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FEAR

Built a hybrid cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: maggotman » Mon Jan 26, 2009 3:38 pm

the problems with just springs is that they provide no dampening effect if you could get hold of a hydraulic ram you could make one to quickly absorb the force and slowly bring the chamber barrel back to position it

http://www.blackartdesigns.com/BAD-How% ... works.html
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Unread postAuthor: Fnord » Mon Jan 26, 2009 3:45 pm

I am open to any ideas people want to throw up about recoil absorption. Remember though, FEAR will weigh in at over 450 pounds and is intended to fire 50+lb projectiles at around 400ft/s.

Bungee cords with a sliding recoil mount on a frame?
It could be made to look pretty if it had to be, but the best part is if you need more absorption, just keep adding them till it works like you want.
Plus bungee cords aren't hard to replace.
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 4:25 pm

Killjoy wrote:I am open to any ideas people want to throw up about recoil absorption. Remember though, FEAR will weigh in at over 450 pounds and is intended to fire 50+lb projectiles at around 400ft/s.

That means that even excluding the effects of recoil from gases leaving the muzzle (which can probably be ignored relative to the projectile momentum), you're dealing with nearly 14,000 ft-lbf of recoil energy.

That's more than could be expected from a old ship's cannon, even something as large 36-lb gun - which although it's firing at a higher velocity, has a lot of weight to absorb that, often a tonne or more.

Your problem is that the projectile is heavy relative to the cannon. Consider a rifle like the M16 - it's projectile weighs about one thousandth of what the rifle does. Compare that to FEAR - your 50 lb projectiles weigh more than a ninth of what the cannon does, and this means that a full tenth of the total kinetic energy is absorbed by the cannon.
Indeed, to put the initial recoil velocity in context, it's recoil velocity will be something like 45 fps - or if you'd prefer, 30 mph.
You now have a fifth of a tonne recoiling at 30 mph.

I suggest you need to be looking less at recoil springs/bungee straps/gas struts (i.e. stopping it in the space of a couple of feet) - and more at advancements on the recoil control of naval cannons. For example, my advice would be to do something like mounting it on wheels and stopping it over several yards with bungee jump ropes.

You'd also benefit from having some ballast you could add to it - if you could fix it's weight in ballast to it, you'd immediately halve the recoil energy.
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Unread postAuthor: Brian the brain » Mon Jan 26, 2009 5:11 pm

Recoil energy will remain the same, according to Einstein.
To every action there is an opposite equal reaction.

Speed will halve.Travel will reduce, but not the energy.
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 5:28 pm

Brian the brain wrote:Recoil energy will remain the same, according to Einstein.

Two mistakes here. Recoil MOMENTUM (not energy) will be the same, and it it's Newton 3rd Law (nothing to do with Einstein).

Equal momentum in both cases does not equal equal energy.
If two things have the same momentum, but one is heavier, it will have less energy.

Double the mass, and it will have half the velocity. Feed that velocity and mass into the kinetic energy equation:
KE = 1/2 M * V<sup>2</sup>
... and it will have half the energy.

To put numbers on it: 1kg at 1 m/s and 2 kg at 0.5 m/s - same momentum (momentum being mass * velocity) but:
1/2 * 1kg * 1m/s<sup>2</sup> = 0.5 Joules
1/2 * 2kg * 0.5m/s<sup>2</sup> = 0.25 Joules

The fact that the halving of the velocity will due to the fact that the velocity is squared, reduce the result by a factor of 4 - however, the doubling of weight will adjust this to a factor of 2.

Double weight of gun = half recoil energy. The important thing is that it's recoil energy, rather than momentum, that needs to be controlled.
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Unread postAuthor: Brian the brain » Mon Jan 26, 2009 5:38 pm

Darn, I thought I had you there... :D

Einstein... :oops:

Wait..So twice the weight, travelling half the speed has less impact?

Somehow the numbers add up, but the explanation doesn't.

Wouldn;t it take you as much force to stop a 2 tonne car travelling 50 km/h as it would a 1 tonne car at 100?

well..going by the numbers I'd say no...I'm confused...
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 6:00 pm

Wait..So twice the weight, travelling half the speed has less impact?

Exactly - it will have half the energy, although the same momentum.

All of this comes from the fact that momentum is a linear relationship with velocity, and kinetic energy is a quadratic (i.e square) relationship with velocity.
A doubling of velocity doubles momentum, but quadruples KE.
A halving of velocity halves momentum, but quarters KE.

What this means, that where the mass and velocity are adjusted to keep one of the parameters constant:
-In any case the momentum is identical, kinetic energy is inversely proportional to the mass of the object.
-In any case KE is the same, momentum is proportional to the square root of the mass of the object.

Wouldn't it take you as much force to stop a 2 tonne car travelling 50 km/h as it would a 1 tonne car at 100?

It would take the same amount of impulse (impulse being change in momentum, and equal to force times time*), but half the energy (energy being force times distance**) to slow the heavier car.

