OK if the shuttle is going the same orbital velocity required to get at into
orbital velocity.
Then cannot it be slowed down while in orbit where you would not need a
massive amount of energy to slow it down from 22700mph if done over a few
days which I take it is its approximate speed while in orbit
Could this work? And if it was slowed down could they not use parachutes to
keep it from reentry problems

wrote in message
oups.com...
Someone please tell me why spacecraft are designed to
reenter the earth's atmosphere at high speed.

The answer is really quite simple when you think about it. Slowing
down from orbital velocity requires exactly the same change in speed as
attaining orbital velocity. It is entirely possible to slow down with
rockets instead of air resistance, but the ISP of those rockets would
have to be basically the same as is required to get into orbit.

You know the Space Shuttle, with that large tank of fuel and those two
huge boosters? All the power from those boosters and that fuel is used
to accelerate the shuttle to orbital velocity. Sure, it's possible to
slow the shuttle down a lot so that it would enter the atmosphere at a
leisurely 200kts, but doing that would require the same power as is
required to get it into orbit in the first place. So basically we're
talking about having the shuttle in orbit with a large, *full* external
tank at least. Getting the shuttle into orbit with a large, full
external tank would require three times the amount of thrust required
to put the bare shuttle into orbit.

So just imagine the shuttle sitting on the launch pad with not one but
three external tanks, and six external boosters. That's on the order
of magnitude of what would be required to get it into orbit with the
fuel to brake out of orbit. That's a larger stack than anything that
anyone has ever launched. That's much larger than the Saturn V or the
Russian Energia. It's much too large to be practical.

And of course there are other considerations, like keeping all that
fuel cooled for the duration of the mission. It's really just not a
workable idea.

Has anyone modeled the idea of unfolding some large
wings to add a lot of surface area

This is similar to the idea of a ballute.

http://en.wikipedia.org/wiki/Ballute

It's certainly helpful, but for a full reentry in less than one orbit
you still need a heat shield. Slowing down more gently in the very
high atmosphere, as you're suggesting, results in a ballistic
trajectory that brings you down into the lower atmosphere before you
can bleed off enough speed to no longer need the heat shield.

Another idea, that I don't know enough about to speak to, is to drop
down into the atmosphere and then pitch up so that you fly out of the
atmosphere like a rock skipping on a pond. You're still on a sub
orbital trajectory though, you don't fly off into space, you come back
down into the atmosphere and repeat the process.

This idea was employed by the X-20 Dyna-Soar.