What I mean is an engine whose thrust is on the order of tens to hundreds of Newtons with an ISP "well" above that of chemical engines. I don't require this engine to be space ready - if it has to plug into a mega watt generator and only works on the ground at the moment that's fine - but it must be buildable with current technology. That means that fusion power or antimatter is out. I'm ok with an engine that has been proposed but not built, or an engine that is low thrust but only because it has never been scaled up.

I do require the engine to at least have been peer reviewed and simulated in a high fidelity simulation.

So, doors such an engine concept exist?

  • 1
    $\begingroup$ I thought I had one: What kind of engine does this Isp = 1600 refer to? Is it cubesat-friendly? but the thrust turns out to be a hundred *milli-newtons." $\endgroup$
    – uhoh
    Sep 14, 2019 at 23:30
  • 1
    $\begingroup$ A cluster of any ion engine we used so far will do - they offer about 50N from your 1 MW generator. $\endgroup$
    – asdfex
    Sep 15, 2019 at 11:20
  • $\begingroup$ It doesn't really make sense to talk about Newtons here. Cluster enough engines of any type and you'll hit that. Perhaps thrust-to-mass ratio instead? $\endgroup$
    – TLW
    Sep 15, 2019 at 17:17

2 Answers 2


How about NERVA?

NERVA (Nuclear Engine for Rocket Vehicle Application) was a nuclear-thermal engine. A reactor heated hydrogen propellant and exhausted it out the nozzle.

The engine was extensively and successfully ground tested.

enter image description here

Thrust ~ 250 kN

ISP ~ 840

(You didn't mention political considerations or thrust-to-weight ratio)

  • $\begingroup$ Nah, the T/W of NERVA is fine while not amazing. This would also fall under "low thrust but only because it has never been scaled up", since there are still considerable scaling gains. $\endgroup$ Sep 14, 2019 at 21:03
  • $\begingroup$ I can live with NERVA, yes. I was hoping for something more modern/politically possible but that wasn't really specified in the question $\endgroup$ Sep 14, 2019 at 21:10
  • $\begingroup$ If you need a data point for how such an engine may scale down, the smaller rd-0410 was also extensively tested. $\endgroup$ Sep 14, 2019 at 21:19
  • $\begingroup$ @MichaelStachowsky Project Timberwind was the next generation nuclear-thermal -- no more political feasible but slightly more modern. $\endgroup$ Sep 14, 2019 at 22:29
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    $\begingroup$ I'm coining a new phrase and claiming ownership: "thrust-to-dose ratio". $\endgroup$
    – uhoh
    Sep 14, 2019 at 23:31

In general, thermal rockets can in theory have very good $I_{SP}$ (around 1000s), and provide high thrust.

Unlike chemical rockets, which by definition produce heavy exhaust gasses like $CO_2$, $CO$ or $H_2O$, a thermal rocket can pick any propellant. That would mean it's possible to eject pure hydrogen gas (which in the hotter cases will start to disassociate into mono-atomic hydrogen). The advantage of this is that lighter gasses have higher exhaust velocities at equal temperatures. A thermal rocket is limited by what temperature the engine can stand.

But eliminating the chemical reaction leaves the engine without an energy source for the heat. Some options:

  • Using a reactor, Organic Marble's example.
  • Concentrated solar power.
  • Lasers from Earth.

On the ground, we could hook it up to the grid.

  • $\begingroup$ factoid: lighter atoms (with +1 charge) also have higher Isp at a given acceleration voltage ($\sim m^{-1/2}$) $\endgroup$
    – uhoh
    Sep 14, 2019 at 23:34

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