Gun Velocity

[pktaske]

Having absolutely no knowledge of the subject matter, I wanted to pose a question to the community: How is a gun's velocity achieved? I say this in relation to how does one 75mm from the next gain significant ft/s improvements?

From a totally ignorant point of view, I can't seem to figure how the length or specific construction of the tube has much of an impact aside from accuracy. It would seem that the amount of propellent (explosive) in the shell would be the main determinant of how fast it travels. Is it that the engineering of the tube itself dictates how 'explosive' a shell it can handle? How much does ordinance have to do with it? Is it the construction of the gun as a whole that provides greater compression and inertia?

What is it that I know I'm missing?

[pktaske]

And what the hell is a Junior Member...?!?!?!

[aka_tom_w]

You have made only 6 posts to the forum so far, until you have made at least 30 posts you will be a "junior member"

I think some others here are more qualified to answer your first question than myself.

-tom w

[Mannheim Tanker]

Are you suffering from "member" envy? Actually, you gain "member" status after 30 posts. Fewer than 30 posts qualify you as a Junior member. So much for that ?

I'll leave the velocity question to the true grogs!

[aka_tom_w]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Mannheim Tanker:

Are you suffering from "member" envy? Actually, you gain "member" status after 30 posts. Fewer than 30 posts qualify you as a Junior member. So much for that ?

I'll leave the velocity question to the true grogs!<HR></BLOCKQUOTE>

Hey MT!

We need to play TCP/IP head to head do we not?

now that v1.1 is out we have no excuses other than the inevitable "I don't have any time"

Are we still on....?

It will take a about a week for me to clear about 4-5 hours for a good sit down head to head match with you

-tom w

[Mannheim Tanker]

Hey Tom! I nearly forgot about that. Yeah, drop me an email and we'll set something up. I think I'm free on Sunday (Saturday is DEFINITELY out for me - the Mrs. has the day off and I'd be mighty unpopular if I spent all day whupping your butt) .

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"Oooh, tough crowd. A real bunch of nihilists.

Let them eat chads..." - Lawyer (who else!?)

[aka_tom_w]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Mannheim Tanker:

Hey Tom! I nearly forgot about that. Yeah, drop me an email and we'll set something up. I think I'm free on Sunday (Saturday is DEFINITELY out for me - the Mrs. has the day off and I'd be mighty unpopular if I spent all day whupping your butt) .

<HR></BLOCKQUOTE>

ok

I have not forgotten but I'm a right off for this weekend.

But I'm still looking forward to the match up.

LEts see if we can get a custom build fair and balanced scenario with Railroad tracks, towns rivers and bridges build for us.

What side do you want?

-tom w

[Mannheim Tanker]

I'll take either side, but I'm always happy to play with some Krupp steel. Even the cheap stuff.

[:USERNAME:]

The propellant can build up pressure for a longer time in a long barrel. This pressure accelerates the projectile to a higher velocity. Having alot of propellant in a short tube weapon will just make a big ineffective flash. Its propellant not explosive btw.

Too long a barrel and not enough propellant is a waste also.

[tss]

pktaske wrote:

From a totally ignorant point of view, I can't seem to figure how the length or specific construction of the tube has much of an impact aside from accuracy.

And back to answer the original question.

When the propellant charge explodes, the resulting expanding gasses push the shot along the tube, accelerating it. Initially the gasses can go only in the barrel but when the shot leaves the muzzle, the gasses spread to all directions. At that point they don't give any more energy to the shot.

So, making the barrel longer lenghtens the time that the gasses accelerate the shot.

However, if the barrel is too long, the drag of the shot overcomes the propelling forces at some point, and it starts to loose velocity even before leaving the barrel. So increasing the barrel length beyond a certain cutoff point is a waste of resources.

- Tommi

[aka_tom_w]

<BLOCKQUOTE>quote:</font><HR>Originally posted by Mannheim Tanker:

I'll take either side, but I'm always happy to play with some Krupp steel. Even the cheap stuff.<HR></BLOCKQUOTE>

OK

I'm happy with the Allies

now all we need is a Scenario

I trust Rune, maybe he has a new one for us?

IS one of Rune's custom scenarios good for you?

Like a New one that we have both NEVER seen before ok?

-tom w

[Mannheim Tanker]

Hey Tom, how about if we finalize this over email, rather than hijacking this thread Thanks!

