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Comment by rmu09 | Hacker News Reader

I'm curious why a crygenic fuel is harder to transfer in space than something like kerosene? Space is cold, so my naive idea is that a crygenic fuel is easier to keep liquid (non-frozen).

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Space isn't "cold", it's a thermal insulator. Objects in space are still illuminated by sunlight whenever they're not in shadow, which heats them up, and the vacuum makes it difficult for them to get rid of that heat.

That's why (for instance) the JWST has a sunshade that is considerably larger than the telescope itself, just to keep it cooled to 50K -- which is still well above the boiling point of liquid hydrogen. It's also why the ISS has large radiators so that it doesn't get too hot for human habitation.

A large sun shade for an object in low earth orbit shouldn't be that much of a problem. Transferring a liquid in microgravity should be equally difficult regardless if it's cryogenic or not.

That sunshade will turn into a big sail in LEO, annoyingly.

The environment in low earth orbit isn't cold, beyond the obvious direct sunlight the earth is warm and fills a lot of the sky. Shielding stuff from it is a challenge. Even near Mars orbit isn't cold enough to store liquid methane, let alone liquid hydrogen.

The issues are: a) boil-off (others have addressed that elsewhere in this these threads), and b) transfer in zero G (something close to an inertial frame of reference). (b) is hard because bubbles of gas form randomly as you drain one tank, so the pump can't work very well. Bubbles of gas are not a problem for rockets normally because they either are always under acceleration (so the gas floats to the top of the tank, while the outputs are at the bottom) or because they have small and full tanks that SpaceX calls "header tanks" to make engine start possible which then resolves the gas bubbles problem in the larger non-full tanks.

It's possible to avoid creation of the bubbles in the first place though. Some tanks could use pistons, and some - flexible membranes between displacing gas and the liquid fuel. Or tanks could be flexible themselves, squeezing the fuel out when their volume decreases and when being fueled, increasing their volume.

If bubbles appear, it's possible to liquify them back. Increase of pressure with possible decrease in temperature (active cooling) could help.

This is an old problem, with quite a few ideas how to solve, some of those ideas even tried in practice.

Retrofitting such things into Starship sounds difficult.

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