Space Transportation - mmmt_transportation.pdf - Moon Society

Mission DV from :L1 to the lunar surface is approximately 2.76 km per second, a little over lunar escape
velocity, (unless you’re in a hurry, in which case it is more), and the flight time is 3 to 4 days (again, unless you’re in a
hurry). Likewise, you can launch from any place on the lunar surface to the L1 base at any time, for the same DV and
flight time.
But an LLO base circles the Moon every 2 hours or so, so it shouldn’t take more than about 3 hours at most to
get down, and the DV cost is only lunar orbital velocity, or 1.7 km/sec. So where’s your advantage?
That 3-hour time span and 1.7 km/sec. DV represent the best case. That best case occurs if the lunar surface
base is in the orbit plane of the orbiting station. Only two situations can guarantee that the surface base will always be
in space station’s orbit plane. One of those situations is if the surface base is on the lunar equator and the space
station is in the Moon’s equatorial plane. The other is if the space station is in lunar polar orbit, and the surface base
is located at either the north or south pole of the Moon.
But what if we want more flexibility in our surface base location? Or what if we want to support several surface
bases at different locations on the Moon? The one type of orbit which can overfly every spot on the Moon is a lunar
polar orbit. Problem is, you have to wait until the Moon rotates under you, to reach any given spot on the lunar
surface. If you are lucky, the surface base may be in your orbit plane now. Otherwise, it could mean a wait of anything
up to 14 days.
Suppose there is some emergency, and you have to get a spaceship down to the surface, or from the surface
up to the space station, as quickly as possible, and you can’t afford to wait 14 days? Then you will have to do a plane
change before descending (or after ascending). Orbit plane changes are very expensive in DV, and thus in propellant.
In the worst case, if the surface base is 90˚ away from the current station orbit plane, the ship will have to do a 2.4
km/sec. burn to change planes and then pay the 1.7 km/sec price to reach the surface, for a total DV of 4.1 km/sec.
So we see that in the best case for a LLO base, it beats the L1 base for wait time and DV to and from the lunar
surface. But in the LLO base’s worst case, the L1 base wins. And we recall that the L1 base gives us a much more
predictable, and at the same time much more flexible, mission scenario. Launch windows are essentially unlimited.
Another comparison of interest is station keeping. Space stations in either orbit will have to perform
propulsive maneuvers from time to time to maintain their required orbits, and will require propellant to be supplied
for that purpose. A station in LLO will have its orbit perturbed by the gravitational tugs of Earth and the Sun. The
eccentricity of its orbit will be changed over time from the initially circular orbit to one more elliptical. The perilune
(point nearest to the Moon) will be lowered and the apolune farthest point) will be raised. L1 is not one of the stable
Lagrange points. The station will eventually begin o drift away from that position if its orbit is not corrected.
According to Farquhar’s estimates, the DV requirements for such station keeping are rather similar for both
LLO and L1 bases: on the order of 120 meters/second/year. The consequences of failing to perform the station
keeping maneuvers, however, are not. For orbital altitudes typically quoted for LLO studies, 100-200 kilometers above
the lunar surface, perilune would be lowered so much that the station would crash onto the surface of the Moon in a
matter of months. An L1 station would drift away from the L1 orbit, but would most likely remain somewhere in the
Earth-Moon vicinity. This gives a much better chance for rescuing the crew, and perhaps even of inserting the station
back into the desired orbit.
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