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I'm doing some research for a book I'm writing, and, as the title implies, I'm looking to understand what happens when a supersonic jet sustains in-flight damage.

Specifically, what happens to the occupants if the cockpit is breached? (I assume that this is why jet pilots wear masks?) Could a jet maintain super sonic speeds (for at least a short time) with such a breach? What would happen to a person in the cockpit without a mask if there was a cockpit breach at those speeds?

Just how weird do things get at super-sonic speeds? Could a conventional airliner ever reach mach one in extreme circumstances?

Many thanks!

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I think the pilots wear masks because a supersonic military jet is not pressurized. A conventional airliner could theoretically match that speed I guess by plunging down from a high altitude... Don't know if any modern plane would survive the stress though. –  Opt Feb 27 '12 at 21:35
    
@sid of course you had the Concorde as a commercial jet at velocities more than 1Mach –  Bernhard Feb 27 '12 at 22:45
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As @Bernhard said, the Concorde flew faster than Mach 1. Commercial airliners have speed limits, because as they approach Mach 1, some of the airflow exceeds Mach 1, and they're not designed for that. The stresses are very high. I'm not sure about modern combat aircraft, but if a cabin is unpressurized (as in ww2 bombers) the mask provides supplemental oxygen, which must be around 2-3psi partial pressure or else brain function suffers. –  Mike Dunlavey Feb 28 '12 at 0:18
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3 Answers 3

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You also ask:

Just how weird do things get at super-sonic speeds? Could a conventional airliner ever reach mach one in extreme circumstances?

Things don't especially get "weird" at supersonic speeds. The aerodynamics is different above the speed of sound than below, and that affects the design that goes into the airplane and its engines: how lift is generated, how to handle drag and shock waves, how the engines consume air. But that's all engineering stuff.

As the above implies, an airplane must be designed to go supersonic. The behavior of flight near, at, and above Mach 1 is different than subsonic flight and the craft needs to be engineered with these in mind. It's not the case that you can just "push" a standard aircraft into exceeding the speed of sound under "extreme circumtances".

The only commercial airliners that were supersonic were the (both now defunct) Concorde and Russian "Charger".

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I seem to remember that before Chuck Yeager, some planes could be dove into supersonic speeds. The problem was that the control surfaces contained hinges, where shock waves formed, causing havoc - flutter, etc. On a fighter jet today, the horizontal and vertical stabilizers rotate as a unit, rather than having hinged rudders and elevators. –  Mike Dunlavey Feb 28 '12 at 0:23
    
Cool stuff. So if a plane not designed to go supersonic attempted to, what would be the consequences? Any uneven part of the surface would cause a shock wave? What's 'flutter', and what's in the 'etc'? –  So8res Feb 28 '12 at 0:40
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Mike is correct - if the engines can't go supersonic, you could always use gravity! But there is an ideal shape for supersonic flight (en.wikipedia.org/wiki/Sears-Haack_body) and any deviation from that shape will tend to cause shock waves. The shock waves cause rapid compression and decompression near the surfaces. This can then lead to resonance conditions like flutter or buffeting, where resonance causes vibrations to grow and grown until things fall apart. –  Mark Beadles Feb 28 '12 at 1:01
    
@So8res: Mark's right. You know how a flag flutters in a breeze? When that happens on an airplane, things break and people die. Just go to a small airport, and with the pilot's permission, take a look at the small weights built into the ailerons and elevators of a small plane. Part of your pre-flight check is to examine those. The surfaces are tuned not to flutter. –  Mike Dunlavey Feb 28 '12 at 21:23
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Fighter aircraft cockpits are pressurized, otherwise you wouldn't be able to breath even with an oxygen mask. When you breathe out you need the external pressure on your chest to force the air from your lungs. For a short period you can breath with less pressure with low efficiency - which is why you need a richer source of air ie. higher oxygen content, in an airliner emergency mask.

The mask on a fighter pilots helmet is also to provide convenient place to put communications gear and to provide an air supply if the cockpit failed or you had to eject. This does lead to another problem, if you have to eject at high altitude you suffer a rapid loss in pressure - this is equivalent to ascending from underwater too quickly and pilots can get the bends. This is the justification for the airforce having a diving research facility.

Flying at very high altitude aircraft (eg an SR71 or U2) the outside pressure would be too low to allow you to breathe, even with an oxygen mask. This and the high temperature and high speed of the airstream if you had to eject means the crew had to wear pressurized suits.

How long the plane could remain flying would depend on the nature of the damage, there is nothing particular special about supersonic on a small scale - there are still numerous openings, intakes, ports and other holes in supersonic aircraft.

The canopies are pretty strong - bullet proof is generally a user requirement - so they wouldn't shatter on impact. There have been pilots killed by canopy failures or accidental ejections but there are also pilots who have survived the disintegration of an SR71 at mach 3

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All right, the mask requirement (and the pressurization requirement) are, according to my understanding, necessary due to the altitude, not the speed. Would you be able to breathe without a mask at super sonic speeds in a plane with a cockpit breach but at relatively low altitudes? –  So8res Feb 28 '12 at 0:41
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I don't expect a cockpit to develop a "breach" at low altitudes and supersonic speeds. The aerodynamic forces on a plane depend on speed (very much higher at supersonic speed) and pressure (higher at lower altitudes). Therefore, I expect a damaged canopy to quickly disintegrate under such circumstances. –  MSalters Feb 28 '12 at 8:51
    
Well, this is fiction we're discussing, so there are other environmental factors (read: bullets) assisting in the development of the breach. Am I correct in assuming that a small hole in the canopy would soon cause the canopy to be destroyed? What forces would the pilots undergo in this scenario? What are their chances of survival? –  So8res Feb 28 '12 at 17:07
    
"Flying at very high altitude aircraft (eg an SR71 or U2) the outside pressure would be too low to allow you to breathe, even with an oxygen mask." This is not true. In the mid 1990ies there was a record breaking hangglider flight, launched from over 20km high of a balloon. The pilots were all wearing masks. Also with those masks breathing works differently: You're no longer sucking air in, you're kind of force fed with air. To support exhalation there's a valve that reacts to the slightest pulmonary contraction, which effectly evacuate the lungs. So you've to relax to inhale. –  datenwolf Feb 28 '12 at 18:05
    
@datenwolf - positive pressure masks are a little different though from the normal aircraft masks. I thought SR71/U2 had a ceiling of around 120,000ft but apparently it's 'only' 90,000 so their pressure suits are mostly for temperature and speed. –  Martin Beckett Feb 29 '12 at 4:43
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Specifically, what happens to the occupants if the cockpit is breached?

Without supplemental oxygen, the occupants will lose consciousness, or lose the ability to think clearly enough to improve the situation. This table from the Pilot's Handbook of Aeronautical Knowledge (free FAA publication; search for "hypoxia") might be useful:

Time of useful consciousness

Also, if you're not wearing a seatbelt, you can easily be sucked out of a window if the window breaks. There are several incidences of this occurring.

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An interesting point is the speed you lose consciousness at very high altitudes. The oxygen partial pressure in your lungs is so low that your lungs essentially 'pump' oxygen OUT of your blood stream. –  Martin Beckett Feb 29 '12 at 4:45
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