Orbital Refueling Depots

[Johno]

I first read about the idea of orbital refueling depots shortly after the Augustine commission presented its findings. Now, cards on the table - I\'m not a rocket scientist. I am a High School Science teacher, meaning I\'m scientifically literate; and I have a very basic understanding of orbital mechanics (which, incidentally, has been enormously augmented by my playing a certain space game involving insane alien astronauts! ).

That simple, straightforward understanding is enough that I appreciate the size of LEO space. It was originally also enough that I thought I understood the utter futility of parking a large amount of fuel in orbit for a future mission to rendezvous with. After all, unless the future mission happens by luck to be on the same trajectory and use the same fuel as has been stockpiled in the depot, it can\'t access the depot. Let\'s face it, there aren\'t that many possible destinations on particular orbital paths - a mission to Mars couldn\'t use a refueling depot set up to send something to Venus, etc. I\'d simply put the Commission\'s ideas down to having watched 'Armageddon' too many times.*

But I have noticed a data point which may well be significant. It\'s that I haven\'t seen a chorus of scientists screaming 'You are so stupid!' to this idea.

Now, this means one of the following:

1) The Commission are so far from understanding ANYTHING relating to the realities of space that the scientists don\'t believe their findings to be worth the paper they\'re written on. Since President Obama (for all his faults) appears to be a fairly clever bloke, surely he picked a commission which had scientists and engineers as well as bean counters, so I include this option only for completeness.

2) The scientists are intimidated into silence (conspiracy theories, don\'t you love \'em?).

3) The idea hasn\'t had enough publicity to arouse a serious response.

4) I don\'t know orbital mechanics as well as I thought I did, and there\'s actually sensible concepts for depots that might work.

I\'m tipping 4, but please discuss!

EDIT:

Just a clarification: I\'m completely au fait with the idea of rendezvous of two or more units in space, giving a larger and more capable spacecraft than a single launch can manage. But that\'s different in my mind - both launches are specifically targeted for a particular purpose, so consequently you deliberately place them on intersecting orbits. In addition, to my mind it\'s ridiculous to refuel a spent tank - wouldn\'t it make more sense to have a complete stage in position? Eliminates a lot of pointless fluffing around, and when you\'re finished with it you just drop the stage, meaning you\'re not carrying all that extra mass all the way to Mars.

* Once is too many times.

[RulerOfNothing]

Correct me if I\'m wrong, but aren\'t the proposed fuel depots considered as part of the mission, rather than going 'Let\'s put fuel in orbit around Mars in case we want to go there in the future'?

[Johno]

Well, that\'s kind of the thing. I can get alongside that. But what I\'m reading (just Wikipedia and similar, of course) is seeming to imply 'huge cache of juice in orbit, just in case'. . . .

[icefire]

Well, given that most planets orbit with low inclinations relative to the ecliptic, most interplanetary missions have a 10 degree variance at most. What this means is that it would be plausible to have an orbital refueling depot at somewhere like the Earth-Sun L2 lagrange point that\'s available for most missions to other planets. With only minor plane corrections needed after refueling and final orbit injection.

So no, course variance isn\'t the largest hurtle for such an installation. It\'s simply finding a cost effective way to get it up there in the first place.

[Johno]

Yes, that one had occurred to me too.

[Saaur]

You don\'t PUT it up there. You MAKE it up there.

Plenty of sunlight to run solar panels, so you make a great big magnetic scoop to gather in free-floating Hydrogen. Bussard would be proud.

Actually, the easiest way (though not so useful for us right now) would be to have something in low orbit over Jupiter, scooping Hydrogen out of his atmosphere and cleaning it up. Be great for missions on the way further out.

[Johno]

Is there any confirmation of that much Hydrogen out there, or is that just an idea at this stage?

[Newman]

No, there isn\'t. It\'s estimated you have about 1 molecule of hydrogen per cubic meter of your run of the mill vacuum of space. Of course, this number will vary depending on where you are, but it doesn\'t change the fact that the vast majority of space is pretty devoid of that stuff. Proposed ship design that uses a bussard ramscoop to collect hydrogen and use it as fuel would need to have the ramscoop dish part that collects the hydrogen at an estimated diameter of 100km, if memory serves. It was mentioned in Carl Sagan\'s 'Cosmos' but it\'s been.. way too many years since I last read it, so it\'s quite possible I got some of the numbers wrong. But the main gist of it was, there\'s not a whole lot of hydrogen molecules in your typical cubic meter of space, and you\'d need a giant collector. The point is - as nice a theory as it is, for all intents and purposes it\'s not a practical idea.

Scooping something off of Jupiter\'s atmo wouldn\'t have that problem since there actually is hydrogen in large quantities over there. But that would prove to be a huge technical challenge for other reasons. First of all, even reaching Jupiter is a non trivial, and very fuel intensive task. Secondly, the high levels of radiation that close to Jupiter would make the technical challenges of such a mission rather high. A much cheaper and easier way would be a Lunar base built over a subterranean patch of ice that converts it to hydrogen. I seem to recall they did find some ice patches below Lunar surface using an orbital spectrometer last time they 'bombarded' the moon, impacting the surface with a special probe and analyzing the dust/debris cloud that formed. Someone correct me if I\'m wrong, I\'m not entirely sure just how much ice was found and whether or not it would be enough for a sustained fuel manufacturing operation.