I'm designing what I call a "fusion thermal drive" for my world.


  • The drive's purpose is brachistochrone-based intra-system travel.
  • The maximum specific impulse of about 16 hours.
  • The top thrust is 0.5G.

Using some online calculators:

  • A 100-ton ship with 65 tons of reactive mass,
  • a specific impulse of 57,600 seconds (16 hours)
  • and an exhaust velocity of ~565km/s,

...would have a delta-V of about 280km/s, which is exactly what I'm looking for on the high end.

Fundamental Design

Water (or liquid hydrogen, I haven't decided yet) will be pumped over a deuterium-deuterium fusion reactor to super-heat it for the purpose of thrust (explosive vaporization).


Is my hypothetical fusion thermal drive capable of achieving these results, at least in theory?

  • You may assume that materials science has significantly advanced, making materials a non-issue for this question.
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    $\begingroup$ Note that fission drives are not limited by the way the produce heat. They are strictly limited by their ability to a) transfer the produced heat to the reaction mass, and b) not to melt in the process. Your drive has exactly the same limitations, and will perform strictly within the envelope that's possible for fission drives. $\endgroup$ Oct 19, 2020 at 22:40
  • 1
    $\begingroup$ @cmaster-reinstatemonica That's a good point about the heat transfer, but if we assume that materials science has advanced to allow more heat to be safely transferred to the propellant, would the performance envelope be pushed forward? $\endgroup$ Oct 19, 2020 at 23:06
  • 2
    $\begingroup$ @JBH Thank you! I'm new here and I'm more used to the likes of the KSP forums, where people eat up that kind of detail. Will keep this in mind as an example for the future. $\endgroup$ Oct 19, 2020 at 23:56
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    $\begingroup$ Cheers. We don't mind the detail, but we prefer it be separated from the question. If you wish to add the narrative back in, it would appear before Characteristics so that people who either can't or don't want to get involved can skip to the point. $\endgroup$
    – JBH
    Oct 20, 2020 at 0:00
  • $\begingroup$ I have my doubts that water flashed into steam exiting into a vacuum will reach 560kmps over the length of a rocket nozzle regardless of how much heat is applied to it. I lack the skills in nozzle design to confirm my doubt however $\endgroup$
    – Ash
    Oct 20, 2020 at 0:04

1 Answer 1


Your concept works for the propulsion numbers.

Where you have a problem is with "Fundamental Design: Water (or liquid hydrogen, I haven't decided yet) will be pumped over a deuterium-deuterium fusion reactor to super-heat it for the purpose of thrust (explosive vaporization)."

The temperature required to reach your stated exhaust velocity are quite staggeringly huge. Exhaust velocity is proportional to the square root of temperature. Particle mass of the propellant also plays a role.

For monatomic hydrogen, your exhaust (and thus reactor temperature) is 10.6 million Kelvin

For water, the required temperature is 50.9 million K. But water would directly disassociate into monatomic hydrogen and oxygen, muddying this calculation as it gives two distinct exhaust velocities.

You need an alternate means of transferring the energy from your reactor to the propellant, simple thermal transfer is absolutely out of the question. Look to something like the plasma rocket used by the VASIMR design.

  • $\begingroup$ Perfect, exactly the answer I was looking for. As a side note, I know VASMIR uses radio waves to heat noble gases. Would that also work for H2, or would, say, microwaves be better? $\endgroup$ Oct 20, 2020 at 12:59
  • $\begingroup$ @JamesHaywood - Keep in mind that there is (more or less) a way around this problem. Rather than postulate a continuous reaction/flow mechanism, which does require rather daunting temperatures, take the issue and run with it. Try a burst-mode reactor - essentially a deuterium-based Orion Drive. Wave your hands real hard and come up with a justification. Consider using a reaction mass (water) which is in part heavy water, and you don't have to treat power generation and reaction mass separately. $\endgroup$ Oct 20, 2020 at 23:08

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