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#22
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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
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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
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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. |
#25
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#26
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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
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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
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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
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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
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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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