SpudFiles

What a great website. I just wanted to make you guys aware of a senior design project that myself and a partner are working on. We are designing an automated tennis ball cannon that is mounted onto a remote control robotic base. We have all kinds of goodies planned for the system but what I'm primarily concerned with at this point is properly designing the combustion chamber, barrel length, and proper amount of propane to use.
So what I've gathered thus far is that SCH40 (or 80) 2.5" PVC pipe fits tennis balls VERY snuggly. But, I've read that PVC SDR 2.5" has a larger inside diameter (ID) that accomodated the tennis balls better. Because we are going to autoload the tennis balls into the barrel through some hatch, and hopper system, I am leaning toward the SDR 2.5". But, many of our professors are concerned that the propane will generate too much heat and pressure for the SDR to withstand. Is this a legitimate concern if the proper propane volume is determined?

Also, we are struggling to determine the proper combustion chamber size. Could we get some suggestions and/or tips? I will continue to search and check this thread.

If anyone has suggestions on to design an autoloading system as well, that would be fantastic!

Great site too!

Autoloading:
Blow forward bolt is one of the easiest.

Chamber size:
Read this already?
http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio
Note that the graphs having an optimum around 0.6-0.8 are the optimal ratios for a fixed chamber size and variable barrel length, I am not sure what the most powerful chamber for a certain barrel is, it all depends on how many spark gaps you've got and how well you will be mixing it with a chamber fan (for more information also check the spudwiki)

Assuming that you have got a fixed barrel volume and a variable chamber then:
I would say use a 0.8 ratio for a good performing and efficient gun.
0.5 or 0.6 for a quiet and even more efficient gun.
1:1 for a slightly more powerful gun, but also louder.
1:5 to 2:1 for even more powerful and loudness, but make sure to use alot of spark gaps to keep the burn time as low as you can.

For the heating of the pvc:
How many RPM will you want to shoot it at?
The higher the rpm, the warmer it will get ofcourse, and you could always switch over to metal pipes, preferably aluminum because of its low weight.

Wouldn't it be alot easier if you made it a pneumatic? I mean it would cost alot less. not as many electrically operated valves. No fans. no need to vent the chamber. How long will the barrel be? I suggest that you go with pneumatic.

Wouldn't it be alot easier if you made it a pneumatic?

I second that

You'll have a little easier time of it if you go with a pneumatic design as Sticky_tape suggests. I would think portable CO2 tanks would be the way to go....really depends on what your goals for the cannon are, how much automation you want, etc.

However, if you want to go combustion, the fans and vent aren't that big a deal. Take a look at either of my combustion cannons in my sig....either would be a great size to shoot tennis balls

As far as SDR 2.5", I haven't seen that for sale anywhere. The SDR 1.5" is hard enough to find. Not that it doesn't exist, it's just going to be really rare. I believe the consensus is that regular 2.5" sched 40, while offering a snug fit on new tennis balls, is probably your best bet.

Agreed with the above, semi-automatic combustions are a bit of a rarity because of their complexity, since you have to bother with a traversing and elevating base I would go with a purely pneumatic system to avoid headaches. Don't worry, even without the flames you'll still make an impression

Ah, no biggie. All you need is a propane source, an air source, some kind of reloading feature, and a microcontroller with a couple of electric solenoid valves.

Pressure rated, SCH 40 PVC should be more than suited to the task.

solenoid valves are kinda expensive.

I am also working on a semi combustion, but mine was going to be hand held. if you have access to machining facilities, it wouldn't be excessively hard. I am still working on drawing rough plans, but I will share them in this thread if you would like. Mine was designed to no use any gas besides propane. It would be very challenging to scale up to tennis ball sizes.

If you have a high volume compressed air source for your launcher, things could get much easier. A centrifugal blower could also probably work. see this for a good design. look at the first post, the one with the animation.

good luck, and I hope you're good with a lathe.

Wow great info everyone! I really appreciate the helpfulness thus far. I will surely reconsider going to compressed air as well, b/c the profs are a little worried about combustible gases and flying shrapnel (if it were to go wrong). I am busting my butt right now studying for a thermodynamics test, so I will be sure to update when i have to reconsider.

