer in space is radiation, which is not a very effective way to cool down a nuclear submarine's reactor.


De-orbit refers to the process of slowing down an object in orbit enough for it to re-enter the atmosphere. The video explores the possibility of using the submarine's ballistic missiles as makeshift rockets to provide the necessary thrust for de-orbiting, albeit with the risk of the submarine breaking up during atmospheric re-entry.

💡Ballistic Missile

A ballistic missile is a missile that follows a ballistic trajectory, with little or no ability to change course during flight. The video proposes the unconventional idea of using the submarine's ballistic missiles in reverse to generate thrust for de-orbiting, as the submarine lacks conventional propulsion systems for this purpose.


Nuclear submarines wouldn't burst in space due to their hull strength, but they would face other challenges.

Submarines use CO2 scrubbers that can run indefinitely as long as they have power, so carbon dioxide buildup wouldn't be an issue.

The inability to manufacture more air would be a problem, as submarines rely on extracting oxygen from water.

The main issue would be overheating due to the lack of a cooling medium like water to dissipate the heat from the nuclear reactor.

Space near Earth is actually warm in terms of the kinetic energy of particles, but this heat is negligible due to the small size and low density of the particles.

The submarine would need to slow down enough to hit the atmosphere to get out of orbit, which it cannot do without rockets.

The ballistic missiles carried by the submarine could be used in reverse to provide enough thrust for de-orbiting, but the submarine would likely break up during atmospheric re-entry.

There is a tiny chance of survival if strapped into an acceleration couch in the right crevice and using a parachute before impact, but disabling the missile detonators would be critical.



This question comes from Jason, who asks: How long could a nuclear submarine last in




The answer is “not very long,” but not for the reason I expected.


The submarine wouldn't burst.


Submarine hulls are strong enough to withstand 50 to 80 atmospheres of external pressure


from water, so they'd have no problem containing one atmosphere of internal pressure from air.


And the hull would likely be reasonably airtight.


Although watertight seals don't necessarily hold back air, the fact that water can't find


a way through the hull under 50 atmospheres of pressure suggests that, when the sub is


in space, air won't escape quickly.


Dangerous carbon dioxide buildup wouldn’t be an issue as submarines use CO2 scrubbers


that can be run indefinitely as long as they have power.


But oxygen is another story: nuclear submarines use electricity to extract oxygen from water.


In space, there's no water, so they wouldn't be able to manufacture more air.


They carry enough oxygen in reserve to survive for a few days at least, but eventually they'd


be in trouble.


The really big problem, though, would be overheating, because space is much warmer than the ocean.


If you're pedantic, that's not really true.


Space is of course very cold.


But if you're even more pedantic (and I am), it is true – in two different ways!


Space in Earth orbit seems cold because it's so empty.


Without a warm environment around you radiating heat back to you, you lose heat by radiation


much faster than normal.


But space in earth orbit is actually warm.


The pedantic reason for this is that temperature is a measure of the average kinetic energy


of a collection of particles.


And in space near the earth, molecules can have average kinetic energies in the thousands


of degrees.


This doesn’t make space feel warm, though.


When I was a kid, I remember watching my dad use a metal grinder.


Whenever metal touched the grinding wheel, sparks flew everywhere, falling in a shower


on his hands and clothes.


I couldn't understand why they didn't hurt him—after all, the glowing sparks were several


thousand degrees.


I later learned that the reason the sparks didn't hurt him was that they were tiny; the


heat they carried could be absorbed into the body without warming anything more than a


tiny patch of skin.


The hot molecules in space are like the sparks in my dad's machine shop; they might be hot


or cold, but they're so small and there are so few of them they don't change your temperature




Instead, your heating and cooling is dominated by how much heat you produce and how quickly


it pours out of you into the void.


And this is the practical reason that space is warm: without air or water around you to


carry heat away from your surface, you don't lose heat by conduction or convection – just


radiation, which isn’t a very effective way to cool down.


For most human-carrying spacecraft, the big problem isn't staying warm, it's staying cool.


And a nuclear submarine isn’t just carrying humans – it’s carrying a 200 megawatt


nuclear reactor.


It’s hard to get good numbers on the efficiency of nuclear reactors in military submarines,


but a conservative guess based on civilian reactors is that around half of the reactor’s


energy – or around 100 megawatts – is lost as heat.


This heat is normally dissipated by seawater, but again, there’s no water in space.


Without cooling, a 200 megawatt nuclear reactor outputs enough heat to warm the entire submarine


by about half a degree Celsius every minute – the submarine would become too hot for


human survivability within an hour.


But the nuclear reactor isn’t designed to warm the submarine in this way, so instead


the heat would build up within it and lead to a reactor meltdown.


It appears a nuclear submarine in orbit is a bad idea.


To get out of orbit, the submarine would need to slow down enough that it hit the atmosphere,


which would slow it down the rest of the way.


Without rockets, it has no way to do this.


Okay—technically, the submarine does have rockets.


The problem is, they're not attached to the submarine; launching the missiles won't meaningfully


propel the sub.


But they don't need to be attached – they just need to be turned around!


If the ballistic missiles carried by a modern nuclear submarine were placed in the tubes


backward, they could each change a large nuclear submarine's speed by about 4 meters per second.


A typical de-orbiting maneuver requires in the neighborhood of 100 m/s of speed change,


which means that the 24 Trident missiles carried by an Ohio-class submarine could be just enough


to get it out of orbit.


Now, because the submarine has no heat-dissipating ablative tiles, and because it's not aerodynamically


stable at hypersonic velocities, it would inevitably tumble and break up in the atmosphere,


the debris disintegrating in the air or plowing into the ground at several hundred knots.


If you tucked yourself into the right crevice in the submarine—and were strapped into


an acceleration couch—there's a tiny, tiny, tiny chance that you could survive the rapid


deceleration in the atmosphere.


Then you'd need to jump out of the wreckage with a parachute before it hit the ground.


If you ever try this, I have one piece of advice that is absolutely critical: remember


to disable the detonators on the missiles.

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