I assume a nuclear thermal rocket with a solid core first. The principle of operation seems simple enough: Bring your propellant close to your nuclear fuel, start a reaction and wait for some heat/pressure to build up due to neutron (and other?) absorption. Once you have enough heat, you release the propellant through your nozzle and get acceleration, right?

However, when you release propellant, the pressure inside the reaction chamber drops, so you will have to refill it with some fresh propellant from your tanks. I consider this a severe practical problem, since the pressure inside the chamber is still quite high (ideally, if you want to achieve constant thrust, you need to work against your target pressure). From my limited understanding, it seems that you need to bring the propellant to an even higher pressure to press it into the reaction chamber, which would be absurd since then you could pump it through the nozzle directly.

So what am I missing here? How do you get the propellant into the reaction chamber? Bonus points for answering the same for the way more complicated liquid or gas core engines.

  • $\begingroup$ I don't know if you may be misunderstanding something fundamental: The engine runs continuously. That is, you don't keep the coolant stationary until you release it, you run it through a cooling channel where it gets heated on the way through. At least in nominal operation, the flow not stagnant anywhere. $\endgroup$ Mar 31, 2016 at 14:09
  • $\begingroup$ Of course it does. But a little discretization helps to understand the fundamentals sometimes ;). $\endgroup$
    – choeger
    Mar 31, 2016 at 14:47
  • $\begingroup$ Interestingly, this is advanced rocket science problem, but the same situation happens in a steam engine boiler, and the fundamentals of their solutions are quite similar. $\endgroup$
    – Pere
    Jan 24, 2018 at 18:13
  • 1
    $\begingroup$ The reactor doesn't pressurize the propellant -- rather, it heats it up. The nozzle then converts hot, moderate-to-high pressure gas into cold, low-pressure, high-velocity gas (same as a chemical rocket) $\endgroup$
    – ikrase
    Oct 6, 2022 at 5:54

2 Answers 2


You have the same problem in any rocket. The pressure at the pump outlet must be higher than the chamber pressure. What you are missing, is that the density of your propellant drops by heating, both in chemical reactions, as well as by being fed through a nuclear core.

For a nuclear thermal rocket some of the heated (and thus expanded) hydrogen is tapped off to drive a turbine, which provides the energy to power a fuel pump.

Diagram of a nuclear thermal rocket

Image credit: Original drawing by Tokino at Wikimedia Commons, vectorized by CommiM at Wikipedia; licensed under GFDL and CC-By-SA 3.0.

  • $\begingroup$ Nice answer. You didn't just create that sketch for me, did you? Do I understand this correctly: the amount of energy required to bring the liquid hydrogen to the required pressure is lower than the amount of energy carried by the heated hydrogen? $\endgroup$
    – choeger
    Mar 31, 2016 at 12:15
  • 5
    $\begingroup$ @choeger The picture is from wikipedia. Yes, the liquid hydrogen is several orders of magnitude denser than the hot gaseous hydrogen, so it requires relatively little energy to pressurize... "relatively" is the key word here. It's still a lot of energy. $\endgroup$ Mar 31, 2016 at 13:05
  • 4
    $\begingroup$ When using pictures from Wikipedia, you really should try to credit the actual author; most of them are drawn by volunteer contributors, just like you and me here on Stack Exchange, and the credit is all they get. (Admittedly, in this particular case, tracking down the original author(s) was a bit difficult, since the specific diagram you picked is a modified version of an earlier one, and the modified copy had been shuffled around various Wikimedia sites before ending up at its current location. But it's worth at least trying.) $\endgroup$ Mar 31, 2016 at 17:25
  • $\begingroup$ I believe the drawing is to basic to warrant copyright protection, so I didn't bother. But thanks for adding it. $\endgroup$ Apr 1, 2016 at 0:59

Yes, you could pump propellant through the nozzle directly without heating it with a nuclear reactor. If you did that, you would discover the propellant losing pressure and temperature very quickly, so that its final velocity was lower than if you had used a reactor. Heating propellant with the reactor allows the volume to increase while temperature also increases.

The key is to understand that only a pump can increase pressure in a non-closed system. A rocket combustion chamber or nuclear reactor increases volume and temperature but the pressure at the exit must be lower than the pressure at the inlet or propellant will not flow in the desired direction.

  • $\begingroup$ Your answer could be improved with additional supporting information. Please edit to add further details, such as citations or documentation, so that others can confirm that your answer is correct. You can find more information on how to write good answers in the help center. $\endgroup$
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    Apr 27, 2023 at 7:15

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