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Normal solid-core nuclear thermal rockets use a nuclear reactor to heat gas (almost always hydrogen, since higher molecular weights cause miserable performance) and expand it through a converging-diverging nozzle. This normally results in ISP of 850 to 1100 and a thrust-weight ratio that is dismal compared to chemical rockets but vastly higher than ion engines, appropriate for point-thrust trajectories and possibly planetary landing on low-atmosphere, low-gravity bodies.

The LANTR design has been proposed to add liquid oxygen to the hot hydrogen in the rocket bell, as a kind of afterburner. This maintains an ISP of about 600, significantly better than chemical rockets, while massively improving the thrust, and also greatly improving the density of the fuel since LOX (or any other oxidizer) is vastly denser than LH2.

I wonder about the possibility of a rocket engine in which both the hydrogen and the oxygen are pre-heated by nuclear reactor(s), and then then, when they are already hot gas rather than cryogenic liquid, burned and expanded. This seems like it might provide rather high ISP, though probably not nearly as much thrust as a LANTR. But I have never heard this proposed.

Why not? Is it because the burning of already-hot gas would simply melt any usable nozzle design?

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    $\begingroup$ So, correct me if I'm wrong here, but isn't the thrust of the rocket provided by expansion of hot gasses? If the gasses are already hot, the additional heat and pressure provided by igniting them is going to be proportionally less even if the end temperature is higher. $\endgroup$ – Starfish Prime Dec 1 '19 at 9:51
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    $\begingroup$ At sufficiently high temperatures water will dissociate back to hydrogen and oxygen, absorbing energy in the process $\endgroup$ – Steve Linton Dec 1 '19 at 10:28
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    $\begingroup$ @uhoh but en.wikipedia.org/wiki/Water_splitting says 3000K. Can anyone clarify? $\endgroup$ – Steve Linton Dec 1 '19 at 13:01
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    $\begingroup$ @uhoh you'll almost certainly get a better answer out of the chemists, but the issue here (I think) is one of degrees of freedom. A monotomic gas only has three, but molecular gasses can store energy in the vibrational modes of their molecular bonds as well as their movement in space. Its part of why the specific heat capacities of monatomic gasses are so different to the regular kinds. $\endgroup$ – Starfish Prime Dec 1 '19 at 14:46
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    $\begingroup$ @uhoh right, on further reflection, I think this might be an average-vs-peak kinetic energy thing. Disassociation and ionisation are progressive processes. At 50000K pretty much every molecule will have disassociated into monatomic hydrogen but still get some disassociation at lower temps because some of the molecules with greater-than-average KE will be able to disassociate. You can model this with the Saha equation, as it turns out. Some possibly relevant graphs here (which also mentions the 50000K temperature). $\endgroup$ – Starfish Prime Dec 2 '19 at 11:49
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If I were to hazard a guess, I'd say there are several reasons why you wouldn't want to do this.

I'll reference A Revolutionary Lunar Space Transportation System Architecture Using Extraterrestrial LOX-Augmented NTR Propulsion (describing the LANTR) a few times below. Here's the important bit of figure 5 from that paper:

LANTR figure 5

Firstly, as you suggest, nozzle temperature is almost certainly an issue. It is suggested that the exhaust temperature exceeds 3500K with the cool subsonic oxygen injection, which is already a fairly punishing environment for any material. Some effort was made to keep the exhaust gas temperature under 3600K in the LANTR design, though they don't say precisely why, but I'll bet that as things get much hotter than that your nozzle or your oxidiser injectors will just start burning up, regenerative cooling or no.

Secondly, the post-throat part of the LANTR is already operating as a scramjet, spraying subsonic oxygen into a supersonic hydrogen flow. There are a whole bunch of technological challenges involved in that, ensuring that the fuel and oxidiser mix and burn whilst they're still within the nozzle. What you're proposing involves injecting a supersonic oxidiser stream into a supersonic fuel stream and hoping it all burns nicely before leaving the nozzle... something that sounds a substantially less simple than making a scramjet which we still seem to be having problems doing. Possibly you were thinking of superheated subsonic oxygen injection which seems less implausible from a combustion point of view, but injectors which could do the job sound like a serious engineering challenge.

Finally, you're faced with the problem of handling a 2-3000K high pressure oxygen stream, and no materials engineer is going to relish that. At that temperature, a small amount of the gas will have disassociated into monoatomic oxygen, making it even more reactive than usual. You'll need a different reactor core to cope with the harsh oxidising environment... the hydrogen reactor core won't do at all, it being optimised for a reducing environment, so you need to design two different complete working nuclear rockets to make just one engine. You can't simply hook up the oxygen reactor to the nozzle of the hydrogen reactor... you'll need some sort of plumbing which needs to cope with even more heat whilst carrying very high pressure superheated oxygen.You can't vent both preheated fuels into a combustion chamber and then blow the result out of a nozzle because the chamber temperature will be too high and you won't be able to run your engine efficiently in hydrogen only mode.

That gives you three nightmarishly difficult engineering problems to overcome, for a what isn't obviously a major improvement over a LANTR. If you can achieve three impossible things before breakfast, why not finish up with making a practical liquid or gas core NTR instead?


Additionally there are other performance issues which aren't quite at the level of complexity of those above. You're having to pack in two nuclear reactors now, and if you're not running both of them at once you've got a big dead weight to haul around until you finally decide to use it, which reduces your thrust-to-weight in non-augmented mode. You can't use both reactors as hydrogen-reaction-mass rockets, because of the impracticality of having fuel elements that can work at 3000K for both an oxidising or a reducing propellant depending on your mood.

If you're not running both reactors, you have to switch your oxygen rocket on when you need it, and off again when you're done. Your NTR will have a limited number of cycles, and you'll have to cool it down (by venting propellant through it) when you're done, which costs you reaction mass. This contrasts with a LANTR which can smoothly shift between normal and augmented mode of operation without having to cycle the rocket at all.

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  • $\begingroup$ Wait, the gas is going supersonic, in a normal NTR, INSIDE THE REACTOR??? $\endgroup$ – ikrase Dec 1 '19 at 20:12
  • $\begingroup$ @ikrase I don't think so; it should be subsonic on the reactor side of the nozzle; just hot and high pressure like a regular rocket would be. (have i written something silly?) $\endgroup$ – Starfish Prime Dec 1 '19 at 20:14

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