Bullet physics question

[Corsair415]

I great illustration of this was the SR-71. On ground it would leak fuel. As it flew it would get hotter and expand (by several feet) and seal up the tanks.

This is probably a myth. F-14's leaked like crazy as well, and this was always explained by the fuel tanks being pressurized in the air. Seems like just one of those things people make up.

[Wicked Pete]

I had the misfortune of having a bullet come my way, about 5 feet away from my head. I could feel the heat of it as it passed. Now that's HOT!

Did I poop my pants?

[UBFRAGD]

I'm too lazy to walk to the library and look it up, but doesn't chamber temperatures spike really high in large rifles, for the briefest of time, to like 52K F? Then add bore friction. The time in flight does not seem long enough to cool effectively.

[BKinzey]

That's a great story that gets passed around but it's not true. I know, I was asked to work on the tank issues. The problem is that they took two dry bays and made gas tanks out of them. They leaked like a sieve. As they flew that fuel would get used up, that is the only thing that kept them from leaking.

I'm fairly certain I read that story at the Blackbird Park out in Lancaster. It's pn a plaque which discusses the expansion of the plane in flight and the leaking tanks due to seals which expand in flight. I also remember the expansion measured in inches, not feet.

[viet4lifeOC]

Interesting information on the SR-71, but which factoid is true?

[krwada]

High speed FLIR camera

I'm too lazy to walk to the library and look it up, but doesn't chamber temperatures spike really high in large rifles, for the briefest of time, to like 52K F? Then add bore friction. The time in flight does not seem long enough to cool effectively.

Go to this website. The answer is on display there.

[five.five-six]

It gets hot from air friction.

If that was true then it would melt right through the ice.

[SWalt]

If that was true then it would melt right through the ice.

Pretty cool video, but it might have more to do with gyroscopic stability than hot or cold. Just guessing

[Lone_Gunman]

That's a great story that gets passed around but it's not true. I know, I was asked to work on the tank issues. The problem is that they took two dry bays and made gas tanks out of them. They leaked like a sieve. As they flew that fuel would get used up, that is the only thing that kept them from leaking.

I'm fairly certain I read that story at the Blackbird Park out in Lancaster. It's pn a plaque which discusses the expansion of the plane in flight and the leaking tanks due to seals which expand in flight. I also remember the expansion measured in inches, not feet.

I'm gonna take the guy that worked on the aircraft's word for it.

[Frisco3Gun]

Anything that travels faster than the speed of sound at standard temperature and pressure is going to get hot.

Mm this isn't entirely true. Think of a vacuum. Or light.

You can think of this two ways.
1. When the air can't get out of the way of the object fast enough, a sonic wave is created. This compresses the air around it causing an increase in temperature, which can gradually be transferred to the bullet (but this goes deeper, learn heat transfer if you'd like more info).

2. Resistance in the air equates to friction. Any friction turns kinetic energy into thermal energy (heat) on the surface of both objects.

Fluid dynamics are the devil.

[as_rocketman]

Go to this website. The answer is on display there.

Nice find.

Be aware, though, that there are problems with IR imagery on bullets, particularly unless you can photograph them at night... solar reflection and refraction through the boundary layer will both confuse the signal.

Having said that, the image shows you qualitatively what's actually going on. The aerodynamic heating at the nose is significant (for supersonic bullets), but away from the nose, that heating is negligible compared to mechanical friction and the powder charge in the barrel.

Reason is simple -- the supersonic bullet develops a strong oblique shock, and the shockwave does significantly heat the air. But only where the shock contacts the bullet, i.e., the stagnation point on the tip, is there strong heating of the bullet. The entire bullet structure lies inside the shock cone, so it is spared most of the heat energy, which goes into heating the wake instead of the bullet.

Now, some of that heat energy will get transferred to the bullet anyway, after the heated air passes over the bullet, but this is much less efficient. There are also many weaker oblique shocks on the bullet ogive -- and another strong shock at the bullet's tail -- but these are significantly weaker than the foreshock and do not dissipate nearly as much energy.

As another anecdote, Mythbusters did an episode once where they fired various bullets into containers of flammable fluids, testing the movie cliche of "well aimed" rounds causing cars to explode and other such silliness. They don't, of course. The cast found that only incendiary rounds, or lucky tracers, would have that effect. If bullet aerodynamic heating was more efficient, they'd have found a different effect.

Final check: Consider a typical hunting round, a lead soft-point bullet in a Mach 2.5+ caliber (say, .270 Win). That lead will lose 50% of its strength by 100 degrees C, and melt completely at 330 degrees C. But since our rounds aren't splashing on impact, it can't be heating up all that much.

[pacrat]

I'm too lazy to walk to the library and look it up, but doesn't chamber temperatures spike really high in large rifles, for the briefest of time, to like 52K F? Then add bore friction. The time in flight does not seem long enough to cool effectively.

