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Reentry at high temperature



 
 
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Old June 21st 06, 03:07 AM posted to sci.space.science
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Default Reentry at high temperature

Anni wrote:
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.




Here's the story in a nutshell: As others have explained, the Kkinetic
energy associated with being in orbit is enormous; the energy per pound
is on the same order of magnitude as a pound of dynamite.

Somehow, that energy has to be dissipated again.

Now, here's a point that others haven't made: As you know, a spacecraft
can stay in orbit -- pretty much indefinitely if it's high enough --
because the force of gravity is balanced by the centrifugal force of its
motion. In reentry, the trick is to bleed off energy without losing too
much of that centrifugal force, too fast.

Given enough thrust (delta velocity, really), you could conceivably stop
the spacecraft in its tracks, cutting its velocity to zero. But then
you'd have to use even more rocket propulsion, at least to lower the
spacecraft to a height such that a ballute or parachute will work.

If you think about the videos of the Columbia coming in, as it was
breaking up, you can see that the effect of the energy is huge; even
after it's broken up, the pieces continue on, pretty much on the same
track as the Shuttle would have followed. It's just hard to
over-emphasize the amount of energy that must be bled off.

There's another point. You might think that the amount of heat energy
that must be dissipated is proportional to the g-forces the astronauts
will feel, but that isn't so. A vertical entry can cause g-forces up to
100 g's or more (which would, of course, squoosh any astronauts into
globs of jelly), but it actually generates less heat than a grazing
entry. So if you want a reentry that the astronauts can survive, you
must deal with lots and lots of heat.

Can we land without a heat shield? Of course. We will be doing it on
pure rocket power when we go back to the Moon. But if there's an
atmosphere involved, it's far better to use it, than not.

Jack
 




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