Nuclear thermal rocket designs have a working temperature of 2000k or greater. Shutting such a reactor down won't stop that heat production instantly - it'll take quite some time to cool.

Do current nuclear rocket designs allow for shut down of the actual rocket while in space and then restart it, without melting significant parts of the engine? If so, how to they plan to do this?

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    $\begingroup$ There aren't any NTR engines "currently" so what are you really asking? $\endgroup$ Mar 2, 2021 at 13:40
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    $\begingroup$ What I'm really asking is "is it feasible to have an NTR that is fired at multiple points, on a long mission, without extensive work". $\endgroup$ Mar 2, 2021 at 14:31
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    $\begingroup$ Also, my concern isn't that the restart temperatures are higher, but that on shutdown you lose the ability to vent heat via propellant. You have to be able to withstand the cooling process without an external heat dump, except radiation. No point shutting down and melting 10 minutes later. $\endgroup$ Mar 2, 2021 at 14:33
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    $\begingroup$ It's worth considering that without convection to carry the heat away, that heat is going to be there for a long time, assuming that it doesn't simply conduct to the rest of the ship and cook the crew. $\endgroup$ Mar 2, 2021 at 16:42
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    $\begingroup$ It's a real issue, and the impact of carrying sufficient coolant or a capable enough cooling system is enough that if mass constraints are a bigger concern than hardware cost, it can be favorable to discard each engine after a single use and carry multiple engines. (osti.gov/servlets/purl/4549178) $\endgroup$ Mar 2, 2021 at 21:35

2 Answers 2


At least for the NERVA, after shutdown, a residual, thrust-generating cooldown flow of propellant was run through the reactor.

From the engine specs Shutdown and Cooldown Shutdown consists of throttling, throttle hold, temperature retreat and pump tailoff, and is initiated by a command signal to depart from rated conditions and is completed upon termination of powered pump operation. During shutdown the engine shall be capable of steady- state hold at the engine throttle point. Cooldown is initiated upon completion of engine shutdown and is completed upon termination of propellant flow or the receipt of a command signal for restart. Cooldown propellant is supplied at tank pressure conditions as defined in, (Propellant Conditioning). The total delivered impulse during shutdown and cooldown shall be predictable within TBD percent of the total startup and steady state impulse as a function of engine operating history and shall be controllable as a function of time after initiation of shutdown. Provision shall be made for a TBD sec steady state hold at the throttle point, and for each shutdown cycle during the engine operating life the thrust and specific impulse shall be controllable to TBD percent thrust and TBD percent specific impulse of instantaneous predicted values from initiation of shutdown to termination of steady state hold at the throttle point. The total impulse delivered after termination of the steady state hold at the throttle point shall be controllable to within 20,000 lb sec at ter- mination of cooldown. The time of termination of cooldown impulse shall be predictable within 15 sec. Cooldown thrust shall be not less than 30 lb and average cooldown specific impulse shall be not less than 400 sec.

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(I added the red arrow)

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    $\begingroup$ I do wish the manual gave an estimate of how much thrust the cool down phase was expected to give, but... ah well. Definitely rules out precision burns midflight though. $\endgroup$ Mar 3, 2021 at 17:20
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    $\begingroup$ @user2702772 yeah, lots of TBDs! $\endgroup$ Mar 3, 2021 at 18:03
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    $\begingroup$ Look at the section number: That's why these things were feasible in the seventies but are no longer feasible nowadays. Word would just go bonkers! $\endgroup$
    – Pavel
    Mar 4, 2021 at 15:36

Is it currently possible to shut down an actual rocket (not test reactor with external cooling) while in space and then restart it, without melting significant parts of the engine? If so, how?

It was certainly intended that NTRs under development were restartable, but more importantly a clean shutdown of the engine is clearly desirable so you get good control over your engine burns.

Keeping the reactor cool after shutdown isn't too difficult... you can just keep venting propellant through it, after all. I'm having trouble finding any reference to the shutdown/post-shutdown process for anyone's NTRs right now, but Project Rho has a translation of some stuff from Glushko about the Soviet NTR program with the relevant quote:

Research into start cycle and throttling was performed, along with the terminal stage and shutdown cycle using the remnant heat of a subcritical reactor; reverse systems were developed to minimize post-shutdown thrust.

Details of what the reverse systems were is missing from that translated excerpt, alas. You can read the rest for yourself on this page under the RD-600 section, which lacks a direct hyperlink.

There is an alternative shutdown strategy to just letting the whole thing cool down to ambient and vent a load of your reaction mass out as coolant: the bimodal NTR. This involves strapping on a radiator, turbine and generator with a closed coolant circuit. When the main propellant flow is shut off, the reactor is powered right down but not actually shut off. The closed coolant circuit is run to cool the reactor and drive a generator to provide electricity to the spacecraft between main engine burns, which facilitates handy things like simplified spun gravity (cos you don't have to keep your solar cells pointed the right way) or even a secondary electric propulsion system for a high-Isp cruise, assuming a suitably lightweight engine could be made.

Below is a diagram of a tri-modal variant that included a liquid oxygen afterburner (originally found in the LANTR design) for even more thrust at the expense of Isp. I don't know why that was deemed necessary, but I'm sure the reasoning is found somewhere in the various LANTR and trimodal rocket papers out there.

Low quality diagram taken from Conventional and Bimodal Nuclear Thermal Rocket (NTR) Artificial Gravity Mars Transfer Vehicle Concepts

Bimodal NTR

  • $\begingroup$ Venting fuel, or propellant? $\endgroup$ Mar 3, 2021 at 6:56
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    $\begingroup$ @ChrisB.Behrens derp, I'll fix that. $\endgroup$ Mar 3, 2021 at 8:17

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