Calculating recoil on 76mm artillery gun

[prefetch]

hey folks. i posted this earlier in the classical physics section, but i think i may have hit a wall over there. I'm thinking engineers might be better suited to this problem.

basically, I'm trying to figure out how to fire a blank round in a 76mm gun and make it recoil like it does when i fire a real round.

i've drafted the basic problem and all the known variables in this document - you can download it here: http://utahmvc.org/blankfiremodel.pdf"

just so you know my background - I'm not a physics guy, or an ME, but I'm a software guy (er, and a 76mm guy. ;-)

any help would be much appreciated, or if you think this is beyond the scope of this kind of forum and you know where i can pay a modest fee for a very small consulting job on this, i'd appreciate a pointer. this is just a personal project for my hobby. (and yes, it's all legal and registered with the ATF.)

 

[xxChrisxx]

Having no real idea about how a 76mm works. Is the 18 tons is fixed in place (not supposed to move when the gun fires) and how is the recoil damped?

EDIT: Just read the original thread, and under the conditions you state I don't think its possible to acieve what you want.

The problem is unless you have the same momentum then you won't have the same recoil. It would be possible to use a larger charge snd 'safe' mass but you arent going to get anything like the recoil you want.

The only reason I say this is impossible (I hate to be defeatist :() is that its stated that it still needs to be able to fire live rounds, so no major alterations can be made, meaning faking the recoil is not a possibility.

 

[prefetch]

i'm not quite ready to admit defeat.. I'm hoping to achieve some recoil - some noticeable fraction of the normal live fire recoil is what I'm going for.

so yes, it seems clear to me that without a modification to the gun mechanism, this won't be possible.

the two possible non-permanent modifications i can think of:

weaken the shocks: the shocks are filled with mineral oil and a very heavy spring. draining the oil would weaken the shocks.

turn the barrel into a rocket that goes back into the tank: this is the 'nozzle' idea that so far i haven't gotten any real information about how it works or if it would do anything. in the classical physics thread i got a couple of "that won't work" comments, but no reason was given, and it doesn't make any sense to me why it wouldn't work.

if i put my thumb in front of a hose and partially block the exit hole, it seems like the water squirts out harder - wouldn't changing the exit hole of the gun somehow create more pressure or force pushing back into the tank, creating some recoil?

 

[xxChrisxx]

Altering the shock aborber properties is a good idea, simply draining the shock absorbers will not weaken them as the spring is what absorbes the momentum under compression. Without the oil to dampen the spring it will oscillate. However a weaker spring would do the job nicely.

I would have suggested that but I would have thought they would be sealed units. If they arent then this is by far the more viable option.

I don't have enough knowledge to say if the second idea will work or not.

Overpressure in the barrel is more likely cause the barrel to fail than cause recoil. Recoil is a conservation of momentum acting in 1 direction (simplified), increased pressure in the barrel acts radially outward.

 

[Topher925]

You're going to need to give us the length of the barrel in order to figure this out.

By doing some just some rough hand calcs, your going to need create a force of ~1.3MN against the barrel to get the recoil you want. This assumes no modification to the dampers and a 2m long barrel. If you greatly decrease the damping system of the barrel, increase the amount of propellant, and use the right kind of nozzle it might work. The calculations to figure out just how well it would work a rather strait forward but rather time consuming. I'll try to do a little bit more if I get some time.

I would look at building a blank that contains a substance that vaporized with relatively low temperatures and has a low latent heat, such as an alcohol. Upon ignition the alcohol would vaporize, no longer being a projectile, but still be enough mass to cause a noticeable recoil.

 

[minger]

I'm not so sure I would put any sort of "cap" with a hole on the end to try and increase momentum. That's way too close to a pipe-bomb.

Topher has a good idea though, of putting some sort of liquid in there that will burn as it fires.

 

[prefetch]

You're going to need to give us the length of the barrel in order to figure this out.

it's about 11' long. all the relevant data is here: http://utahmvc.org/blankfiremodel.pdf"

removing the springs might be difficult, but replacing or removing the oil would work. of course, the blow back from the recoil might damage the recoil stop as it wasn't designed for a slap back w/o oil.

 

[Equate]

I'm not so sure I would put any sort of "cap" with a hole on the end to try and increase momentum. That's way too close to a pipe-bomb.

Not really. Such "caps" (Blank-Firing-Adaptors) are used routinely. http://en.wikipedia.org/wiki/Blank-firing_adaptor"

Although it will take away from the "looks" of your gun, it shouldn't be too hard to manufacture/fit such a device on your 76mm.

 

[xxChrisxx]

Blank fire adapters are used to allow cycling in automatics, as this is an artillery piece that is not an issue.