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« Why NASA? | Main | A Real Paper Rocket »

Lunar Transportation Infrastructure

Tom Cuddihy (to whom congratulations on his upcoming marriage are owed), inspired by some musings on the subject by Jon Goff, runs some numbers on reusing lunar landers, and finds that (unsurprisingly), it doesn't make sense. At least with the assumptions that he uses.

The utility of reusable space transportation elements is heavily dependent on the cost of propellants in all of the transportation nodes through which they operate. If we are going to deliver all propellants from earth, to the surface of the moon, using chemical propulsion, then it's not possible to justify reuse of the lander (and in fact it would be impossible to justify reuse of the crew module itself, except for the fact that we have to return crew, anyway). If we are to have a cost-effective cis-lunar transportation infrastructure, it's not sufficient to get the cost of LEO delivery down (though it is certainly necessary). We also either need to manufacture propellants on the moon, or deliver them to L1 via low-thrust high-Isp tugs from LEO, or both.

This was discussed (I believe--at least I wrote a lengthy input to it) in the final Boeing report on the CE&R contract (a document that NASA apparently never even bothered to look at once Steidle was fired and they came up with ESAS).

OK, enough space blogging for a while. I've got to get back to work.

Posted by Rand Simberg at February 13, 2006 06:23 AM
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Tracked: February 13, 2006 10:39 PM

Take home message? Lunar LOX is critical regardless of Earth-to-LEO cost levels.


Posted by Bill White at February 13, 2006 07:27 AM

Mazel tov to Tom on the wedding thingy.

I saw a tether study once that suggested that after 14 flights you were better off with a tether cisuluar infrastructure, and no lunar lox manufacture. But the flight windows were narrow

Posted by Jane Bernstein at February 13, 2006 07:44 AM

You should be able to get some savings with a reusable ascent stage. You'd have to bring out an expendable descent stage for each mission, but the ascent stage with its avionics anf life support probably represents half the hardware cost of the lander.

Posted by Will McLean at February 13, 2006 08:02 AM

That seems unlikely, at current launch costs, Will. The cost of bringing the extra propellant all the way to the moon to bring back the lunar ascent stage is probably still higher than the stage cost (particularly when you factor in refurbishment costs for low flight rates).

Posted by Rand Simberg at February 13, 2006 08:05 AM

Jane is correct that tethers will change everything.

A MXER can drastically reduce the costs of sending bulk goods from LEO to either Luna or EML-1. I like the idea of tossing bulk goods to the lunar surface in air-bag cushioned packaging. Just let the packages bounce for a while.

But if we blend lunar LOX with tether delivered fuel (hydrogen, methane, kerosene, alcohol, whatever) the net available fuel delivered increases quite substantially.

11 tonnes of H2 plus 89 tonnes of lunar LOX equals 100 tonnes of propellant. 20 tonnes of CH4 plus 80 tonnes of lunar LOX equals 100 tonnes of propellant. Just the ticket for shipping back platinum bearing asteroid fragments. ;-)

Posted by Bill White at February 13, 2006 08:16 AM

I'm not proposing to bring back the ascent stage. Rather, it would stay at a depot in lunar orbit after ascent until the next mission. This saves the propellant required to bring out a new one.

There's an added bonus to doing it this way. NASA proposes to carry extra fuel in the CEV SM for anytime return in an emergency. In a nominal mission with no emergency, it is offloaded at the depot, providing fuel at the depot at no extra cost.

Posted by Will McLean at February 13, 2006 09:39 AM

Actually, the Boeing proposal was similar to that, except that they had a single-stage LSAM, so it required more propellant. It's kind of moot for now, though, since Mike Griffin's NASA seems to have dropped all plans for L1 (and it would be hard to find any other location that would be convenient for rendezvous for each lunar mission).

Posted by Rand Simberg at February 13, 2006 09:49 AM

If you had a polar lunar outpost, polar orbit would work for orbital rendezvous, yes?

