Re: Slow Re-entry

On Sun, 07 Aug 2005 22:20:47 -0400, Jim Burns <burns.87@xxxxxxx>
wrote:

>How about Spaceship One, though? When I saw them on 60 Minutes,
>my impression was (1) they're using a revolutionary design
>to passively orient the ship for proper re-entry, and (2)
>they don't look like they're using NASA-style ceramic-foam tiles.
>OK, so all they've done so far is pop out of the atmosphere
>and drop back in,

That's the secret. I didn't see 60 minutes, but I wouldn't be
surprised if they glossed over the difference between a suborbital and
orbital flight, and the differences in re-entry (it being TV, I'd be
very surprised if anyone other than Miles O'brien got it right). Space
Ship One, when coming back down from essentially a dead stop 60 miles
high, reached a speed of (just a ballpark guess) Mach 2 to Mach 3.
Something coming into the atmosphere from orbit is going at about Mach
23. It's a huge difference in speed and the amount of kinetic energy
being turned into heat.

>but I would think their engineers are
>designing for a future with true orbital flight -- and
>return from orbit. Do they have a Clever Plan to make the
>heat of re-entry not quite so hot?

I doubt it. Their plans are probably just like everyone else's:
figure out how to deal with the heat.

Maybe a super-duper rocket engine that operates in outer space will
be developed that gives a large enough thrust to slow down something
with the mass of the Shuttle so that it can just 'drop' into the
atmosphere, and have much less stringent hull designs for re-entry.
The engine would have to go down with the spacecraft, or perhaps it's
detachable and can then accelerate itself back to orbital speed to
rendevous with and decelerate another craft. Perhaps a nuclear powered
device would have enough power, but it's going to need reaction mass
from somewhere, and it's too expensive to to send it up from Earth.
Reaction mass might come from the Moon or the Astroid belt.
But making such an engine is surely beyond current budgets of
organizations such as NASA, much less the Space Ship One people. It
also has its pitfalls. If it fails after firing for 10 percent of what
it normally would take to 'stop' something in orbit, that something
will enter the atmosphere at near full orbital speed, perhaps without
being designed to do that.
To bring something like Space Ship One out of orbit, the engine
would also have to bring it down to 60 miles and leave it at a
standstill. If the craft were dropped from the usual LEO height (110
to 300 miles), it would have had at least 50 extra miles of freefall
acceleration due to gravity before hitting the atmosphere.

It takes something like ten or 20 times the energy to get something
going horizontally at orbital speed than it does to get it 100 miles
above the Earth's surface. Likewise, you have to slow down from the
orbital speed to come back (unless you want to make a supersonic
landing).

>Jim Burns

-----
http://www.mindspring.com/~benbradley
.

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