piston troubles

[urgle the danish cow]

I used it in an exact diameter pipe, I greased it up and it was fine, You could always sand it a little. I tried it with hot glue and had issues but its been done plenty of times before. this is the how I made the stick guns piston, ill take a picture later if you want.

its fine im doing the epoxy casting now

[SpudBlaster15]

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[Technician1002]

True, but with large diameter pistons and/or high chamber pressures (High opening forces), the effect isn't as significant as intuition would suggest. I've ran calculations for all of the high pressure copper piston cannons I've built, and in most cases, doubling the piston mass only increases the valve opening time by ~0.5-1ms. Probably not enough to significantly affect launcher performance.

This is true for higher mass projectiles. With low mass projectiles and shorter barrels, the low mass piston does make a difference. My goal with most pistons is to have them no more than twice the mass of the projectile. As pistons become more massive, a light projectile can be much further down the barrel by the time the valve is fully open. The lighter the projectile and the shorter the barrel, the more important this is.

Below is an example setup where a light piston pays off. Projectile was a piece of candy. The barrel is short. The piston and projectile are about the same mass. The valve is fully open in the same amount of time the projectile takes to move it's length in the barrel. For the rest of the travel, the valve is wide open.

The cannon below is only moderate pressure <100 PSI. Low pressure or high pressure, the distance the projectile moves as the valve opens scales to the rate the valve opens, so high or low pressure, the mass ratio does play an important role in how far the projectile moves before the valve is fully open.

[SpudBlaster15]

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[Technician1002]

Nice job. I'll have to get a paintball barrel and do some comparison shots at the same pressure.

Considering a valve is considered fully open in 1/4 the diameter, the piston can be up to 4X the mass of the projectile and still be considered fully open in less than 1 diameter of projectile travel.

With a large diameter piston, the area increases so high performance can still be reached with larger diameter and therefore heavier pistons. Attaching a 1 inch barrel onto my 2 inch cannon would be an extreme example. The smallest barrel I attach on the 2 inch is a golf ball barrel. On the 1 inch the smallest barrel is a 1/2 inch barrel for AA batteries. I did put a AA battery through the steel shell of a washing machine with only 100 PSI.

[SpudBlaster15]

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