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Modified QDV for hybrid use

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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Modified QDV for hybrid use

Unread postAuthor: Fnord » Fri Jan 07, 2011 1:00 pm

Since I haven't been contributing anything but sarcasm lately I figured I might as well post one of my non-classified designs to see if anyone can find any bugs or critical flaws I missed.


This is a hybrid version of a Quick Dump Valve that I think may have some advantages over SB15 and Crowley's piston valve launches.

1. More compact depending on the design.
2. Possibly easier to seal.
3. Piston is probably a little lighter, depending on the materials used
4. Probably sturdier if built correctly(super-high mix capable)

disadvantages:
1. Takes longer to make(not all off-the-shelf part)
2. Might require a drill press
3. The piston's housing will require some modding if standard pipe fittings are used. Note that the diagram shows a largely custom housing that I intended to build eventually, but I'm fairly sure off-the-shelf fittings can be substituted.
4. Piston reset rod may tend to fly through your arm if you get in the way.


How it works:
As the mix is ignited, pressure on the inner stepped-piston rises.

The pressure overcomes the piston spring's resistance, and the piston tries to move.
Since it can't move upward(the rod in the spring is pushing against the rod fixed across the barrel port), it pushes the main piston downward- fast.

Once the main piston has moved enough, the orings no longer seal the port, and the pressure is release. The valve is then pushed open the rest of the way by the pressure build-up in front of it, like a standard QDV.

The opening pressure can be controlled by adjusting the spring pressure, rather than the pilot mix pressure. This can be either an advantage or disadvantage depending on how you look at it. It makes fueling easier but makes changing opening pressure more difficult.


By posting this I do, of course, give anyone and everyone permission to use the design in their own projects. This may even be simple enough to make a scaled-down version from epoxy and pharmaceuticals. hint.
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1 square = 1/4"
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(Do not ask why this hybrid needs 1/2" thick steel/fiberglass composite walls)
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Unread postAuthor: POLAND_SPUD » Fri Jan 07, 2011 1:52 pm

I don't get it... where is the chamber ?? It doesn't make sense to me - the system is balanced so what causes the valve to move ?
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Unread postAuthor: Fnord » Fri Jan 07, 2011 2:02 pm

The area inside the yellow is the chamber, not the barrel. There is no barrel shown.

QDVs are natually balanced. In a normal QDV you start the valve opening by hand, in this design the small piston gives it a kick to open it.
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Unread postAuthor: POLAND_SPUD » Fri Jan 07, 2011 2:09 pm

yeah I guessed that but it still seems 100% balanced to me
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Unread postAuthor: Technician1002 » Fri Jan 07, 2011 2:16 pm

To prevent accidents with the rod, it may be best to be unattached to the piston and simply used for the initial reset and then removed.

My ABS cannon uses a rope pull to open the valve and a removable rod which is used for closing the valve.

Nice design. The combustion pressure rise moves the small piston in the center which tries to extend and pushes on the piston stop rod which displaces the main piston to the rear.

The port does not need to open the same distance as the barrel diameter as shown in the drawing. The total piston motion and therefore the total kinetic energy of the piston can be reduced with a shortened stroke.

Try shortening the port to about 1/2 the barrel diameter and then shorten the piston to reduce mass, and then shorten the pilot area to match.
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Unread postAuthor: POLAND_SPUD » Fri Jan 07, 2011 2:31 pm

lol but I still don't get it... what opens the valve ??
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Unread postAuthor: Technician1002 » Fri Jan 07, 2011 2:34 pm

When the inner piston pushes out due to a rise in pressure, the piston can't extend due to hitting a rod crossway in the barrel. As a result the inner piston extending pushes the main piston toward the pilot.
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Unread postAuthor: Fnord » Fri Jan 07, 2011 2:34 pm

Poland, here:

Imagine we have this completely balanced valve(image 1). You can increase the pressure as much as you want and it's not going to move.


Now lets say we put a spring and a little piston inside the big piston.
If you increase the pressure, the little piston will move and compress the spring, but nothing else will happen.


