SpudFiles

Alright guys, this is no discussion for my thread. Take it elsewhere.

@Tech

Back on topic, The videos are great. Have you tried higher pressure? I like the idea of the 1/2 tennis ball. I may try one to see how well it holds up.

Haven't tried that high pressure yet, only 60. It is really loud and I might be making the neighbors angry. Maybe I'll try it in the backyard or something, but not today, the storm is causing terential downpour so not much shooting done today. The piston face is blowing out also because I forgot to buy a washer big enough for the front when I was getting parts so I just threw a smaller one on for testing. I will get a new one soon though so I can test it more. The tennis ball is doing very well, especially for such a large piston.

Do you think the force on the piston will become too strong for the bumper when the pressure is higher and a barrel and projectile is used?

I think it will be fine, but if the back plug breaks I can just replace it with a metal one... then it won't break again. Can you explain how the piston will be under more force with a barrel and projectile?

Also today I realized that the piston doesn't move back much from the seat, less than half an inch, so I will cut some of the threaded rod off so I can get some better flow.

Half an inch would be ideal piston movement, but a smaller lighter piston is good too

Gun Freak wrote:Can you explain how the piston will be under more force with a barrel and projectile?

Because the pressure accelerating the piston falls off faster without a projectile to "plug" the system.

However, the difference won't be that large (in fact, I'd usually consider it fairly insignificant). After all, the chamber can only vent through the valve so fast.

Lockednloaded wrote:Half an inch would be ideal piston movement, but a smaller lighter piston is good too

It will be easy to shorten it plus I think I will get more power.

@Rag, thanks, I understand now. I think it will hold up just fine.

I have noticed on my valves that the weight of the projectile makes a fairly large difference on the piston depending on shooting marshmallows or D cell batteries. The Marshmallow Cannon broke the PVC endcap while shooting AA batteries with the 1/2 inch barrel. It has a different sound from the tank. It has more of a CLICK when it opens with a restrictive load. With light projectiles and a larger diameter barrel, the click is much softer and the muzzle pop is more pronounced.

Oops, sorry for the double post..

Ragnarok wrote: Why exactly? You might as well tell me that this is a hopeless dead end, with no chance of standing up to modern designs in practicality or performance - it won't stop me liking it.

Hmm, It uses a piston, the projectile is captive, runs at above 100 PSI, uses external combustion for a power source, and is hugely massive but can be operated by a couple guys, uses steam instead of air to drive the piston, the valve looks to be much larger than 2 inches, I like it.

That double post was totally worth it

You should see how far it can take a whole bucket of golfballs all at once. 8)

I gotta stay on topic somewhat..

Technician1002 wrote:I like it.

As do I. I find it sounds better than its modern equivalents.
(Still, in my opinion, there is better. A heavily loaded 57xx class up a gradient.)

Back on topic regarding the force on the piston when shooting various projectiles.

For every action, there is an equal and opposite reaction. In the barrel sealing piston valves, the reaction force is applied to the piston that causes the recoil force on the cannon. Launching lightweight items provides a low reaction and launching heavy projectiles provides a high reaction. The reaction force is the force the piston presses into the bumper, and thus pushing the cannon back.

Technician1002 wrote:The reaction force is the force the piston presses into the bumper, and thus pushing the cannon back.

Yes, but force isn't energy. After the piston has hit the bumper, what remains is force - and at maximum, it can only push as hard on the piston bumper as the air in the pilot was doing before it fired.

The force transferred through a stationary piston is pretty irrelevant as far as damaging the "back piston fitting". The kinetic energy is the relevant part, and the difference in that between a "dry shot" and one with a projectile is fairly small.

Hmm.. When I have time I'll have to clock a couple shots with my ABS cannon. I can use a magnet/coil pickup to measure the relative piston velocity between a dry shot and a heavy projectile. The fairly small difference can be quantized. I have clocked a piston, but didn't compare dry vs full load shots. I'll have to add it to my to do list.

I do know that the time I broke the PVC female adapter on the ABS cannon and the pipe cap on the Marshmallow cannon were both with larger projectiles. It has been too long to recall what I was shooting when the Mouse Musket broke it's PVC pistons. All failed parts have been replaced with more substantial impact resistant parts.

Technician1002 wrote:The fairly small difference can be quantized.

It can also be roughly calculated.

The valve on "that cannon's sequel" (apparently, some people think I talk about "that cannon" too much, so it will not be named) has a flow at full pressure (40 bar) of about 2000 litres per second if venting to atmosphere.

Calculations show that, with an infinitely heavy projectile, it takes the valve a little less than a millisecond to open.
In that time, if the valve were fully open (which, obviously, it isn't), about 2 litres of air would flow through. For the 600 cc chamber, that would result in a fall of about 3.3 bar.
That's a ~8% fall in pressure (and thus acceleration) - even if we assume that it's affected by that for the entire travel (which, again, it obviously isn't), opening time would be reduced by only about 4%, and impact energy by that same 8%.

That's hardly a big difference - and I was rounding up the losses each time. It will be different in other cannons, but the same principles will hold - piston valve opening times are short enough that the pressure loss, even when venting straight to atmosphere, is not great enough to have a big effect on the forces on the piston.

So yes, there's a difference, but does it make a difference to that kind of thing? I'd say not, unless you're very marginal on bumper performance.

While the new chamber is not as powerful as the old one, it still packs a punch.
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Another video is uploading. Gosh.... YouTube uploader sucks!

If you miss the target, what is down range? I have some concerns this could cause problems with the neighbor. I have had stuff fly in odd directions after impact and need retrieval from the neighbors. I have had a rolled up t shirt and a 4 inch poof ball go over the fence, oh and a bunch of marshmallows.

This marshmallow landed in the tree in my back yard. Other ones miss the tree and didn't stop for the fence.
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