MMM Classics Year 10: MMM #s 91-100 - Moon Society

When people think of putting staging bases for lunar
missions near the Moon, they are often thinking of bases in low
lunar orbit (LLO), a few tens to a few hundreds of kilometers
above the lunar surface. Why do I want to put a space station
so far away?
There are advantages for having an L1 space station
over an LLO base that apply to Earth-Moon traffic. There are
others that pertain to manned interplanetary flights.
L1 Advantages for Lunar Base Support
To begin with, an L1 base offers great adaptability for
supporting lunar surface operations. From L1, it is possible to
launch at any time to any location on the lunar surface for a
similar delta-velocity (DV) and flight time interval. For those
not familiar with the term, DV [“delta-V”) is the sum of the
velocity changes for all the maneuvers a spacecraft must
perform in order to accomplish a given mission, or a major
portion of a mission. It allows planners to estimate propellant
requirements for the mission. Propellant requirements tend to
go up much faster than DV. For example, doubling DV for a
mission would more than double the propellant requirements.
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.
L1 also appears to offer more advantage from lunar
derived propellant than LLO. As the companion article to be
published in next month’s MMM discusses, transportation
costs for supporting a lunar base can be reduced if at least part
o f the propellant for the spacecraft can be produced on the
Moon. It will be a lot easier if we have gas stations at both ends
of the run.
It has been widely publicized that oxygen can be
Moon Miners’ Manifesto Classics - Year 10 - Republished January 2006 - Page 41