Explosive decompression

too slow

We've discussed the Mythbusters episode;

http://www.thehighroad.org/showthread.php?s=&threadid=59004
http://www.thehighroad.org/showthread.php?s=&threadid=58892


And we've discussed decompression several times here is one of the more recent;

http://www.thehighroad.org/showthread.php?s=&threadid=56764

Seems like bogus testing

I am an aerospace test engineer, have trained at USAF and Navy high altitude chambers many times. The outside air pressure at 35,000 is, shall we say, weak. I've experianced it and am very glad the airplanes are kept pressurized to maybe 5 to 8 thousand feet. The result is big pressure inside, low pressure outside, and the airframe is resisting the urge to explode outward.

This test? Pressurize the inside to 35,000 feet and run the test with the aiplane sitting on the ground, so it is trying to keep from imploding inwards?? How did they suck that much air out of the airplane? Were they inside the airplane (on oxygen) when they did the shooting? How can they tell us that this give results that are comparable to the situation during real flight?

Mind you, I agree with the conclusion, but I didn't need any TV testing to get there.

Bart Noir

Bart, the show isn't made for us gunnuts or for aerospace engineers, the science is very simplified and entertaining and it is designed for the audience of the Discovery channel.

I'm just glad to see a major anti-gun myth dispelled in a manner that the average person watching it can understand.

I think they determined the pressure differential between 8,000 feet and 35,000 and then replicated that differential. I think they were increasing pressure in the plane, not decreasing.

- Gabe

"equivalent to 35000 ft"

I think Rickstir meant to imply that the aircraft cabin pressure was at 8 psi differential, which might be what the aircraft maintains at a 35,000 ft. altitude ...

Now for the big mystery. If I'm riding in an SR71 Black Bird, flying at the speed of a bullet, and I shoot my gun inside the aircraft in the direction of flight, does the round leave the chamber ???

lol Autolite....

of course it does, because the bullet is ALREADY travling bullet speed, so when you shoot it, it will be going twice bullet speed

EXACTLY! I've been saying this all along. The business of bullets causing modern jets to fall apart is absurd. Your average commercial jet is already operating with the equivalent of many bullet holes in its hull. But pressure is controlled by a self-regulating positive pressure system. More holes just means more pressure gets pumped in to replace the air leaving the aircraft. You'd have to blow out a wall to cause a failure in the system.

If you watch the episode, they do pressurize (positive pressure) the interior of the plane.

I once was on a Continental flight from Tokyo-Houston-Newark. Someplace over the Pacific the pilot's windshield cracked. We landed at Houston and the ground crew changed the windshield. They had to pressure test the entire plane before we were allowed to reboard. Took a couple of hours.

Later I found out that windshields frequently crack. Something to do with a malfunctioning windshield heater system.

No, I didn't feel a breeze from the cockpit going down the 1st class aisle.

I am an aerospace test engineer, have trained at USAF and Navy high altitude chambers many times. The outside air pressure at 35,000 is, shall we say, weak. I've experianced it and am very glad the airplanes are kept pressurized to maybe 5 to 8 thousand feet. The result is big pressure inside, low pressure outside, and the airframe is resisting the urge to explode outward.

This test? Pressurize the inside to 35,000 feet and run the test with the aiplane sitting on the ground, so it is trying to keep from imploding inwards?? How did they suck that much air out of the airplane? Were they inside the airplane (on oxygen) when they did the shooting? How can they tell us that this give results that are comparable to the situation during real flight?

Mind you, I agree with the conclusion, but I didn't need any TV testing to get there.

Click to expand...

Why do the two bolded sentences not equate?

We are involved here with three pressures; psia, psid, and psig. Those would be absolute, differential, and gage. Absolute is measured from absolute zero, gage is measured from one atmosphere (14.7 psia), and the differential is the difference between the reference pressure and the pressure in psig. In this case, the reference pressure is one atmosphere (~14.7 psia) or 0 psig. The differential pressure is 8 psid.

The plane was on the ground and the pressure was pumped in, not sucked out. The pressure was a differential pressure on the inside of the plane that was 8psig greater than one atmosphere or ~14.7 psia + 8psig = ~22.7 pisa depending on the altitude at which the test was performed. That is a positive pressure inside the aircraft.

If I'm riding in an SR71 Black Bird, flying at the speed of a bullet, and I shoot my gun inside the aircraft in the direction of flight, does the round leave the chamber ???

Click to expand...

The real question is how much of a reduction of speed of the aircraft will be experienced by the counter force of the bullet leaving the barrel.

The Mythbusters test involved pumping air INTO the fusilage.

Which is why they had a problem with the plywood covering the missing cockpit windows blowing OUT and across the lot.

The real question is how much of a reduction of speed of the aircraft will be experienced by the counter force of the bullet leaving the barrel.

Click to expand...

I'd be more interested in how long until the SR71 overtakes the bullet--assuming plane is flying in the same direction and gun is angled for appropriate trajectory.

I think I'm gonna pop this one on my physics professor(who should make short work of it).

