Artillery recoil for a 76mm tank gun

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this isn't a homework problem - and i am not a student!

i have a 76mm tank gun (it's legal and registered w/ the feds in case you are wondering) and i am interested in firing blanks - but here is the problem: i want the gun to recoil like it does when firing live rounds.

here is how the gun works: there is a long barrel (about 10' or so) and it is mounted on a set of shocks (like car shocks) that are on either side of it (see picture below.)

------ (oil/spring recoil piston)
-------------------------------------- (barrel)
------ (oil/spring recoil piston)

^that is a picture of the gun and shocks.

ok, so when a real round is fired out of it, the barrel pushes back ("recoil") and the shocks compress as the bullet comes out. in case anyone cares, the projectile is about 11 lbs or so and it is fired with about 3lbs of gun powder (actually, it's referred to as propellant, but it's technically just made of smokeless powder, just in a different format to make it burn slower.)

so - back to the problem. if you don't have a bullet, but rather just an empty shell with gunpowder in it and you fire the gun, then all the 'energy' or force or whatever it's called just goes straight out of the barrel, and there is no recoil. when firing blanks, one typically uses about 1/2 lb of gun powder.

my question is this: if i place a 'cap' over the front of the barrel with a very small hole in it, will this make it so a blank round will cause the gun to recoil on the shocks, or just create lots of pressure and blow out the cap? obviously i'd have to carefully experiment with the size of the hole in the cap - but principally, would this work?

i'm not excited about experimenting with this thing without at least having some advisement on how it should theoretically operate under this 'blank' setup.

it seems like anyone with a good grasp of elementary newtonian physics would be able to answer this (clearly, i don't.)

note, i did initially post this in the homework section because i thought it was too elementary, and a nice person posted some guidance, but i'm looking for more detail. any insights would be appreciated.
 
I'm no expert in physics, but unless my understanding is incorrect, you're going to need to shoot something out of the hole if you want any recoil.

Recoil occurs due to conservation of momentum. If P = 0 kg m/s initially, P = 0 kg m/s in the end, as well. Ergo, after you fire the gun but nothing gets shot out, the gun remains stationary. So even with a small hole, the only thing that would be escaping is the 1/2 pound of gunpowder / smoke / hot gas, and this would - even if it's moving just as fast as the shell would have - cause a recoil equal to (1/2)/11 of the original recoil, or around 5%.

If the cap was blown off, you'd get more recoil. I don't see how the hole is going to help...
 
I don't see how the hole is going to help...
on rifles, the blank firing adapters are basically caps with small holes in them. somehow this small hole creates enough pressure for the rifle to cycle the next round and still function, even without bullets.

also, if you shake up a 2 liter bottle of soda, i think if you had an extremely large hole compared with a relatively small one, it would somehow effect the 'thrust' of the explosion. obviously it cant be too small though.

anyway, that's kind of why i'm posting on a physics forum - i'm looking for a physics guy (yeah, i'm a software engineer myself) to get me clued in here.

it'd be great if i had an equation to mess with on this. the backend of the tank weighs 18 tons, so my thinking is if the bullet weight 18 tons also, the recoil force and the bullet would eb the same, right? so if the bullet weighs 11 lbs, the recoil is a lot less -- or if the "bullet" weighed as much as a tube of air (ie. an empty barrel) then the recoil would be even less.

and the next part is about making the exit hole smaller, and if that will somehow increase the force on the back of the gun, which will create more recoil. :confused:
 

FredGarvin

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If you put a restriction on the end of the barrel, you'll probably blow the barrel apart. Like was mentioned, the recoil comes from the momentum of the projectile.

The blanks in your gun example work because you are using the overpressure in the firearm to actuate the bolt assembly. Obviously, you don't have a bolt in this case.
 
Well, prefetch, I was trying to be tactful. My area of expertise is in scientific computing, so I'm not really so great a novice as I was making out.

What you're saying is not possible. If you want recoil, you are going to need to move something heavy. Another option is to use more fuel as a propellant - this is how rockets work, obviously.

And if you want expert advice, I suggest you pay for it and not go to forums where you are not qualified to tell good advice from bad.
 

Danger

Gold Member
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You could add a hidden mechanism, such as a hydraulic ram, to 'fake' the recoil. If it were triggered by the firing system, I doubt that anyone would notice the trickery.
 
