What happens when an aircraft hits something that is not inclined to move?

[R.Mackey]

So there was almost invisible shredded material all over the place? That is why it looks like so little "survived" the crash? In a form that was recognizable?

"Almost invisible" from television-quality images, most of which were through telephoto lenses, yes.

As has been reported here many times, 95% of Flight 93 was recovered, by mass, though there were only a few fragments weighing over 100 kilograms (sections of engine cores, also both black boxes were recovered more-or-less intact). The majority of the aircraft was in small pieces. The other three crashed aircraft were mixed in with building debris and suffered much more fire damage accordingly, and are harder to evaluate comprehensively.

I have to agree with the CT people on the lack of photos or information at times. As I hinted, I would bet you have an official site that has this information, right? I'm interested in reading about it.

You may agree with them at first glance, but that only serves to separate their armchair, utterly worthless opinions from those of actual investigators.

Pictures of the tiny little pieces of aluminum, steel and titanium would be cool. It really is amazing how little obvious-jet-crash material there was. In the few photos and videos.

It would be far more "cool" if each of them wasn't associated with multiple deaths.

But then, watching the video of the crash test, it is obvious that there isn't going to be much left after that kind of impact. I still want to know what happens when you slam a fighter full of fuel into that concrete test wall.

It was outside the scope of the test. The test was to verify that the wall had enough strength to handle the impact. Fuel deflagration after the fact would add virtually nothing -- an additional pressure of at most a few PSI -- provided any fires that resulted were controllable. Nobody really cared whether the impact left the F-4 in chunks the size of a softball versus shreds the size of a postage stamp. It's meaningless minutiae.

 

[R.Mackey]

Here is a description of the Purdue "potato cannon" validation experiments, including a photograph of the can afterwards. Ripped up, but nowhere near vaporized -- it's practically still in one piece. Impact velocity is 80 m/s, or 288 km/hr.

 

[pomeroo]

I would be a "Greenian" if he were not so darn crooked and nasty to the NISTIANs all the time. I believe in his science, I applaud his refusal to side with the CTs (although I think at times he wants to) because the science forbids him, I admire his persistence....

I won't go into yet again the things that I dislike...let just say..."Can anyone say beat the dead horse".

TAM

TAM, the official term, I believe, is "Greeningian" (pronounced "Gree-ning-ee-an").

 

[robinson]

Impact velocity is 80 m/s, or 288 km/hr.

This got me wondering. Does the speed of an object increase the energy in a linear way? Or is it logarithmic? Is that the correct question? I'm not sure.

What I am wondering, does speed matter? As in, if I shoot a piece of metal at a concrete wall at 40mph, is that half the damage as 80mph? Or to ask another way, does an object hitting at 80mph sustain 4 times the damage as the same impact at 40mph?

It doesn't seem to be the case, based on practical experience. For example, hitting a hardened steel hammer against concrete. You can bang all day at normal speed, and the metal won't break. But if you hit the same metal against the same immovable concrete at a very high speed, the metal will shatter.

Is 400 mph a magnitude higher than 100mph? Or is it just four times as much damage? Would the same object hitting four times at 100mph be equal to one impact at 400mph?

Does this matter? Because getting a can to go 500mph is looking like a serious task.

 

[Corsair 115]

This got me wondering. Does the speed of an object increase the energy in a linear way? Or is it logarithmic?

The formula for kinetic energy is:

KE = ½mv²

That is, half the mass times the square of the velocity. Which means for an object of the same mass, if you double the speed, you quadruple the kinetic energy.

 

[robinson]

Thanks. I thought that was the case. So an aluminum can full of jet fuel hitting at 500 mph is going to be a different order of magnitude than 190mph. Right?

It certainly seems to a whole lot more energy is needed to get it to move that fast. Just going from 200 mph to 300 mph seems to be a lot more power required. I don't think 500mph is going to be possible with a potato gun. Rockets are going to be needed.

 

[BenBurch]

Here is a description of the Purdue "potato cannon" validation experiments, including a photograph of the can afterwards. Ripped up, but nowhere near vaporized -- it's practically still in one piece. Impact velocity is 80 m/s, or 288 km/hr.

About what an illegal-in-Illinois M-80 cracker would do to a can. (I won't say how I know this.)

 

[robinson]

How about a trash can?

http://www.youtube.com/watch?v=0kWyldLgpbs

 

[R.Mackey]

This got me wondering. Does the speed of an object increase the energy in a linear way? Or is it logarithmic? Is that the correct question? I'm not sure.

What I am wondering, does speed matter? As in, if I shoot a piece of metal at a concrete wall at 40mph, is that half the damage as 80mph? Or to ask another way, does an object hitting at 80mph sustain 4 times the damage as the same impact at 40mph?

It doesn't seem to be the case, based on practical experience. For example, hitting a hardened steel hammer against concrete. You can bang all day at normal speed, and the metal won't break. But if you hit the same metal against the same immovable concrete at a very high speed, the metal will shatter.

From your questions, I'm guessing you don't have a strong background in physics. Not criticizing, just trying to understand where you're coming from.

As noted above, the kinetic energy scales with the square of velocity, so a ~800 km/hr impact like Flight 93 would be about nine times as energetic as an impact at the speed of the test soda can in the Purdue example, moving at 288 km/hr.

Kinetic energy doesn't tell the whole story, though. Fracture mechanics of solids and solid-liquid interactions are extremely complicated. Just because the can has nine times as much energy pre-impact doesn't mean that it will suffer nine times as much damage. Clearly most of the energy absorption isn't going into deforming the can, but somewhere else, and at higher impact speeds the fraction might be even more biased against the can. The can just isn't that dense on its own.