I'd be more interested in how long until the SR71 overtakes the bullet--assuming plane is flying in the same direction and gun is angled for appropriate trajectory.

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The effective airspeed at the Blackbird's cruising speed and altitude is surprisingly low--I would guesstimate 250 kt or so--due to the extremely low air density. So the bullet wouldn't lose speed as rapidly as one might imagine at first . . .

BTW, I believe the effective airspeed at the speed/altitude Columbia was traveling (~12,500 mph, 200,000 ft, IIRC) when her left finally wing folded, putting her into her final spin, was something like 150-200 kt. (It's in the CAIB report, but it's on my other computer.)

BenEzra

I think you are misunderstanding IAS and TAS and GS

IAS - Indicated airspeed. What the gauge in the cockpit tells you. It is basically a function of how many air molecules can be stuck into the system which pressurizes the system giving you a reading. (Really quick and dirty explanation) I also doubt that the Shuttle was reading actual IAS.

TAS - The actual speed of the aircraft. Although the shuttle may only read 250KTS, it is still truing out too extremely high velocities.

GS - The Speed the thing is moving across the ground.

So the basic calculation is IAS, corrected for altitude. The higher the altitude the more TAS for a given IAS due to density differences. The factor in any headwind/tailwind, which gets you GS.

I don't know what GS the Shuttle was moving at, bit that thing seriously trucks.

BTW. Quick and Dirty explanation. Please do not knit pick.

I think that Ben is pointing out that there would be comparitively little drag placed on the travelling bullet as a result of the thin atmosphere. This brings me to MY physics question. What effect would such atmospheric conditions have on the effective RANGE of the bullet? and what would it do with the tragectory?

FireInTheHole

I'd be more interested in how long until the SR71 overtakes the bullet--assuming plane is flying in the same direction and gun is angled for appropriate trajectory.

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There is a record of an aircraft having shot itself down.

When the jet age came about, planes began flying fast enough to fly into the decaying trajectory of their own rounds when they would test fire their guns. After the one guy managed to shoot down his own plane, the standing orders to this day is for aircraft to change direction after test firing their guns to avoid another incident.

benEzra

The effective airspeed at the Blackbird's cruising speed and altitude is surprisingly low--I would guesstimate 250 kt or so--due to the extremely low air density. So the bullet wouldn't lose speed as rapidly as one might imagine at first . . .

Click to expand...

Are you talking about the avian variety of Blackbird?

I think you need to go HERE and do a bit of reading.

On September 1, 1974 Major James V. Sullivan, 37 (pilot) and Noel F. Widdifield, 33 (reconnaissance systems officer) (photo inset), flashed across the starting line (radar gates in New York) at approximately 80,000 feet and speed in excess of 2,000 miles per hour. Exactly 1 hour 54 minutes and 56.4 seconds later, they had set a new world speed record from New York to London England.

Click to expand...

If the Blackbird cruises at 250kt then why does it take 600 miles to turn one around?

Link above has details, testimonials, pictures, videos, screensavers, the works.

Cool stuff like this:

I'd be more interested in how long until the SR71 overtakes the bullet--assuming plane is flying in the same direction and gun is angled for appropriate trajectory.

Click to expand...

Not to contribute to the hijack of any thread, but I would be interested about a shot fired from the SR-71 in the opposite direction of flight.

If the pilot were to fire a round possessing the same velocity (remotely, I suppose) from the rear of the bird after flying past a broad line painted on the ground, would the round simply drop on that line?

If the round were slower than the velocity of the aircraft, would the slug continue past the line travelling backwards, base forward (without taking into account any effects of backwash, turbulence etc)?
I can't find any reason why it wouldn't, but my brain doesn't want to accept it.

Snowdog

I was conversing with a steward on a flight a year ago and asked him if he realized that when he traverses the length of the aircraft that he is going 1-2mph slower than the aircraft going aft and going 1-2mph faster than the aircraft going forward. He thought it was interesting and hadn't considered it.

By your premis, a bullet being fired aft from an aircraft that exceeded the speed of the bullet it would merely fall out of the end of the barrel. The bullet has its own velocity relative to the firearm and does not lose any of that velocity due to the motion of the firearm. If the plane was going 900ft/sec and you fired a firearm that had a muzzle velocity of 900ft/sec, the aircraft and the bullet would, for a short time due to bullet decay, be moving apart at 1,800ft/sec.

A good place to bring this question up is at http://boards.straightdope.com/sdmb/ . There's a hell of a lot of brain power there -- maybe enough to power a small city.

Enjoyed that episode, but was more than a bit disappointed by the chicken gun segment. Anyone with a basic knowledge of chicken ballistics knows that a frozen chicken has much greater penetration than a thawed one (maybe even good enough for bear).

Another cool SR-71 site is HERE. They have the SR-71 operating manual online.

Enjoy.

Back on topic, there was a guy on TFL, I believe, who worked on airframes and said that the skins get punctured on a regular basis and have to be repaired.

Did not one of the early varients of the SR-71 have a .50 and manage to run into its own projectiles? the A-12 or something like that