Question for those scientifically inclined...

If one has a potato cannon of fixed volume V...
and one introduces a corresponding stoichiometric volume of gas G..

and ignites the mixture...

What determines the pressure of the gas which expells the potato round?

Trying to remember my physics, it seems like it's purely P₁V₁/T₁=P₂V₂/T₂

So... assuming 70* air temp (what is that like 400K?), and a flame temperature of 1200* (like...1500K?) and the same volume... You get about 56psi. (assuming the volume doesn't change). So even if you increase the volume of the combustion chamber to a 50 gallon drum, you still get 56psi.

This doesn't seem to make sense, so anybody care to tell me what's wrong here?

Unless........ the combustion by products create an extra "volume" of gas, pressurizing it further... But how much is it... hmmm.........

gotta remember your chemistry, too. Newton [was it Newton?] says that material cannot be created or destroyed, only transformed. It's quite possible that the combustibles are producing a type of gas that has a greater volume, but unless you know what is reacting, I don't think you can tell... right?

Janell

I think it makes sense that the pressure created by the expanding gas would be consistent as long as all other variables are constant. What you're forgetting is the increased surface area of the projectile. if you apply 56 PSI to a projectile with a pressure surface area of two inches, you're actually applying 112 pounds total pressure, where if you have a surface area of around say, 500 square inches (my guess at the surface area of the top of a 55 gallon drum since I don't know the dimensions off the top of my head) then you're actually applying 28,000 pounds total pressure to the projectile.

[This message has been edited by Taijiguy (edited 11-23-2004).]

quote Originally posted by atleastitruns: It's quite possible that the combustibles are producing a type of gas that has a greater volume, but unless you know what is reacting, I don't think you can tell... right? Janell

That's what I'm thinking... anybody a chemist???

I mean I haven't measured, but it *has* to be higher than 56psi... I'm thinking 100-120. So there's something that isn't adding up.

Taijiguy is right.

It's true that the PSI of the reaction doesn't change. What you're doing is changing the VOLUME and the SURFACE AREA of the reaction, and that is what changes the amount of force acting on your projectile.

Doubling the volume of your combustion chamber should quadruple the force acting on the projectile. (more or less.)

If you raise the pressue too high, though, at some point the potato would just come apart and you'd have a potato-bud shotgun. But that's an engineering concern; we're talking scientific theory, here.

Ed

PS Perhaps a wax-jacketed potato would withstand higher propellant pressure...

Y'all are thinking too damn much! Just go out, find some small critters, and shoot the damn thing at them...

My buddy had a potato gun. He used to load it, and then tell his son to start running....

quote Originally posted by edhering: Taijiguy is right. It's true that the PSI of the reaction doesn't change. What you're doing is changing the VOLUME and the SURFACE AREA of the reaction, and that is what changes the amount of force acting on your projectile. Doubling the volume of your combustion chamber should quadruple the force acting on the projectile. (more or less.)

I think I understand what you're saying, but not quite sure. Now focusing on the *initial conditions* (lets pretend the spud is glued into the tube ) ........

In these two scenarios, different volumes of gas will explode, and both will be at 56 psi... The force on the potato will be the same in both. Right??? (assuming the same barrel and potato area)

quote Originally posted by ryan.hess: That's what I'm thinking... anybody a chemist??? .

ok, it happens all the time. Example: ignite 1 liter of a flammable gas. Think of the gas as a collection of molecules dispersed in the air. Lets say when this particular gas burns, it break up into three smaller molecules (decompostion product). You now have three times the volume of gas once it's burned. Even though the three by-product molecules are smaller, they each still take up as much space as one starting molecule.

It's basic chemistry. Hydrogen gas molecules are smaller than propane gas molecules, but the same number of molecules of each material (at the same temperature and pressure) will occupy the same volume of space.

So, the more by-product gases, the more volume you will create. In the case of a spud-gun, there is a fixed volume in the chamber, so pressure builds instead and eventually...boom.

quote Originally posted by ryan.hess: I think I understand what you're saying, but not quite sure. Now focusing on the *initial conditions* (lets pretend the spud is glued into the tube ) ........ In these two scenarios, different volumes of gas will explode, and both will be at 56 psi... The force on the potato will be the same in both. Right??? (assuming the same barrel and potato area)

No, the force on the potato won't be the same. More volume=more ooomph. Think of a hydraulic jack. You pump oil from a small cylinder into a large cylinder. The small cylinder may exert 10 psi, but if you spread that 10 psi across 100 square inches, you get 1000 pounds of force. This is why you can lift your car with the force you can apply with your hands.

It's the same way with the potato gun. While it's true that the actual per-square-inch pressure is the same, the force is much greater because you have more area acting on the potato. You're funneling (say) 200 square inches of force (at 56 PSI) into a 2 square inch area. So the actual pressure on the potato is 11200 pounds of force, total, or 5600 PSI. And at that point you probably no longer have a potato, but hash browns.

Ed

EDIT corrected math error

[This message has been edited by edhering (edited 11-23-2004).]

Basically, what that translates in to, in simple terms is the AMOUNT of pressurized gas available to propel the potato. In the first drawing you have a given area, which would allow for a certain amount of gas. In the second diagram, you have more space, so room for more gas. The pressure would theoretically be the same measured per inch, but you would exhaust the available explosion more quickly in the first diagram, say, enough of an explosion to actually provide a pushing force for ssay, the first few inches of travel. In the second diagrm, you would have enough force to actually push for probably the entire length of the barrel. Make more sense?

quote Originally posted by Taijiguy: Make more sense?

Yes it does! And after reading that the max pressure seen on that one website was 45psi.... that makes me feel better with my math... Man... just goes to show you the power that's waiting to be harnessed in your tires

its the same amount of force on the potato no matter what size the chamber I'm no chemist but i know if you have a chamber of say 10 cubic inches and one of 100 cubic inches with 56 psi and the potato is say 2 square inches then you will have a total force of 112 dosent matter what you do as long as the psi is the same in both chambers your not going to have more force on the potato now it would seam to be easier to increase the pressure in a small chamber (less volume required)

12,000 dollers a year for tuition am I puttin it to good use?