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Old 07-11-2012, 08:33 PM
s0meguy s0meguy is offline
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Spaceport on small moon more efficient? Physics question.

If our space development ever gets this far: it seems to me to cost a lot less energy to first propel an object relatively slowly to a small moon and from there propel it at high speeds into space to its destination on another planet or moon for example, because of the lower gravity of that moon and counteracting the gravity of that moon costs a lot less energy. Do you guys think that almost all inter-planetary travel and cargo transport would happen through a spaceport on a moon because of that?
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Old 07-11-2012, 08:58 PM
Trinopus Trinopus is offline
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Ideally, the spaceport would be in interplanetary space, and/or at a Lagrange point such as the famous L-5. Why go down into a gravity well again once you've fought free of one?

However, a moon-base makes *some* sense, because you can get away from it again so much more easily than from the earth.

(Compare the Lunar Ascent Module to even the minimum rocket needed to get two men off the earth!)

A honkin' big fuel depot *somewhere* makes sense, and also a construction yards.

It isn't known yet if zero-g industrial-scale manufacturing is more feasible than a factory on the Moon's surface. The simple ability to set things down is remarkably helpful to us -- as we stand now. (Pun.) Once we start developing zero-g factories, this may well change.
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Old 07-11-2012, 09:13 PM
Simplicio Simplicio is online now
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Quote:
Originally Posted by s0meguy View Post
If our space development ever gets this far: it seems to me to cost a lot less energy to first propel an object relatively slowly to a small moon and from there propel it at high speeds into space to its destination on another planet or moon for example, because of the lower gravity of that moon and counteracting the gravity of that moon costs a lot less energy.
I don't think that's true. It would take more energy to climb out of two gravity wells then just the one.

One can kind of imagine a scenario where we'd use the moon to assemble things despite the extra energy costs. I'm not sure that's particularly likely though, and even if it was, it wouldn't be because it "cost a lot less energy".
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Old 07-12-2012, 06:17 AM
robert_columbia robert_columbia is offline
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There's also the advantage of a small moon's lack of an atmosphere. Say we are making or repairing stuff on the Moon. Since there's really no atmosphere to speak of, there's no real need to protect the stuff against rain or snow. Also, since few or no microbes will grow out there, there's less of a need to disinfect stuff, so you could have an unpressurized, sterile factory out there on the lunar surface that you don't need to continually douse in alcohol and bleach to keep clean.
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Old 07-12-2012, 06:25 AM
Der Trihs Der Trihs is offline
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If one builds a space elevator on the moon in question (much easier with the lower gravity), then the energy demands for leaving its gravity well would be lower (or even zero if there's as much going down as up). You can also use such an elevator as a launcher for spacecraft; extend the tether past the geostationary orbit and anything riding it will be flung outward, since it'll be moving faster than orbital velocity. That alone could make sending cargo from a moon (or a smaller planet, like Mars) useful.
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Old 07-12-2012, 06:59 AM
kanicbird kanicbird is offline
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If we can manufacture rocket fuel on the moon, hopefully He3 fusion based, then it makes sense - we can't do that in orbit. It takes a lot of fuel to get to the moon, having the ability to refuel there would appear to give a great advantage.
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