Nuclear Thermal Rocket Engines are an alternative to chemical propulsion, and can offer far better specific impulses. Real world example: NERVA.

The basic gist of an NTR is that, though they have about the same Thrust Power as a chemical engine, they can use different propellants and get specific impulses which are far better than chemical propulsion, without having the abysmal TWR of ion engines.

A handy property of NTRs is that depending upon the propellant you use, you can trade off TWR for Isp. So if you use the stock standard LH2-Only propellant, you can get Isp of around 900 seconds, although the TWR will be less than one. If you use Water, however, you can get the same specific impulse as a Hydrogen/Oxygen rocket. That's because the maximum temperatures for an NTR's nuclear reactor core and that of a chemical rocket combustion chamber are about the same, and water, when split up at those temperatures, has the same molar mass as burnt hydrolox. The NTR will also have about the same thrust as the Hydrolox chemical rocket for the same reason. However, water is far, far denser than Hydrolox propellants, so you can get away with less dry mass.

Solid Core NTRs are not the only kind. There are also Gas Core NTRs, both in open cycle and closed cycle modes, which can get much larger thrust powers than chemical rockets, meaning better Isp AND better thrust. Open Cycle Gas Core NTRs are practically overpowered torchdrive monstrosities, but they also spew radioactive waste out of their bottoms. Closed Cycles, meanwhile, are less powerful but are far safer.

NTRs will need shapeable radiator parts and/or deployable radiators to allow them to cool down, else they'd overheat and stop working/explode.

Suggestion Done Fixed in 0.7.0.5

17 Comments

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    4,358 Kell

    @AnotherFireFox @AndrewGarrison I agree! Great job!

    +1 one month ago
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    @AndrewGarrison Nah, you can't say that - everybody has their role and yours is making this awesome, sparkling shiny and unbelievably well optimized game! If you make a total smarter-dumb-o-meter your score would be far much smarter than ours ;)

    +1 one month ago
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    @AnotherFireFox @mjdfx150529 Players around here are way smarter than I am, but I'm learning! I've really enjoyed digging deeper into how this stuff works.
    .
    I had a realization during dinner of a mistake that I made with my calculations. I was (idiotically) using the gamma and molecular weight of actual water in the calculation, but that's obviously not right. I switched it over to use the same g and M from hydrolox (1.21 and 11.8 respectively) and now the Isp is much better (around 430s) and now the thrust doesn't drop. Mass flow increases, but exhaust velocity decreases and it ends up resulting in the same thrust as an NTR with LH2. The density of water over Hydrolox and LH2 does make this configuration have some interesting advantages, as @Gregrox pointed out in the original post.
    .
    I'm looking forward to getting a beta out to you guys so you can sanity check my calculations ;D

    +2 one month ago
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    4,358 Kell

    @AnotherFireFox OwO

    +1 one month ago
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    @AndrewGarrison I suggest you to hire some serious rocket nerds like @mjdfx150529 or Scott Manley ;) So called LANTRN NTR, which is not identical to Water Propellant NTR, is a way to get higher thrust (and lower Isp) with Hydrogen NTR: adding Oxygen, they combust, having higher chamber pressure - more thrust (and heavier exhaust gas - lower exhaust velocity (=lower Isp)

    +1 one month ago
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    @AnotherFireFox also, it doesn’t make sense to me that it could have similar thrust to a hydrolox engine unless their chamber pressures were equal.

    +1 one month ago
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    4,358 Kell

    @AndrewGarrison according to Wikipedia, a solid core NTR can heat up LH2 to 3000K before having substantial risks of structural failures. Solid Core section of this article


    On Atomic Rockets, examples with temperature ranging from 2750K to 3200K were given.


    I also hope that rocket engines can stop having a net heat change of 0W while operating. (They are always at room temperature for some reason.)

    +1 one month ago
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    @mjdfx150529 thanks!

    +2 one month ago
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    @AnotherFireFox what kind of chamber temps do NTRs have?

    +1 one month ago
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    4,358 Kell

    @AndrewGarrison I also suggest this amazing site

    +2 one month ago
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    @AndrewGarrison Basically speaking, Water NTR should have comparable thrust & Isp with Hydrolox Chemical engine since it works in the same way - the tremendous heat of Nuke Core dissociates water into Hydrogen and Oxygen and burns it at the same time. The benefit of doing this over Hyrdolox Chemical engine is way denser fuel to reduce fuel tank volume & mass.

    +2 one month ago
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    4,358 Kell

    @AndrewGarrison Water is more accessible than liquid hydrogen, more dense (and doesn’t seep out the tank like hydrogen in real life). The drop in performance is expected.

    +1 one month ago
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    I've been experimenting with solid core NTRs a bit today and looking into using water as a propellant and it turns out that it doesn't perform very well. It makes sense given the equations I'm using.

    Chamber pressure and the radius of the nozzle's throat are the biggest driving factors in thrust. The only data point I can find regarding NTR's chamber pressure is the NERVA at 3.1MPa, which is quite a bit worse than liquid chemical combustion engines. The H2O propellant does result in a huge drop in Isp compared to the LH2 (as expected), but it also results in a drop in overall thrust as well.

    I'm still throwing a lot of code around so it's possible I've got a bug in there somewhere, but I just wanted to share my findings with you. I'm disappointed, as I was hoping to have water as a propellant, but it doesn't make much sense if this is how it performs.

    +2 one month ago
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    Does this come with nuke reactors and radiators?

    +2 2 months ago
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    542 Insanity

    Nice, in developement!!!

    2 months ago
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    In development!

    2 months ago
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    4,358 Kell

    In development!

    +1 2 months ago

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