SpudFiles

I'd say if you're predicting how fast the launcher will shoot by modeling it, you're in pretty deep.

You could calculate acceleration as a function of position in the barrel given the barrel diameter, length, chamber volume, projectile mass, and atmospheric pressure. However, the first real leap comes with getting this into a function of time, not position. You would integrate, (calculus function), "something or another", (my old robotics coach told me all about how to do it, but I've since forgotten), to obtain the correct function with respect to time, and then it would be a simple process of anti derivation to obtain a position function with respect to time, which you could use for muzzle velocity, which can be used for all kinds of things.

Maybe D_Halls over from Spudtech would be able to help you, if you could get a hold of him. He wrote the "Gas Gun Design Tool". It is unknown to me how the program calculates muzzle velocity, but given my computer's inept ability at performing most basic differentiation problems, I would say his program uses some sort of limit process. This would be good, as it means he actually knows the real equations for solving your question.

I think the point here is to obtain with trial and error what GGDT would give you through theoretical calculation.

The golf ball barrel made with sdr 21 should be fine by itself in lengths under 3 feet, but you should sleeve it over that, and therefore, sleeve any/all of the barrels to keep every thing constant for your experiment.

Going through a plumbing specialty store will get you the best deal on the pipe. I just picked up a 20 foot stick of sdr 21 for $13.70. It cost a bit more per foot than sch 40 2 inch, but that is the nature of everything specialty.

You should be able to get cheap sch 80 2 inch from lowes, home depot, or menards. Look at electrical conduit. It is not pressure rated, but the barrel sleeve doesn't need to be, it is for structural strength, not pressure. You will save a ton of money. If you can find a plumbing shop that has sdr 21 they will carry sch 80 as well.

Like I said, I had to get a 20 foot stick of sdr 21. If you find yourself in the same boat, I would stick with your original experiment with several barrels since you will have the pipe to do so. If by some outstanding luck you can buy it in a 5 foot length, consider the idea of a smaller chamber and reducing the barrel length repeatedly, as money saved is money for the next cannon

A moded valve will eliminate the human element, good thinking. You should be able to find a 1 inch valve relatively easy, and there are mod programs in the how-to-database for most major brands. You can find 2 inch valves on the internet, check ebay, amazon, or just hit up google. I know there have been a couple of guys use them. A 2 inch valve will have around 1 1/2 inches of flow. A 1 inch valve will have 3/4 inches of flow. Both are adequate for a golfball gun. A 1 inch can be bought and moded for around $20, a 2 inch will run you probably $50 then another $5 to mod it.

Take lots of pictures, too. Science fairs love pictures and so do we.

How about doing something that'll give info spudders could really use?

Pick a reasonable chamber and barrel (perhaps 2"Dx12"L chamber, 1"Dx48"L barrel). Probably want a 1" or less diameter barrel to keep the cost of ammo down. A golf ball or tennis ball barrel would be great but they are more difficult to get the parts for. Get a 3/4" (or 5/8" or 1") sprinkler valve ($15), a couple 9V batteries and clips, and a switch to control the valve.

1. Fire several shots with the unmodified valve. Record the range or muzzle velocity or whatever is easiest. If you do range then you need consistent ammo and a constant launch angle. It is pretty easy to make a homemade chronometer which lets you measure the actual muzzle velocity. Or, you could just use a microphone and PC and a hard target fairly close to the muzzle, I believe there is a spudwiki article about doing it that way. (If not search these forums.)

2. Port the valve a bit by drilling out the vent hole(s) for the solenoid. Just unscrew the solenoid and find the next larger drill bit size for the small hole under the solenoid. Fire some more spuds.

3. Port the valve some more and fire more spuds.

4. Port the valve some more until the solenoid starts to leak. Oops, that's too far. If you have to, you can just fill the hole up with epoxy to do the next mod.

5. Install a blowgun trigger on the valve and fire some more spuds. (You can leave the solenoid on the valve if it isn't leaking.)

Question: How much of an affect does porting and/or blowgun triggering have on the gun's performance? (You are mod'ing the valve to try to get it to open faster, a faster valve should give better performance.)

For each gun calculate the average velocity and the standard deviation. You will need at least three shots for each configuration. Science fair judges love projects that have some mathematical and/or statistical analysis. Is the difference in performance greater than the shot to shot variability? Judges really love that, shows you understand that your measurements have some error, all measurements do. Is the error greater than or less than the affect of mod'ing the valve?


Pictures will be an absolute requirement. It is very likely that you will not be allowed to bring the launcher itself anywhere near a school.