Gentlefolk,
With a GLOW of around 200 tons and a mass ratio of 20+, my feeling is that an
orbital vehicle is an entirely different thing than a fighter or an X-prize
vehicle in the HTHL discussion. If a single vehicle, I don't think you're
wheels up at anything like 70 m/s; the wings to do that would be too big to lift
to orbit, and if not, the thing would fly like a paper airplane coming back.
To save gravity losses and wing area, it would want to be going as fast as
practical before lifting off; more like 120 m/s, which a .9 g initial acceleration
will achieve some 800 m down the runway.
With a steep climbout, it would get a lot of potential energy quickly and
also have a high angle of attack. Because of that, in an all-engines-out
scenario maybe 2 km from liftoff, I think it would lose ground speed quickly and
come back fast, hard, and horizontally slow, with something like 1.2 km out of
a 4 km runway left. My first thought is that, for an orbital vehicle,
undercarriage strength, and not runway length, will be the main concern in such a
circumstance. (Randall might say "Splat!" ;-) )
But this would depend on many specific trades, and one can imagine other
flight profiles.
An x-prize vehicle with a lower mass ratio and wing loading would much
more plausibly resemble a souped up jet. By the same token, the x-prize may be
won by a vehicle that represents a technological dead end as far as space
access is concerned. I would much rather have seen the money go to a 90%
reusable orbital mission of any size. On the other hand, it's probably more
important for the x-prize to produce positive publicity than technology.
Without spending all day on this, I think HTHL is much more suited to
carrier aircraft or in-flight fueling scenarios than for single vehicle SSTO, a
scenario marginal enough to make me sympathetic to the "wings are dead mass"
argument. Engines out at 200 m/s and 5-10 km altitude gives one many more
survival options.
For SSTO, I like the VTVL "flying fuel tanks" that come back with lots of
area per unit mass. Catastrophic early abort could be handled by a separable
capsule design that saves the crew/avionics/payload. With clever, creative
design, in a fully reusable vehicle, that could even double as part of the VL
system, without separation.
--Best, Gerald
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