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I think the choice of Helium rather than Hydrogen may have been dictated by the cost of insurance
Felix Baumgartner is planning to skydive from a balloon at 23 miles altitude, 121,000 feet next week, October 8, 2012. He is hoping to break the speed of sound, 690 mph (1,110 kph) in freefall.
The data from the IRVE-3 experiment gives a pretty good idea of the conditions involved in such a jump. The IRVE-3 was an inflatable mushroom shaped heat shield about 3m in diameter, A=7m2,m=309kg, Cd=0.42. Surprise (to me), CdA/m for IRVE-3 is equal to that of the skydiver in the first air drag calculation above. Highest point for IRVE-3 was about 450km (280 miles), for the first part of the fall the velocity is pretty much v=t*9.8m/s2 and d=1/2 vt, so falling from 450km to 100km takes about 270 seconds and the speed at 100km is about 2.6km/s. At 100km you can switch to the free fall with air drag equation, speed increases a little then a rapid deceleration with a max of about 20 times g at a height of 31km, this agrees with the NASA press release data. NASA reported a max temperature of 540oC (1000oF), I do not know how to calculate surface temperatures yet. The IRVE-3 was constructed from Nextel Ceramic fabric and pyrogel insulation. 6 minutes up, 14 minutes down.
I tried to look IRVE-3 experiment up but had some trouble due to the government shutdown.I don't understand why a heat shield would be needed for a halo jump.