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A memory of hexane and water in a separatory funnel brought to mind the issue of helium and LNG in the header tank while the Starship flips and does its powered landing.

There was discussion of the possibility that helium ingestion contributed to the SN 10 hard landing.

Would alcohol separate faster from helium than LNG? Could it be used as "landing fuel" with the Raptors.

Considering the issues of keeping LNG cold for 6 months on the way to Mars, could the Starship fly completely on alcohol?

The energy per volume seems close enough, and there are only seconds on the "flip" to get the thrust right.

The Falcon-like design of the BN booster features a long vertical drop under drag G's, plenty of time for the LNG fuel to "settle". Could another fuel be used for the Starship if LNG/helium in the header tank doesn't work out?

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    $\begingroup$ The short answer is no, and helium is not intended to be use long term anyway. The characteristics of hexane or ethanol would be different from LNG (for example density, energy content and boiling point) and would require a complete redesign of the engine. $\endgroup$
    – Slarty
    Commented Mar 30, 2021 at 19:23
  • $\begingroup$ @Slarty you may note liquid or gas fuel can be used in turbines without "complete redesign of the engine". Indeed, both LNG and ethanol enter the combustion chamber as liquids. $\endgroup$
    – Robert DiGiovanni
    Commented Mar 31, 2021 at 13:57
  • $\begingroup$ yes a good point perhaps for aviation or other turbines, but these are rocket engines and operate under extreme conditions. A Raptor engine will consume more propellant in 4 seconds than a 747 engine does in an hour. Raptor turbines operate under hundreds of bar pressure and both the oxidant and fuel enter the combustion chamber as supercritical fluids. The size and shape of the pipe work really matters. $\endgroup$
    – Slarty
    Commented Mar 31, 2021 at 18:15
  • $\begingroup$ @Slarty Agree 100%. That's where they seem to be having problems. A (poor) fallback would be to fly it with a much higher boiler (like the SR71) and accept the temp change leaks, but I hope they can fix it and carry on with LNG. We know what gas leaks can do to houses and Hindenburgs. $\endgroup$
    – Robert DiGiovanni
    Commented Mar 31, 2021 at 20:58

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Adding another fuel, would mean NOT using a header tank as meant by the usage in the current Starship design.

The current header tank is the drain/vent through which all fuel to the engines current flows on its bottom side. It then closes its upper valves when it is time to 'transfer fuel to the header' as they call out on the web casts. Now it is a full header. With its associated issues.

To use another fuel is to move to a very different design. It is still early, and who knows what they will do.

But switching fuels is not likely to happen, since the design is predicated on a couple of things, but one is that Mars is a primary target, and on Mars making CH4 from CO2 and H2O is essentially a known problem that is semi-solved. (The mechanism/processes are all well known. Actually doing it on an industrial scale on a different planet, not so much).

Thus the short answer is no to two of your questions.

Split out the rest into individual questions for better answers to those.

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    $\begingroup$ could. If the LNG header does not work. Keep in mind combustion chamber temperatures quickly vaporize ethanol, perhaps even faster than LNG. Complete redesign? Perhaps not. And we know how to make ethanol (from the first Martian vineyard). And it could be pressurized indirectly by using a bladder. We may yet see SN 12. $\endgroup$
    – Robert DiGiovanni
    Commented Mar 30, 2021 at 20:19

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