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It seems that all nuclear propulsion concepts extract fission energy as heat, and somehow convert a portion of that heat to energy in the desired form, such as electrical (e.g. to power an ion thruster). It always amounts to creating heat in one part of the system and dumping waste heat somewhere else. For example: using a working fluid that is heated, perhaps discharged through a turbine, cooled, and sent back to the heat source. As the turbine (for example) extracts mechanical energy from the working fluid, the equivalent heat energy is removed. Nevertheless, there is still a lot of waste heat to be dissipated somewhere.

My question is: in the higher power concepts, how is all that waste heat removed from the spacecraft? In the vacuum of space, it can only leave by radiation. So wouldn't that require a very large array of radiators? Wouldn't that add a significant amount of mass to the spacecraft, affecting the overall mass efficiency?

As a secondary question: would the radiation pressure from "waste" heat for a nuclear powered electric engine be enough to make a worthwhile contribution to spacecraft thrust?

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    $\begingroup$ $\sigma T^4$ is your friend. Higher temperatures lead to much higher radiated heat from the same surface. $\endgroup$ – Deer Hunter Nov 4 '15 at 7:19
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    $\begingroup$ Visit the site with realistic (more or less) interplanetary spacecraft examples. Find some of the ones that use nuclear power - "Gasdynamic Mirror", francisdrakex's version of Hermes, HOPE/FFRE, Stuhlinger Ion Rocket - their common feature is ludicruous radiator area. $\endgroup$ – SF. Nov 4 '15 at 12:48
  • $\begingroup$ FFREs actually do not primarily extract fission energy as heat at all — that's their central selling point. Still, about 20% of their overall fission power must be radiated away as heat. (cc @SF.) $\endgroup$ – Nathan Tuggy Nov 4 '15 at 21:37
  • $\begingroup$ @DeerHunter Realistic, long-term working heat radiators can't have a temperature much bigger as around 2000K, which can be a serious limit if we are talking about the cooling of a realistic, long-term working nuclear reactor. $\endgroup$ – peterh - Reinstate Monica Nov 6 '15 at 19:08
  • $\begingroup$ @peterh - agreed, and I'd say even working fluid temps greater than 600K are unhealthy (depending on the choice of alloy). $\endgroup$ – Deer Hunter Nov 7 '15 at 3:24
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As you stated, radiation is the only method of removing excess heat from a conventional spacecraft in vacuum. Designing a spacecraft to radiate heat appropriately is important and yes, it does add mass to the overall thermal control system. The best way to reduce that mass is to try and ensure the spacecraft doesn't need to radiate as much heat. For example, heat is desirable for maintaining electronics, propellant, or other components within required temperature ranges, so taking advantage of a nuclear source to provide that heat makes it an asset instead of waste (or at least some of it).

On the other hand, if you are talking about a nuclear thermal rocket engine then most of the heat generated by the nuclear source is ejected along with the exhaust. (See Deer Hunter's comments below.)

For your secondary question: using heat radiation pressure for thrust has definitely been considered (check out nuclear photonic propulsion). Also, that kind of effect has been observed as a perturbation on spacecraft already (see the Pioneer effect).

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  • $\begingroup$ Not quite. Heat may be removed with the propellant - liquid hydrogen. $\endgroup$ – Deer Hunter Nov 4 '15 at 7:14
  • $\begingroup$ Do you mean with the exhaust? Or do you mean the liquid hydrogen is cold so it absorbs heat? The latter is simply moving the heat somewhere else -- which is another solution but does not remove heat (similar to phase change materials). $\endgroup$ – Brian Lynch Nov 4 '15 at 7:19
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    $\begingroup$ The exhaust. Cooling becomes a problem after reactor shutdown - residual decay heat must be removed, and most designs I've read emphasize keeping LH2 flowing to cool the core even if specific impulse falls drastically. $\endgroup$ – Deer Hunter Nov 4 '15 at 8:05
  • $\begingroup$ Now I am more confused. What engine are you talking about? "Reactor" and "decay" suggest you are talking about an RTG power source, "keeping LH2 flowing to cool the core" sounds like a closed system -- which, like I mentioned, cannot remove heat. Are you referring to the method of wrapping propellant lines around the engine nozzle for thermal control? $\endgroup$ – Brian Lynch Nov 4 '15 at 11:09
  • $\begingroup$ en.wikipedia.org/wiki/Nuclear_Thermal_Rocket $\endgroup$ – Deer Hunter Nov 4 '15 at 11:14
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At higher power levels, the radiator comprises 70% of power plant mass.

Image Source https://books.google.com.mx/books?id=fmIrAAAAYAAJ&printsec=frontcover&hl=es-419#v=onepage&q&f=false

enter image description here

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    $\begingroup$ Is this figure 15 valid for power plants on Earth surface or for spacecrafts in the solar system? $\endgroup$ – Uwe Feb 13 '19 at 11:00
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    $\begingroup$ The figure is from an article on nuclear reactors for space propulsion.You can check the link to the book to make sure. $\endgroup$ – Patrick Tibbits Feb 14 '19 at 4:58

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