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Comment by burfog | Hacker News Reader

Looking at the list of cycles on Wikipedia, I'm surprised that nobody seems to have used preburners to pressurize the tanks. Use a fuel-rich preburner to pressurize the fuel tank, and an oxygen-rich preburner to pressurize the oxygen tank.

Mixing should be limited even if nothing special is done, due to the temperature and phase of matter and short timeframe. One could of course pay the weight penalty of a piston (need not have a perfect seal) or collapsing bag.

Doing a heat exchanger (to boil and thus pressurize) is another option, but then you're back to needing a place for the exhaust. It would let you do a sort of full-flow engine without turbopumps however, which is great. All those issues with cavitation and lubrication and stress cracking just go away.

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> I'm surprised that nobody seems to have used preburners to pressurize the tanks

This combines the worst of both worlds. You get the weight and complexity of turbopumps. And you get the weight, explosion risk and leakiness of pressure vessels.

Skip the turbopump. That is the whole point of pressurizing.

Pressurizing has weight advantages. You can use a balloon tank. The tank no longer has to have the rigidity to support itself.

Rockets with typical cycles have used balloon tanks. Even modern ones like the Falcon 9 are partially that way, with just enough rigidity to be erected empty on the pad. The Falcon 9 uses helium to pressurize; that could be changed to preburner exhaust even if you did keep the moderate pressure and the turbopumps.

This is one of the reasons SpaceX uses supercooled fuel and oxydizer (and one of the reasons for going methalox in the first place) - the tanks self-pressurize, so they can skip the Helium system.

Pumps are still good because you really don't want the tanks to pressurize at the same pressure as the combustion chamber. If you do, you'll see a rapid disassembly, but your claim on it being unscheduled will be questioned.

This is completely, completely wrong. Really honestly confused nonesense. Supercooling lowers the ullage pressure, if anything increasing the need to pressurise (self or otherwise), because the self pressurisiation (the vapour pressure) is lower. So you have to do additional work to feed the pumps. The only reason to supercool is to increase the density. Your comment is quite incorrect.

I really appreciate your expertise and that you're taking the time to correct the sometimes overeager amateurs in this thread. But can I suggest you tone down the unnecessary invectives? It's true that there ought to be some social penalty paid when people give confident wrong answers -- it's a pet peeve of mine -- but I think it's sufficient to just say something like "Your comment is incorrect. Here's why, in detail. Please don't project so much false confidence next time." That still stings for the other person to hear, as it should, but it leaves them much more likely to want to learn more. And it's much less likely to escalate into a fight that distracts from the main subject.

All fair, valid and noted. It’s the eve of Opportunity’s demise, and i worked on mars EDL, and the infinite tide of javascripters making statesmanly-yet-quite-mistaken proclamations on physics can feel quite disrespectful to the actual engineers who work on this stuff now, worked on the stuff on both sides in the cold war, and everyone back to goddard, tchiolkovsky and moore. But you are right. More whisky and no more HN.

I think you're underestimating the pressures involved here by several orders of magnitude.

To drive the flow of fuel and oxidizer into the combustion chamber, you must overcome the pressure inside. To drive this flow, the turbopump output pressure will be of comparable magnitude to the combustion chamber. This pressure is in the realm of a few 10s of megapascals, or a few thousand PSI. This is a hard design requirement.

If you're proposing to "skip the turbopump", then necessarily you'd have to pressurize the entire fuel and oxidizer tanks to these high pressures. This is, to put it very mildly, utterly infeasible.

> nobody seems to have used preburners to pressurize the tanks

IIRC, Block D taps some preburner exhaust for that...

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