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In Zero G, can excess heat be used propel a spaceship forward?

I was looking for a similar answers online, and have bounced around a few places, I found this question, and it's responses thread useful Why do some spacecraft have heat radiators? Why not repurpose the energy? I've been looking generally at this from sci-fi perspective, and have absolutely no qualifications that relate to science, this is simply my looking at whether I abandon the Avatar realism of the The Interstellar Vehicle (ISV) and stick to the realms of fiction inherent in Star Trek, Star wars or futurerama.

Essentially I'm wondering about the loss of heat and how, this seems wasteful. From what I've read this is necessarily so, but I've wondered after thinking & doing some limited google research, if hypothetically heat could be used in conjunction within an environment where a limited gravity field had been created using an anti-matter & matter field, that if within that field of stasis, if heat could be vented to turn magnetic bearings and turbines thus repurposing the energy? Creating some degree of sustainable energy.

As well as this could that heat then, be used in some way to propel a spaceship forward, in some way - perhaps similar to that of a rail gun?

Given the fictional, hypothetical output of the research, hypothetical responses are welcome.

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  • $\begingroup$ In principle, yes. Heat can be converted into other forms of energy (at least if you have a temperature differential), which can be used to propel a spacecraft. In practice, it won't be worth the cost. $\endgroup$
    – gerrit
    Dec 15, 2023 at 9:16
  • $\begingroup$ Thanks, do you think that it could be stored, to be released in concentrated bursts, rather that a constant release? $\endgroup$ Dec 15, 2023 at 9:21
  • $\begingroup$ Same answer. Theoretically yes, but not to get any meaningful or cost-effective amount of kinetic energy. The answer you linked yourself covers it pretty well. $\endgroup$
    – gerrit
    Dec 15, 2023 at 9:29
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    $\begingroup$ Just to go in another direction--"heat" isn't something that can be vented from a spacecraft, but a hot spacecraft can emit photons. This answer about photonic rockets teaches us that 300MW (roughly the thermal output a nuclear submarine's reactor) of photon emissions will give us 1N of thrust (1 kg * 1 m/$\text{s}^2$, i.e. essentially nothing). And that's only if you can make the photons all go in one direction, which they generally don't want to do. $\endgroup$
    – Erin Anne
    Dec 15, 2023 at 9:54
  • $\begingroup$ "a limited gravity field had been created using an anti-matter & matter field". What does that mean? $\endgroup$
    – PM 2Ring
    Dec 16, 2023 at 5:02

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You can always stick a heat engine between the source of the heat and the radiator dumping the heat into space, but that will make the heat source hotter and reduce the amount of heat you can radiate to space. Things like electronics and passengers are sensitive to high temperatures, and you're more likely to put energy into actively cooling them than you are to try to scavenge energy from them.

In short, waste heat is waste heat because you don't have an effective way to make use of it.

As for propulsion, you could arrange your radiators to radiate heat in a preferred direction, but the actual propulsion this gives you will be nearly immeasurable. Something like this was behind the Pioneer anomaly, and took years to notice. Things like rocket engines do frequently use regenerative cooling, using fuel to cool engine components before burning it, which both keeps the engine from melting and scavenges that heat for propulsion, but this requires using propellant for both cooling and reaction mass.

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It really depends on a few factors. Just having heat does not do anything. In order to get an object to move you got to have some force. So in theory if you radiated the heat away or if you had so much heat that is was radiating then the answer is yes, it can propel the ship. Since small particales being pushed in one direction will have an equal and opposite reaction

However, keep in mind that you need it to be radiating, so shooting small particles in one direction to push it in another will not have much thrust. It would be comparable to a simple ion thruster. Just that it will probably not have as high as an Isp.

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In rocketry you always have to take mass into account. Lots and lots of very cool ideas of how one could use otherwise wasted energy die on the vine for the simple reason the machinery required to utilize that energy would weight more, and the recovered energy would be less, than if you just packed more fuel instead.

There's a number of ways waste heat could be utilized for propulsion. So far every single of them requires equipment that would add so much weight to the craft you'd actually lose delta-v instead of gaining it.

In sci-fi everything is possible, create fantastically light radiators out of handwavium, add thermoelectric cells, and you can use waste heat to create electricity to power ion engines. Or generate so much waste heat unidirectionally and you get a photon drive that isn't beyond useless. But if you stick to realism, radiating that heat away into space through regular radiators and using the saved mass for more solar panels, more RTGs, or more propellant, is currently the mass-optimal solution.

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