Aerobraking vs. Rockets Comparison
We doubt that aerobraking is a viable option for the first
one of the questions that kept coming up in the earliest days of the program
because aerobraking fans (especially the folks working in that area) have
worked a lot of publicity for the concept, so it wound up in the list of
mission options. However, it might turn out that aerobraking won't be worth
taking seriously until there are SSTO rockets, with built-in heat shields,
flying home from the moon.
There are lots of problems with aerobrakes. The first one is
don't exist, so we'd have to do a lot of fundamental research and development,
with test flights at several hundred million dollars a launch.
Weight is another important factor. It might seem that an aerobrake wouldn't
weigh nearly as much as the fuel needed for 10,000 ft/sec, but we'd need a
very large, massive structure; it could be several thousand pounds by the time
we got all the details worked out.
Aerobraking drives the weight of the lunar transfer vehicle off the
scale. In the reference mission, empty weight of the LTV is about 5,000 lbs,
perhaps even less. That's based on using a single Spacehab module and
designing the equipment mostly for zero-g operation. The LTV has to be able to
take about 3 g's for its initial launch, but it doesn't have to be operational
during that phase of its life. If we add aerobraking, it will have to be
stressed for 12 g's in an operational configuration, with nominal operations
of 9 g's.
All that said, someone might prove us wrong. If someone comes up with
some hard numbers that show aerobraking gets us to the moon faster, then we'll
go with it; but it'll have to be hard numbers with a detailed business plan to
back it up. It's worth looking at in a little bit more detail to see if we're
missing a major point, and to be able to provide an even more solid answer
when the question arises.
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