And if you want expert advice, I suggest you pay for it and not go to forums where you are not qualified to tell good advice from bad.
lol. no need to get touchy! when you said "I'm no expert in physics, but unless my understanding is incorrect..." that sounded like you weren't super confident. sorry for taking you at your word!

i'd happily pay for good advice, i just don't know where they sell it. if google answers was still open, i'd put a $100 bounty on this. i would think an expert physicist could figure this problem out in less than an hour, and $100 an hour is pretty good money.

but, like i said, i don't know where they sell such advice, which is why i posted it here.

fredgarvin - wouldn't the shocks act like a bolt in this case? the bolt is simply a 'weak' wall to the pressure chamber because it has a spring behind it, right?
 
It could be possible to fire some "safe" dummy mass. I'd imagine water would instantly vaporize, but the mass would still be there.
 
That's actually a good idea.

I wonder if sand wouldn't be enough? I guess that could be bad for the gun barrel... If it wouldn't be too messy, it would probably dissipate fairly quickly, while having high mass.

Water should be good too, in fact, probably preferable.
 

LURCH

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There was a Russian design for a recoilless rifle that used palstic flakes forced out the back as a counterweight for the bullet going out the front. Tha might work here.

I don't think you want to put sand or water in your barrel, unless it's in a container. Even then, you have to worry about the container and the counterweight exiting at high velocities. If they hit anything, they can do serious damage.

It should be pointed out that the gasses exiting the barrel do have some force (what rocket scientists call "specific impulse"). This exhaust of gasses is much like the gasses exiting the back of a rocket, and do provide some propellent force. It might be simpler to soften up your recoil pistons, so they don't provide as much resistance. Then, you can let the escaping gasses propell the gun backward. After all, you won't need the recoil pistons at full strength unless you fire real proectiles.
 
ah. those are good ideas. it sounds sort of silly, but i could put a couple of small, 3" diameter water balloons in the shell. maybe i could get a couple of lbs worth of water in there. small bits of rubber coming out the front end would be relatively harmless at a safe distance.

now - i'm still stuck on this "propulsion nozzle" thing (ie. making the 3" hole at the end of the barrel smaller through a secured cap.)

also, just fyi, we do shoot real projectiles out of this gun also (yes, this is legal and we are authorized by the feds) so we cant do anything permanent to the gun to facilitate the blank recoil.

first off - is it true that the hole at the end has an effect on the recoil?

if so, is there a relatively simple way to calculate the optimal hole size?

btw - i'm totally serious about $100 - i know everyone is helping out here pro bono, and i really appreciate it (:smile:) but if someone wants to actually do some predictive math for me related to this project (i'll give you all the exact weights and measurements) i'm happy to send them a $100 amazon gift certificate or paypal or whatever amount they think makes sense. you can shoot me an email prefetch@gmail.com if you are interested!
 
Well, you already have my opinion. For my money, a cap with a hole on the end isn't going to do anything for you.

If you're allowed to actually fire this thing, why not just invest a day making a mound of dirt to shoot the thing into? Or shoot it into the ocean? How many shots are we talking? There's no better way to simulate something than actually doing it.
 
What's the muzzle velocity?

Say you have, when you actually fir the thing, 5 kg of mass moving at 100 m/s. Then this is 500 kg m/s of momentum.

The gun will experience an equal and opposite impulse. Assume the shocks stop the gun's recoil in 0.5 seconds. Then the force exerted by the shocks on the gun is, on the average, 1000 N, or 200 lbs. If the shocks stop it in 0.1 seconds, the average force would be 5 times as great, or 1000 lbs.

It may be more realistic to talk about how far the shocks work over. Say they stop the gun in 10 centimeters. Then, for a gun weighing 100 kg, the impulse gets the gun moving to 5 m/s, and that means that F = 12500 N ~ 2500 lbs.

Don't know if you find any of this interesting.
 
csprof - yes, that's interesting to me. i've drawn a couple of pictures with what i think are the relevant variables included. it's a small pdf you can download here: http://utahmvc.org/blankfiremodel.pdf" [Broken]

firing the live gun with live shells is no problem - that's what it was made for, but conducting experiments with blanks is a little scary without actually having some kind of model that i believe in to guide me, so hopefully i can get this figured out.

anyone who checks out the model in the link above, if there is data missing that you think i might know, i'll see if i can get a hold of it.
 
