jet plane flight height - what's the determining factor?

Factual Questions
21

The bends are not the same thing, but I see what you’re getting at. A liquid under pressure will absorb more gas into solution than a liquid that’s not under pressure. When SCUBA diving, you are breathing compressed air, and it’s the high pressure of the air being forced into the lungs that allows the nitrogen gas to go into solution. Remove the pressure, and the nitrogen starts to bubble. Just like popping open a bottle of coke. (In SCUBA, we learned a little ditty when studying gas laws to remember that this is a demonstration of Henry’s Law: ‘The best way to remember Hank, the gas dissolves like in the Coke you drank!’).

I think it’s the compressed air you breath that causes the bends. And yeah, the pressure of the water on your body is transferred to your lungs, which then shrink unless pumped back up with compressed air. There’s a science toy called a ‘Cartesian Diver’ which illustrates this. You can make one yourself - find something that has a little air bubble in it and and is compressible, and weight it so that it just barely floats. Now put it in a a plastic pop bottle, and fill it with water and put the cap on. Squeeze the bottle, and the little ‘diver’ will sink because the pressure causes the air bubble to shrink. Release the bottle, and the diver will come up again.

Would you get the bends if you were suddenly tossed out into space? I don’t know. The pressure differential is one atmosphere. That’s like going from 33 ft deep in water to the surface quickly. Normally, that wouldn’t cause the bends. But if it happens instantly? I dunno. An air embolism is much more likely. Remember kids - if your airplane explosively decompresses at 30,000 feet, exhale. Don’t try to hold your breath!

Anyway, I don’t think your blood will boil, because the body does act as a pressure vessel. It’ll expand a little, and the pressure on the blood might drop a little, but I don’t believe it’s enough to cause it to boil. Remember 2001? There’s a scene in which a guy has to leave an airlock without a spacesuit and float across open space into another one. A lot of scientists I’ve talked to think that that is possible. I have my doubts, because I think the brain might become oxygen starved really fast and knock you out. But if you were pulled across unconscious, and were outside for no more than a minute or two, I think you might be okay.

22

T’ain’t nuthin’ that simple in aviation… why just getting home from breakfast last week involved quite a bit more than I expected, but that’s another story. Was a bit strange flying back with none of the electric gizmos working, though. The guy I was with was such a stitch! “What if the remaining fuel pump goes out?” Indeed :rolleyes: “Tighten your seatbelt, buddy, 'cause in that case landing is about 5 minutes away.”

Well, actually… naw, probably better to just leave it at that :slight_smile:

The problem isn’t designing planes to fly higher - we’ve got those already. The problem is designing them to fly higher economically.

IIRC, planes like the SR-71 and U-2 actually leak on the ground. In order to allow for the expansion due to atmospheric friction and internal pressurization, parts of the planes have significant and visible gaps on the ground. Once up and flying various panels expand and then fit tightly together. The SR-71 had to be built of titanium to withstand friction heating. The space shuttle has to withstand enormous stresses and pressures - it’s what, something like $100,000 a pound to fly something on it, or some other awful figure?

Even a passenger jet experiences a certain amount of actual expansion in the pressurized part of the hull during flight, something like a half an inch to an inch in diameter. Which may not sound like much, but when you consider that you’re talking about a metal object your life depends on doing that… I mean, my car wouldn’t survive even one pressurization cycle like that.

Oh, great, so we can kill twice as many people per crash? You know, I thought I read something about them having test flown that thing.

It’s all a matter of cost-effectiveness. If it can be made profitable, they’ll do it. Otherwise, they won’t.

Easy to change regulations? Ah-ha-ha-ha… Arnold, you’ve never dealt with the Effin’-A-A or its international counterparts. Alright, let’s assume we can change this regulation… should we?

There’s a saying that the regulations are written in blood. A lot of the rules are there because somebody - or several somebodies - died. The real question is, if you de-pressurize at 40,000 feet do you have time to put on a mask? The Lear jet Payne Stewart was on rapidly de-pressurized at considerably less than 40,000 feet and apparently no one managed to get a mask on. There is a case of a military pilot ejecting at high altitude, while he was already wearing a mask, and he spent part of the following fall unconcious (he also sustained some injuries from the depressurization on the rest of his body - he had to empty the nose blood out of his mask several times, he was bleeding from his eye sockets and ears, it was pretty gross - and a great deal of pain due to intestinal expansion). Get much above the ground it’s a hostile environment, one that we frail, unprotected humans can’t survive without considerable help. While not officially outer space, above 40k it might as well be as far as your ability to breathe is concerned. I mean, maybe we shouldn’t make pressure suits mandatory on spaceships, either.

