ST401385@BROWNVM.BITNET (02/12/86)
A recent posting by Rick McGeer asked about trips to the moon before
2000, and suggested Apollo vehicles. Coincidentally, I recently looked
into this question (strictly as an amateur), and the following article
is a summary. Comments and criticisms are welcome, to
ST401385%BROWNVM.BITNET@WISCVM.ARPA <Geoffrey A. Landis>
SHUTTLE TECHNOLOGY TRIPS TO THE MOON
For a one-shot, it might conceivably be worthwhile to build
some stuff mostly according to the specs from the Apollo
project (although you'd want to take them to Low Earth
Orbit [LEO] in the shuttle). For example, if somebody saw
something on the moon worth taking a good look at this might
be a fast way to do it. Mods to make it fit the shuttle
would be extensive, but I think it could be done so that
most of it would be just rearanging the parts, not
engineering new stuff. Apollo Command module weighed 6
metric tons, the service module 25 tons, and the LM 16 tons.
To boost that much stuff from LEO into trans-lunar orbit
would take, let's see... Delta V=3.2 km/sec, shuttle
specific impulse 460 sec... something like 47 tons of fuel.
And a booster rocket, say maybe 10 tons?, that's a total
mission mass of 104 tons. Not bad at all: the shuttle
carries 26 tons payload, so it's only four shuttle flights.
For anything like a real capability, you'd want to design
new stuff using shuttle-era technology. After all, Apollo
used 1967 technology: twenty years obsolete! There are
three things which can make the mission a lot more
reasonable without too much unproven speculation.
First, if you have a lunar base, its most important mission would
be to manufacture oxygen. This isn't that hard: the lunar
crust is silicates, alumina, titania, etc: all oxides. It
takes a lot of power to break off the oxygen, but presumably
you'd have a power source anyway. (maybe you only
manufacture fuel when the sun shines). Shuttle engines burn
about eight kilograms oxygen per kilogram hydrogen: it helps
a lot if you don't have to bring the oxygen with you. The
other thing you'd do is aerobraking to get rid of excess
velocity at the earth. This idea got a big publicity boost
when the movie 2010 came out. The idea is that to get rid
of excess velocity, dip down into the atmosphere and throw
it away into friction. (probably wouldn't be big balloons
like in 2010, though. Most aerobrake schemes I've seen,
although I'm no expert, look like rounded-nose cones.) This
paper by Woodcock and Priest says you can do it for 2.78
shuttle flights per lunar landing without lunar-oxygen, 1.25
shuttle flights per landing with (for a four person landing
party, compared with Apollo's two). Last, you can make a
new, unmanned launch vehicle using shuttle technology; a
"Heavy Lift Launch Vehicle (HLLV)" or, as Woodcock and
Priest call it, a Cargo Launch Vehicle (CLV). The idea is
to make a thing using shuttle technology, forget the wings,
forget the crew compartment, and optimize for just getting
mass into orbit. W&P figure it could lift about three times
as much as the shuttle, cutting costs by about a factor of
two.
OK. Ready for a scenario?
Agenda for Lunar Trip :
(1) The shuttle (or HLLV) carries liquid oxygen and hydrogen
into orbit. If done by the shuttle, this is done by
carrying it in a slightly enlarged external tank [ET] (not
very much enlarged, either--about 3%) which is carried into
orbit. The shuttle itself flies virtually empty. It's
probably used to rotate space station personnel, give
astronaut candidates training in zero-gee, and give NASA
personnel free trips to keep their moral up (also probably
congressmen, schoolteachers, and whatnot).
(2) In orbit,
the propellant is pumped into an ET designated for orbital
propellant storage. There's probably a seperate tank for
hydrogen and for oxygen. To pump, the new tank is tied to
the old one with a tether, and the two set spinning slowly:
this is to get the liquid down to the bottom of the tank to
allow it to be pumped. Each of these fuel depot tanks is
shaded from the sun by a large parasol of aluminized mylar.
(It may be worthwhile to shade them from the Earth, too: the
Earth is warm, and will radiate to the tanks.) This depot
is co-orbital with the space station, but trailing behind it
by maybe a hundred miles, so that outgassing etc. from the
tank doesn't affect whatever it is people are doing in the
space station.
(3) When you're ready to head out, you fuel
up at the depot, and head for the moon. The first stage is
an orbital transfer vehicle called ELI (Earth-Lunar
Injection Vehicle), the second stage gets you into lunar
orbit and back, and is called the t-ship. It carries four,
plus cargo. (The rest of what you need waits in lunar
orbit). ELI thrusts at about one g for about 5 minutes.
It is basically a Centaur, or something very much like it.
reconfigured with an aerobraking shell and adaptors to dock
with the t-ship. It
coasts for about three days en route to the moon. ELI
seperates off, whips around the moon in the familiar
Apollo-8 figure-eight (interesting numerical coincidence there)
returns to Earth. It zips through the atmosphere at
11 km/sec, shedding excess energy like mad, possibly making
several passes, and eventually makes a small orbital
correction burn to rendezvous with the space station to
ferry the next passengers.
(4) The t-ship, also using a Centaur engine or derivitive,
fires its Lunar Orbit Insertion motor to put you into Lunar
orbit, with a burn of about a minute and a half at one g (like
all my burns). Here you rendezvous with the surface "ferry
shuttle".
(5) The people waiting to go to Earth get off the
ferry, and you get on. You fuel the ferry shuttle up with
the Hydrogen you brought along for it. It already has
oxygen, in fact it has oxygen to give to the t-ship. You
then make a de-orbit burn (leaving the transfer ship in
orbit) and, 18 or so minutes later, a landing burn.
(6) You pop the Champagne. You're on the moon at last.
(7) Before leaving, at the surface you fill the ferry shuttle's tanks
with LO2. Then you take off, and rendezvous with the next
transfer ship in orbit. You pump the t-ship's tanks full,
and wave bye-bye.
(8) The t-ship's Lunar Orbit Insertion
motor has mysteriously been renamed the Trans Earth
Injection Motor. (actually it
was probably called a LOI/TEIM all along). You fire it
(another burn of about a minute and a half), and leave Lunar
orbit for the trip back home, three or four days.
(9) At Earth, you do the aerobraking trick to slow your 11 km/sec
down to orbital velocity, then make a correction burn to rendezvous
with the space station. If there is no lunar base, the
difference is that between missions the ferry shuttle waits
in lunar orbit, and does not keep LO2 on board while
waiting--you have to bring it from home (it would all
evaporate if you tried to store it for long periods between
missions. Unless, maybe, the ferry had a parasol like the
one in earth orbit.) Also, the orbit is quite a bit higher
(orbits around the moon tend to decay due to the
perturbation of the earth.)
--Geoffrey A. Landis