Space Transportation - mmmt_transportation.pdf - Moon Society

At perigee 3 : Initiate Mars transfer = 0.79
Total near-Earth DV = 1.14
At Mars Entry into low Mars Orbit = 2.08
TOTAL one way DV = 3.22
LEO Launch
At LEO: Initiate Mars transfer = 3.65
Total near Earth DV = 3.65
At Mars Entry into low Mars orbit= 2.08
TOTAL one way DV = 5.73
DV saved by launching from L1 = 2.51
1 : maneuver 1 of Figure 5; 2 : maneuver 2; 3 : maneuver 3.
One study I was involved with at Lockheed assumed a three-stage Mars exploration ship launched from LEO
[3]. When we considered launching from L1, we found we could omit the entire first stage.
Part of the DV for the return trip could be accomplished by aerobraking in Earth’s atmosphere, rather than by
a propulsive burn. But here, too, return to L1 would have an advantage over return to LEO. The perigee maneuver at
Earth would only have to cut the velocity by 0.79 km/sec to put the spacecraft on a trajectory to L1. This is a lot less
than the 3.65 km/sec needed to brake into LEO. That means a lot less kinetic energy converted to heat, so the
aerobrake could probably be made much smaller and lighter (which in turn means less propellant to haul it to Mars
and back).
Similar DV and propellant savings will apply to any interplanetary flights launched from L1 (including to
asteroids). The reason is that most of the mass for an interplanetary mission does not consist of people or payload. It
consists of propellant ... and as we have already noted, propellant requirements for a mission increase much more
rapidly than DV requirements.
But you may be saying to yourself, I still have to get my ship, crew, payload and propellant from Earth up to L1
to begin with. When you raise this question, you are on to an important point. The scheme of using L1 as Earth’s
interplanetary port only makes sense if the ships can use lunar derived propellant, and if we plan on repeated
interplanetary voyages. If we plan on making only one Mars mission with people on board, or two or three, then we
might as well assemble and fuel them in LEO and forget L1 basing.
However, we have already discussed some possibilities for getting propellant from lunar sources. Any
propellant made at the Moon will not have to brought up from Earth. If the interplanetary ships use hydrogen/oxygen
propellant, lunar oxygen could make up 5/6ths of the propellant mass (using Shuttle combustion ratios). That’s quite
a saving.
We have also discussed lunar aluminum as propellant for lunar landing vehicles and possibly for Earth/Moon
ferries. Aluminum and oxygen alone may not have the specific impulse designers would want for a Mars ship. (Specific
impulse is a performance measure for rockets somewhat analogous to miles per gallon. It is often given in units of
seconds, meaning the number of seconds that one pound of propellant could produce one pound of thrust, before it is
consumed.) However, as discussed in more detail in the sequel companion paper, a concept for an aluminum/
hydrogen/oxygen tripropellant combination has been put forth by Andrew Hall Cutler [4]. At an H:O:Al mass ratio of
1:3:3, such an engine is expected to have a specific impulse exceeding 400 seconds - only slightly poorer than
hydrogen and oxygen alone. An engine of this type might be worth considering for interplanetary vessels as well as for
vessels on the LEO-Moon run.
The second requirement for L1 launches to be advantageous is to have repeated interplanetary trips, and to
return to L1 as well as start voyages there. That way, you build a ship once, and only once have to lift the structure
from LEO to L1. For all subsequent voyages of that ship, you will only have to ferry people, payloads (the less massive
elements of the [loaded] vehicle) and (maybe) propellant hydrogen from Earth. The major portion of the propellant
(and the majority of the initial mass of the [loaded] ship) will be Moon-derived.
Establishment of an L1 space station to support both lunar surface operations and interplanetary voyages
would offer significant advantages in flexibility and efficiency to each, and an opportunity to “kill two birds with one
stone.”LJF
References:
[1] Keaton, Paul W. “A Moon Base/Mars Base Transportation Depot,” Lunar Bases and Space Activities of the 21st
Century, W.W. Mendell, ed., Lunar & Planetary Institute, Houston, (1985) pp. 141-154.
[2]Farquhar, Robert W. “A Halo-Orbit Lunar Station,” Astronautics & Aeronautics, (June, 1972), pp. 59-63.
[3]Friesen, Larry Jay & Bridget Mintz Register. “Space Station Accommodations for Manned Lunar and Mars Initiatives,”
Lockheed Engineering and Sciences Co. for Advanced Projects Office, NASA-JSC Houston (1989).
[4]Cutler, Andrew Hall. “Aluminum Fueled Space Engines for Economical Lunar Transportation.” Lunar Bases and Space
Activities of the 21st Century, W.W. Mendell, ed., Lunar and Planetary Institute, Houston, (1985) p. 61.
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