This is, after all, why Purdue is doing this experiment in the first place -- there isn't much experimental data out there with which to test our models. It seems simple, but it's not.

Does this matter? Because getting a can to go 500mph is looking like a serious task.

It's not that hard, actually. It would be difficult with a compressed-air cannon, but it's trivial to get that kind of muzzle velocity with gunpowder...

But also keep in mind that speed isn't the only quantity being scaled. The can is also totally full of water, making it far more dense sectionally than any aircraft. The size is also scaled differently. It's not clear that tests like this actually do scale, in size, mass, speed, or strength of materials. Instead, its true value is to check the simulations, and those can be easily scaled up to the problem of interest.

Still, I give Purdue full credit for checking their simulations against real physical cases. It's good experimental design, if a difficult problem overall.

 

[defaultdotxbe]

About what an illegal-in-Illinois M-80 cracker would do to a can. (I won't say how I know this.)

hmm, then i probably shouldnt mention how i know what a half stick will do to a 55 gallon drum (and in another incident, a concrete basement sink)

 

[robinson]

Hey! I saw that on YouTube. That was you???

Yes, obviously my Physics has faded with age. I was pretty sure it was an inverse square or something, but not certain, so I asked. Obviously an aluminum can isn't an airplane. But what happens to anything hitting a solid object at 500mph? That has a quantity of jet fuel involved?

Based on the video of the fighter, I am imagining at first just a lot of fuel expanding quickly, it being like water, it won't compress. Then the flame front trying to catch up, a possible fuel air explosion, with the un-vaporized fuel creating the fireball.

Gunpowder won't work, the bottom and top of the can blows out, and you get a water cannon with shrapnel. Don't ask me how I know this.

 

[R.Mackey]

Obviously an aluminum can isn't an airplane. But what happens to anything hitting a solid object at 500mph? That has a quantity of jet fuel involved?

The fuel will probably ignite. But you won't get an explosion, most of the time. The fuel isn't sufficiently well mixed, and there isn't enough oxygen around to get it done. This is why in the case of UA175 and AA11, NIST estimated that only 20% of the fuel cooked off in the initial fireballs.

Instead of an explosion, you get a fairly boring deflagration, followed by oxygen starvation.

If you crank up the speed even more, you will change the mixing of the fuel, and at some point you may hit a magic speed where you actually detonate that fuel. Still probably won't affect the debris much, though, apart from throwing it around. If you drop a bomb on a pile of sand, you don't create smaller sand grains, you just spray it around (until we're talking about enormous bombs).

Gunpowder won't work, the bottom and top of the can blows out, and you get a water cannon with shrapnel. Don't ask me how I know this.

Not if you treat the projectile and barrel with Teflon, and load it pop-top towards the breech. Or insert a wad column (discs of Delrin are ideal, but expensive). Don't ask me how I know this.

 

[jaydeehess]

The trouble with the scaling would have to do with the ratio of cross sectional area to circumference would it not?

 

[BenBurch]

If you put the round in a sabot, the gunpowder won't touch it.

 

[jaydeehess]

Robinson's experiment using fuel would be modeling the fuel tanks rather than the fuselage. To model the fuselage one could tape balls of plasacine inside the can to simulate bodies, seats and other items in the cabin, though I venture it would not make much difference and they have to be able to stay inplace during the accellerative phase while coming free of the can wall in decelleration.

to make the acceleration less and still atain a high muzzle velocity you could use a magnetic rail gun.

 

[R.Mackey]

You don't need a rail gun. HPA simply doesn't work well because we want velocities that approach Mach 1, and you're likely to choke any dump valve before achieving enough flow.

Gunpowder is fine. Use a wad column (or sabot), increase your chamber size, etc. The science of cannons has been perfected over many hundreds of years for exactly this purpose.

 

[rwguinn]

You don't need a rail gun. HPA simply doesn't work well because we want velocities that approach Mach 1, and you're likely to choke any dump valve before achieving enough flow.

Gunpowder is fine. Use a wad column (or sabot), increase your chamber size, etc. The science of cannons has been perfected over many hundreds of years for exactly this purpose.

It can be done with air...We did 1100 FPS with a turbine blade using a 6"ID CRES tube and 150 PSI air. Used a Rupture disk instead of a valve, and it took a 10000 in^3 tank as a reservoir to maintain pressure down the barrel.
It did require a sabot (with a stripper at the muzzle end). You could actually se the shock onthe high-speed film...

 

[R.Mackey]

Yeah, it can be done. I simply mean that your typical ball-valve setup won't do it.

At GALCIT we have a hypersonic shock tunnel. It uses a "burst disc" of about 3/8" aluminum plate, with a massive piston compressing the driver gas on the other side until it cuts loose... Doesn't fire any projectiles, though.

 

[BenBurch]

It can be done with air...We did 1100 FPS with a turbine blade using a 6"ID CRES tube and 150 PSI air. Used a Rupture disk instead of a valve, and it took a 10000 in^3 tank as a reservoir to maintain pressure down the barrel.
It did require a sabot (with a stripper at the muzzle end). You could actually se the shock onthe high-speed film...

Yikes. I bet that was dry-ice cold when you were done with it.

 

[jaydeehess]

Yeah, it can be done. I simply mean that your typical ball-valve setup won't do it.

At GALCIT we have a hypersonic shock tunnel. It uses a "burst disc" of about 3/8" aluminum plate, with a massive piston compressing the driver gas on the other side until it cuts loose... Doesn't fire any projectiles, though.

By evacuating the 'muzzle' and keeping the outlet end sealed with a thin disc you could fire a projectile that would lose very little KE in rupturing the outlet seal. i don't know what the effect on the projectile would be as it went from a vacuum to atmospheric pressure at hypersonic speeds.

Didn't NASA or JPL do something like this though?