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Idea for metal piston valve.

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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Idea for metal piston valve.

Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 4:22 pm

Has anyone made an metal piston valve spud gun that uses a spool valve like on piston hybrids. Because it would work great.
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 4:47 pm

Well there's no reason to use that type of valve. With a hybrid, you can't ignite the gas and sit around as long as you like while the combustion stays at 1000PSI; the pressure dissipates in a matter of milliseconds due to heat transfer I believe. This meant that a piston hybrid had to have an "automatic" piston valve that would open once combustion pressure peaked; this resulted in the spool design.

In a pneumatic, you can wait around all day while the pressure in the chamber stays pretty much the same. This allows a pneumatic piston valve to be opened whenever you like. This is why pneumatic piston valves have pilot valves that are triggered manually by the operator and not automatically due to pressure differential.

While your proposed idea would work, and has worked (as I've run my piston hybrid as a pneumatic on occasion), it would be unnecessary. Piston hybrids are more difficult to build seeing as they require o-rings to isolate the pilot area. The only advantage I could see is being able to have the chamber at a higher pressure than the pilot, which could help with venting. But if venting is an issue in your pneumatic piston valve, all you need is a bigger pilot valve.
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 4:51 pm

I was thinking it would avoid having a bumper inside the gun and there be almost no pressure limit on the gun. It was just an idea to make them more efficient I guess.
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 4:55 pm

Well you still need a bumper in a piston hybrid. Granted, an all-metal piston valve design with metal fittings and such would allow higher pressures to be used, the specific spool piston hybrid design that SB15 and I use is not essential to such a design. You could just make the gun out of metal fittings with a decent piston valve but it wouldn't need o-rings to isolate the pilot or a spool to vent the valve.
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 6:03 pm

What do use as a bumper on your hybrid?
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 6:29 pm

I keep forgetting what they're called but it's a little like this except it's all the same diameter and has more 'rings'. You can see it at the back of my pilot housing in the latest video on my youtube channel.

There are much better alternatives, especially for pneumatics. SB15 and I have just found that these weird rubber plumbing things are alright as bumpers.
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 7:19 pm

Looks tough!!!

How did you figure out how much pressure to add to the pilot when fueling your piston hybrid?
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 7:27 pm

8tonsemi wrote:How did you figure out how much pressure to add to the pilot when fueling your piston hybrid?
Force = Pressure x Area. What more do you need to know?
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 8:15 pm

Im sorry i'm going to sound like a real nag with this but a haven't gotten a really good answer for this what is the formula for manometric metering?
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 8:55 pm

what is the formula for manometric metering?

Research + Spudfiles = Hybrid Knowledge

Freakin' hell... how many times do we have to give this link to you? You basically want us to spoonfeed you the exact calculations, right?
http://www.spudfiles.com/forums/hybrid- ... 13602.html

Part 2: Using chamber pressure to achieve stoichiometry

DYI brought this up in another thread as he intends to use it on a hybrid launcher, so I figured the "chamber pressure meter" had a place in this thread.

As you could have guessed, the chamber pressure in a closed vessel will rise proportionally to the amount of gas added to it. This forms the basis for an accurate metering setup using only the chamber pressure as reference. Propane is injected until the chamber reaches a certain pressure, then the supply is cut off, and air is added.

To determine the chamber pressure required to obtain a specific volume of fuel, we must first calculate the volume of fuel required. Out comes the 100ci chamber, which will be using a 10x mix (lower mixes really aren't very practical with such a metering setup). The required fuel volume for stoichiometry is;

VP = (100*10)/0.958-(100*10)
VP = 44 cubic inches

Now, after adding the 44 cubic inches of propane to the 100 cubic inches of air, your chamber pressure will has risen. By how much exactly? To figure that out, you must first figure out what fraction of the chamber's volume you have added. In this case, it is 44/100 = 0.44. Because one atmosphere is equal to 14.7 psi, multiply the fraction by 14.7 to obtain your result.

PC = 0.44*14.7
PC = 6.44 psi

This pressure will be constant no matter the chamber volume, but will vary depending upon the fuel:volume ratio.

Hopefully this guide has been helpful, and will prevent people from inaccurately fueling hybrids in the future. As I said before, if there is something you don't understand, read it again until you do, or if that fails, ask about it.


8tonsemi, this may sound harsh but... if you cannot figure it out from that link, YOU SHOULD NOT BE BUILDING HYBRIDS. Everything you need to know, calculation wise, is in that link. If you do not understand it, forget about building hybrids until you do understand it. No will spoonfeed you this information.

Same thing goes with the pilot pressure calculations. Don't build a hybrid if you don't know how to do the math.

Edit: To other members, this is the reason for my response:

http://www.spudfiles.com/forums/viewtop ... tml#352283
http://www.spudfiles.com/forums/manomet ... 24172.html
http://www.spudfiles.com/forums/viewtop ... tml#350324
http://www.spudfiles.com/forums/viewtop ... tml#350323
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 9:17 pm

The reason I don't understand most of these calculations is probably cuz I'm in the 8th grade. I've heard that you need to know ninth grade math to figure it out. Sorry for bothering you(8tonsemi).
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Unread postAuthor: MrCrowley » Wed Jun 20, 2012 9:26 pm

Not sure if that requires 9th grade math; then again I can't remember what I would have done in the equivalent of 8-9th grade math. Anyhow, if you're in the 8th grade you probably shouldn't be building hybrids.

You're barely a teenager, I wouldn't expect someone like you to know everything there is to making hybrids so why are you so intent on trying to make one when you don't really grasp everything? Give it a few years. I was building basic pneumatics and combustions at that age. Although I didn't make a hybrid until I was about 16-17, it would've been possible when I was 15. Other members have made hybrids at age 15 as well. Just give it time.
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Unread postAuthor: 8tonsemi » Wed Jun 20, 2012 9:42 pm

Thanks. I'm going to have to do a lot more research before making one. I have never actually attempted making one. I'm just trying to gather up enough knowledge. so when I build one I will able to safely.Thanks again for the help :lol:
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Unread postAuthor: mattyzip77 » Wed Jun 20, 2012 10:22 pm

Im 35 and I barely understand it. Thats why I havent even attempted a hybrid yet. That and the fact that air is free and fuel isnt!! :lol:
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Unread postAuthor: D_Hall » Wed Jun 20, 2012 11:31 pm

MrCrowley wrote:Piston hybrids are more difficult to build seeing as they require o-rings to isolate the pilot area.

Not necessarily. Or at least, I hope not as I'm preparing to build one sans o-rings to isolate the pilot area.

Solution: A labyrinth seal and a non-combustible mixture in the pilot area.




edit: And as an aside... The pressure decay of a hybrid due to energy loss at the chamber walls is highly variable upon chamber design. Spherical is better. And bigger is better. For an extreme counter example (yes, I know, I'm good at those), Vera's burst disk doesn't go until nearly 80 ms after ignition. That's when the disk goes. We had shots on her demonstrating high pressures all the way through 250 ms (at which point projectiles were clear and such so pressure dropped regardless). It's all about the ratio between surface area and volume. The higher the ratio, the less you have to worry about pressure decay.
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