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What are the tradeoffs between LOX/methane and peroxide/kerosene for a Mars landing mission?

In which ways are each better or worse than the other?

I think the second one would more attractive for storability for example since LOX is cryogenic, but I’m not sure if that's as true for a deep space mission.

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    $\begingroup$ It's a tradeoff. The second option (HTP-K) is storable/non-cryogenic, generally easier to use and handle, and has a better impulse-per-volume (specific energy, I think?). LCH4-LOX gives better impulse-per-mass (specific impulse) and is more easily synthesized from local martian chemicals. It really comes down to your other mission constraints. I don't feel confident recommending a "better" option, hence comment rather than answer. $\endgroup$
    – Infinite_Maelstrom
    Commented Jun 8, 2023 at 22:30
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    $\begingroup$ per Infinite_Maelstrom's comment, better/worse is frequently only defined in context in engineering, and even then it can be fluid. To decide which one to use, you'd do a trade study, which is a tool engineers use both to help make a decision and document why a decision is made. $\endgroup$
    – Erin Anne
    Commented Jun 9, 2023 at 0:08
  • $\begingroup$ @Hartsfield Welcome to Stack Exchange! I've made some edits to your question to replace "better" with "tradeoffs". SE questions should be written such that they can potentially have a right answer, and asking for what's "better" in a complex situation can't do that. Have a look and feel free to edit further. Thanks! $\endgroup$
    – uhoh
    Commented Jun 10, 2023 at 0:51

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High-test hydrogen peroxide (HTP) can be stored at room temperature, but is not long-term stable. Decomposition of hydrogen peroxide is what limits the Soyuz to a bit over 6 months of operating lifetime. HTP/kerosene has a lower specific impulse to begin with, and its performance will be even lower by the time the craft reaches Mars due to decomposition of the oxidizer.

Maintaining even that low of a decomposition rate requires extreme care to avoid contaminating the tanks, plumbing, or peroxide oxidizer. In LEO, discovering unexpectedly high decomposition might mean coming home early, for a Mars mission it would mean not having enough performance left to land. The hazard isn't limited to reduced performance either, HTP is capable of runaway thermal decomposition and its vapors can decompose explosively. On contact with some substances, it can cause serious corrosion or even spontaneous ignition.

LOX and LCH4, if you have active cooling systems, can be stored indefinitely, without many of the hazards of HTP. They can also be produced on Mars from local materials. If you don't have active cooling or a mass budget for passive storage of cryogenic propellant, more traditional hypergolic systems have better long-term stability.

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  • $\begingroup$ However we know that the Soyuz's peroxide does not contain stabilizing agents and the answer suggests they have a risk of interference with the catalytic exothermic decomposition as a monopropellant thruster. Is there any reason to believe that chemical stabilizing additives would substantially interfere with it's performance as an oxidizer in a bi-propellant scheme where there is no catalysis? Also, wouldn't mild cooling (shielding from sunlight, exposure to deep space) also extend its lifetime - not sure the case against H2O2 is so simple. $\endgroup$
    – uhoh
    Commented Jun 10, 2023 at 3:16
  • $\begingroup$ The first part of this answer could also apply to space.stackexchange.com/q/63826/6944 $\endgroup$
    – Organic Marble
    Commented Jun 10, 2023 at 22:17
  • $\begingroup$ @uhoh I'm not aware of anyone ever seriously attempting to store HTP for really long duration spaceflight, with or without stabilizers. As for chilling, it has a freezing point similar to water...somewhat depressed when mixed with water, but well above that of hydrazine at ~90%. Apart from blocking flow, freezing would tend to separate stabilizing agents from the peroxide ice, which might be an issue. Also, the stabilizing agents are in solution, and don't stabilize the vapor. $\endgroup$
    – Christopher James Huff
    Commented Jun 11, 2023 at 1:09
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    $\begingroup$ There's a long history in rocketry of people trying to use HTP as an oxidizer because it seems like it should be easy to deal with. So far, it's only had a few small-scale successes, with most attempts either not getting anywhere or switching to another oxidizer. Its biggest success was the Black Arrow, a small launch vehicle which made a single successful orbital flight (out of two orbital attempts) and was canceled because the Scout was cheaper. It's also been involved in numerous accidents in other fields. I doubt just adding some stabilizers will "fix" it. $\endgroup$
    – Christopher James Huff
    Commented Jun 11, 2023 at 1:29
  • $\begingroup$ @ChristopherJamesHuff Okay I'll go research storage of high concentration H2O2 (I'll drop the "HTP" monicker since this chemical is in widespread use outside of rocketry.) Let's see what's actually known about it. If I ask a Chemistry SE question I'll ping you here with the link. Would you say any concentration above 90% is relevant? Does HTP have an official concentration spec? (I have a small bottle of 35% at home - should probably get rid of it come to think of it) $\endgroup$
    – uhoh
    Commented Jun 11, 2023 at 3:36

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