In other words, if subjected to a constant force of the same magnitude, both cars would stop in the same time (because it has the same momentum), but the slower, heavier one would stop in half the distance (because it has half the energy)

*As well as mass times velocity.
**As well as Half mass times velocity squared.
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Unread postAuthor: Combustion Monkey » Mon Jan 26, 2009 6:55 pm

Just a noobs two cents. I think I've seen examples of reward vented gasses at a muzzle break on .50 cal rifles that actually make the gun travel forward a small bit. Something like that may help here, but not sure of all the operating mechanics.
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 7:28 pm

Combustion Monkey wrote:Something like that may help here, but not sure of all the operating mechanics.

Afraid not. Bear in mind a .50 BMG uses around 16 grams of powder for a bullet about three times that weight. It can cut recoil over just directing the gasses forward, but the numbers can't fit this:
I think I've seen examples of rearward vented gasses at a muzzle break on .50 cal rifles that actually make the gun travel forward a small bit.


Even though muzzle brakes have some uses, the problem is that FEAR is firing very heavy projectiles with relatively light gasses - so the gasses do not have the momentum required to have anything like the recoil reduction required. Actually, I would be highly surprised if a muzzle brake made any notable change at all with that kind of projectile mass.

Similar thing for the .50 cal - although the results are far more notable, they don't have the momentum to completely nullify recoil.
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Unread postAuthor: Combustion Monkey » Mon Jan 26, 2009 7:38 pm

I understand now that the mechanics don't apply. I wish that i could find an online link to the article. As close to the original as I can remember "the gun began to recoil then it drifted forward. The new break was a great improvement." I believe it was tested on a Barret. Think it was from Guns and Weapons for Law Enforcement a few years back..
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Unread postAuthor: Killjoy » Mon Jan 26, 2009 10:15 pm

Ah good, I see Ragnarok has picked up on the problem I have been toying with on and off since I started pursuing this.
I knew with such massive recoil means I need not only a very sturdy frame (and I mean very F*ckin sturdy frame), but also a long travel distance and a recoil dampener, which has limited my choices.

I like Rag's idea of increasing weight, that could be a viable option (I have lots of access to scrap metal). I also figured that a long travel distance would be needed so I'm glad Ragnarok also confirmed it.

I think from the input y'all have given, I'm gonna lean towards working with my old design which involved a sliding recoil mount with 5 feet of travel; However, I will also increase weight by several hundred pounds and work on some sort of dampener system, and I did a little research and I think I might also be able to work out a hydraulic return system.
Any comments on this path?
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Unread postAuthor: Ragnarok » Mon Jan 26, 2009 10:51 pm

Killjoy wrote:I knew with such massive recoil means I need not only a very sturdy frame (and I mean very F*ckin sturdy frame)

You don't say. Bear in mind, it will be accelerated up to it's full recoil velocity in a matter of a few dozen milliseconds (inside a tenth of a second)
That represents an huge average recoil force you're going to need to be able to transfer those forces into your frame and carriage.

I like Rag's idea of increasing weight, that could be a viable option (I have lots of access to scrap metal).

Just make sure it's (sort of) fixed - doesn't matter if you've got a large hopper you can chuck the metal into, but you don't want it sliding around (or off) your carriage.

I think from the input y'all have given, I'm gonna lean towards working with my old design which involved a sliding recoil mount with 5 feet of travel; However, I will also increase weight by several hundred pounds

Ignoring the damping system for now - let's say you make it weigh 1000 lbs. That's still a generous 6300+ ft-lb of recoil, so to stop it over 5 feet, that's an average force of 1260+ lbs, which is about half the weight of a small car, which is a very serious force.

I suggest that it's going either to have to weigh a lot more and recoil somewhat further - or a combination of both.

At the same time, as a precaution against recoil containment failure, I recommend not standing anywhere behind it when firing - although that is normally where you want to be when firing something, most things don't have quite this much recoil.
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Unread postAuthor: Killjoy » Tue Jan 27, 2009 3:08 am

Hahaha yeah pieces of flying steel definitely strikes me as a bad thing (no pun intended). I'm thinking a combination of thick steel plates (or what ever I can find at the scrap yard) bolted on, and maybe for or five water filled 10' 4" tubes which I can bolt to the cannon.

Yeah and I wasn't intending to stand real close to this thing during the testing phases. As sure as I generally am in my work, forces like these could very easily humble me.
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Unread postAuthor: Lentamentalisk » Wed Jan 28, 2009 1:40 am

Any chance of making a similar cannon facing the other direction, that fires at the same time? Similarly, you could make some sort of active recoil absorbing system, out of compressed springs and weights, or something.
Just brainstorming ways to not have to fight 1/2 the normal force of your average sized car on the ground...
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Unread postAuthor: Killjoy » Sun Feb 08, 2009 8:08 pm

In theory I could but I feel it could get to complicated if I tried to do that with timing and all.
The active recoil system sounds like a possible idea though. Hmm....
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