Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: lyner on 16/02/2007 11:47:04
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Here's a question for someone who really knows what they're talking about.
Spacecraft returning to the Earth are always subject to high thermal shock due to the rapid dissipation of Kinetic Energy into Thermal Energy as they lose height (and, hence, Gravitational Potential Energy). SO, if the atmosphere at high altitude is 'thick enough' to produce a retarding force, why can't it produce a 'lift' force to limit the rate of descent. If the rate of descent were reduced by a factor of, say ten, then the power dissipated would be reduced pro-rata. You wouldn't even lose radio contact as there would be no ionisation problem.
If the craft could be, effectively, flown down rather than just 'braked' from its low orbit, it could glide down under complete control and, as it would have decent sized wings, it could land where it liked - even in high latitudes. (Gatwick?)
I appreciate that the control would have to be precise to avoid the 'bouncing off' problem which was always talked about in the early days of manned spaceflight. In those days, however, they didn't have fly by wire.
Is there any significant reason why this system wouldn't work?
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If you were not orbiting I think you could actually parachute down from space with an appropriately sized parachute because the atmosphere does start very slowly, however because you are orbiting at 24000km/hr there is an immense amount of energy to loose.
You are right if you spread out the process you would limit the heat loading. Although aerodynamic shapes are normally quite pointy, which would concentrate heating at the sharp edges/points, which is why the shuttle is so rounded.
I think hitting air molecules at 2400km/hr is going to cause ionisation whatever happens although you may be able to keep it to a limited amount and you may be able to get radio signals through it.
Fundamentally I think the problem is that hypersonic air flows are difficult to deal with and not entirely understood - you can't test your theory in a wind tunnel, so doing as you suggest would be very expensive in R&D or very risky, and I don't know how difficult the control problem would be as the angle of your craft would be very critical to the lift/drag/possibility you will break up into little glowing pieces.
Also I guess that the centre of lift is going to move as you slow down, and as you slow down you are going to have to generate more lift to stay at the same rate of descent... It all gets quite nasty, and hence expensive
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May be wrong, but I thought heat was proportional to the square of the speed, not pro-rata.
http://books.nap.edu/openbook.php?record_id=9251&page=7
The skin temperature drops initially as the aircraft climbs due to exposure to decreasing ambient air temperatures. As the speed increases above about Mach 1, where the temperature begins to increase, the skin temperature reaches a maximum of 120°C (248°F) after exposure while cruising at Mach 2.2. (At Mach 2.0, the skin temperature would stay below 100°C (212°F); at Mach 2.4, it would reach 150°C (302°F.)
As you can see in the above example (for Concorde), a 10% increase in speed produces a 5% increase in temperature, while a 20% increase in speed is producing a 13% increase in temperature (I have converted the temperatures to kelvin).
There are also quite different aerodynamics at supersonic and hypersonic speeds, as well as the issues of very low density atmospheres which behave very differently to high density atmospheres. The Reynolds number of an aerofoil will drop as atmospheric pressure drops - this could be compensated for by creating super-giant aerofoils to correct for the reduction in atmospheric pressure, but I don't know what other factors would then come into play.
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May be wrong, but I thought heat was proportional to the square of the speed, not pro-rata.
OK, then it would be proportional to the rate of descent because it would be proportional to the rate of change of Potential Energy? The heat (power) produced would be proportional to the speed times the drag force and the drag force would be proportional to the speed raised to some power - dunno what, in such a low pressure.
How's that?
I like daveshorts comments about aerodynamics and the problems of hypersonic flight. Experiments with models could be tried if you launched them from existing orbital vehicles. That could solve the wind tunnel problem.
As for possible dangers of frying on the way in, you could come down with a series of skips , rather like dabbing the brakes on an old car with drum brakes. I wouldn't like to be on the first test flight - but the Shuttle isn't exactly a stress free way to re-enter, either.
The center of lift problem would, I suppose, call for swing wings.
No one said it would be easy!
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I like daveshorts comments about aerodynamics and the problems of hypersonic flight. Experiments with models could be tried if you launched them from existing orbital vehicles. That could solve the wind tunnel problem.
As for possible dangers of frying on the way in, you could come down with a series of skips , rather like dabbing the brakes on an old car with drum brakes. I wouldn't like to be on the first test flight - but the Shuttle isn't exactly a stress free way to re-enter, either.
The center of lift problem would, I suppose, call for swing wings.
No one said it would be easy!
I think that Space Ship One (Virgin Galactica) uses reconfigurable wings, but it is not flying to the same altitude as the Shuttle, and claims only to go supersonic, not hypersonic.
In terms of super-giant wings I was thinking of, I was thinking more in terms of large inflatable, or otherwise thin membrane wings that can be unfurled in space. While such wings would be highly vulnerable at atmospheric densities increase, the question is whether a wing of such super-giant size could at higher altitudes move the vehicle from ballistic to aerodynamic flight in a thin enough atmosphere that it could survive (although descent time could be quite large for such a vehicle); and then maybe discard the wing once air density increases.
The other option that was used on early space re-entry was to maximise the shock wave ahead of the craft to give the craft some protection from the airstream. Again, the question is whether this could be used on a larger scale (possibly by giving the vehicle outriggers that could create overlapping shock waves well ahead of the craft, but by constantly repositioning those outriggers, you can change the shape of that shock wave as required, and reduce the wear on any one outrigger by giving it a limited duty cycle).