All of which is not something folks on airlines want to think about. Just an inch or two of metal and plastic between you and death. What a charming thought.

But, by God, it’s beautiful up there.

23

The Concorde is 8 inches longer at cruise speed than it is on the ground, because the airframe heats up so much and expands.

If you look in the cockpit of one sitting on the ground, you can see gaps between all the panels. At cruise, there are no gaps.

Friction heating is one of the biggest limiters, BTW. At cruise speed, parts of the Concorde heat up to over 130 degrees celcius (almost 300 degrees F.), despite the outside air temperature being about -70F. The fuselage and windows on the inside of the pressure vessel will get quite warm to the touch.

The temperature increases very rapidly past those speeds. The Concorde goes about Mach 2. By Mach 3, the temperature can be over 200 degrees Celcius, and by the time you get to SR-71 speeds, the temperature will be over 300 degrees C. Those temperatures will destroy conventional metal, so you need special titanium skins and high-temp ceramics and stuff. Expensive materials, hard to work with, not very malleable, easy to crack, etc.

And BTW, the Concorde is so expensive to fly that even though it’s often full, and a ticket on it costs thousands of dollars, the fleet loses money hand over fist. The British and French governments subsidize the Concorde as a point of national pride, because those two governments worked together to build it, and there was too much political pressure to let the program die when it became clear that the thing would never make money.

24

Actually, no; although Airbus has greenlighted the process, a the Airbus website says that a prototype won’t be ready until 2004. Also, the BBC reported that the A380 will have a 3-class capacity of 550; the current 747 has a capacity of 412. It’s possible, of course, that future versions might have an increased capacity - but the current 747-400 carries about the same number as did the 747-100 33 years ago. The advances have been in efficiency and range.

The monster you may have been thinking of is the Antonov 225, the world’s largest aircraft, which made a much-delayed appearance at Le Bourget this year. Six engines, 250-tonne freight capacity, etc.

25

When I was learning to fly I was also taking SCUBA lessons. My instructor told me that it’s okay to fly and then dive, but never dive and then fly. Now when you’re diving, the air is carried compressed in tanks (normally about 3,000 psi when full). At depth, your body is compressed with one atmosphere of pressure at 33 feet as Sam Stone says. There is no way your muscles can inhale at even a few feet underwater because the pressure is so great. Try it in a swimming pool with a six-foot hose as a snorkel. The regulator delivers the air so that you can breathe underwater. The pressure differential in your lungs, however, remains the same as if you are at the surface. Note: I’m saying the differential and not the absolute pressure. (Oh, by the way… I’m going on memory and common sense here, but I’m sure any misstatements will be corrected.)

But the “bends” isn’t cause by the air in the lungs. The bends is caused by the gasses dissolved in the body. These gasses were accumulated under pressure. If the pressure is suddenly removed it is like popping the top off of a bottle of Coke. The gasses are no longer dissolved in the body and escape. When I went through the alitude chamber at Edwards (BTW: I just read that it’s been closed) we had to “prebreathe” pure oxygen for 45 minutes before simulating altitude. This is to purge the body of accumulated nitrogen. We only “went up to” 35,000 feet and they were taking these precautions. It seems to me that the problem would be greater in a sudden decompression at 50,000 feet (that’s about 17,000 metres to you, Arnold :wink: ).

I think that it is very likely a passenger, who is not breathing 100% O[sub]2[/sub], would get the bends in a rapid decompression.

What about “boiling blood”? Well, I was going on memory. Sam says the body acts like a pressure vessel so it won’t happen. But the body is very flexible. Ever had gas? You can expand quite a bit. And it hurts. If the body acted as a pressure vessel, why not just breathe pure oxygen and go out into space without a pressure suit? (I love hyperbole.) But let’s see if we can turn to an authority. Here’s an artical called Life Support: Flight Physiology from the University of Idaho:

So I was wrong about 50,000 feet, but it looks like 63,000 will do the trick.

26

Well, it says that fluids will boil at that pressure - it doesn’t say that blood in your veins is AT that pressure.

I’ll see if I can do some digging and come up with the straight dope on this.

Well, I’m back, and it turns out the master himself had something to say about this: http://www.straightdope.com/classics/a3_147.html

And it looks like the truth is somewhere in between. Apparently the blood will boil at some point in the process, but not immediately.