SpudFiles

So my physics class is building a potato cannon as a project and I need some advice and help. We have all of the pieces now and all we need to do is put together our air chamber and we are ready to go. Before we cement the pvc in place i was wondering where the best place is to put our schrader valve. We have a 6" x 10' sch 40 solid core pipe for our chamber. Also should we drill a hole and pvc the cement out of it or thread it in or both or idk. Any help would be great.
I will see if i can get some pictures up, but right now they say the pictures are too large

UPDATE: After testing the cannon with 10ft of barrel we achieved a velocity of 335fps (or 102m/s). Therefor, in a perfect world, our cannon would shoot 3480feet (1.061km).
Thanks

Do yourself a favor and don't use a Schrader valve if your chamber is 6" x 10' !!!

At the very least, try and use a 1/4" NPT quick discconnect in combination with a ball valve to seal the chamber. Then just directly connect your air compressor (which I hope to steezus krist you have - given your chamber size!!) like you would any pneumatic tool!

Hahaha yes we have quite a large air compressor, something like a 50 gallon compressor. I am not very inclined to these sort of things though. We do have a ball valve that we plan to use to release the pressure from the tank to the barrel. With the 1/4 inch npt how would we attach it to our tank?

You'd have to get some bushings to reduce the 6 chamber size down to 1/4 inch. Most of these will be pvc and can be cemented into the chamber. After you get down to 1/2 inch or so, the last bushing (1/2 inch to 1/4 inch) will probably be brass and can be threaded in. Then, just screw in a 1/4 inch nipple, a 1/4 inch ball valve, and a 1/4 fill valve.

What if I were to drill a hole, cut threads into the hold with a tap, then screw the 1/4 quick disconnect into the hole and cement it in place?

Yes please do that instead of excesseive reducers (and possible failures due to all the glue joints!!)

Just make sure you drill through at a spot where a coupler is overlapped with the pipe underneath - i.e. a glue joint on a union/reducer. Try and put the hole halfway on the overlap if you can. Make sure you don't burry the 1/4"NPT tap into the pvc pipe or else the threads will be too large - only go about 1/2 - 3/4 of the tap's threads. This will allow a nice tight seal and minimal thread sealing tape to be used.

Thanks for the input, I did do exactly that. Seems to work great. Had a small leak at 60psi from the front coupling and reducer, but i took care of that. I think we may be testing today! I'll let you know how it holds up at 80psi and how far we can launch the potato! In 2009 a class built one like ours only smaller. Theirs was a 18 foot, 4"x10' chamber with an 8' barrel and they managed about 970 feet. I dont know what ours will do but im hoping for well over 1500 feet

Update: Had our first test fire today, turned out awesome. The first shot -- with about 30ish psi and only 10 feet of barrel attached -- launched....well I don't know because we couldn't find it. It landed in the middle of a muddy corn field and we will need to look tomorrow for it. The second one launched with about 50 psi and looked like it went twice as far. The third launch was with 70psi and was launched so fast I don't think anyone in our class was able to follow it. Our teacher is planning on using a radio transceiver or something so that we can know how far it traveled. Anyone got any ideas on that? Besides having someone stand at 1000 feet and 1500 to see how far it goes.
Does anyone have any suggestions on projectiles we could use of 2" diameter that would be more aerodynamic and have a better range? Section of a 2" dowel rod sanded to a point with some weight behind it? I have looked at compound rounds but they are just too expensive and would take a long time to ship. Anyone got any ideas? Good or bad i will take em'
Lastly i would like to thank you for all the input and help on the cannon.

You may want to revisit the 30 foot barrel design for a couple of reasons.

1 Flow drag. -- This increases with length
2 Acceleration distance -- How far does the projectile travel before it goes no faster
3 Depletion of source pressure -- Loss of chamber pressure combined with flow resistance may end acceleration sooner than expected.
Combining the two above, excess barrel length impacts both terminal velocity and can slow the projectile after it reached it's peak
4 Another advantage of a shorter barrel is greater mobility.
5 A longer barrel is more likely to buckle and break
The optimum length will vary with projectile weight and operating pressure so any trim will be best case compromise. Trim for your most common operating conditions and call it good.
Heavier projectiles will use a longer barrel due to slower acceleration. Use a short one for marshmallows and a long one for canned goods.

If you have the resources such as some small magnets, a computer with a Mic input, and some wire, you can measure your launch in the barrel, then cut off the unnecessary length.

When trimmed your new length should have even higher launch velocity.

Details on how to take your measurements are here. https://inteltrailblazerschallenge.wiki ... rim+method
I wrote the above page. If you have questions, feel free to message me. I am sometimes away for days at a time, so be patient.

Graph below shows the end of a 10 foot barrel where acceleration ended. By comparing the distance between each 0 sensor crossing, the loss of shortening of distances between them indicate a loss of further velocity gain.

Measure, trim, and enjoy.


A couple of more notes, to improve valve opening time and lowering flow resistance (CV) i would have advised with going with a larger valve such as 3 inch then reducing to your barrel. This can apply higher flow as the valve opens partially assisting with early acceleration.
A launched dowel will tumble. Launched a 3 foot long 1 inch dowel straight up and watched it tumble back down. We were not sure where it would land as it would glide back and forth as it tumbled.

Thanks for the info! I had wondered about the barrel being to long and actually slowing down our projectile but I figured that because we had such a large pressure chamber with a lot of air behind it, it would be okay. I have gotten a hold of a chronograph so we can measure the muzzle velocity of the potato with 10 and 20 ft of barrel