The U2 guys wear suits.


On Wednesday, November 9, 2016 at 4:46:27 PM UTC-8, Dan Marotta wrote:
It has to do with the partial pressure of oxygen in the blood compared
to ppO2 in the ambient (cabin, space suit, etc.). PpO2 outside the body
has to be higher than PpO2 in the blood or no oxygen will get in.
Pressure breathing helps to a point, but it won't help at U2 or Perlan
altitudes. They have to be pressurized, either suit or cabin.

On 11/9/2016 1:42 PM, Duster wrote:
It's all about getting oxygen to brain tissue (oxygen exchange efficiency), and avoiding embolism (aka "the bends"), not so much % O2 that's available to breath. At those altitudes, breathing 100% oxygen is about as efficient as breathing air at around 12,000ft (someone correct me if I'm off). Hence the need to increase "cabin" pressure in the Perlan 2 and as the USAF is doing now with the U2's. Remember Robert Harris and his 1986 49,009ft altitude record was done in an unpressurized Grob 102. He was forced to come down when his O2 system started to fail, apparently suffering from medical issues for some time thereafter. Now it kinda makes sense why Perlan 1 (Fossett) spent all that time and money to best Bickle's flight by only 1,600ft wearing NASA spacesuits with Enevoldson.

--
Dan, 5J