Short answer.......NO........All smokeless powders burn at close to the same temp. 3,000 to 3,400 deg F. The major difference is the rate they burn at and the amount of plasma they generate when burning.

Another big factor in BTU, Watts, Calories, heat transfer is time. Everything that happens in a firearm between primer ignition and muzzle clearance is done in a span of a few thousandths of a second. There just isn't time for the heat of the burning powder to transfer much energy to the bullet.

Look at the FLIR pics in the links posted. Hottest part of the bullet after muzzle clearance is a small portion of the base, but it is cooler between there and the grooves where friction was highest.

Whether it contimues to heat up during flight. I have no idea, but believe it very well could from friction. Just as the tip of the bullet in the pic is hotter than body of bullet.

Would be cool for similar pics showing bullet after 500 yd flight just before striking target for comparison.

As to leaky fuel tanks on airplanes.

Really? It was/is acceptable to have a bunch of F-14 Tomcats puking fuel all over the flight deck? Damn. Not in the US Navy it isn't. Fuel tank QC must have certainly gone downhill since the late 1960's when I was building F-4 Phantoms at Douglas Aircraft. they had "self sealing multi layer bladder tanks" way back then. And even as far back as WW11. Even a hit from a 20mm would seal itself in just a few seconds.

Ask yourselves.

Would Uncle Sam build [32] SR-71's at several billion dollars a unit and put lowest bidder unsealed fuel tanks in them? That sure didn't work out very well for the Challenger crew.

just say'n

[krwada]

Speed of sound in a VACUUM???

Mm this isn't entirely true. Think of a vacuum. Or light.

You can think of this two ways.
1. When the air can't get out of the way of the object fast enough, a sonic wave is created. This compresses the air around it causing an increase in temperature, which can gradually be transferred to the bullet (but this goes deeper, learn heat transfer if you'd like more info).

2. Resistance in the air equates to friction. Any friction turns kinetic energy into thermal energy (heat) on the surface of both objects.

Fluid dynamics are the devil.

In a vacuum ... there is NO speed of sound. Heck ... there is NO sound. Sound requires a medium in order to have any presence!

I suppose you are an expert at fluid mechanics and heat transfer? Given your comments about speed of sound in a vacuum ... well ... let us leave it at that.

Or perhaps you have already found the closed form solution for the Navier Stokes equation ... If you have ... then a very big prize is awaiting you!

Heh!

[krwada]

Nice find.

Be aware, though, that there are problems with IR imagery on bullets, particularly unless you can photograph them at night... solar reflection and refraction through the boundary layer will both confuse the signal.

Having said that, the image shows you qualitatively what's actually going on. The aerodynamic heating at the nose is significant (for supersonic bullets), but away from the nose, that heating is negligible compared to mechanical friction and the powder charge in the barrel.

Reason is simple -- the supersonic bullet develops a strong oblique shock, and the shockwave does significantly heat the air. But only where the shock contacts the bullet, i.e., the stagnation point on the tip, is there strong heating of the bullet. The entire bullet structure lies inside the shock cone, so it is spared most of the heat energy, which goes into heating the wake instead of the bullet.

Now, some of that heat energy will get transferred to the bullet anyway, after the heated air passes over the bullet, but this is much less efficient. There are also many weaker oblique shocks on the bullet ogive -- and another strong shock at the bullet's tail -- but these are significantly weaker than the foreshock and do not dissipate nearly as much energy.

As another anecdote, Mythbusters did an episode once where they fired various bullets into containers of flammable fluids, testing the movie cliche of "well aimed" rounds causing cars to explode and other such silliness. They don't, of course. The cast found that only incendiary rounds, or lucky tracers, would have that effect. If bullet aerodynamic heating was more efficient, they'd have found a different effect.

Final check: Consider a typical hunting round, a lead soft-point bullet in a Mach 2.5+ caliber (say, .270 Win). That lead will lose 50% of its strength by 100 degrees C, and melt completely at 330 degrees C. But since our rounds aren't splashing on impact, it can't be heating up all that much.

What you have just so clearly elucidated ... I believe ... is pretty close to the correct answer.

There is some heating observed at the tip. Most of the heating, (base and lands) looks to be due to heating from the powder burn and especially the friction of the lands on the projectile. Of course ... the observed image on the base could be due to a thermal reflection as you point out... I believe that this may be the case too.

The heating at the tip, while overall negligible ... still appears to be significant. 100 to 150 Celsius is hot enough to boil water ... but will not have any effect on the metallurgy of the jacket or bullet.

So ... in a manner of speaking ... the projectile does get 'hot' due to air resistance ... but in a relative fashion ... not by much.

Just as I have said before ... You push anything faster than the speed of sound ... especially for any length of time .... it will get hot!

[pacrat]

From the text accompanying the pics posted.


"Recent experiments have demonstrated the camera's capability to generate thermal images of bullets in flight. Figure 1 shows a .30-caliber rifle bullet that has traveled about 3 feet out of the muzzle