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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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