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A crossbow pneumatic idea

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Re: A crossbow pneumatic idea

Unread postAuthor: velocity3x » Sun Jun 28, 2009 11:36 pm

boyntonstu wrote:
Imagine a .375 piston 6" long.

The 80 pounds on a .375" diameter (0.110 square inches) would create 727 PSI
6" x .11 = .66 cu-in.


IN A CLOSED CHAMBER:
727 psi is 49.5 atmospheres Your 6.0" x .375 cylinder, if a closed tank, would have to be pressurized with 32.67 in3 of gas in order to reach 727psi. Or, to state it another way....Your 6" x.375 cylinder would have to be pressurized to 49.5 times it's empty volume to reach 727 psi.

I assume you won't be using a closed tank and will be sending pressure from the piston directly into the open barrel breach with only one stroke of the piston from the bow string. In this case I doubt you'll achieve 727 psi but....I could be wrong.
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Re: A crossbow pneumatic idea

Unread postAuthor: boyntonstu » Mon Jun 29, 2009 7:02 am

velocity3x wrote:
boyntonstu wrote:
Imagine a .375 piston 6" long.

The 80 pounds on a .375" diameter (0.110 square inches) would create 727 PSI
6" x .11 = .66 cu-in.


IN A CLOSED CHAMBER:
727 psi is 49.5 atmospheres Your 6.0" x .375 cylinder, if a closed tank, would have to be pressurized with 32.67 in3 of gas in order to reach 727psi. Or, to state it another way....Your 6" x.375 cylinder would have to be pressurized to 49.5 times it's empty volume to reach 727 psi.

I assume you won't be using a closed tank and will be sending pressure from the piston directly into the open barrel breach with only one stroke of the piston from the bow string. In this case I doubt you'll achieve 727 psi but....I could be wrong.


The piston compresses the air during the stroke and the chamber cylinder is closed until the piston almost reaches the end of its stroke.

At that point a relief valve or a valve actuated by the piston rod extending outside the chamber quickly opens a valve to feed the barrel.

Lengthen 6" stroke to 24"; increase the force to 150lb (typical crossbow), and reduce the .375 to .300 and you will have some mean pressure.

BoyntonStu
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Re: A crossbow pneumatic idea

Unread postAuthor: Technician1002 » Mon Jun 29, 2009 7:30 am

boyntonstu wrote:
velocity3x wrote:
boyntonstu wrote:
Imagine a .375 piston 6" long.

The 80 pounds on a .375" diameter (0.110 square inches) would create 727 PSI
6" x .11 = .66 cu-in.


IN A CLOSED CHAMBER:
727 psi is 49.5 atmospheres Your 6.0" x .375 cylinder, if a closed tank, would have to be pressurized with 32.67 in3 of gas in order to reach 727psi. Or, to state it another way....Your 6" x.375 cylinder would have to be pressurized to 49.5 times it's empty volume to reach 727 psi.

I assume you won't be using a closed tank and will be sending pressure from the piston directly into the open barrel breach with only one stroke of the piston from the bow string. In this case I doubt you'll achieve 727 psi but....I could be wrong.


The piston compresses the air during the stroke and the chamber cylinder is closed until the piston almost reaches the end of its stroke.

At that point a relief valve or a valve actuated by the piston rod extending outside the chamber quickly opens a valve to feed the barrel.

Lengthen 6" stroke to 24"; increase the force to 150lb (typical crossbow), and reduce the .375 to .300 and you will have some mean pressure.

BoyntonStu


The simple solution is to omit the valve to omit an energy loss point. Put the projectile in the cylinder so at the end of the crossbow stroke, the space between the piston and projectile contains 2% of the cylinder volume (this space will be traveling) so the piston comes to a stop and the projectile picks up speed. The compressed air will only be a spring newtons cradle style to transfer kinetic energy from the bowstring and piston to the projectile with high efficiency.

