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Not sure if this would be best placed here or in World building SE, but I'll try here. Let's say I have a Dueterium-Helium 3 fusion pulse engine (similar to what is in the Expanse) that will use water as a propellant to increase the thrust for the engine. **

  1. Would it be feasible to use heated water as the propellant?

** The basic idea is to use the water as a waste heat reservoir and then use that heated water as the propellant.

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  • $\begingroup$ I'm pretty sure you have to use more than just waste heat to get good performance. $\endgroup$ – ikrase May 9 at 21:31
  • $\begingroup$ It's not about performance so much as it is about removing the excess heat from the ship and making use of the "hear sink". Hitting two birds with one stone $\endgroup$ – Markitect May 9 at 21:49
  • $\begingroup$ I think by the time you're getting to high power fusion thrusters (anything similar to what they have in The Expanse is far more powerful than fusion thrusters planned for "whenever we figure out fusion") that doesn't help. $\endgroup$ – ikrase May 10 at 0:30
  • $\begingroup$ Remember a efficient rocket needs to make good use of the mass it projects. If the exhaust velocity is slower than typical chemical engines you might just use LH2/LOX. $\endgroup$ – user3528438 May 10 at 1:15
  • $\begingroup$ @Markitect: You really don't want a low performance steam rocket. Your fusion propulsion system will be doing almost all of the work, and you'd be making that system's job harder by making it move many, many times your vehicle's mass in water to be sputtered out at waste-heat temperatures. Even a NTR purpose-built to get the propellant as hot as possible can't really get significantly better performance out of water than you can get with a plain old hydrolox chemical engine, that's why they all use liquid hydrogen. You're better off just throwing away the heat with radiators. $\endgroup$ – Christopher James Huff May 10 at 1:50
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Ideally, you might not even need that much of a waste heat removal, since D + He-3 is aneutronic fusion, you can directly use the fusion products, high energetic charged alpha particles as a very efficient thrust by using electric fields to guide particle beam out of the ships magnetic nozzle. Then the thrust is provided by work done by fusion energized particles against the electric field, which will gently provide the "damped" momentum transfer to a relatively large surface area of the ship.

D + D -sidereactions which would produce poorly usable neutron radiation and wear the engine, can be minimized by running the burning plasma lean on deuterium, rich on He-3. If additional thrust is needed because of the poor energy/momentum -ratio of very high velocity exhaust, shouldn't any atoms collected from where the ship travels could be used to increase momentum by injecting them in to the exhaust, much like a turbofan jet engine is superior to rocket engine when flying in a dense medium?

With magnetic fields created by superconductors, the whole thing might not need that much of a waste heat removal after all.

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It's hard to know without numbers (which pobably makes this not a great SE question), but a few thoughts:

  1. Your engine is producing an enormous amount of energy. Most of this had better be going into accelerating your reaction mass to a very high velocity indeed, but some will inevitably end up as wate heat in your engine and surrounding systems. A spacecraft also produces waste heat from its own systems, and possibly from absorbing sunlight. You can move that heat around, and end up dumping it into a tank of water if you want, but eventually you have to get rid of it.
  2. If your engine design involves injecting water somewhere, heating it into a very hot plasma with fusion heat and then ejecting it through a magnetic nozzle, then starting with hot water or steam instead of cold will not make any significant difference, but it will get rid of the hot water.
  3. If your engine does not do that, then the thrust you get from just ejecting the water as steam will be negligible and you will run out of water too quickly. (more technically the $I_{sp}$ is too low). You are better circulating the water through radiator panels and getting rid of the heat to space.
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  • $\begingroup$ It's a highly hypothetical engine based off of items I have seen at Project RHO. But some basic stats are: Specific Impulse is around 1,900,000 seconds. Thrust is 109MN, Thrust power is around 1,200 TW. Exhaust velocity is 19,000km/s. And the MDot is 5.38kg/s. $\endgroup$ – Markitect May 10 at 13:02
  • $\begingroup$ One additional note is that it is using a pulse system where the fuel pellet is sent out the back of the engine and hit with a laser to create the fusion process. The distance anywhere between 100 to 300m behind the ship. $\endgroup$ – Markitect May 10 at 13:09
  • $\begingroup$ For comparison, a low temperature steam rocket will get more like 100 s specific impulse. Possibly even lower, you're running on waste heat after all. It'd be more of an evaporative cooling system than a source of propulsion. If parts of your ship get hot enough and you have radiators sufficient to keep things from overheating, you might be able to use steam thrusters for RCS, but if you can get fusion rocket fuel, you can get something better for your RCS thrusters. (Something that doesn't stop working when you turn your main engine off would be nice.) $\endgroup$ – Christopher James Huff May 10 at 18:24

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