[jasoncawley@ameritech.net]

The velocity of any particular type of round out of a particular gun varies with a number of factors. It may help to go through the actual physical process to understand these. The round is loaded into the breech of the gun, which is then closed and sealed with a rotating lock, like on a safe.

Except for larger artillery pieces, the ammo comes in one piece with powder already inside the shell - larger artillery (155mm and up, generally) varies the powder charge with seperately loading powder, depending on the range. Intermediate artillery pieces like 105mm sometimes have the powder charge varied by the loaders (at the same time they add the fuse to the shell), by literally opening the round and removing some of the full charge if necessary. Those exceptions are basically only important for indirect fire, but it gives you an idea of the factors the long range arty has to play with, in order to get a fire mission to the right grid square on the map, 5-10 miles away.

After the shell is in the tube and the breech is closed, the round is primed to be ready to fire (on smaller cannons this isn't necessary). Small shells will be fired by a firing pin directly, just like a bullet out of a rifle - the impact of the firing pin sets off a detonator cap at the back of the round. Larger cannons use a priming round, like a small shotgun shell, inserted into a hole in the breech (itself then locked off) which is fired by the pin (itself pulled by a trigger or lanyard). The explosion from the primer hits the back of the shell or powder to set it off.

By one of the mechanisms mentioned, the powder charge at the back of the shell is ignited. It then burns, chemically reacting. The burning ignites the portions of the powder charge adjacent to the existing fire, and rapidly (microseconds) spreads forward through the rest of the charge. As this burning is taking place, the material of the powder charge and the air packed in with it is tranformed into a hot gas, vaporized by the heat of the exploding powder.

Ahead of the powder charge sits the payload of the shell, or the shell proper. This is the part being thrown at the enemy. It is meant to rest snugly in the tube it sits in, and sometimes its width will change very slightly in the course of the shot. Normally, this is a slight expansion of the round which causes it to "grip" the grooves cut in the side of the tube, the "rifling", which will impart spin to the round as it moves down the tube. Some special rounds narrowed as they went down the tube, by collapsible fins on the side of the round getting squeezed down to a narrower exit opening than the breech at the back of the gun, a technique meant to raise the final velocity imparted to the shell.

What is pushing the shell down the tube is the expansion of the hot gas behind. Just like the force of a car's piston, the underlying phenomenon is a hot gas creating a strong pressure on one side of a moving part, stronger than the pressure on the other side. The difference creates the force that drives the shell forward. That is the thermodynamic description. At the molecular physics level, what is happening is that the molecules of the gas in the firing chamber are moving extremely rapidly - that is the high temperature. They hit the back of the shell and push it forward, rebounding off of all the other walls of the chamber and breech-block and hitting the back of the shell over and over again.

The shell is the only wall that is moving easily, but the barrel as a whole will also recoil from the "equal and opposite" reaction, created by these many little collisions. The barrel is very heavy and only moves back a few feet, with a recoil mechanism transfering much of the recoil force to the entire tank or gun, which is held in place by its weight, spiked-in trails for field guns, etc.

As the gas expands in the chamber behind the shell, it loses pressure. The same number of molecules are flying about, but they have more space to roam so they hit the back of the shell less often each. The powder charge of the round is designed to burn progressively up the charge as the expansion continues (it starts right away), to keep up the pressure behind the shell as long as possible. See, more space behind the shell is lowering the pressure. But more powder having burned is raising the temperature to match. The maximum overpressure behind the shell will occur while it is still in the tube, some ways down the tube, when the powder is all burned.

As soon as the shell leaves the muzzle, other stuff starts mattering. The pressure of the gas behind the shell drops enourmously, as now it can "vent" around the shell - since each little molecule in the gas is moving much, much faster than the round itself. Some guns, especially larger ones, have a "muzzle brake" just before the exit of the barrel, to channel this venting to the sides. This reduces the recoil from the whole shot.

Oops, one other thing to understand while the round is still going down the barrel. For many antitank rounds, the round goes supersonic inside the tube. That means that the air in front of it is not pushed out of the way by a preceeding wave of force from the shell through air, but is instead being pushed by the shell itself. (Sonic, the speed of sound, just means the speed waves of force can travel in the air by air pushing on air).

So when the round finally leaves the tube, it is often going several times the speed of sound, and is therefore preceeded by a bow shockwave just like that which creates sonic booms from jet planes (though smaller). As the overpressure behind the shell drops, it instead starts to decelerate as it meets air resistence, plowing through the air - the pressure in front of the shell is now much higher than the pressure behind it, so it begins to slow down just about the instant it leaves the tube. The "muzzle velocity" is therefore (usually) the peak velocity of the round.