In order for the projectile to be in that location at the end of the piston stroke, the unrestrained projectile will need to have an initial position further back towards the piston as it will start to move as the piston compresses the air. Energy transfer will be an elastic collision with air as a spring.
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Re: A crossbow pneumatic idea

Unread postAuthor: boyntonstu » Mon Jun 29, 2009 9:08 am

Technician1002 wrote:
boyntonstu wrote:
velocity3x wrote:
boyntonstu wrote:
Imagine a .375 piston 6" long.

The 80 pounds on a .375" diameter (0.110 square inches) would create 727 PSI
6" x .11 = .66 cu-in.


IN A CLOSED CHAMBER:
727 psi is 49.5 atmospheres Your 6.0" x .375 cylinder, if a closed tank, would have to be pressurized with 32.67 in3 of gas in order to reach 727psi. Or, to state it another way....Your 6" x.375 cylinder would have to be pressurized to 49.5 times it's empty volume to reach 727 psi.

I assume you won't be using a closed tank and will be sending pressure from the piston directly into the open barrel breach with only one stroke of the piston from the bow string. In this case I doubt you'll achieve 727 psi but....I could be wrong.


The piston compresses the air during the stroke and the chamber cylinder is closed until the piston almost reaches the end of its stroke.

At that point a relief valve or a valve actuated by the piston rod extending outside the chamber quickly opens a valve to feed the barrel.

Lengthen 6" stroke to 24"; increase the force to 150lb (typical crossbow), and reduce the .375 to .300 and you will have some mean pressure.

BoyntonStu


The simple solution is to omit the valve to omit an energy loss point. Put the projectile in the cylinder so at the end of the crossbow stroke, the space between the piston and projectile contains 2% of the cylinder volume (this space will be traveling) so the piston comes to a stop and the projectile picks up speed. The compressed air will only be a spring newtons cradle style to transfer kinetic energy from the bowstring and piston to the projectile with high efficiency.

In order for the projectile to be in that location at the end of the piston stroke, the unrestrained projectile will need to have an initial position further back towards the piston as it will start to move as the piston compresses the air. Energy transfer will be an elastic collision with air as a spring.


I see a problem.

As the pressure builds, the projectile will move and a new volume will be created.


Imagine a long barrel and a piston traveling to half the barrel length.


It seems to me that the best results could be achieved if the projectile sees the highest pressure before it moves.

BTW A multiple speargun rubber system would be a sleeker alternative to a bow.

The rubber straps could be cocked one at a time onto a triggered sliding shuttle.

I can visualize 250-300 pound of force and a 24" stroke.


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Unread postAuthor: THUNDERLORD » Mon Jun 29, 2009 7:13 pm

Why not invert it then?,
And use the pressure to cock the bow string...
(Van Helsing style) ...Dry ice...Co2 @ 800psi pressing piston in a chamber, to a spring (for mechanical spring action, like bow)???
How large spring or bow could a 1" surface area @800+psi cock back???8)
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Re: A crossbow pneumatic idea

Unread postAuthor: Technician1002 » Mon Jun 29, 2009 9:25 pm

boyntonstu wrote:
I see a problem.

As the pressure builds, the projectile will move and a new volume will be created.


BoyntonStu


Not a problem if designed properly. Imagine a chamber with a Negative volume. Kind of hard, but the chamber starts inside the pump and during the piston stroke, moves toward the barrel, AKA moving chamber mentioned earlier.

If the chamber did not move, the piston would go through it making the negative volume and smashing into the projectile.

It only remains positive in volume because the projectile does move during the piston stroke. This highly compressed trapped air is the small chamber. The chamber contains the air spring. As the spring pushes back on the piston, it slows to a stop and the projectile accellerates.

Your peak pressure may be less than the ideal. But This is important, as the piston follows the chamber air pocket and projectile, the pressure applied has a LONGER dwell time at pressure for energy transfer to the projectile. There is no reason to try to valve, store and release the compressed air.
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Last edited by Technician1002 on Mon Jun 29, 2009 10:36 pm, edited 1 time in total.
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Unread postAuthor: Hawkeye » Mon Jun 29, 2009 9:31 pm

A fire piston diesels with the force that you can depress with one hand.
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