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"In the world of spuds today"

A place to ask general spud cannon related questions.
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Unread postAuthor: jackssmirkingrevenge » Sat May 07, 2011 6:45 am

ramses wrote:And I still need advice on a bumper for a 4" toolie valve. I was thinking multiple tennis ball halves.


As I suggested before, air bumper ;) the only disadvantage in this case is a moving pilot valves but there are other possbile configurations.
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Unread postAuthor: mark.f » Sat May 07, 2011 9:26 am

ITWOST, and related to the topic at hand, I'm recalling something SB15 said that made quite a bit of sense.

With the same size piston, no matter what its weight (to an extent), the same amount of work is performed on it by the compressed gas in the chamber. You could argue that since a heavier piston moves slower, less work will be done on it by the gases since more gas will escape out the barrel before the piston is at the end of it's travel.

Therefore, shouldn't the energy of the piston at the end of its travel be about the same for heavier or lighter pistons (of the same diameter)?
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Unread postAuthor: Technician1002 » Sat May 07, 2011 9:43 am

The weight of the piston if adjusted properly will efficiently transfer energy to compressing gas and then to the projectile.

In physics you are looking to achieve an elastic collision with an air spring. You are looking to achieve close to 100% energy transfer. This would result in the first piston losing all energy (stopped) at the same time the second (projectile) has taken all the energy (joules) so the smaller projectile has the same energy the heavy piston, but at higher velocity to contain the same energy.

Ideally the piston should slow rapidly at the end of travel and come to a stop with no rebound or forward motion into the end of the chamber with all energy transferred to the projectile.
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Unread postAuthor: saefroch » Sat May 07, 2011 11:51 am

ITWOST: An arbitrary amount of epoxy will cure in about 5s when you set the toaster oven to "broil."

mark.f wrote:Therefore, shouldn't the energy of the piston at the end of its travel be about the same for heavier or lighter pistons (of the same diameter)?
Yes. The discussion starts here. It wasn't just SB15 :wink:.

You are looking for an elastic collision of sorts, between the piston and the bumper. You want to convert all the K<sub>e</sub> to U<sub>s</sub>. Simply put, mv<sup>2</sup>=kx<sup>2</sup>
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Unread postAuthor: Fnord » Sat May 07, 2011 12:16 pm

Therefore, shouldn't the energy of the piston at the end of its travel be about the same for heavier or lighter pistons (of the same diameter)?


Yes- Remember that in the case of projectiles, low mass is innefficient because of the speed at which the propellant gas can travel; pistons never come close to this speed and therefore (energy = force * distance) is still fairly accurate.

However! Keep in mind that a heavy piston can still do more damage due to momentum conservation effects. Shooting a barrel full of water and a lead slug will yield similar energy efficiency, but the kick from the water shot will be much more painful to you, just as a heavy piston will kick your bumper 'harder'.
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Unread postAuthor: Labtecpower » Sat May 07, 2011 1:36 pm

Today I tweeked the sights on my copper gun, and tried out a new projectile. I got 1" grouping on a distance of 8 meters. Pretty cool for a smoothbore gun I think!

I like the projectiles because they don't tumble in flight. All the shots made nice round holes in the furniture board.
Anyone wants pics?
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Unread postAuthor: ramses » Sat May 07, 2011 2:00 pm

The momentum is what concerns me. The air bumper is not feasible, since I am under a big time crunch. Not only do I have to finsh the cannon, but I have to make the robot that will hold it, and attach a sight and webcam, and write (easy) code to control it. We're using the TETRIX platform with RobotC.

I think I'll just fill the pilot area up with rolled up rubber scraps, tennis ball halves, etc.
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Unread postAuthor: saefroch » Sat May 07, 2011 4:12 pm

The kinetic energy of the piston when fired at 15psi should be manageable, no problem (I estimate 22J). A fairly stiff spring could even be used as a bumper, but at 220psi(320J :?)... :roll: bits of rubber are as good as anything.
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Unread postAuthor: ramses » Sun May 08, 2011 10:56 pm

Was your estimate based on d/4 and uniform acceleration thorugh that time frame?
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Unread postAuthor: Lockednloaded » Sun May 08, 2011 10:58 pm

Labtecpower wrote:Today I tweeked the sights on my copper gun, and tried out a new projectile. I got 1" grouping on a distance of 8 meters. Pretty cool for a smoothbore gun I think!

I like the projectiles because they don't tumble in flight. All the shots made nice round holes in the furniture board.
Anyone wants pics?


Sure, why not?
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Unread postAuthor: mark.f » Sun May 08, 2011 11:03 pm

Lollipops make pretty neat projectiles. Can put one through tin roofing with a spray n' pray combustion, so... :D
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Unread postAuthor: Technician1002 » Sun May 08, 2011 11:29 pm

Hmm, a jawbreaker on a stick. I like it. Jawbreakers, lemonheads, and gumballs all make great projectiles.
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Unread postAuthor: Labtecpower » Thu May 12, 2011 10:12 am

60 gram potato slug @ 80 psi

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Unread postAuthor: saefroch » Thu May 12, 2011 3:13 pm

ramses wrote:Was your estimate based on d/4 and uniform acceleration thorugh that time frame?
Sorry I didn't see this until now. No, actually it wasn't. I grabbed enough points from the GGDT graph to do a quartic regression on my graphing calculator, then assumed the piston would only retract far enough to open to full flow before it stopped gaining energy, and used my calculator to take the derivative of the graph when the piston would be retracted fully.
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Unread postAuthor: ramses » Thu May 12, 2011 9:12 pm

Yay! Someone else who knows how to use a calculator! Although I'm pretty sure GGDT just plots something like 10 points on the graph, and connects them with some kind of spline that fits them.
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