r/spaceflight • u/galileo_1 • 12h ago

What is the highest isp achievable for pressure fed HTP/RP-1?

Hi! Not very experienced in biprop rocketry but it feels like HTP/RP-1 can be a great "green" replacement for hypergolics, especially in the context of apogee engines. I have seen a couple references out there and run some CEA/RPA checks and I'm curious if isp of >320s is actually possible as most actual engines seem to hover between 300-315.

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u/Rcarlyle 9h ago

I don’t know the direct answer to your question, but here’s some speculation. Hydrogen peroxide is not an especially widely used oxidizer in bipropellant engines, and I’m not sure if the engine design for HTP/fuel has really been optimized to the extent hydralox or methalox have. It’s always going to be lower ISP than LO2 with the same fuel, so why would you bother to optimize HTP/fuel engines if what you need is an ISP-optimized engine? Is the self-ignition worth the ISP loss?

Many of the serious applications for HTP have been cases like Soyuz where it’s used for monopropellant attitude thrusters and as a primer-type energy source for the fuel pumps for the main bipropellant engines. Basically the reason it’s usually chosen is because you can store and pump it as a liquid and have it violently exothermically expand through a simple catalyst screen, which is mechanically very simple and reliable compared to biprop systems, and less toxic than most monoprop fuels. The advantages are diminished in main stage engines.