How Can We Efficiently Transport Hydrogen and Oxygen to Space Stations?

[Colby]

Eventually, if we want to establish outposts in our solar system, starting with the Moon, we will need a continuous source of propellants. A space station in LEO and in lunar orbit would be needed. Let's say the propellant of choice is hydrogen and oxygen. What would be the best way of getting these propellants to the space stations?

I figure the hydrogen would have to come from Earth, since no other body in the inner solar system has a sufficient amount. What would be the best way to bring this up into space? Would storing it as liquid hydrogen or storing it as water be the best method? If I did my figures right a tank of H2O would have 1.5X more H2 in it then a tank of LH2 of similar size, but it would weigh 14.4 times more (not taking into account active cooling to prevent boil-off). I also figure that LOX would come from the Moon, since it is pretty heavy to bring up from Earth and the Moon is 45% oxygen by mass.

What do you think?

 

[enigma]

Hi Colby!

Welcome to PF!

It's not as easy at it would seem. Storing fluids in space is not as easy at it might seem.

Every day, approximately 1% of the atmospheric gases manage to leak out of spacecraft , whether it's the Space Shuttle, the ISS, the Mir, or any other craft. You just can't make the thing perfectly sealed everywhere against a darn-near-zero outside pressure. This applies to piping relating to the stored fluids as well. You can't have it perfectly leak-proof.

Additionally, you have to deal with boil off. Liquid Hydrogen and Oxygen are really cold. Frostbite your face off in an instant type cold. It's really hard to keep something that cold at a steady temperature. There is no air in space, so all heat transfer has to happen through radiation. When we're in orbit, almost half of the sky is blocked by the Earth, which radiates at approximately 298K. You need really big radiators to get rid of all of that PLUS the extra temperature you'd need to dump to keep the stuff liquid. What real spacecraft do is just boil off the fuel. If you watch a launch, you'll see what looks like steam escaping from the rocket. That's actually boiling oxygen escaping. That is one reason why the Shuttle uses hydrogen/oxygen for its main engines, but room temperature fuels (Nitrogen Tetroxide/Monomethyl-hydrazine or N2O4/MMH) for on orbit maneuvers.

 

[Colby]

NASA Ames Research Center is working on active cooling to eliminate boil-off. They did some testing and it seemed really promising. As for the leaking, we really just need to invest some more money into that area. It will have to reduce the leakage, eventually, somewhat... hopefully?

Well, perhaps for launch the propellants can be stored cryogenically, but after launch they can be converted to water through fuel cells. That may be easier to store at a space station for any given length of time, or on a transfer vehicle, because the transfer vehicles that would be best for the job probably would use ion propulsion. It may take weeks, but it would be economic, since it has quite the high Isp. So, perhaps all the propellant for the transfer vehicles and the fuel for the landers should come from Earth, while the oxidizer for the landers should come from the Moon. It would be ideal if a lunar lander would be able to store enough propellant to land and take-off without refueling, and still have a large payload.

 

[drag]

Greetings !

Would storing the water as ice work ? Maybe wrapped in large bags
of reflective material or something and rotating with respect to the sun ?
If so, a reusable rocket could bring many of them up into low lunar orbit
for further use.

Live long and prosper.

 

[hitssquad]

The economics of storing expended fuel in space

Well, perhaps for launch the propellants can be stored cryogenically, but after launch they can be converted to water through fuel cells.

Converting hydrogen and oxygen to water expends their capacities as chemical fuels.

That may be easier to store at a space station for any given length of time

It would certainly be easier to store expended fuel, but what might be the point?

It may take weeks, but it would be economic, since it has quite the high Isp.

If water has a high Isp, one might imagine it might be pretty dangerous to drop a match into an ocean.

 

[drag]

Converting hydrogen and oxygen to water expends their capacities as chemical fuels.

Nothing that a couple of solar panels couldn't fix in a couple
of days.

 

[sigma]

Nothing that a couple of solar panels couldn't fix in a couple
of days.

That's the spirit of it!