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vacuum assisted pilot?

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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vacuum assisted pilot?

Unread postAuthor: drex » Wed Jun 04, 2008 4:01 pm

i was just wondering if you could increase a piston valves performance by hooking up a vacuum to the pilot like this

piston->pilot->vacuum


when the pilot valve opens the vacuum sucks all of the air out of the pilot volume.

i think it should work but i don't know if it would be worth it.

any thoughts?
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Unread postAuthor: SpudFarm » Wed Jun 04, 2008 4:06 pm

should not matter at all (a tiny bit)
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Unread postAuthor: D_Hall » Wed Jun 04, 2008 4:08 pm

Won't help. The flow coming out of the pilot is already choked.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 04, 2008 5:07 pm

D_Hall wrote:Won't help. The flow coming out of the pilot is already choked.


Ah, but won't the greater pressure difference mean that the air coming out will do so at a somewhat higher vecloity and therefore vacate the chamber quicker?
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Unread postAuthor: potatoflinger » Wed Jun 04, 2008 5:11 pm

I think it will help, but it won't really help nearly enough to be practical, especially if you have to carry around a vacuum.
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Unread postAuthor: drex » Wed Jun 04, 2008 5:13 pm

thats what i thought, and maybe have a vacuum chamber connected to the pilot
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Unread postAuthor: Eddbot » Wed Jun 04, 2008 5:32 pm

if it was too effective, it could suck the air out of the chamber before the piston actuates (theoretically)
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Unread postAuthor: Lentamentalisk » Wed Jun 04, 2008 6:09 pm

Eddbot wrote:if it was too effective, it could suck the air out of the chamber before the piston actuates (theoretically)

no. A vacuum is only -14.7psi.

it will not have a noticeable effect, except possibly with cannons working at 30psi or less. Any cannon running at much more than that it would be too small of a fractional difference to really change anything. As D_Hall said, the flow is already restricted.

Then you run into the issues of how you will create this vacuum. To have a vacuum you need some sort of containment, to keep the rest of the atmosphere from rushing in. That means that your pilot will have to be venting into this container, so the pressure will go up more quickly than it would when venting into the ambient air, reducing its effectivity.

it would not make much of a difference, and take up much more space than a bigger valve, so it is not practical.
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Unread postAuthor: DYI » Wed Jun 04, 2008 6:48 pm

A good tool to keep confusion at bay when talking about vacuums is to remember that the atmosphere doesn't suck, it blows. :roll:

And as has been said already, with a cannon operating at any useful pressure, it won't make a noticeable difference.
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Unread postAuthor: MrCrowley » Wed Jun 04, 2008 11:28 pm

You seem to have got your answer. It's good that you're thinking of new ideas though. Keep it up :)
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Unread postAuthor: dongfang » Tue Jun 10, 2008 2:10 pm

Hi,

I agree that sucking air out of the pilot probably will not make a difference. Besides, beyond a certain opening speed (which is not difficult to achieve), the returns for even better speeds start to diminish.

BUT if you have some serious vacuum, I have an idea: Place a thin burst disk at the muzzle, and evacuate the barrel! That could be interesting.

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Unread postAuthor: jimmy101 » Tue Jun 10, 2008 5:27 pm

dongfang wrote:BUT if you have some serious vacuum, I have an idea: Place a thin burst disk at the muzzle, and evacuate the barrel! That could be interesting.

Regards
Soren

That's been done, google "vacuum canon".

I doubt the increase in performance would be all that great considering the amount of effort required (vacuum system + burst disk). Vacuum in the barrel would do two things; reduce the mass that has to be accelerated and increase the net pressure on the ammo.

The density of air is only ~1g/L. A 2" x 10' barrel is about 6L, so the air in the barrel is ~6g. Compared to say a 100g projectile the mass of the air is insignificant. If you were firing a very low mass projectile, like a ping pong ball, it would be a much larger affect.

And, as with the "vacuum pilot volume" you are talking about adding ~15psi to the net propelling force on the shell. Probably a lot easier to just boost the pressure in the chamber or increase the speed of the valve.
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Unread postAuthor: D_Hall » Tue Jun 10, 2008 6:10 pm

jackssmirkingrevenge wrote:Ah, but won't the greater pressure difference mean that the air coming out will do so at a somewhat higher vecloity and therefore vacate the chamber quicker?

No.
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Unread postAuthor: jackssmirkingrevenge » Wed Jun 11, 2008 12:35 am

D_Hall wrote:No.


Don't trample over my dreams, you'll be telling me Santa doesn't exist next :?

por qué?
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