Some special ammo types also change again on clearing the barrel. These are rare in WW II, used by Brit TD units in particular, but much more common today as the kinks have been worked out of the idea. These are called "sabot" rounds, APDS or varieties thereof. A sabot round sheds the outside of the round just after leaving the barrel, leaving a much thinner, long, arrow-like core of denser metal as a penetrator. Like shooting a metal arrow at somebody. The narrower cross section after leaving the barrel means less air resistence in front of the round - while the full width of the round while still in the barrel means full "push" from the pressure of the gas before leaving the barrel. Early sabot rounds had problems with the aim, as the sabot exterior falling away did not always happen uniformly on all sides, which could deflect the direction of the remaining "arrow" and thus throw off the aim. But APDS is rare in WW II anyway.

That is the basic physical process. The length of the barrel enters because how long the pushing is going on, before the slowing down after clearly the barrel, will certainly change with different barrel lengths. That does not mean, however, that the longer the barrel the higher the velocity without limit, as though a 1 mile barrel would shoot super-fast. There is a point down the barrel where maximum overpressure is achieved, as explained before. The shell is still speeding up, but not by much after that. If the barrel is too long, it can even slow down as the friction with the sides of the barrel outweighs the remaining overpressure behind in the expanded space. So getting the right length was and is an engineering problem, not a simple more-is-better affair.

But generally, the guns designed for longer lengths are higher velocity guns. They may be using larger charges behind the shells (diameter the same, but length different e.g.) - they may change the composition of the powder to time how it burns down the tube better, to meet a designed length - the barrel itself, or the breech block to close it, or the recoil mechanism to withstand the discharge, may be different and thus allow a heavier or a lighter powder charge for the same diameter of shell, without wrecking the gun or dismounting it. Tighter rifling can impart more spin to stabilise the round in flight, and it can "fly" better by wobbling less, like a tight spiral in football, and thus keep more of its muzzle velocity farther into its range.

There are enough factors that the ballistics question is basically an empirical one. You have some gun, and some round, and you see what they in fact achieve together. In the design process they are certainly doing more than that, and trying to find the right length, rifling, powder charge, and other gun characteristics to get the best performance. You only know the result of their efforts by testing, but the general principles are enough to see that longer, physically heavier barrels of the same diameter will have been designed to achieve, and usually will achieve, much higher muzzle velocities than shorter, physically lighter barrels.

Indeed, to a gun designer the overall gun weight is more of a constraint than the caliber. He can pick between a larger diameter and greater length and other features that equally increase the weight, and allow higher velocities to be achieved. That is a trade off, with both heavier options able to provide increased hitting power if the overall weight can go up. The designer picks the one he thinks will work better in practice. So do not be surprised that a long 75 on a 40+ ton late-war tank is a more serious weapon than a short, light 75 on a 20 ton early-war tank. The caliber may be the only thing they have in common.

I hope this is helpful.

[Theron]

<BLOCKQUOTE>quote:</font><HR>Originally posted by pktaske:

Having absolutely no knowledge of the subject matter, I wanted to pose a question to the community: How is a gun's velocity achieved? I say this in relation to how does one 75mm from the next gain significant ft/s improvements?

From a totally ignorant point of view, I can't seem to figure how the length or specific construction of the tube has much of an impact aside from accuracy. It would seem that the amount of propellent (explosive) in the shell would be the main determinant of how fast it travels. Is it that the engineering of the tube itself dictates how 'explosive' a shell it can handle? How much does ordinance have to do with it? Is it the construction of the gun as a whole that provides greater compression and inertia?

What is it that I know I'm missing? <HR></BLOCKQUOTE>

I don't know this by reading it, but from a physics point of view a longer barrel on the gun allows more chemical (expanding gasses from the gun powder being ignited) energy to be imparted on the bullet. For example if one places a bullet in a vice and whacks the primer with a hammer the bullet won't go nearly as far because the gasses will rapidly escape. Don't do this experiment since the brass casing of the cartridge is very likely to explode and become shrapnel. Making a longer and longer barrel for the gun will lengthen the time the expanding gasses can push the bullet. Of course there are limits to how long a barrel should be made. It can become too cumbersome for a tank or person to carry. Eventually a long barrel will slow the bullet down because of friction. Therefore in general a longer barrel increases the rounds velocity in general, but there are several factors that limit the desired length.

