On Wed, 21 Sep 2005, James Humes wrote:

> I liked the philosophy of the plane that won the X-prize. I'm not clear if
> it made it far enough up to get OUT into space, but I think it proved the

Getting out into space and staying out there are 2 different things. The 
first requires 1e6 J/kg and the second 3e6 J/kg, roughly three times 
more. Disregarding small problems such as navigation, communications and 
(remote) control well over about 2000km (the flyer will be beyond the 
local radio horizon in 15 minutes or less), you have to handle reentry 
during which the 3e6 J/kg will turn into heat.

To put this into more normal figures, a coke can considered of 
negligible weight a third full of water coming down from orbit with say 
2e6 J/kg will weigh about 100 grams and if it reenters in about 6 
minutes it will be equivalent to being heated with ~550 Watts for that 
amount of time. Its temperature will increase with about 1.4 degrees / 
second. The water will boil after about 60 seconds (asuming a valve 
keeps the internal pressure at 1 atm and starting at 20C) and then ~80% 
of the water will evaporate to absorb the heat leaving about 20 grams of 
water in the can. Thus you expend 80% of the starting mass for cooling 
purposes. Not good. With a heat shield it could probably return with 80% 
payload. You get to design the heatshield.

Peter
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