Bullets from a Jet Fighter Mach 3.2

[jackman]

The F-86 fired 50 cal. guns in flight but only flew at about 550 mph. Keep in mind that to push the shock wave cones on the engines of an SR-71 it takes 14-16 tons of pressure. Also, at Mach 3+, forward surface temps will be as high as 450 degrees. The SR-71 was never designed to fire a gun at speed as the mind set at the time was missles only.

 

[LURCH]

I was just wondering what happens at the exact moment the bullet exits the gun barrel and hits air that is moving Mach3 faster than the air inside the barrel? Could this be like hitting a wall, causing the bullet to just disintegrate a few inches out of the muzzle?

Also, regarding the impossibility of hitting one's own ammo; when a bullet leaves the barrel at a muzzle velocity of about Mach1.5, and then encounters a headwind of Mach3+, couldn't it be shoved back into the gun? That might be where the concern about "exploding inside the magazine" originated.

 

[russ_watters]

No, it certainly wouldn't be like hitting a wall, it's not even like splashing into a pool. There is no surface interface and the transition is relatively smooth. Nor could the bullet be shoved back into the gun. Remember, whatever pressure the bullet is going to feel in front of it, it feels vastly more behind it, so there is no chance of it disintegrating and deceleration is much smaller than the acceleration was. Even if the bullet decelerated at 10 g's (which would be an awful lot), it would still take more than 10 seconds for it to get back down to the speed of the plane.

 

[LURCH]

No, it certainly wouldn't be like hitting a wall, it's not even like splashing into a pool. There is no surface interface and the transition is relatively smooth. Nor could the bullet be shoved back into the gun. Remember, whatever pressure the bullet is going to feel in front of it, it feels vastly more behind it,

Not true. Once out of the barrel, the bullet feels no pressure behind it. It is coasting, and only feels the pressure in front of it.

...and deceleration is much smaller than the acceleration was. Even if the bullet decelerated at 10 g's (which would be an awful lot), it would still take more than 10 seconds for it to get back down to the speed of the plane.

Yeah, now you say it, that's a sure thing. I don't know what typicall accelerarion is for a bullet, but even 1,000g for 1/10th of a second would make your numbers accurate, and the chance of "blow-back" essentially zero.

But I also wonder; wha would be the aerodynamic effects of putting a hoolow metal tube (a gun barrel) on the front of an aircraft going Mach3+ with the open end facing forward? Any effect, or would the turbulent buffer zone form an envelope that prevetns direct interaction between the tube and the on-rushing headwind? Probably the latter, yes? BTW; did the Blackbird use Petot tubes?

 

[Cyrus]

But I also wonder; wha would be the aerodynamic effects of putting a hoolow metal tube (a gun barrel) on the front of an aircraft going Mach3+ with the open end facing forward? Any effect, or would the turbulent buffer zone form an envelope that prevetns direct interaction between the tube and the on-rushing headwind? Probably the latter, yes? BTW; did the Blackbird use Petot tubes?

It would melt.

The pitot tube is on the nose.

In fact, the tires were made with aluminum powder and filled with nitrogen or they would heat up and explode inside the wheel wells in supersonic flight.

 

[russ_watters]

Not true. Once out of the barrel, the bullet feels no pressure behind it. It is coasting, and only feels the pressure in front of it.

You misunderstand. I mean that before it leaves the gun, it will feel vastly more pressure behind it than it feels in front of it after leaving the gun. The point being that the acceleration will be much smaller and the possibility of deformation is much smaller.

Running some quick numbers, I'd say the peak acceleration of a rifle bullet would have to be on the order of 10,000 g's in order to make it up to, say, 2,000 fpm in the span of a 3' rifle barrel. A rifle bullet is essentially explosively accelerated while an airplane is using conventional engines (so the force of drag can't be quite so spectacular as people were initially thinking here).

 

[Cyrus]

Who cares about shooting a bullet?

Fire a friggin' missile!

http://roadrunnersinternationale.com/ursini.html

http://upload.wikimedia.org/wikipedia/commons/b/b4/AIM-47.jpg