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TryingNot2BeToxic t1_j5xew3p wrote

Reply to comment by electric_ionland in Why do sample return missions such as OSIRIS-REx use their own reentry vehicles instead of just going to the space station for pickup and return with ISS equipment? by PromptCritical725

Oh this brings up an interesting problem when it comes to an intermediary moon base/launch platform. Would the propellant needed in order to reduce speed to land back on the moon offset the propellant saved from launching in lower gravity/no atmosphere?

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waylandsmith t1_j5xtcqe wrote

Naively, on paper, yes. Launching from the Moon vs Earth saves you about 9km/s in delta-v, more than making up for the 4km/s to slow down, and then another 2.4km/s to actually land on the moon. But the problem is a lot more complicated than that. Without aerobraking, the propellant needed to land would need to be sent with the spacecraft on its entire journey. Fuels that are stable for long, long journeys are typically significantly less efficient than those that can be refreshed/topped-up until the moment of launch, so more of that stable fuel is needed, requiring more launch fuel to get it into space. The delta-v budget would be turned on its head and the vast majority of it would be needed to be spent right at the end, instead of at the beginning. And this, of course, ignores the problem of how to get the spacecraft onto the moon and the payload back to Earth.

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TryingNot2BeToxic t1_j5zwnns wrote

Aight, new question. Would the space elevator concept be viable on the moon? Like we'd have a moon base, an elevator, and then a station in orbit connected.

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waylandsmith t1_j60w9pt wrote

A lunar space elevator is possible, but would be quite different than one that could be built on Earth. But the moon's lower gravity makes a lot of the benefit of a space elevator moot. A lack of atmosphere on the moon also makes some sort of railgun launch possible and economical, at least for cargo.

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electric_ionland t1_j5xte0y wrote

Yes stopping on the surface of the moon is a pretty terrible idea. However you could envision missions where lunar ice is mined to create propellant and that fuel is sent to a convenient orbit that is more "on the way".

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