Thanks!

The only place that I have been able to find SDR PVC pipe is at some type of John Deere Irrigation store; you might want to try those types of places for the pipe. Be warned, the pipe is expensive; you buy it in 20 foot lengths, and I think the price was like $80 (can't be sure, this was a few years ago). Needless to say, I did not purchase it.

Team PSU wrote:Wow great info everyone! I really appreciate the helpfulness thus far. I will surely reconsider going to compressed air as well, b/c the profs are a little worried about combustible gases and flying shrapnel (if it were to go wrong).

You have just as much if not more chances of "shrapnel" with a pneumatic as you do a combustion. However, proper construction, materials and usage will not produce shrapnel except maybe in your target....

Team PSU wrote:the profs are a little worried about combustible gases and flying shrapnel

Somthing you might want to point out to them is that at atmospheric pressure, you can only fit a certain amount of air/fuel mix into the chamber, and therfore there is a limit as to how much energy can be produced. This limit (under absolutely ideal conditions I've heard it's around 120 PSI) is well below the pressure rating of (some) pvc pipes. In fact, a pneumatic is probably slightly more dangerous because if the valves controlling the pressure (when running off somthing like high pressure co2) were to fail, the pressure could just keep rising until somthing failed (of course, safety pop-off valves etc can help avoid this)

As for the fuel, the worst that could happen would be the whole thing catching on fire, and if the fuel tank was somwhere in the middle of this, the heat could cause it to burst, but again, the chances of it catching alight are pretty darn remote.

Didn't we see a video of this a while back on this site?

If so, and it's the video I'm thinking about... WELCOME!

Like insomniac said, the maximum closed chamber pressure (1 ATM initial pressure) for propane in air combustion is about 120 PSIG.

Team PSU: But, many of our professors are concerned that the propane will generate too much heat and pressure for the SDR to withstand. Is this a legitimate concern if the proper propane volume is determined?

You, and perhaps your prof's, might need to do a bit more studying.

Some facts about propane combustion in air at 1 ATM:

1. Maximum pressure in a closed chamber is ~120 PSIG. The maximum pressure occurs with a stoichiometric (or nearly so) mixture of propane and air, roughly 4% by volume propane. Greater amounts of propane in the chamber will reduce the pressure the gun generates. If the propane is more than about 9% (volume) it won't even ignite. So if you grossly over fuel the gun the energy it will release when ignited is zero.

2. In an operating gun the pressure rarely approaches the 120 PSIG theoretical closed chamber limit. With very tight fitting ammo the peak pressure is generally more like 60 PSIG.

3. Pressure rated PVC is good to at least 200 PSIG. The actual limit depends on the pipe diameter and will be printed on the pipe. 3" PVC is rated to 260 PSIG. Note that that is the safety rating and not the actual fail pressure. The fail pressure will be a fair amount higher than the rating.

4. The pressure rating for the pipe is a static rating. An operating gun generates a dynamic load which should be rated differently than a static load. I don't think anyone has a real good feeling for exactly what, if any, derating is needed for the shock (dynamic) load. Combustion of propane in air is a fairly slow process. For a typical sized gun the time from the click of the igniter to the boom as the round exits the barrel is about 50mS. That's a pretty "long" time period and is well outside the time domain where shockwave type affects would be an important consideration.

5. The adiabatic combustion of propane in air at 1 ATM does indeed produce a very high temperature gas, roughly 2300C (4200F). But the mass of the combustion products is so small (roughly 1.2g/L) that there is very little actual heat. A typical sized PVC combustion gun will only warm the PVC by a couple degrees when fired.

You might want to get copies of GasEq (a thermodynamics calculator for gases including combusting gases) and HGDT (a simulator for combustion spud guns).

Great information everyone. I've been doing some significant research lately and I believe we are going to switch our approach to a pneumatic launcher. Look for some questions/suggestions in the pneumatic section...

Thanks!