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Reentry at high temperature



 
 
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  #22  
Old August 2nd 05, 07:49 PM
Ray Saffin
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In the long term, if we ever get to the moon could we manufacture fuel there
and ship it (relativelyy easily) by space tug to LEO. Ships wishing to
return to earth could then top up with fuel prior to a massive reto burn
enabling them to return earth at a much lower speed.

The orbital "gas station" (as you call them in the US) could also be used
for manned or unamanned planetary probes.


  #23  
Old August 4th 05, 03:46 AM
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Henry Spencer wrote:
Correct. The high-temperature metals, unfortunately, tend to be really
dense. Things like niobium and molybdenum are nearly as dense as lead;
tantalum and tungsten are substantially *denser* than lead. They have to
be kept thin or the weight just goes out of sight.


Right...but even 1000 square meters of 1mm nickel superalloy plate is
only 9 tons. 0.5mm of tungsten would be about 10 tons, too, and that's
not quite foil gage. (Obviously, the heat shield wouldn't be a simple
plate, that's just a convenient approximation.)

Using the approximation of heat shields being 12.5% of the re-entry
vehicle, that would leave something like 12- to 15 tons, that should
allow a metallic heat shield on the order of 0.5mm to 1mm thickness for
a vehicle like the shuttle (with roughly 500 square meters
underside/leading edge area.) That should also leave tonnage for
insulation (like the X33's saffil), shouldn't it?

Mike Miller

  #24  
Old August 4th 05, 06:50 AM
Ron Webb
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"Henry Spencer" wrote in message
...
In article ,
Ron Webb wrote:
- While orbiting at 17000mph or so to dip down into the upper atmosphere -
the very edge -then deploy a large parachute similar to the modern sport
chutes that forms an airfoil.


It's been proposed, actually.


I assume anything practical has been studied on a simulator, as well as
mathematically, in aerospace engineering classes all over the world as well
as at places like NASA.

That's what's cool about newsgroups like this one. We non-aerospace
engineers get to ask the questions, instead of retaining our misconceptions,
or having to do the calculations ourselves. But who knows, once in a while
the kernel of a new idea may pop out!

Thanks for the reply!




Alas, here you propose a numerical impossibility. *It can't be done.*

snip

In fact, when you study the details, it turns out that the large surface
area of something like a parafoil doesn't really make any difference to
how *quick* reentry is. That is determined almost solely by the L/D ratio
of the shape, and there are real limits to how good that can be.

A large surface area does buy you something: you decelerate earlier, in
thinner air, and the heat is spread out over a larger area. This lowers
temperatures and makes materials problems much easier. But things still
happen just about as quickly.


OK- it seems half my idea is practical (large surface area spreading the
waste energy over a large area, thus making the thermal stress on any given
part a lot less) and half is not very useful (can't slow the re-entry down
much using aerodynamic lift).

How about slowing the re-entry using active thrusters? It would take a lot
less thrust to keep the craft up in the thin air for an extra half hour -
while friction slows us down at reduced temperatures - than it would to try
to actively decelerate using thrusters.


  #26  
Old August 9th 05, 07:50 AM
George
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Mike Lepore wrote:

Someone please tell me why spacecraft are designed to reenter the
earth's atmosphere at high speed. Isn't there some way to come down
slowly, so the heat shields wouldn't be needed? Has anyone modeled
the idea of unfolding some large wings to add a lot of surface area,
or using propellers to resist falling, or parachutes? Thank you.


why not desent on/near poles (Nort or south) where atmosphere is not so
dence, and then fly like normal aircraft to Texas
  #27  
Old August 11th 05, 12:00 PM
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George wrote:

why not desent on/near poles (Nort or south) where atmosphere is not so
dence,


The heating experienced by a re-entering spacecraft is a result of the
spacecraft's speed and mass. When you hit any sort of terrestrial
atmosphere at those speeds, you're going to dump a lot of energy.

There's also the problem that few manned spacecraft are interested in
orbiting the over poles except on rare missions. It is a very difficult
challenge to go from a normal orbit (tilted 20 - 50 degrees with the
equator) to an orbit that can intercept the poles. When I say very
difficult, I mean, "It's easier to go to the moon from low orbit than
to change to a polar orbit."

and then fly like normal aircraft to Texas


That would call for a lot of jet fuel, perhaps in excess of 10-20 tons
(depending on the size of the spacecraft). That's much heavier than any
savings you'll get on the heat shield.

Mike Miller

  #28  
Old August 11th 05, 12:21 PM
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Although there may be less ozone near the poles :-), atmospheric
density is not drastically different there.

Secondly, getting into an orbit over the poles from a typical orbit
that might inclined 20-60 degrees to the equator will require *lots* of
propellant.

Thirdly, once you've completed re-entry, you will need propulsion and
fuel to fly from the pole to Texas or wherever. That propulsion system
and the fuel for it will have to be carried all the way to orbit and
back.

Finally, why would thinner atmosphere be an advantage in the first
place?

  #29  
Old August 11th 05, 02:28 PM
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George wrote:
Mike Lepore wrote:

Someone please tell me why spacecraft are designed to reenter the
earth's atmosphere at high speed. Isn't there some way to come down
slowly, so the heat shields wouldn't be needed? Has anyone modeled
the idea of unfolding some large wings to add a lot of surface area,
or using propellers to resist falling, or parachutes? Thank you.


why not desent on/near poles (Nort or south) where atmosphere is not so
dence, and then fly like normal aircraft to Texas


Earth's atmosphere is plenty dense at the poles. People are even
known to breath there. And besides, returning spacecraft start to
experience drag induced heating even in the extreme upper atmosphere,
where the air really is rather thin. Furthermore, spacecraft entering
the Martian atmosphere also require heavy heat shields, even though the
Martian air is only gets up to about 1% the pressure on Earth at sea
level. That's how gawd-awful fast spacecraft travel.

But even aside this, if every Earth orbital spacecraft were, for
whatever reason, required to re-enter the atmosphere over the poles, it
would mean every such spacecraft must be placed on a polar orbit. Polar
orbits aren't always maximally useful. Plus, it's more expensive per
pound of payload to launch polar rather than eastward, because polar
orbits don't make use of free velocity contributions from Earth's
eastward rotation.

Thus, for satellites returning from non-polar orbits, they'd need to
have their orbital planes adjusted to polar before returning. Problem
is, that orbital plane maneuvers are among the most costly in terms of
propellant. For an adjustment of anything over a few degrees, it's
impossible for a typical satellite to even carry enough propellant.

-Mark Martin

  #30  
Old August 11th 05, 05:23 PM
Ian Stirling
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George wrote:
Mike Lepore wrote:

Someone please tell me why spacecraft are designed to reenter the
earth's atmosphere at high speed. Isn't there some way to come down
slowly, so the heat shields wouldn't be needed? Has anyone modeled
the idea of unfolding some large wings to add a lot of surface area,
or using propellers to resist falling, or parachutes? Thank you.


why not desent on/near poles (Nort or south) where atmosphere is not so
dence, and then fly like normal aircraft to Texas


The atmosphere is practically as dense at the north or south poles.
It would take several hundred times more fuel than is available to change
to an orbit that would reach the poles.
It would take signficant alterations to be able to fly that far.
Gliding isn't an option.
 




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