Posted by Will McLean at February 13, 2006 11:16 AM

I have a quick question for you. All the numbers I had previously seen for LEO-L1 delta-V were in the ~3.7-3.8km/s range. Tom seems to think that it's closer to 4.2km/s, and the Boeing study supposedly was claiming 5.1km/s according to Mark Wade (who I think screwed something up). I checked the Boeing CE&R powerpoint, and it was a little bit confusing if they were saying 4200m/s +900m/s to get into L1, or if it was 4200m/s to L1, and then 900m/s for something else entirely. Do you have any idea where they got these numbers? an extra 1-2km/s round trip makes a huge deal when it comes to chemical propulsion, and I was just trying to do some fact checking on my assumptions.


Posted by Jonathan Goff at February 13, 2006 11:25 AM

Well, I assume that they got them by calculating them. ;-)

I can't answer your question without seeing the chart at issue. I do know that they were looking for disposal orbits from L1, so that 900 m/s might be to L4 or L5. I suspect that the number is the 4200, but as I said, it's hard to know for sure without knowing what you're looking at (and perhaps even then). The Boeing folks who did that are in Huntington Beach, and we could probably get in contact with them if it can't be resolved otherwise.

Posted by Rand Simberg at February 13, 2006 11:32 AM

If you had a polar lunar outpost, polar orbit would work for orbital rendezvous, yes?

Occasionally. But not really. The question is, which polar orbit? If there was one that naturally precessed once a month, it might work fine, but I don't think there are any like that.

For orbital mechanical consistency in terms of getting in and out of it from both LEO and the lunar surface, it's tough to beat L1.

Posted by Rand Simberg at February 13, 2006 11:39 AM

From CEV Boeing per astronautix (as linked by Tom Cuddihy):

From L1 to any point on the lunar surface, the LSAM would have to make a 907 m/s delta-V to move out of L1; 2199 m/s to land on the moon's surface, 2133 m/s from the surface back towards L1; and 999 m/s to brake into L1.

Getting to EML-1 is one thing. Stopping at EML-1 is another. Getting to EML-1 and docking or just passing through? Minimun delta V might extend delta t (elapsed time) to an unacceptable level.

I seem to recall reading that one or more Apollo missions passed very close (or through) EML-1 on the return leg to Earth but they were travelling close to 4000 kph relative to Earth, Moon and EML-1.

That would make it tough to dock with an EML-1 facility. ;-)

Posted by Bill White at February 13, 2006 12:22 PM

Another useful link for lunar trajectory information.

Posted by Bill White at February 13, 2006 12:52 PM

Link to Jon's post was borked when I tried it in Firefox.

Posted by Jeff Dougherty at February 13, 2006 09:47 PM

Perhaps the best cislunar architecture which uses lunox would be a spacecraft similar to the t/Space proposed lunar-lander. That is, a reusable lunar-lander that travels from LEO to the lunar surface and back again and that refuels at each arrival point. That way the lunar-lander only needs enough propellent for a one way trip.

The reusable lander would use methane/LOX engines. Traveling to the moon, the lander would carry an excess of methane fuel, enough for the return trip to Earth. Once on the moon the lander would refill it's oxygen tanks with lunar derived oxygen. Earth orbital capture would mainly use aerobraking supplemented by a small amount of propulsion.

This cislunar transportation system might not require any offworld infrastructure other than the moon base. I imagine a LEO propellent depot would be usefull though.

Posted by Brad at February 13, 2006 11:40 PM

How would something like a lunar space elevator figure into all of this?

Posted by BravoRomeoDelta at February 14, 2006 10:03 AM

I wrote:

If you had a polar lunar outpost, polar orbit would work for orbital rendezvous, yes?

Rand Simberg writes:

Occasionally. But not really. The question is, which polar orbit? If there was one that naturally precessed once a month, it might work fine, but I don't think there are any like that.

