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Some quick questions

Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Some quick questions

Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 5:16 pm

I'm building a QEV semi, but i like customizability, so i decided to make it able to fire full automatically and as a powerful hybrid. To do this I'm just removing the 3-way valve and putting on a a pop-off and and a quick connect fitting. with an externally metered propane mix I hope to go up to 4x.

My question is, what kind of pressure will this create? And will a 3/4" QEV be able to handle it? lastly, what pressure pop off do I need?
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Unread postAuthor: POLAND_SPUD » Mon Jul 12, 2010 5:26 pm

you don't need the pop-off and QEV... all you need is a plain piston valved hybrid... there have been at least a couple of such guns

what kind of pressure will this create
use HGDT

Also if you want to build a semi hybrid then you need to find a way to vent the chamebr after each shot... well not to mention that it neds a loading mech too

have a look here -> http://www.spudfiles.com/forums/spring- ... 20439.html
you might find some of it useful
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Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 5:29 pm

sorry, I was unclear. A NORMAL piston hybrid, single shot
can't download HGDT, I have a mac :oops:
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Unread postAuthor: POLAND_SPUD » Mon Jul 12, 2010 7:04 pm

IIRC someone posted a similar thread yesterday...
rag said that 1X mix is able to generate about 100 psi... so 4X = 400psi
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Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 8:28 pm

so the QEV piloted by a 250psi pop-off will work @ 4X?
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Unread postAuthor: Ragnarok » Mon Jul 12, 2010 8:42 pm

Probably. But 100 psi per X is only a rough rule, and is more to be considered the probable maximum than necessarily a value that will be reached.

As far as theoretical maximum, it's about 120 psi at 1X... I think. <s>I'd check it in GasEQ, but for some (presumably stupid) reason, I can't get it to work today.</s>

Okay, maybe I can. 121.8 psi, apparently, for a 1X mix. 541.9 psi for a 4X mix. (It may not seem perfectly linear, but the absolute pressures are reasonably thus.)
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Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 8:46 pm

so 1" galvanized chamber with a 3/4" QEV will be safe?
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Unread postAuthor: Ragnarok » Mon Jul 12, 2010 8:46 pm

Define "safe". :tongue3:
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Unread postAuthor: Lockednloaded » Mon Jul 12, 2010 8:48 pm

safe for anything not down range :lol:

edit: so i'd probably triple the performance of my current 200 psi launcher right?
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Unread postAuthor: Ragnarok » Mon Jul 12, 2010 10:06 pm

Lockednloaded wrote:safe for anything not down range

Again, define safe. There are different degrees of safe - unlikely to cause injury, unlikely to suffer failure, weapons testing range level of safety...

I guess it's probably safe by the normally perceived standard of safe the forum seems to hold, but ultimately, most launchers are somewhat kludged contraptions that use parts that were never intended for the purpose.

so i'd probably triple the performance of my current 200 psi launcher right?

Probably not. Double... maybe. Triple... well, that depends on how many flow and transonic losses your other launcher is getting.
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Unread postAuthor: Lockednloaded » Tue Jul 13, 2010 7:14 am

I want the launcher to be safe to fire from my hands over and over again without any flukes. I want the peak pressure to stay at or below the pressure rating on my chamber.

has anyone ever built a piston hybrid with a QEV piloted by a pop-off
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Unread postAuthor: Technician1002 » Tue Jul 13, 2010 9:22 am

A pop off is a limited volume device. It has a rated flow rate when open to release energy in a safe mannor.

Due to the pilot area volume and finite time to vent it, the delay in the design may be too slow to protect you from peak pressures.

A regular piston hybrid would be much faster opening. The proper selection of the piston ratio could set the desired pressure rise at opening.

The pop-off could be still used in the pilot area to act as a dashpot relief valve to remove some of the energy stored in the pressure in the pilot when the piston does open. This would reduce the force the piston would have to reclose when the chamber pressure no longer holds it open.
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Unread postAuthor: Lockednloaded » Tue Jul 13, 2010 10:32 am

with a lower pressure pop-off, I don't see why it wouldn't open and stay open at the ignition point. I may lose power by having the pop-off pop before the pressure can build, but safety first.
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Unread postAuthor: Technician1002 » Tue Jul 13, 2010 11:46 am

If you depend on the pop off to pilot the valve, there is an obstruction between the chamber and it's pressure rise and the pilot area with the pop off valve. If the sudden rise in chamber pressure does not lift the QEV, the pilot area will have to wait for the chamber pressure to bleed into the pilot area before the pop off valve opens to pilot the main valve. By the time the pilot area has risen enough in pressure, the main chamber may be over pressure.

Remember the pop off on a QEV is not responding directly to the chamber in real time. There is a delay for the pilot to rise too.

Most likely what will happen is the chamber pressure rise will create enough of a pressure differential on the QEV and it will pilot itself, then the pilot area will rise in pressure and the pop off will open. By the time this happens, the projectile is well on it's way down range. The difficulty is the pop off is not governing the pressure the QEV will open so the pressure and safety is unknown.
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Unread postAuthor: ramses » Tue Jul 13, 2010 11:53 am

The solution would be using the exhaust of a pop-off connected to the chamber to open a pilot valve on the QEV.
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