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Also see HTP as oxidizer for main propulsion (for more info on history of H2O2/HTP in rocketry)
Note: HTP and H2O2 will be used interchangeably

I know the Black Arrow LV (The first orbital lipstick) used H2O2 and catalysed it's decomposition for make O2 gas to react with the fuel, but the fuel itself wasn't the greatest.

My idea is as follows: A non-cryogenic HTP(95%) and cryogenic LH2 mix.

Reasoning: H2O2 has great bulk density, and isn't expensive, with the only real drawbacks being a lower ISP than LO2, and relative toxicity of the HTP itself. LH2 would raise the ISP to make it a more effective mix.

Pro's:

  • Less cryogenic tech, pumps for coolant, etc.
  • Better overall bulk density.
  • Products (H2O, H2, O2, H2O2, O3) are less toxic and more energetic than using most oxidisers with comparable performance.

Con's:

  • Probably wouldn't be used on ascent stage due to increased likelihood of explosive combustion and instability due to variance in temp during atmo-ascent.
  • More toxic than LO2 and more dangerous to handle.

My Question(s):
Has an engine of this type ever been planned/fired/flown?
Would such an engine be useful for any specific circumstances?
Are any of the assumptions or assertions I have made incorrect?

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    $\begingroup$ Unlikely. Somewhat decrease difficulty of the easier half (oxidizer) while making the fuel part as difficult as they go. Also H2O2 can decompose explosively, O2 can't. $\endgroup$
    – SF.
    Commented Sep 9, 2023 at 8:32
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    $\begingroup$ How much improvement in bulk density do you get by switching the oxidizer while still using the incredibly un-dense LH2? $\endgroup$
    – Organic Marble
    Commented Sep 9, 2023 at 12:14

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While denser than liquid oxygen (1.4 to 1.1 g/cm3) Hydrogen peroxide contains less energy than LOX because it has Oxygen already party reacted with Hydrogen. This means the energy available per unit is less. Hence the ISP of Kerosene+Peroxide in Black arrow of around 250 vs closer to 300 for RP1+LOX.

This means that in any project where the problems in handling cryogenics are acceptable swapping Peroxide for LOX generally makes sense on performance grounds.

The Peroxide stability issue is not just a problem for safety, it also means you cannot quite tell what the performance if a given tank will be when fired, since it will constantly be turning itself into water and O2. While LOX evaporates and has other exciting issues if you keep the tank full you at least know how 'go' is in there.

A problem peculiar to Peroxide + Liquid Hydrogen is that the freezing point of Peroxide is quite high, -0.4 degrees C. Apart from the problems of ice forming and getting into pumps, valves and injectors the water in the Peroxide will freeze out leaving pockets of highly unstable pure Peroxide that either detonate in situ or when trying to pump through the engine, especially if you are using catalytic decomposition prior to the combustion chamber. It would seem especially problematic trying to ignite a Peroxide/Hydrogen engine in space with a complex warm+cooling cycle needed to get plumbing 'just right' that makes burn planning of say a de-orbit+landing interesting.

This means that Peroxide really only pairs sensibly with non cryogenic fuels, in places where lower performance is acceptable - an orbital runabout craft that used a Peroxide RCS system boosted with liquid hydrocarbons (ethanol?) in the main engine might make sense.

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