SpudFiles

SpudBlaster15 wrote:Throw even a 2X hybrid mix in the combustion model, and it will outperform the pneumatic by a fairly significant margin.

That's the point of pneumatic power potential vis a vis a non-hybrid combustion, as long as your chamber and valve can take it you can up the pressure and up the power, while a combustion is limited to a certain maximum pressure so the only option for more power (once ideal combustion is achieved) is upping the dimensions.

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jackssmirkingrevenge wrote:combustion is limited to a certain maximum pressure so the only option for more power (once ideal combustion is achieved) is upping the dimensions.

To see a size comparison, I just tried to model a combustion gun in hgdt. Of course I have no idea of what I'm doing. Can some knowledgeable person please model a combustion cannon to the following specs and tell me the chamber size, etc., required to drive a projectile 850fps using the following:

Barrel....72" x 1.25"
Projectile ...1.25" 134 grams

This is a great discussion..........

I agree that combustion that exceeds my tank pressure level would, indeed, outperform my cannon for a number of reasons.

#1 Restriction through my valve.

#2 Pneumatics have a pressure loss as the air expands due to the cooling effect of the sudden drop in tank pressure. So, 250 psi is not truely 250 psi of pressure seen by the projectile.

I do not know the math to give an idea of the loss due to each factor (and I am sure there are more factors at play, here), but I understand the general principals at play here.

Keep up the good info. This is great! I am learning alot.

Matt

Hey Guys,

Sorry I have been gone for so long. I have had alot going on.

Anyway, I finally finished the cannon, other than some cosmetic details and a laser sight.

I just wanted to let you know I have fired the cannon for the first time with a decent projectile. I went to a campground this weekend with a firing range. I got 3 good shots fired at 230psi firing UHMW compound rounds. The recoil was intense! Also the gun is quite accurate. I did not bring my chrony. But, the muzzle velocity is obviously very high. I was surprised that I could not see the slug in flight at all. It went through numerous layers of plywood, hit the hill behind them and destroyed the slug from the impact.

Intense!

I will post some finished pictures and a video soon.

Oh, it does not take long to figure out the method of holding and firing this beast. The first shot knocked me backward. The second was a bit better, but by the third shot, I learned to put one leg far behind me, squat down a bit and lean forward. In that stance, the gun (though it really kicks) is relatively easy to handle.

I plan on testing the tank (by filling it with water to reduce overall energy) to 600 psi soon so I can begin shooting it at 300 to 400 psi safely. Either that or I will make some heavy slugs for it to see how much power I can achieve with this thing.

Oh, the gun is nearly 40 pounds at this point. It is a bit heavy to handle, but not too bad. I can tell the weight is actually a very good thing when firing with so much energy.

Freaking awesome!

Matt

We love videos

Here is a youtube video.



The upload isn't that great. It was made from my Droid. The audio kind of sticks. The cannon is much louder than the video represents. But, this gives you an idea of what it is like.

I will post pictures in a couple days.

Matt

Holy crap...
That thing is beast.

Well done Use extreme caution if you attach a larger diameter barrel. The larger projectile cross section will increase the kick. I have to shoot from the hip and lean into it to fire cans of soda pop or bottles of water.

Will do.

I have to thank you guys for all the advice. It has been a lot of fun.

This thing is a freakin wild ride, I must say. I do not plan on a larger barrel. What I do plan on doing is upping the pressure and projectile weight. Oh, I also have a 6 foot barrel to try out as well.

Matt

When I had problems with recoil I employed a cross pipe that went across my waist to help spread out the force. Shoulder firing would be far too difficult with the cannon though.

Before
and
After

You can see the difference in my ability to handle the ~6000J shots in this video. The last 3 shots are without the cross pipe, all the other handheld shots are with the cross pipe.

High-PSI wrote:This is a great discussion..........

#2 Pneumatics have a pressure loss as the air expands due to the cooling effect of the sudden drop in tank pressure. So, 250 psi is not truely 250 psi of pressure seen by the projectile.

This applies to combustions too. Same law applies. In addition in a combustion, radiant heat adds to the rate of cooling.

In a warm chamber pneumatic, radiant heat and conductive heat contributes to heating the gas.


In a combustion the heated gas does have the light gas effect, so with lighter projectiles, the SOS figure is higher. For heavier projectiles the heat loss is more dramatic in a combustion. The loss is more apparant with longer barrels for longer launch times.

A 100 foot barrel for pumpkin chuckin would be good for an air cannon but would suck for a combustion.

Good to see this thing is still in operation I reiterate my previous suggestions of adding a muzzle brake if you intend to shoot heavier and faster.

jackssmirkingrevenge wrote:Good to see this thing is still in operation I reiterate my previous suggestions of adding a muzzle brake if you intend to shoot heavier and faster.

Now that I have it firing reliably and consistantly with repeatability and have a good grasp of handling it, I can focus on other aspects. So, yes, I will be looking at a muzzle brake, a laser sight, a stand to ease tank filling and reloading, as well as other details.

Honestly, though the recoil is high, it is soft. It is hard to describe. Basically, there is alot of rearward travel of the cannon when fired, but it is not a sharp blow to the shoulder like a 12 guage shotgun would be, kind of like the difference between being hit in the shoulder with a slow moving sledge hammer versus a fast moving fist.

At any rate, now I just need to find a good place nearby to take this thing on a more regular basis. That and looking into increasing tank pressure!

Oh, one quick question, (and I know this is a vague description), at 210 psi, it had a bit of kick. However, at 230 psi the recoil felt like the pressure was far more than a mere 10% higher. It is almost like the recoil energy increase is exponential, not linear as the pressure rises.

I am not sure if I am communicating my thought properly............

Lastly, any suggestions on this thing?

Matt

High-PSI wrote:Honestly, though the recoil is high, it is soft. It is hard to describe. Basically, there is alot of rearward travel of the cannon when fired, but it is not a sharp blow to the shoulder like a 12 guage shotgun would be, kind of like the difference between being hit in the shoulder with a slow moving sledge hammer versus a fast moving fist.

It's down to the relatively low acceleration compared to a 12 bore, the same reason why pneumatics were used to fire dynamite until desensetised explosives were developed to enable them to be fired from powder burning artillery.

Oh, one quick question, (and I know this is a vague description), at 210 psi, it had a bit of kick. However, at 230 psi the recoil felt like the pressure was far more than a mere 10% higher. It is almost like the recoil energy increase is exponential, not linear as the pressure rises.

Knowing the weight and velocity of your projectile, and the weight of your launcher, you can calculate the recoil energy. In this case, you should also add the mass of air in the chamber to the mass of the projectile, as it makes a significant difference.

Technically, recoil energy should be roughly proportional to pressure, I would put the perceived exponential increase down to your impression - though it would be interesting to mount the launcher on a free-moving trolley and quantify the results.