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I know that most aircraft have skins that are either aluminum or an aluminum alloy variant. I'm trying to figure out the minimal amount of kinetic energy that would be required for a projectile to penetrate/perforate an aircraft.

Does anyone knows of any useful papers about this? Or, perhaps, can anyone offer useful guidance or advice?

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  • $\begingroup$ It depends on the size of the projectile. A standard 9mm pistol bullet (~400-500J) is definitely considered unsafe if fired inside a pressurized airplane. Some purposed-built pistols for onboard security agents has a muzzle energy as low as 60J (Chinese type 84 pistol). $\endgroup$ Commented Apr 24, 2022 at 5:06
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    $\begingroup$ I have a pellet rifle that can shoot through a couple steel cans, so it isn’t a lot…. How exact of an answer are you looking for? $\endgroup$ Commented Apr 24, 2022 at 6:06
  • $\begingroup$ Size and shape. One innovative design for tank projectiles was a very skinny high density dart shaped projectile with fins at the back, without any explosive at all. By reducing the cross sectional area of the projectile, not only did it reduce drag enormously, it also allowed the kinetic energy to be applied to a much smaller area of the impact surface, so the energy per square inch at the point of impact was significantly higher. $\endgroup$ Commented Apr 24, 2022 at 12:28
  • $\begingroup$ Thank you, @user3528438. $\endgroup$ Commented Apr 24, 2022 at 15:22
  • $\begingroup$ @Michael Hall, I'm not looking for an EXTREMELY accurate answer. I'm just wondering if there is any officially documented article or paper that provides an estimate for the limiting kinetic energy that can potentially cause damage to the skin of an aircraft. $\endgroup$ Commented Apr 24, 2022 at 15:22

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Only a few joules. This isn't about energy. One could perforate an aircraft's skin with a millimeter-thick punch.

The theoretical deformation energy of 1 mm^3 of metal is UTS*elongation*volume. For 7475 aluminum (one of the strongest alloys used in aircraft), that comes to 0.53e9*0.12*1e-9 = 0.06 J/mm^3. This amount of energy can be slowly delivered even by hand, e.g. with a hand drill.

For a projectile, you'll need to use an armor penetration equation. The Lambert-Zukas formula is the most suitable here.

Plugging the values into the online calculator, a 10*10 mm, 10 gram projectile can penetrate 1 mm of aluminum at 40 m/s. That's an energy of just 8 joules. 1 mm is about the thinnest an airliner's skin can be at any point, usually only in the crown. Most are thicker, and GA aircraft can have skin as thin as 0.5 mm.

To penetrate 1/8" of aluminum, the high end of the range for airliner skin thickness in passenger areas, the same projectile would need 118 m/s. That's an energy of 70 joules. Energy requirements will scale roughly linear with reducing or increasing the hole's diameter, with 7 J for 1 mm, 35 J for 5 mm.

Note that a 10mm hole will not compromise the aircraft's integrity, airworthiness, or anything else. It will only make a small amount of noise.

Airliners aren't airtight: there is already a much larger hole in the fuselage. They take fresh air in from the engines and dump used air through the outflow valve.

enter image description here

What matters for damage is the size of the hole. Blowing out a window will generally cause some winds in the cabin. It's dangerous to the passengers and there was one case when a blown window caused a fatality, to specifically the passenger next to it.

Some amount of damage will cause worse effects. This amount is unclear and depends on the aircraft. At low altitudes, pressurization isn't necessary, so one can fly open-cabin all the way. As an extreme example, this airplane actually landed.

enter image description here

But most of the time, an aircraft with this amount of damage would not end up landing safely. There was some luck involved. The area torn off was the crown, which doesn't carry much load - the thick and heavy lower fuselage between the wings and the cockpit held it together.

The threshold hole size to risk the entire aircraft is somewhere between these points. Unless it hits something critical.

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    $\begingroup$ this is getting more confusing the further I go. What, then, would compromise an aircraft's integrity or airworthiness. $\endgroup$ Commented Apr 24, 2022 at 21:01
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    $\begingroup$ @HannahSanders, that's because there isn't an easy answer. People pierce their ears and other parts all the time with no ill effect, but shove that same spike through your aorta or frontal lobe and you will have trouble. Airplanes are kinda the same way... If a bullet punctures the skin of the wing it may be no big deal, but if it severs a hydraulic line, control cable, or fuel tank it would be a problem. Help us understand the root issue and you might get a better answer. $\endgroup$ Commented Apr 24, 2022 at 21:09
  • $\begingroup$ @HannahSanders Overall, simple skin punctures punctures aren't a danger in terms of completing the flight. Large holes may overcome the pressurization system's capabilities, but the plane can fly lower to compensate. If there's not enough fuel, the pilots still have enough emergency oxygen to carry on at an intermediate altitude, even if everyone else will get incapacitated. $\endgroup$
    – Therac
    Commented Apr 24, 2022 at 21:34
  • $\begingroup$ You won't be able to punch a hole in a .040" to .063" thick 2024 T3 skin (typical for fuselages - 7 series alloys are typically used for wing planks) with a hand punch unless you are able to drive it with a fairly hefty hammer. $\endgroup$
    – John K
    Commented Apr 25, 2022 at 3:11
  • $\begingroup$ @JohnK True, though mostly because it's thick and dull - the energy is there. I'll edit to clarify. $\endgroup$
    – Therac
    Commented Apr 25, 2022 at 8:58

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