[amsat-bb] Re: Since We Are Off Topic Somewhat....

Bruce Bostwick lihan161051 at sbcglobal.net
Fri Feb 15 07:00:37 PST 2008

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?

"We're going to shape the future of jurisprudence, the laws that  
sustain our whole society.  Or shove somebody in there to strike down  
those God-awful excuses for laws the Republicans are passing." -- Toby  

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