In this post, Jay Manifold comments:

...my (possibly incorrect) understanding is that LH2 can only be stored for a few hours.

It is incorrect. There's no intrinsic limit on how long you can store LH2. It's just a matter of how much weight and power you're willing to devote to insulation and/or refrigeration systems.

In fact, the concept I have for a cis-lunar infrastructure is a series of combination depots/tankers. You'd have at least four of them (probably five, for backup purposes). One would be sitting in LEO, being filled up. One would be sitting at L1 to provide propellant for returning and lunar-bound vehicles. There would always be (at least) two in transit, one heading toward LEO, and the other heading toward L1. When one arrived, the one already there would depart to the other destination.

I'm envisioning them with plenty of power, both to run high-Isp thrusters, and to keep propellants continuously chilled. You might be able to do it with solar (though the panels would take repeated beatings going through the Van Allen belts). The obvious technical solution would be nuclear, but that's probably still politically unacceptable, despite its reasonableness.

[One further evening thought]

If they were powered with nukes, there'd be plenty of power to not only keep the hydrogen chilled, but to actually crack it from water. The marginal cost of doing so, given the initial investment of the nuclear tankers, would be pretty low, and it could dramatically affect the cost of delivering the propellants to orbit, since they could be delivered in a more dense form that doesn't require cryogenic tankage, and is much safer. Of course, the vehicles would either have to operate at a stoichiometric ratio of 8:1 oxygen/hydrogen (which is suboptimal in terms of specific impulse--ideal is 6:1)) or throw away or find other uses for the excess O2.

Do not forget EML-2.

Over at nasaspaceflight a poster named vanilla has done fabulous work data mining various 1960s papers by Dr. Robert Farquhar which show the delta v advantages of EML-2.

Also, the ability to add ISRU lunar LOX creates tremendous additional leverage.

You can't make effective insulation for LH2,
that being defined as lasting days, not hours.

You need to actively cool and have a way to circulate it so
the material does not stratify.

It's tough stuff to work with.

If you had the space nukes, why not just go for nuclear thermal propulsion and cut out the middleman, so to speak?

Tethers are nice, also.

Anonymous,
That's really not necessarily true. In vacuum, where your only source of heat transfer is radiation, proper use of sunshades and MLI, and careful stage design can get LH2 losses to less than 0.1% per day, and possibly as low as 0.01% a day, *without* needing cryocoolers. The Centaur guys I've been speaking with lately think that it's perfectly feasible to have LH2 stored in space, without active cooling on the several months to one year time-frame. The problem does take careful work, but the difficulty is vastly exaggerated (at least from what info I've gotten from people actually working directly on the problem).

~Jon

Jon,

Though the advanced Centaur looks to have fabulous potential, I wonder if it is truly applicable to a reusable lunar lander. I would think the thin skinned Centaur design wouldn't stand up to the kind of beating a lunar landing could inflict. That's one reason why I think the Centaur might be better employed during lunar landing as an expendable crasher stage.

Rand,

If nuclear power is exploited for spacecraft propellant depots, why not just use ammonia propellant and nuclear thermal propulsion for the spacecraft?

Ammonia would work great for a pressure fed storable NTR propellant, and even though it would deliver half the ISP of hydrogen propellent NTR, ammonia NTR could match the ISP of LOX/LH2 chemical rockets.

Compared to chemical rockets ammonia NTR could be the best of both worlds -- high-density non-cryogenic propellant easily handled, stored and transfered in space, while still having high ISP. Ammonia propellant NTR propulsion could make the entire concept of orbital propellant depots and dry launched deep space rockets easily practical.

Solar-dynamic power systems (e.g. the Brayton-cycle engine work done at Glenn) would be less subject than photovoltaics to radiation damage from repeated Van Allen passages, and more politically acceptable than fission power.

To follow up on Stellvia's point, I suspect passive solar power will prove to be the better value for lunar surface applications when compared with nuclear. Temperature gradients between sunny areas and shady areas will be substantial and a modest amount of Lowe's grade mylar -- fashioned into the appropriate parabolic shape -- will be able to heat a working fluid (ammonia? supercritical CO2?) to well over 1000 degrees C.

Stick the hot end of a heat engine (Sterling cycle? Braxton or Brayton?) at the focal point of a mirror array and the cold end in shaded regolith and that fluid should circulate quite nicely.

Night time? Various ideas come to mind including using heat sinks and Sam Dinkin's glass ball idea.

I am not against nukes (David Poston has terrific ideas) but a simple heat engine may trade much much better.

OK, some interesting possibilities here, to say the least, among them: if LH2 losses can be pushed down to 1 part in 10^4-10^5 per day, then high-ISP, low-thrust (ie solar-electric or nuclear-electric), very high payload-fraction tankers become feasible.

The other is the use of EML2. Turning to T.A. Heppenheimer's Colonies In Space (pp 115-116), we find:

By computing orbits to be followed by the payloads, it has been found that if they are launched from a suitable point on the lunar surface and aimed at a target, they will hit that target even if the launch velocity is slightly off. That is, the gravity of the earth and moon acts to focus the trajectories so they arrive at the target despite slight errors in velocity. The best such launch site is at 33.1° east longitude, near the craters Censorinus and Maskelyne. Then, the target can be chosen as a point in space 40,000 miles behind the moon, known as the L2 point. A catcher or target located there will stay on station, since there the gravity of the earth and moon is cancelled by the centrifugal force due to the orbital motion of the catcher.

Oops, shoulda said "1 part in 10^3-10^4 per day."

Oops, shoulda said "1 part in 10^3-10^4 per day."

Hey, its just teh inatrweb -- no worries, mate.

In astronomy, getting within one order of magnitude of the correct value is often considered adequate. ;^)

That's why they plot everything on log/log, makes the error bars look tiny.

"The marginal cost of doing so, given the initial investment of the nuclear tankers, would be pretty low..."

I realize it was an "evening thought", but this isn't an especially appealing argument. You could make the same argument, for example, that the marginal costs of operating a fusion powerplant will be "pretty low" given the hellish amount of money that's been spent (and will be spent in the future) to develop them.

I take it as a given that development costs for space nuclear reactors aren't that high, Reed. It's not like we've never built one before. The Russians did it decades ago. A comparison with fusion is inappropriate.