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You would just increase pressure in the barrel if you caped it with a small hole.
I think there is a way of solving your problem. The recoil system is like a shock absorber.
There is an orifice inside of the recoil system that allows only so much fluid to pass through it. If you could replace or modify the orifice size to make it bigger: that would make it recoil much easier. This would weaken your recoil system but might work for you.
 

Q_Goest

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The intent of the orifice is to turn this into a rocket, so to make it more effective you'd need a convergent/divergent nozzle. The charge might even be made from a slower burning propellant so the pressure builds up to some maximum safe value and holds it there for half a second -give or take.

Perhaps with the right nozzle and the right propellant, sufficient force (thrust) could be produced to get the cannon to recoil properly. The problem with all that is in the analysis. The rate of burn for the propellant is one major hurdle and the second is to accurately model the recoil mechanism since this would recoil more slowly so any hydraulic resistance will be less (that's a benefit actually).
 
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The orifice I was talking about is located in the recoil system.
The recoil system uses a hydraulic fluid that passes through a hole that is metered to let a measured amount through over time thus absorbing the shock of the recoil.
 

Q_Goest

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The orifice I was talking about is located in the recoil system.
The recoil system uses a hydraulic fluid that passes through a hole that is metered to let a measured amount through over time thus absorbing the shock of the recoil.
Yes, I was also thinking perhaps this orifice could be removed, the port plugged, and then an external line routed with 2 separate orifices which can be valved off. That way it's as easy as switching the valve to determine which orifice to use.
 

LURCH

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Yes, that's basically what I was talking about, too. Essentially, you could disconnect the gun from its normal recoil system and connected to a much weaker system when firing Blanks. This could be as simple as the position of a single selector switch. You would want to include some sort of failsafe to ensure that the gun is not using the secondary recoil mechanism when firing live rounds.

I think this would work a lot better than the "cap and nozzle" arrangement. When this arrangement is used in rifles firing Blanks, the "kick" of the rifle is not significantly increased. This system only increases pressure inside the barrel so that the action of the weapon can function properly, loading the next round and keeping the weapon a semiautomatic (or automatic, as the case may be). I'm afraid that if you attempt to use this method with a tank gun, you will get the same results as firing the cannon with the barrel plugged. Hardly the desired result, although it would make for an impressive show (your last show).
 
Yes, that's basically what I was talking about, too. Essentially, you could disconnect the gun from its normal recoil system and connected to a much weaker system when firing Blanks. This could be as simple as the position of a single selector switch. You would want to include some sort of failsafe to ensure that the gun is not using the secondary recoil mechanism when firing live rounds.
weakening the recoil mechanism might be something i could look into, but it wouldn't be easy. the recoil mechanism is made of armored steel and weighs several hundred lbs. -- any modification would require major machining and surgery.

i may experiment with draining the oil from it, and testing to see the weight required to pull it back. maybe with a come-along cable/crank and some kind of a fish scale to measure the weight.

what i'm gathering from all these posts is that calculating actual data from this is very difficult, which sort of surprises me. i guess i'll have to go the scary empirical route!
 

Q_Goest

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what i'm gathering from all these posts is that calculating actual data from this is very difficult, which sort of surprises me. i guess i'll have to go the scary empirical route!
It's not so much that calculating all this isn't possible, but calculating propellant burn rates is a specialized area. I worked at Picitinny Arsenal in NJ for a summer and I know they have the expertise to calculate the reaction of various propellants, but most people don't have the data. You might see if there's someone you can dig up that might have that expertise and of course, the data needed to do the analysis. Maybe contact http://www.pica.army.mil/PicatinnyPublic/index.asp" [Broken]?
 
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Another thought on this is that if you could drain the oil and replace it with thinner oil it would let the gun recoil easer. I don’t know what this involves and it may not be good for the recoil system.
 
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There are two relevant equations here; momentum conservation and energy conservation. You will get the most recoil if the projectile were heavier than your 76-mm gun, the least if the projectile were very light. If you had a 500-mm gun, and shot 19.6-inch diameter pumpkin projectiles, you will get a lot of recoil, and perhaps a few hundred meters (or more?) of range.

Your explosive charge represents about 5.4 megajoules at 4000 joules per gram (typical for explosives). If a 19.6-inch diameter pumpkin weighed 65,400 grams (144 pounds), its maximum velocity would be about 400 meters per sec if the gun were infnitely heavy, or about 287 meters per sec if the gun weighed as much as the pumpkin. Still a dangerous projectile.
 
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