In terms of your other questions the amount of explosive does influence how much kinetic energy can be imparted to the round. The size of the round itself also makes a difference. One can generally shot a small round at a high velocity or a large round at a lower velocity with the same amount of powder. Both rounds will get the same amount of kinetic energy from the gun powder, but the smaller round has less mass therefore will have a greater velocity. I how that this helps.

Theron

[tss]

I just found an online picture showing a gun with definitely too long barrel.

It's the Queen's Pocket Pistol, from 16th century. Some enterprising king or queen (can't remember right now who, and my sources are at home) tried to shoot over the English Channel with it. Well, the shot landed several hundreds of meters from the muzzle. The same range could have been achieved with 1/3 barrel length.

Oh, and the actual link: http://www.riv.co.nz/rnza/shrapnel/qepp/qepp1.htm

- Tommi

[This message has been edited by tss (edited 01-10-2001).]

[jgdpzr]

Jason,

Impressive stuff. Very detailed and well- structured explanation. Methinks you may be an engineer of one sort or another? Thanks for the time to explain so thoroughly!

[Gremlin]

Jason, regarding breeches, from my reading, they weren't all screw breeches (typically used for bag charge guns), as sliding block breeches were also common. The screw breech sealed the barrel with an obturator pad, which expanded on firing from the hot gases, whereas the block breech guns used the rounds' propellant cartridges to seal the back of the barrel.

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To refrain from imitation is the best revenge. --Marcus Aurelius

[Olle Petersson]

Jason, what can I say more than that this is the best explanation I've ever seen on inner ballistics.

You didn't miss out on any single aspect!

<BLOCKQUOTE>quote:</font><HR>Originally posted by jasoncawley@ameritech.net:

1) If the barrel is too long, it can even slow down as the friction with the sides of the barrel outweighs the remaining overpressure behind in the expanded space. So getting the right length was and is an engineering problem, not a simple more-is-better affair.

2) Indeed, to a gun designer the overall gun weight is more of a constraint than the caliber.<HR></BLOCKQUOTE>1) I think it was in WW1 that the Germans built a gun with about 100m barrel length to shoot into Paris. This was fixed in the ground and had several chambers for propellant along the tube. The different propellant charges were supposed to ignite in sequence as the projectile passed by, thereby keeping up the pressure.

2) This is very true. One way to keep the weight down is to get rid of the heavy breach and recoil compensators, and instead let some of the propellant gasses expand rearwards to compensate for the recoil. This is the principle of "recoilless" or backblast weapons. They have a neat muzzle velocity, although not the very top. The weight is much lower than for a normal gun with same ballistics though.

Cheers

Olle

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Srategy is the art of avoiding a fair fight...

[Dan Robertson]

The multichambered gun was constructed by the Germans in WWII to shell London because by the late war German planes had a life span of minutes in British airspace.

It wouldn't have worked though because the gass pressure would have chosen the path of least resistance which is to compress the air down the tube rather than move the shell up the barrel.

As for the balistics, that was quite a good explaination.

With the WWII projectiles the objective was to ahieve the highest energy per area to achieve the maxium penetration.

So gun power to penetrate armour is proportional to it's volume and pressure, and inversly proportional to the square of it's diameter.

Thus the engineer will try and produce the best combination of bore and lenght to produce the required power. However most usually the bore of the weapon is set, by president or the vehicle it must fit it.

In which case the engineer tries to get the highest velocity possible out of the weapon.

This can be done by making the weapon longer, and giving it more power. Or by increasing the pressure and using the propellant more efficiently to produce a steadier push up the barrel.

A good example of the is the British 17pdr.

It was a 76.2mm L 55 gun (barrel is 76.2 X 55 long) and had similar power to the German 75mm L70, and vastly superior power to the American 76.2mm L53.

Gun power itself it slightly more complicated to measure, than it would seem.

The Kinetic energy imparted to a round is not the true measure of this. This is because the gun must also push gass up the barrel at speed too.

Thus it is best to standardise weapons performance by comparing what weight of shot a weapon would throw at a given speed.

I choose 1000 meters persecond.

((Muzzle velocity/1000)^0.45)*mass of round

You can use this to predict what will happen if you fire a differnet shell from the same gun. By finding the power of the intial gun and then working back throw the formula with a different mass.