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Reentry at high temperature
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. -- Mike Lepore in New York - email with the 5 deleted |
#2
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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? They are initially traveling very fast, since they are either in orbit or are coming from far away and have fallen into Earth's gravity well. Slowing without drag in the atmosphere would mean using rockets, which would require a prohibitively large quantity of propellant. 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. Certainly. The time required to brake during reentry is increases as the lift/drag ratio increases, so this can be used to prolong the reentry. The altitude also is dependent on the ballistic coefficient (mass/area) of the vehicle, allowing a broad, light vehicle to slow higher in the atmosphere, spreading the heat over a larger area. But the energy still has to be dissipated somehow. Paul |
#3
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Mike Lepore wrote: Someone please tell me why spacecraft are designed to reenter the earth's atmosphere at high speed. Because they orbit the Earth at high speed and the amount of fuel needed to slow down for a gentle re-entry is heavier than a heat shield. 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. Yes, most of those ideas have been modeled. However, they all have to deal with the same total energy release per kilogram of mass in orbit. Larger wings or other drag systems (like parachutes or ballutes) can spread out the heating, but they're not a perfect answer and usually add weight for little gain. It usually ends up being easier (or lighter, or more proven) just to use a plain vanilla heat shield. Different re-entry profiles can help, too. The original civilian designs for the US space shuttle used metallic heat shields. When the USAF signed on, it had requirements for the shuttle that included more demanding re-entries (a lot more steering, or "cross-range", than the civilian shuttle designs needed) and materials with higher temperature tolerances were called for, like the fragile ceramic tiles of the current shuttle. It'd be interesting to see a flight-proven metallic heat shield on a shuttle. Mike Miller |
#4
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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. |
#5
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"Mike Lepore" wrote in message ... Someone please tell me why spacecraft are designed to reenter the earth's atmosphere at high speed. The only reason any spacecraft remain in orbit to begin with is because of their high speed. Imagine, if you will, that you could build a 200 mile high ladder and could climb up to a platform at the top with a scale on it. If you were to weigh yourself on that scale you would only weigh about 5% less than you would on the ground. A space shuttle might just zip on by you at 17,000+ mph with all the astronauts on board in freefall and feeling mighty weightless - but if you were to step off the platform you would would drop like a rock. In order for an orbiting spacecraft to return to earth it has to do something with that excess 17,000+ mph. It has to lose it somewhere. Using the atmosphere as a brake is the least expensive way to go. Isn't there some way to come down slowly, so the heat shields wouldn't be needed? The amount of fuel needed to slow it down enough in space would be more than it could carry up there in the first place. 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? You betcha. As the saying goes: Google is your friend. Be fearless, look it up. |
#6
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In article ,
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. There is a LOT of energy in an Earth orbit, and altitude makes little difference. Propellers would be useless until one got into thick enough air, and probably even then; the speed is about 18,000 miles an hour, and propellers are not much use even at 1000. I presume shallower angles of entry have been considered, but the air friction gets rather high before wings or parachutes can be used; parachutes have been used for reentry after the speed is low enough. Until the speed is low enough, just keep the heat outside the critical part of the vehicle, unless you have your antigravity device available. Mike Lepore in New York - email with the 5 deleted -- This address is for information only. I do not claim that these views are those of the Statistics Department or of Purdue University. Herman Rubin, Department of Statistics, Purdue University Phone: (765)494-6054 FAX: (765)494-0558 |
#7
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In article ,
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? Not literally, no. The spacecraft *is* arriving at the outer edges of the atmosphere at high speed; the only way to change that is with lots and lots of rocket fuel, totally ridiculously impractical amounts (unless you are talking about advanced nuclear rockets). Has anyone modeled the idea of unfolding some large wings to add a lot of surface area... There has been a little bit of exploration of the idea of very large surface areas. It doesn't solve all the problems; you do still decelerate relatively rapidly. But the deceleration happens higher up, in thinner air, and the heating is spread over a larger surface area, so the materials problems are easier. It's an interesting idea, but there hasn't been funding for full-scale testing, which is what's really required. (In fact, there are at least half a dozen unorthodox reentry concepts which *might* work, and look promising, but will become credible to mainstream designers only after a full-scale demonstration. This is the sort of technology R&D that NASA should be doing, and isn't.) The idea of doing a very gradual reentry, spreading the heating out over a long *time*, is appealing in principle, but nobody knows a way to make it work. There is just no way to *stay up* in thin air that long. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
#8
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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. A spacecraft enters the atmosphere at such high speed because its orbital speed is so high to begin with. On orbit a satellite has a certain (large) orbital speed, and if some of that speed is lost, for instance by firing its rocket motor forward, then the shape of the orbit changes so that it dips downward, closer to Earth. If the satellite loses enough speed, then the path dips right down deep into the atmosphere. This is, in fact, how a returning spacecraft is made to lose enough of its speed to allow it to land safely, by plowing through the atmosphere. One may ask, "So why not just lose more speed before even entering the atmosphere, eliminating the need for heat shielding?" First, think of how much energy the spacecraft loses by its passage through the air. In the case of the space shuttle it goes from the neighborhood of 17,000 mph to only a few hundred mph by aerodynamic drag alone. That's nearly as much change in orbital speed as it aquired at launch. But launch required an enormous quantity of propellant. In order to lose all that speed by rocket would require the shuttle to have onboard nearly as much propellant as it took to get it aloft to begin with. But that original investment of propellant was just enough to get the shuttle to orbit *without* all that extra fuel. To place it on orbit with enough fuel to lose all or most of its orbital speed would then require VASTLY more propellant at launch. It's quite possible it couldn't even be gotten off the ground. The most practical & economical method is for the shuttle to have just enough propellant available to change its orbital path to dip down into the atmosphere, whereat it loses most of its speed by drag. Keeping this in mind, it's possible to see that folding wings won't do much in the spacecraft's favor. The shuttle would still have to plow through the air at the same speed, and in fact extra large wings might even be a liability, since they'd increase the lift, keeping the ship from dropping downward. All the extra wing area would do is increase the orbiter's weight at liftoff, reducing the payload it can carry. -Mark Martin |
#10
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In article .com,
wrote: 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. Skipping doesn't really help with the fundamental problem, that there isn't *enough* aerodynamic lift available to stay up in the thin air where deceleration is gradual. Sure, the heating is concentrated in brief periods, but so is the lift -- there is no net gain. -- "Think outside the box -- the box isn't our friend." | Henry Spencer -- George Herbert | |
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