SpudFiles

If this is a science project, I'm guessing you will want consistent, controllable shots. Unless you're set on combustion, I would go pneumatic, because its hard to make a propane meter good enough to fire as consistently as a pneumatic will. Also, with a pneumatic, you can easily vary the power to suit your needs.

Fan direction doesnt really matter much for mixing.
Though for venting, one may pick a certain direction to suit his needs.
Most fans are indeed directed to the front, but it does not matter that much.

I was referring to his advice of reversing battery polarity to get his computer fan to run in reverse. Computer fans don't work that way.

Direction for mixing is irrelevant. However, if you want to draw in fresh air from the rear and expel spent gases from the breech in a timely manner, you need to setup the fan to blow forward, from intake to breech.

FishBoy wrote:If this is a science project, I'm guessing you will want consistent, controllable shots. Unless you're set on combustion, I would go pneumatic, because its hard to make a propane meter good enough to fire as consistently as a pneumatic will. Also, with a pneumatic, you can easily vary the power to suit your needs.

Not really. Indeed, a $3 syringe based meter is simpler and just as precise (reproducable) as the $10~$20 regulator that the pneumatic will need.

We tested but it went to fast to record The radar gun didn't pick up the speed. I am now stretched for time so were taking foam golf balls and using different of amounts of propane to see how the amount of propane affects the distance. And BTW i have a pneumatic cannon the combustion cannon is funner More powerful!

RabidDuck wrote: I am now stretched for time so were taking foam golf balls and using different of amounts of propane to see how the amount of propane affects the distance.

It really doesn't work that way. More propane does not equal more distance. For any specific chamber, you have a fairly narrow window of propane amount that will work...period. If you add more propane and want to extract its energy, you need to find a way to add more O2 or air, thus creating a hybrid.

You will need to find a way to meter propane to the equivalent of 4.2 - 4.4% of your chamber space. That will be your optimum. Less propane and it will be less energy or not fire at all, more propane will likely fire but will be too rich and will actually exhibit a less energetic firing.

RabidDuck wrote:We tested but it went to fast to record The radar gun didn't pick up the speed.

Radar guns don't work well with cannons. It's too hard to get a fix on the projectile, and by the time you do, the projectile is out of range anyway (which is why chronometers are used.)

starman wrote:

RabidDuck wrote: I am now stretched for time so were taking foam golf balls and using different of amounts of propane to see how the amount of propane affects the distance.

It really doesn't work that way. More propane does not equal more distance. For any specific chamber, you have a fairly narrow window of propane amount that will work...period. If you add more propane and want to extract its energy, you need to find a way to add more O2 or air, thus creating a hybrid.

You will need to find a way to meter propane to the equivalent of 4.2 - 4.4% of your chamber space. That will be your optimum. Less propane and it will be less energy or not fire at all, more propane will likely fire but will be too rich and will actually exhibit a less energetic firing.

The amount of propane is still a very good experiment. You should be able to find the optimum amount of propane, which should be about 4%. More or less propane will decrease performance. Propane less than ~3% or greater than ~9% won't even ignite. It would make a very nice Science Fair project since most people think more fuel = more power. A result that is counter to "common sense" is always a good result. Especially when you can give a scientific explanation for why the system does not behave like "common sense" predicts it should.

The tricky part (assuming you have a reproducable way to measure the fuel and consistent ammo) is measuring the performance of the gun. Range is a terrible way to measure performance. There are just to many factors that affect range.

A chrono is needed to get decent muzzle velocity data. Got a laptop or old PC? Just use a mic to record the firing of the gun and the sound of the ammo hitting something hard (and strong enough to survive the impact). Its pretty easy to extract the velocity from the sound recording if you know the distance from the gun to the target. Becareful though, a golfball may well be coming back off the target at nearly the same speed it hit the target. A golfball at a couple hundred feet per second will do a lot of damage to things, particularly at short range.

You might even be able to just use a piece of newspaper hung from a suitable support as the target. The sound of the paper ripping might be loud enough. Put the mic near the paper. You can correct for the different distances from the muzzle to the target fairly easily.

Science Fair judges (that know what they are doing) love replicate measures with statistical analysis. It ain't science without replicate measures and stats. Fire the gun several times at each fuel ratio so you can do the average and the statistics.

If you are pressed for time and want a surrogate measure of performance then I would do hang time instead of range. Just shoot the gun nearly straight up and record the time from bang until the ball hits the ground with a stopwatch. That'll give much better data than will range. Just be sure that you have a safe place for the round to come back down.

Sorry guys i hadn't said much to this recently. We did do different amounts of propane to see how far the foam golf balls would go. It was a super cold day and the cannon wasn't being consistent. We recorded the data though. We ended up winning our school competition and going onto the state competition. We took first in that too. We could of done a better question to figure out or a more creative experiment. I think we should of tried the chrono but since we were outside and a ways away from our house we didn't have a computer or mic. Thanks for all your guys help! I want to make some more cannons soon! Thanks again.
~Danny
p.s. sorry for the late response

What state are you in? When the judges asked what the practical applications, what did you say?

I didn't even place with my 3kj electromagnetic ring launcher!

RabidDuck wrote:Sorry guys i hadn't said much to this recently. We did do different amounts of propane to see how far the foam golf balls would go. It was a super cold day and the cannon wasn't being consistent. We recorded the data though. We ended up winning our school competition and going onto the state competition. We took first in that too. We could of done a better question to figure out or a more creative experiment. I think we should of tried the chrono but since we were outside and a ways away from our house we didn't have a computer or mic. Thanks for all your guys help! I want to make some more cannons soon! Thanks again.
~Danny
p.s. sorry for the late response

inconsistency is often due to improper venting. Left over combustion gas from a prior shot deprives the next shot from needed oxygen even when the propane is metered very carefully.

Most radar guns are out of range at above 170-200 MPH and simply don't register higher speeds. Check your radar gun specifications. Spud guns typically launch in the 150-600 FPS range.

Often a combustion is perceived as more powerful due to the late termination of combustion outside the muzzle. Don't judge power by noise.