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The Business Case for Reusable Launch Vehicles
An anonymous reader writes "Remember the failures of "shuttle replacements" like VentureStar? A Space Review article argues that even if VentureStar succeeded technically, it and other proposed big RLVs would never have made it financially: they cost too much to develop and wouldn't have made it up through increased launches. What's the solution? The author says that suborbital RLVs, like what Carmack, Rutan, and the other X Prize contenders are working on, will create a business cycle that will eventually lead to orbital vehicles."
The problem with incremental development of RLVs is that there's a huge
leap between the size and difficulty of putting something into space
for five minutes (as in the current X-prize contenders) and putting it
into orbit (as in the shuttle). That will make it difficult to evolve our
way into a commercial space program.
I often find myself pointing out that just getting into space isn't
all that hard. Lifting yourself up 100km requires about a megajoule
(that's the energy equivalent of a stick of dynamite, or about 1/12th
of a gallon of gasoline (about 1/4 kg or 1/2 pound of gasoline), or a
jelly doughnut, or running a hairdryer for 2 minutes) per kilogram of
mass.
By contrast, orbital speed is something like 7000 meters per second,
(or 16,000 miles per hour for you provincials). Getting going that fast
requires an additional 24 megajoules per kilogram of mass (for a total of
25).
In short, the difference between the amount of energy you need to
get into orbit and just into space is a factor of 25, for the same
mass. That ratio of 25 is about equal to the difference between the
latent chemical energies of broccoli and gasoline.
Except that, in the case of space travel, you better be burning
something at least as energetic as gasoline to start with, or you'll
never even hoist yourself up 100km.
The way we've traditionally gotten into orbit is to concentrate the
kinetic energy into ever smaller bits of the vehicle: you use a huge
rocket motor and tanks to get everything started moving, then ditch the
empty tankage and rocket motors for the first stage -- that lets you
concentrate on moving a smaller amount of stuff even faster.
Realistic reusable designs are usually not staged designs,
because it's hard to recover and reuse the first stages. The problem is
that you have to have incredibly lightweight tankage and engines to make
everything work. But pushing stuff to lighter weight makes it more
flimsy and less prone to being reusable. Darn.
The VentureStar, IIRC, ran into problems with exactly this technology --
they were using lightweight carbon fiber tanks to hold their propellant,
and they couldn't make the tank light enough to boost itself into orbit.
The shuttle is NOT a reusable vehicle in any but the most technical
sense of the word: it requires constant skilled redesign and intelligent
(rather than scripted) maintenance, and the engines have to be overhauled
after every flight.
Ed, one of the guys aboard the ISS currently, wrote his take on the future of spaceships, which i thought was a good read.