# Ballistics Question?

Discussion in 'Rifles, Bullets, Barrels & Ballistics' started by scsims, Apr 23, 2008.

1. ### scsimsWell-Known Member

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This is an odd question but I have always wondered when a bullet is fired from a fast moving vehicle such as a fighter jet, what happens?

If the jet is traveling nearly as fast a bullet, then fires a bullet the bullets would then be traveling twicw as fast as one would from a stationary gun.

Does the construction of the bullet need to different to handle the friction? How does it differ ballictically?

2. ### WildcatBWell-Known Member

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I don't know about the bullet construction, but the speeds will add, so if the plane is going the same speed as the bullet, and the bullet is shot in the same direction as the plane then the overall bullet speed will be twice as fast.

3. ### Long Time Long RangerWell-Known Member

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I'm a rocket scientist and not a jet fighter scientist so I don't know at what speeds they fire bullets out of jets. A bullet shot at 3000 fps is about 2045 mph which is faster than our fighter jets. Back in ww2 the prop fighters relied on machine gun fire instead of missiles but they didn't go very fast. Even at 500 mph which adds about 733 fps on the bullet for a total of 3733 fps isn't a problem. Even in Vietnam with the F-4 phantoms they were not as fast as todays jets but certainly they used a lot of machine gun fire. I don't know at what speeds in a dogfight they were accurate, but at 1000 mph the combined bullet velocity is up to 4466 fps. I think todays jets at 1500 mph primarily use missiles.

With missiles at high speed, heat off the nose cone represents a big problem because we must design it around all the junk we have to get inside the nosecone. We can't just build a sleek streamlined nosecone in other words. With a bullet you can, which should keep the heat down to workable levels. But even at that and considering the full metal jackets they shoot I still bet that metal gets a bit soft in the front. Looking at the bullets they use I wonder if that different colored tip where most of the heat would be is a metal much more resistant to heat. What happens to that plastic, nylon tip popular on a lot of bullets when fired at 4000 fps or about 2727 mph? That is pretty close to equal of a 50 caliber shot out of a F-4 phantom.

Good question.

4. ### goodgrouperWell-Known Member

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Oh how I love these types of questions. Sure beats the heck out of "uumm what's the best load for my 30-06". I know I'm not normal but who is?

Ok, here she goes:

This is all relative to the Point of View as to what you "see" but in absolute reality, the same thing happens everytime. Physics teaches that the velocity of the fired round is added to the velocity of the jet. However, since the bullet rapidly decelerates due to the increase in air resistance, several things appear to happen. If you are the pilot in an SR71 Blackbird (which holds the currently disclosed airspeed record for a manned jet) zipping along at 3.2 mach or 2200 mph or 3226.74 feet per second and you fire a bullet (hypothetically of course as the Blackbird was never fitted with guns) at just over that speed, you will see the bullet go out ahead of you for a time and speeds similiar to a slow bb gun then as your plane catches up, the bullet will come right back at you given a no wind situation of course. A bystander on the ground would not see either the bullet or the jet because of their extreme speeds and how much ground they cover in a short time but if he could, he would see the bullet leave the jet at double the speed, then see it slow, then see the jet pass it, and then see the bullet trailing behind the jet until it's velocity couldn't overcome the effects of gravity and if fell from the sky.

Now, how about a gun fired from behind the jet backwards? If the bullet is slightly faster than the velocity of the jet, then it will go away from the jet at the difference. If the bullet was slower than the jet, it would appear to trail the jet from the pilot's point of view as it decreased in velocity from air resistance and got farther and farther from the jet. The opposite vectors either add or subtract and it would depend on where you were at in the scenario as to what you would actually see as an effect.

The speed of the jet would be added to the MUZZLE velocity but because of increased velocity, the air resistance would be greater and quickly begin to decelerate the bullet much quicker than the bullet fired from the stationary. Remember Newton's laws? For every action there is an equal and opposite reaction. A bullet fired at double the normal speed will encounter double the air resistance. Yep, you read that right folks. A 221 fireball shot from a handgun at 2000 fps encounters 1/3 less resistance than a 223 fired at 3000 fps if bullet shapes are same. Or half the resistance of a 220 swift at 4000 fps.
The melting point of lead is 621.5 degrees F. The melting point of copper is 1984 degrees F. An alloyed copper will be higher. The melting point of depleted uranium (which is what a lot of anti-aircraft air to air bullets are made with) is 2069 degrees F. The melting point of plastics and polymers varies greatly depending on their crystalline structures and theromplastic or thermalite properties but it is around 600 degrees F. for some.
A 223 bullet fired at around 3000 fps is heated by gas, friction on bore, and then friction against air to a maximum of 500 to 550 degrees F. with most of the heating occuring from the gas and friction in the bore. Air resistance is a very distant third.

