3 ms·

One of the problems with using nuclear reactors in spacecraft is that it's difficult to get rid of excess heat in a vacuum. How do nuclear thermal rockets deal

by elihu 6y ago

One of the problems with using nuclear reactors in spacecraft is that it's difficult to get rid of excess heat in a vacuum. How do nuclear thermal rockets deal with that? Presumably a lot of the heat is transferred to the propellant, but is that energy transfer efficient enough that there isn't an excessive amount of leftover heat?

m4rtink 6y ago
IIRC the wast majority goes out with the propellant & propelant being cold liquid hydrogen also helps. Still after a burn you would definitely need some cooling until the fision byproducts decay. How much & for how long I have no idea. If a couple minutes/ hours just running some more hydrogen through the shutdown reactor might be enough. Otherwise passive or active cooling loops and radiators could be needed. Or maybe not enough fission byproducts form during the tens of minutes the reactor is active and cooling after a burn is not a major concern?
- valuearb 6y ago
  Heavy radiators are definitely needed if you don’t want your space ship becoming uninhabitable. And running extra propellant through the engine to cool will further reduce usable ISP.
godelski 6y ago
In this case you're passing the heat into the fuel. Excess heat you can use for ship or to power your vehicle.
Tuna-Fish 6y ago
A simple way to think of a nuclear thermal rocket is that it's a nuclear reactor where the coolant is vented into the vacuum. (At very high temperatures, through a rocket nozzle, therefore it produces thrust.) So yes, it pretty much by definition has no cooling issues. If you'd stay too hot, you'd just pump more fuel to cool your reactor better, and get more thrust. (One flipside to this is that you cannot just turn it off. After a long burn, there will be decay heat you will have to dispose of, potentially for weeks. NTRs design around this with complex, slow shutoff sequences, or in the case of disposable stages, by making sure the stage will not be near anything important as it melts down.)

by elihu 6y ago

m4rtink 6y ago

IIRC the wast majority goes out with the propellant & propelant being cold liquid hydrogen also helps. Still after a burn you would definitely need some cooling until the fision byproducts decay. How much & for how long I have no idea. If a couple minutes/ hours just running some more hydrogen through the shutdown reactor might be enough. Otherwise passive or active cooling loops and radiators could be needed. Or maybe not enough fission byproducts form during the tens of minutes the reactor is active and cooling after a burn is not a major concern?

valuearb 6y ago
Heavy radiators are definitely needed if you don’t want your space ship becoming uninhabitable. And running extra propellant through the engine to cool will further reduce usable ISP.

godelski 6y ago

In this case you're passing the heat into the fuel. Excess heat you can use for ship or to power your vehicle.

Tuna-Fish 6y ago

A simple way to think of a nuclear thermal rocket is that it's a nuclear reactor where the coolant is vented into the vacuum. (At very high temperatures, through a rocket nozzle, therefore it produces thrust.) So yes, it pretty much by definition has no cooling issues. If you'd stay too hot, you'd just pump more fuel to cool your reactor better, and get more thrust. (One flipside to this is that you cannot just turn it off. After a long burn, there will be decay heat you will have to dispose of, potentially for weeks. NTRs design around this with complex, slow shutoff sequences, or in the case of disposable stages, by making sure the stage will not be near anything important as it melts down.)