Tuesday, August 30, 2011

EML1 Buildup

Today's space launch market is used to place satellites - commercial, scientific and military - into orbit, with the majority going to the geostationary orbit. In all such cases, the launch vehicle does not perform the final maneuver to circularize the orbit. The satellite is dropped off and circularizes its own orbit using on-board propellant. This is a significant delta-v change of about 1.6 km/s, and the remaining fuel is used to maintain the orbit, usually for 25 years or more.

Launch to Geostationary Transfer Orbit, circularize using on-board propellant. This is the standard model for how satellites are deployed into space. It is a mature process which has served us well for decades. However, when planning an exploration architecture, it has always been treated as irrelevant.

Here is a list of some current (and one near future) launch vehicles, their listed throw mass to GTO and the calculated mass that can be placed into the first Earth-Moon Lagrange point using a 312 second specific impulse storable propellant thruster (GTO to EML1 delta-v is 1.27 km/s).

Launch vehicleMass to GTOMass to EML1
Falcon 94680 kg3090 kg
H-IIB 3048000 kg5282 kg
Long March 3B/E5500 kg3632 kg
Proton6360 kg4199 kg
Atlas V 5518700 kg5745 kg
Ariane 5ECA10050 kg6636 kg
Delta IV-H12980 kg8571 kg
Falcon Heavy19000 kg12546 kg

What should be obvious is that there is quite a healthy international stable of launch vehicle providers, and they're all geared up for sending payloads to GTO. What is perhaps not obvious is that by going from GTO to EML1 I am seriously cheating myself. I don't mind because throwing to a lunar transfer orbit is something all of these vehicles can also do and, in all cases, the subsequent transfer to EML1 will be less than a transfer from GTO. As such, we can accept the numbers above as accurate, even if they are overly conservative.

So what does this mean? Suppose we want to land a payload on the surface of the Moon. One option is to simply pick the biggest one of these rockets and fly it directly to lunar orbit and start our descent. The total delta-v for such a mission is likely to be about 3.2 km/s, which means we can land a maximum of 6676 kg.

Suppose, instead, we fly to EML1 and pick up fuel. The table above indicates we can put a maximum of 12546 kg to EML1, and the delta-v from EML1 to the lunar surface is 2.52 km/s, so we need 16043 kg of fuel to make the trip. Because we're using storable propellants, this can be delivered over a long time using whichever provider offers the best price, or over a short time by engaging as many providers as become available.

Although this is just a rough analysis, it shows that we can land twice as big payloads by building up EML1 with propellant, without the need for any new launch vehicles, new technologies or even new ways of doing business, and we could start doing it right now.

6 comments:

  1. Why do you propose going from GTO to EML1 instead of straight to EML1 or from LEO to EML1?

    I can see some advantages, you are already in a high energy orbit which reduces the penalty of using storable propellant from there onwards, and GTO is cheaper to reach than EML1, which reduces throw weight constraints on your launcher at the expense of additional total delta-v. It does require refueling in GTO.

    Then again, a Centaur with minor modifications could do EOR with a storable payload in LEO if you launch the payload first. This requires the cryogenic stage to loiter no more than a couple of days at most.

    A fully fueled Centaur can transport large payloads (~16mT I believe, I can look up the numbers if you want) to EML1/2 and can itself be launched fully fueled on an EELV Heavy, as a quasi-third stage.

    This throw weight from LEO should be enough for most purposes, especially since you could remove most of the propellant from the spacecraft. 16mT dry mass is a lot.

    Using EML1 as the first destination beyond LEO would reduce the total delta-v penalty compared to a detour to GTO as well as allowing you to do a larger fraction of the total delta-v with cryogenic propulsion, still without requiring depots.

    In addition it would allow the use of 3.2km/s three body trajectories from LEO to L1/L2. This actually *reduces* the total delta-v to the lunar surface compared to the traditional LEO -> LLO -> lunar surface approach. It's been a while since I did the sums, but I found that the added efficiency of using three body trajectories almost makes up for the use of hypergolics for LLO -> moon. Not quite, but almost.

    That's why I have long argued that insisting on cryogenic depots first is unwise if we want to achieve cheap lift and open up the solar system. As I say in my NSF signature:

    People should stop obsessing over launch vehicles, cryogenic depots and other infrastructure and focus on spacecraft, missions and destinations instead. Market forces will then establish any necessary infrastructure, especially cheap lift ($100-$1000/kg launch prices). But being selfish they won't.

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  2. Hang on, GTO wouldn't actually be much of a detour delta-v wise. That happens with nearly circular orbits, which you are avoiding to get a closer staging orbit.

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  3. Well, yes, we can land twice as big payloads by building up EML1 with propellant.

    Two questions, however:

    (1)

    What fuel? Yes, I know it doesn't matter much but a specific fuel does need to be chosen and that will drive future options for lander engines.

    RP-1? Traditional hypergolics? NOFBX?

    (2)

    Yes, I do agree we should be stockpiling fuel at EML1, starting yesterday.

    Now, define this "we" of which you speak.

    NASA? Collaborative team of national space agencies? Elon Musk?

    Again, which answer "we" pick doesn't much matter so long as someone does it.

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  4. Bill, for the purposes of this post, let's say it's unsymmetrical dimethylhydrazine, and for oxidizer, nitrogen tetroxide.. as is found in many commsats. NOFBX sounds pretty neat too. RP-1 is only half interesting.

    By "we", I was referring to a fictional international community that actually gave a damn about going somewhere and getting value for taxpayer money.

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  5. You specified a 312 second fuel which seems enough info.

    It cost extra to take fuel beyond LEO. Would it be enough cheaper to the surface of the moon or mars from EML1 than from LEO to justify?

    I know I'm being too lazy to do the calcs myself. One of these days I'll write a little calculator.

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  6. Say you wanted a lunar ice development architecture that also doubled as a lunar colonization plan. So, say you wanted to have the same lunar lander for both so that you could first use it to land teleoperated robots, equipment, a habitat, greenhouse, and supplies before landing the humans. This way you could significantly retire the risk of the lander before sending people on it.

    How much final payload (not the lander) could you land on the lunar surface starting with those two Falcon Heavies?

    Thanks

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