I reply:

Then consider this option. The system resembles the NASA baseline, but with a reusable ascent stage. You are supporting a polar outpost

The first mission is much like the NASA baseline. The CEV retains enough fuel for anytime return, and when the mission goes acording to plan and no emergency return is required, transfers the propellant to that mission's ascent stage after it returns to lunar orbit.

The next mission brings out a fully fueled CEV, again with propellant for anytime return, a fully fueled descent stage, and a logistics module similar to the Russian Parom, capable of carrying five tons of propellant for an ascent stage.

For any object in Lunar orbit, there are two optimal launch windows a month. If all goes according to plan, the launch windows are met and no early return is required. The ascent stage is mated with the descent stage and makes a round trip. The anytime return contingency fuel
is used to refuel the ascent stage, and the logistics module is retained as a reserve.

If the last optimal launch window is missed, the ascent stage makes the necessary plane change for rendezvous, and must be refueled from the logistics module. Unless an emergency return is required, the service module can refill the logistics module. If emergency return is required, another logistics module must come on the next trip.

Posted by Will McLean at February 14, 2006 06:57 PM

Hey Jon, it looks like the 900-1000 m/s (4.2 + 0.9 = 5.1 km/s) number appears to be to drop into LRO (Lunar Rendezvous Orbit?) orbit from EML-1, or climb from LRO to L-1, if I'm reading the chart correctly. The paper that Bill notes shows an ~700 m/s transfer from EML-1 to LLO, so it's not inconsistent.

4.2 km/s into L-1 does seem a bit high. A 900 m/s dV to park in L-1 seems counterintuitive to me, as you're talking something close to L-1's geocentric velocity. Ah, the joys of orbital mechanics.

Posted by ken murphy at February 14, 2006 09:34 PM

For any object in Lunar orbit, there are two optimal launch windows a month.

Again, in what lunar orbit? There are many lunar orbits that will be expensive/impossible to get to. Are you assuming that a lunar polar orbit is inertial, and that it will present an in-plane opportunity twice a month? I'm not sure that's the case.

Posted by at February 15, 2006 08:52 AM

It's my understanding that a lunar polar orbit is inertial, as a reasonable first order approximation.

What I don't know is whether you can tailor an efficient parking orbit for EOR that will let you fail the first opportunity and still be able to launch into the the second without plane changes.

Posted by Will McLean at February 15, 2006 02:12 PM

Well, I'd assume that if the answer to that were affirmative, it would have been described in the CE&R studies, and the attractiveness of L1 would go away.

L1 really, really simplifies operations, at the cost of delta V, but if we were designing for low-cost operations, it would be worth it. Unfortunately, NASA is locking itself into a high-cost transportation infrastructure for the foreseeable future.

Posted by Rand Simberg at February 15, 2006 02:17 PM

Call me crazy but how could it cost more to leave the Lunar Module ascent stage at the L1 platform and just bring fuel and a decent stage the next trip?

Even better design the system with a light descent stage so that it can stay with the module and eliminate the need to bring up additional descent stages. Then the weight currently used in the stack to get the lunar module up can be used to bring up supplies.

Of course you would want to bring up two lunar modules to ensure redundancy and be able to steal parts when required but that would still be cheaper than bringing up a new lunar module each launch.

You might also want to bring up a garage. A transhab that can zip open on one end and an open interior to park the module in between missions but if we're gonna get the L1 station working sending up transhabs from time to time would tend to be in the plans anyway.

Posted by rjschwarz at February 23, 2006 01:42 PM

See, now that's what I call a brilliant insight. Given that you would probably want two or three descent/ascent stages for the L-1/Luna run, over ten missions you'd save the mass of at least seven of them being shipped beyond LEO. Boy does that open up a lot of possibilities (ship more fuel!). A garage is a good idea too, and I'm becoming more and more fond of Homer Hickham's suggestion of ballon landers for the crew.

It also suggests to me that we're better off doing this whole thing in lot's of smaller, different packages than one super-mega-package.

Posted by ken murphy at February 25, 2006 02:07 AM

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