On Feb 19, 3:23*am, Dan G wrote:

As has been said I would think some kind of novel, lightweight, glider-
borne pressure cabin would be at least as likely to fail as the MIT-
developed and tested suit...

I suppose that's so. The art, science, and technology of pressure
vessel design and fabrication is quite mature, literally centuries
old. Of course, the pressure vessel containment failure at Rutan's
Scaled Composites a couple years ago that killed two or three people
shows that there is still something to be learned as we extend
composite construction into ksi pressures. But down around the 8.5 psi
pressure level like Perlan will use and like SpaceShip One and Global
Flyer have already applied, there's a pretty well-established
technology for making composite pressure hulls and securing the
viewing ports and hatches that it takes to make them useable.

As an aside, it's interesting to see that the Perlan guys are looking
for 8.5 psi pressures. As another poster points out, you can get away
with a lot less pressure if your pilots breathe 100% O2 in accordance
with the law of partial pressures. Since air is about 1/5 oxygen, at
100% oxygen you can achieve the same partial pressure of O2 across the
semipermeable membranes of the alveoli with 15/5 = ~3psi as you would
get at sea level under normal circumstances.

Of course, the lessons of Apollo 1 inform us that 100% oxygen can be a
real fire hazard, especially at sea level pressures and when mixed
with various machinery and its lubricants and also a bunch of
electronics. One solution to that issue is the one they used with the
X-15 program: They filled the pressure suits with 100% O2 to a few
psi, and then pressurized the cabin with nitrogen to the same
pressure, resulting in a limp pressure suit and a non-flammable cabin
atmosphere. That's the solution I think I might have chosen for a
similar mission profile to that of Perlan.

Based on their 8.5psi cabin pressure, I'm guessing that what they have
planned is to pressurize the cabin with regular air (20% O2, 80% N2,
or maybe a bit richer) to 8.5psi, which replicates the environment
you'd see down around 10,000 or 12,000 feet. They'll put the crew in
David Clark 1030-series suits pressured up to the same 8.5psi as the
cabin, and feed supplemental oxygen into the helmet enclosure to get
blood oxygen saturation up to what you'd see at sea level. The suits
will be limp and relatively easy to wear except in the case of cabin
pressure failure.

BTW though as I understand it the MIT suit works by applying
mechanical pressure rather than inflation, so there's not a lot to
fail in the first place.

True enough. But when I estimated mission profiles for a Perlan-type
program, it looked like you'd want to allow for a flight time of up to
12 hours. I'm not sure how the MIT suit works, but if its based on
mechanical pressure I can imagine that it might be quite fatiguing
after a few hours. Adding into the equation the work required to fly a
rather heavy 20-meter glider with unpowered controls probably makes it
a lot worse pretty fast.

Thanks, Bob K.
www.hpaircraft.com