Re: Since We Are Off Topic Somewhat....
Is it not possible to engineer a craft with a much lower re-entry speed thus reducing friction?
Is it not possible to engineer a craft with a much lower re-entry speed thus reducing friction?
Yes. The problem is slowing from 17,000 MPH to 0.
Remember, it took the entire billion gallons of fuel to accellerate the shuttle from 0 to 17,000 MPH. It takes just as much energy to slow it from 17,000 MPH back to zero. Rather than taking along a second External fuel tank with a billion gallons of fuel into orbit and a retro-rocket, just to slow down, it is more cost effective to just let the air slow you down. You get all that "slow down" energy from the air, and that generates heat.
So yes, you could retro fire a slow-down rocket above tha atmosphere, slow to zero speed and then just simply fall like a rock and keep cool all along the way. But it takes an awful lot of fuel...
Bob
The initial re-entry speed is strictly a function of where the spacecraft is coming from. For LEO spacecraft like the Shuttle, that speed is almost always going to be a little less than 17,500 mph or thereabouts, and for spacecraft arriving from outside Earth orbit, such as on an earth transit from a lunar mission, the initial re-entry speed will be much higher, close to Earth escape velocity of around 25,000 mph or so. There's no way to change those numbers without changing laws of physics.
What *is* possible is to change the re-entry profile to spread out the drag deceleration over a longer distance, staying in the higher altitudes longer and doing more aerobraking in the thinner air at high altitudes. The Shuttle does this somewhat, using the lift of its wings to control its descent profile, partly to keep the re-entry deceleration within comfortable limits for the crew, but partly to reduce heat load on the heat shield components exactly as you suggest. This isn't too hard for a LEO spacecraft, which is guaranteed to return to the earth's surface one way or the other. For an extra-orbital spacecraft, it's considerably more difficult because approaching at too shallow an angle doesn't reduce the spacecraft's velocity enough for a re-entry capture and it skips off back into space, ending up in a highly elliptical HEO at best and escaping into solar orbit in worst case scenarios. Even so, the Orion crew vehicle is planned to use at least some controlled-deceleration techniques to reduce the re-entry loading as well as to enable land recovery in CONUS from an approach origin below the equator .. which is considerably more sophisticated than the direct-reentry water recovery methods used for Apollo missions ..
On Feb 15, 2008, at 5:06 AM, David Barber wrote:
Is it not possible to engineer a craft with a much lower re-entry speed thus reducing friction?
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At 10:06 PM 2/15/2008, David Barber wrote:
Is it not possible to engineer a craft with a much lower re-entry speed thus reducing friction?
That means carrying enough fuel to slow down to the desired re-entry speed. Hardly economical (remember, you have to get that fuel UP there in the first place!).
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participants (4)
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Bruce Bostwick -
David Barber -
Robert Bruninga -
Tony Langdon