Finally we put a bolt in the way of the little piston's rod. When the pressure increases the rod will hit the bolt and shove the whole thing backwards, opening the valve.


Tech: Good point with the porting size and travel distance. I guess I can make the hole closer to a square shape than a circle for better flow.
I suppose it would be wise to just leave the reset rod "floating" also. Maybe with a weak spring to keep it in the open position.
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Unread postAuthor: POLAND_SPUD » Fri Jan 07, 2011 2:40 pm

ohh yeah somehow I missed the rod in the pic..

it should work but it's practical only for large bore guns - something tells me it might not work well with a 10mm valve
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Unread postAuthor: Lockednloaded » Fri Jan 07, 2011 7:22 pm

On the internal piston, It would probably be better to use a stepped piston so it will really pop open
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Unread postAuthor: Technician1002 » Fri Jan 07, 2011 7:52 pm

QDV valves pop open when they are cracked open. Don't let the trigger piston fool you. It is there just to get it started.
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Unread postAuthor: saefroch » Fri Jan 07, 2011 7:54 pm

I like the design :D Innovative, certainly. I do have one possible issue with it though... it uses a conventional spring. I don't like those, because it means that if one were to significantly change the mix number, one would also have to change the spring. In addition, it may be difficult to make it work with available springs.

I do see an opportunity to make it adjustable though, thread the other side of the internal piston, place a nut or two on it, and have the piston stop with the nut, not the entire internal piston. Changing the position of the nut(s) could change the popping pressure.
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Unread postAuthor: Fnord » Sat Jan 08, 2011 12:41 pm

LnL: Actually, the internal piston IS a stepped piston. Note how the sealing face is tight against the little port. Once it moves a little, BAM. This design has a surface area increase of about 20 times.

saefroch: Well, the initial area exposed to pressure is only .012"^2, so if you wanted it to open at say, 2000 psi, you'd only need 24 pounds keeping it closed. Since I wouldn't want much more than 2kpsi in this anyway I don't think multiple springs would be necessary.

I doodled a little earlier to make a design with optimized dimensions and came up with one that allows the threaded rod to be screwed in further, so in the end anywhere from say, 5 to 25x could be used with one spring.

Basically it'll work like what you suggested, but you don't have to take the small piston out. You still have to take out the big piston, but it requires no tools since there're no air-tight threads to worry about.

The only thing I don't like about air springs in hybrids is that hybrids are kind of hostile towards orings. Believe me, I don't like the fact that this valve already depends on 2 of them to seal. If I ever get around to building it I'll probably use the hardest high-temp silicone orings I can find.
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Unread postAuthor: saefroch » Sat Jan 08, 2011 3:39 pm

I've been thinking about ways to replace o-rings, and since a hybrid QDV would seem to produce almost unidirectional wear on the seals, it might be possible to make a pseudo o-ring using a UHMWPE ring and belleville disc springs to stretch it a little and form a robust seal. The UHMWPE could be made much thicker than a normal o-ring (1/16"), and shouldn't wear nearly as much.

Fnord wrote:I wouldn't want much more than 2kpsi in this anyway
What about lower mixes? Or are we assuming we have no interest in less than maximum power :roll:....
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Unread postAuthor: Fnord » Sat Jan 08, 2011 4:16 pm

Not sure about UHWMPE. Everything about it sounds great except the lack of heat resistance, which is my primary concern. Yes, low mixes won't be very much trouble, but after 10x the high density of gases tend to transfer a lot of heat to everything they come in contact with. I remember seeing a few plastic barrels wherein the flames found a sharp-angled edge to carbonize and it all got worse from there. DYI had one I think. I've never messed with UHWMPE but I don't think it streches very much so replacing the o-rings may be difficult.



Or are we assuming we have no interest in less than maximum power

I AM AN AMERICAN!

[youtube]http://www.youtube.com/watch?v=ereEhTuBlwA[/youtube]
But yes, this was intended for high-power low-weight designs. If I ever actually build it the way I intend to, I'm not sure where hand-helds will go from there. This is approaching the limit of what can be achieved without special tools and materials, as well as the limit of my collar-bone's impact strength.
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