So you can see that none of those commonly used materials will melt from being fired from a gun at normal speeds. However, a lead tipped bullet fired from a supersonic jet WOULD melt itself right off but you won't find any lead tipped ammunition being fed through a gun on a supersonic jet. It is depleted uranium or solid copper which would need nearly four times the temp of melting lead to melt.

Incidentally, the leading edges of the SR71 Blackbird jet under maximum velocity of mach 3.2 soared well over 600 degrees F.. This was at 75,000 feet elevation which is where the top speeds were reached. Lower elevations heated up the plane to even higher temps of around 900 degrees F. because of the higher density of air at lower elevations. Mach 3.2 was never achieved at much lower elevations.

Because of the superheating that occurs in hypersonic flight, the Blackbird was designed to be very "loose fitting" during construction. As it sits on the runway, it leaks oil, fuel, and the hinged parts wobble. But when she's ripping through the atmosphere at normal operating speeds, the parts of the plane swell from the thermal expansion of supersonic flight and everything "self seals". And they built this thing in the SIXTIES!!!!

Last edited: Apr 23, 2008
5. ### P KUNDAWell-Known Member

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A balistic engineer couldn't give a better explanation

However, it doesn't mean that one has to agree with it.

Peter

6. ### roundssWell-Known Member

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Any speed over the speed of sound (Mach 1), which is approximately 1,087 ft/s or 761 mph in air at sea level, is said to be supersonic.

I just thought that was interesting. I never knew what fps the speed of sound was. I had looked that up a few months back when I was curious what mach my bullets were traveling at.

7. ### jwp475Well-Known Member

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For the speed of sound to be 1087 fps the temerature would have to be 32 F. at 70 degrees the speed is 1129 fps and 1142 fps at 80 F

According to this web site with a speed calculator;

Speed of Sound

8. ### goodgrouperWell-Known Member

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As Jwp pointed out, the speed of sound changes with temperature regardless of the medium. Sound actually travels faster in water than air (nitrogen, oxygen mix) and it will be faster in warm air than cold air.

So here's a brain teaser, if the temperature at 12,000 feet is 70 degrees and the temperature at sea level is 70 degrees, what will be the speed of sound at both elevations??

9. ### britzWell-Known Member

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GG, according to JBM calc, the speed of sound is the same at both altitudes and 70 degrees. To be honest, at first I found that hard to believe, but upon reflection, the only difference in altitudes would be the force of gravity causing compression on the molocules (very very little difference). The biggest change (for the elevation that you mentioned) is going to be temp which affects the distance between molocules which has a direct affect on the time it takes for vibrations to travel from one molocule to another. Air has very little mass, so gravity will play a smaller role as long as we are talking only a few thousand feet.

Thats a good question though, 14.7 PSI is standard atmosphere at sea level and it takes how many feet of air to create that much pressure??? how high is the atmosphere anyway??? So actually the higher we go, the slower the speed of sound would be because as the air gets thinner it will be less "packed down". Sound doesn't travel at zero gravity so basically the higher we get, the lower the speed of sound will be with all other factos being equal.

OR AT LEAST I THINK This one's kinda like the saturday night live skit "deep thoughts"

10. ### WAMBOWell-Known Member

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I'll take a crack at it, it would be about the same at both elevations.
Guess I was a little slow on figuring it out, britz beat me to it.

Last edited: Apr 23, 2008
11. ### goodgrouperWell-Known Member

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Well, actually it was a trick question because there is debate among scientist on this very subject. Some literature says there will be no difference between the two elevations because the sound waves are solely influenced by temp alone. But then there is another side that says gases under varying pressures will also contribute to the sum of the velocity in addition to the temperature. But for the purposes of trajectories, the difference in pressure is so minute that it wouldn't show up on any ballistic program that I know of.

12. ### britzWell-Known Member

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Sound has to have matter to travel through. The more dense the matter is, the quicker the sound will "jump" from molocule to molocule. That is why sound travels very quickly and reliably through water for Sonar to work so well for submarine navigation. That is also why you can put your ear to a railroad rail and hear a train from many miles away. This is very interesting since higher temp is caused by compacted molocules as well. Compacted molocules will transmit sound faster. Space is very cold because there is nothing to heat in a vaccume.

I started to type my response to argue with you, but after thinking it through again I can see your side with this one

13. ### esshupWell-Known Member

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The knowledge base of the members here never ceases to amaze me. Maybe that's why I like this place over others that I've participated in.

14. ### wayneborngesserActive Member

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"A bullet fired at double the normal speed will encounter double the air resistance." My understanding was squared resistance when doubling speed, but my memory isn't as good as it used to be. I forgot which law it was as well.