Bullet physics question

[tjg]

When the bullet leaves the bore, it's hot from powder charge and bore friction. It travels through the air at a high speed.

So, is it traveling fast enough for the air friction to keep the bullet hot, or does the air dissipate the heat and cool the bullet?

[russ69]

It gets hot from air friction.

[TimRB]

The times I have had occasion to handle just-fired bullets, they were quite hot.

Tim

[09rubicon]

If its a rifle round it is possible that it can be traveling upwards of two times the speed of sound. It is getting hotter as it flies. 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.

[border.bandito]

When the bullet leaves the bore, it's hot from powder charge and bore friction. It travels through the air at a high speed.

So, is it traveling fast enough for the air friction to keep the bullet hot, or does the air dissipate the heat and cool the bullet?

good question! you should test that out...have someone shoot you in the buttocks and report back if the bullet was hot or cold.

[krwada]

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

[RobertMW]

Ok, some quick calculations gave me this.

A .308 Winchester 180 gr. round will be dumping energy at a rate of about 5000 watts between 0 and 100 yards. If the copper jacketed bullet left the gun at 90* C (probably a low estimate so I rounded my answer up by quite a bit), the bullet will be dumping 10 Watts of thermal energy into the surrounding air.

At that rate, if more than 0.2% of the energy dumped by the bullet (I'm guessing most goes into moving the air around the bullet, not into heat via friction) is transferred into heat, the bullet warms up.

I am willing to guess that if I did a detailed study, I would get roughly the same answer. But this only took 1/4 piece of paper. I do not want to write a program that will calculate the actual temperature the bullet will reach during its flight

[russ69]

...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.

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.

[stix213]

I wonder if the answer is different for a subsonic round like a .45ACP.

[unusedusername]

If you pick up a just fired bullet you will find that a pistol bullet will be quite warm, while many rifle bullets will be hot enough to cause a burn.

As far as if the temperature is higher or lower then when the bullet leaves the muzzle, I'm not sure.

[MisplacedTexan]

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.

From Wikapedia: (and before you say it - yeah it's user supported it could be wrong - just pointing out the information from an oft quoted source - rightly or wrongly)
The high temperatures generated during flight required special design and operating techniques. For example, major portions of the skin of the inboard wings were corrugated, not smooth. (Aerodynamicists initially opposed the concept and accused the design engineers of trying to make a Mach 3 variant of the 1920s-era Ford Trimotor, known for its corrugated aluminum skin.[26]) The heat of flight would have caused a smooth skin to split or curl, but the corrugated skin could expand vertically and horizontally. The corrugation also increased longitudinal strength. Similarly, the fuselage panels were manufactured to fit only loosely on the ground. Proper alignment was achieved only when the airframe heated up and expanded several inches. Because of this, and the lack of a fuel sealing system that could handle the thermal expansion of the airframe at extreme temperatures, the aircraft would leak JP-7 jet fuel on the runway. At the beginning of each mission, the aircraft would make a short sprint after takeoff to warm up the airframe, then refuel before heading off to its destination.

[krwada]

High speed FLIR

Someone took a high speed infrared camera and did a study on this.

You can see the results here

[russ69]

From Wikapedia:
The high temperatures generated during flight required special design and operating techniques. For example, major portions of the skin of the inboard wings were corrugated, not smooth. (Aerodynamicists initially opposed the concept and accused the design engineers of trying to make a Mach 3 variant of the 1920s-era Ford Trimotor, known for its corrugated aluminum skin.[26]) The heat of flight would have caused a smooth skin to split or curl, but the corrugated skin could expand vertically and horizontally. The corrugation also increased longitudinal strength.

True.

Similarly, the fuselage panels were manufactured to fit only loosely on the ground. Proper alignment was achieved only when the airframe heated up and expanded several inches.

False. Panels are fastened at the edge with many fasteners, they had a normal manufacturing gap but just so you could assemble them, no special design for different expansions of panel and airframe.

Because of this, and the lack of a fuel sealing system that could handle the thermal expansion of the airframe at extreme temperatures, the aircraft would leak JP-7 jet fuel on the runway.

It always leaked because the bays were not sealed very well.

At the beginning of each mission, the aircraft would make a short sprint after takeoff to warm up the airframe, then refuel before heading off to its destination.

They take off empty (relatively) because loading all that fuel on the ground just eats up more fuel before you top up the tanks during air refueling.

[Manolito]

Russ69

Than You for what you worked on and accomplished.

Did you perform your work in Lancaster CA?

[highpower790]

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.

Go see the Blackbird in Omaha.It sits on a single point,with a walkway spiral around it so all can see.