Liquid Rocket engine design is quite complicated and idealised and assumed theories rarely get sufficient validation. In an effort to verify/validate newer/untested fuels to be used in engine designs, can one adapt data of present engines from similar fuels? If so, what are the parameters most likely to be affected and which are the parameters insensitive to the fuel being burnt(if any)? I see that the engine cycle is quite important, but to compare the performance of cryogenic methane for example in comparison to existing engines (gas generator, closed expander, staged combustion atleast)

TL:DR I'm looking for a sensitivity analysis for rocket engines for various design parameters across different fuels.

P.S. Has anyone used Rocket Propulsion Analysis (RPA) for such an analysis? How accurate is it?

  • $\begingroup$ related to space.stackexchange.com/questions/38377/… $\endgroup$
    – Rajath Pai
    Commented Sep 9, 2019 at 6:19
  • 1
    $\begingroup$ @uhoh RPA appears to be a software suite called Rocket Propellant Analysis. propulsion-analysis.com/index.htm $\endgroup$
    – Ingolifs
    Commented Sep 10, 2019 at 4:18
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    $\begingroup$ @uhoh I apologise for not providing the specifics. I hope that the scope is now limited to get some answers. $\endgroup$
    – Rajath Pai
    Commented Sep 10, 2019 at 18:55
  • $\begingroup$ @RajathPai looks great, thanks! $\endgroup$
    – uhoh
    Commented Sep 10, 2019 at 23:50

1 Answer 1


I think the RL10 is a biprop cyro system that was also tested with methane in the 60s, to get you started with something. Generally your isp takes a big hit when switching fuels, along with max thrust and general mass ratio.

In general though, you would have to change your whole prop system assembly to fit a different fuel configuration, not just your engine.

Edit: To elaborate, I think you should pick a couple discriminating factors for rocket engines. This could be the ones I identified: Isp, max thrust, and thrust to mass ratio. Then look at getting public data points from different engines to be able to plot curves and see what role the fuel plays in changing those values. From memory of the RL10 information I've seen, a cyro RL10 configuration has much higher Isp than the "storable" methane configuration. If you can find information like this for multiple engines, then you can plot their relationships wrt the discriminating factors and maybe come up with a general model. Then use that model to scale unknown engines to different configurations.

I'll also add, it's a little difficult to look at these things in a purely engine view. A cyro lox/lh2 configuration is really great for it's Isp, but boils off very quickly and cannot be stored in space for long periods of time. There are trade-offs like these that expand beyond engine and more into conops and propulsion system architecture.

Sorry this isn't all that more detailed, and I'll dig around for a textbook I still have on Aerospace Propulsion.

  • $\begingroup$ I would really appreciate it if you could elaborate more on this. $\endgroup$
    – Rajath Pai
    Commented Sep 10, 2019 at 18:56
  • $\begingroup$ Another example to look into would be the LR87 and LR91 engines used on the early Titan missiles. Titan I was kerosene/LOX, Titan II was Az50/NTO. I don't know what changes were made to the engines to facilitate that. Apparently a version of the LR87 was also modified to use hydrogen/LOX as a possible engine for the Saturn upper stages, but was rejected in favor of the J-2. $\endgroup$ Commented Oct 22, 2020 at 16:51
  • $\begingroup$ Storable methane? $\endgroup$ Commented Oct 23, 2020 at 10:53

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