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New Guy with some Qs

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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New Guy with some Qs

Unread postAuthor: BigBang J » Thu Jul 08, 2010 12:23 pm

First of all hello everybody, I'm new here just signed on today.

I have been working on a hybrid on and off for probably 2 years now.
I am constructing a pellet gun so it is a little on the small side compared to a spud gun. My chamber volume is about 54.9 ml. I wont go into detail now, hopefully I'll get a post up when I'm done with specs, pics and everything. I quick thought, I have done some beta testing with it and can propell a steel bb through both sides of a tin can.

Alright now to the questions

1st I have been considering what is the best type of valve to use for the escaping high pressure hot gasses that propell the projectile. The valve I tested with was a 175 psi pop valve off of a air compressor, I soldered a tube from the exit port of the valve and directed it to the barrel. But what are you guys using for this valve?

2nd what do you guys mean when you say 2x or 4x for a hybrid.
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Re: New Guy with some Qs

Unread postAuthor: jackssmirkingrevenge » Thu Jul 08, 2010 12:28 pm

BigBang J wrote:1st I have been considering what is the best type of valve to use for the escaping high pressure hot gasses that propell the projectile. The valve I tested with was a 175 psi pop valve off of a air compressor, I soldered a tube from the exit port of the valve and directed it to the barrel. But what are you guys using for this valve?


Most people use either burst disks or piston valves, have a look round the hybrid section and you'll get the idea.

2nd what do you guys mean when you say 2x or 4x for a hybrid.



1x is say 4% conc. propane at atmospheric pressure - an ordinary unpressurised combustion cannon.

2x is say 8% conc. propane at 15 psi (so internal pressure is twice atmospheric, hence 2x)

3x is say 12% conc. propane at 30 psi (so internal pressure is three times atmospheric, hence 3x)

4x is say 16% conc. propane at 45 psi (so internal pressure is four times atmospheric, hence 4x)

etc.
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Unread postAuthor: Technician1002 » Thu Jul 08, 2010 1:08 pm

A pop off valve by design has a safe flow rate for relieving overpressure in a compressor. They do not open for wide open flow, so they are not the best valve for a high flow application. They work with a pressure differential on them at all times. It is preferable to use a valve that will pop open with as low a pressure drop on the valve as possible for the highest possible pressure behind the projectile. Pistons, burst disks, and diaphragm valves are much better suited for the application.


In the above post, the 8% and 12% examples are misleading. The ratio of fuel to air remains the same. The fuel is doubled for a 2 X shot and the air is also doubled. When done you have 2 times atmospheric pressure in the chamber for a 2X shot. With more air in the chamber you need more fuel to keep the ratio the same. The propane air mix is most often close to 4.2%.

Pressure in absolute pressure is used in this application. In PSI at sea level pressure is about 14.7 PSI absolute. Doubling that is 29.4 PSI absolute or 14.7 pis gauge (above local pressure) This will vary some with your location and altitude.

Other ways to measure absolute pressure include barometric using inches of mercury, bar, k pascals, Torr, or other measure.

Most spudders simply assume the local barometric pressure and use a relative pressure gauge and use PSI or Kilograms/sq cm depending on the local unit of measure. A relative pressure gauge is much easier to find at the local tool or hardware store. Absolute gauges are harder to find and cost more.
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Unread postAuthor: BigBang J » Thu Jul 08, 2010 2:23 pm

So where is a good place to find: Pistons, burst disks, and diaphragm valves?

Thanks for all yalls help.
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Unread postAuthor: Technician1002 » Thu Jul 08, 2010 2:35 pm

You are in the right place. Most are home built. Dig in the forum and build one. Most are built with items found in the plumbing section. For example a pipe union with a few layers of aluminum foil sandwiched inside becomes a burst disk. A modified pipe cap becomes a piston. A lid from a coffee can becomes a diaphragm etc. Not all designs work as well as others. Some are easy to build, some are more difficult. There are designs for all skill levels here. Safety is encouraged. Share your ideas and our crack team of safety experts will rip bad designs apart for you. Don't be discouraged when that happens. Learn from others mistakes and build something that will knock your socks off. There are plenty of examples in the showcase. Most will be beyond your building level, but given time and practice you will work up to them. Don't try one beyond your ability. A chain of failures is no fun.

Oh, Welcome to Spudfiles. Looking forward to seeing you around.

Just a quick note on where you posted the question, this is the showcase for finished cannons. In the future, post the question in the discussion section. I'm sure a moderator will move this thread to the right forum for us this time. Be careful where you start a thread in the future.
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Unread postAuthor: BigBang J » Thu Jul 08, 2010 3:22 pm

Sorry, Yeah I just realised I posted in the wrong spot. :oops:
I am finding my way around the site now.

Burst disks seam to be pretty popular here and I like the simplicity and inexpensive nature of it. But I was wanting to make this a semi-auto gun, not a big neccesity but I thought it would be nice if I did not have to worry about replacing burst disks after every shot. I just don't want to spend $100 on a good pop valve.

Also does anybody know of a place like McMaster Carr? The problem I have with McMaster is that alot of the times you have to get a lot more than what you need for materials.
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Unread postAuthor: jackssmirkingrevenge » Thu Jul 08, 2010 3:25 pm

That's quite an ambitious first project, I don't believe anyone has actually built a practical semi-automatic hybrid.

Also, with the kind of power hybrids put out, one shot is usually enough ;)
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Unread postAuthor: POLAND_SPUD » Thu Jul 08, 2010 5:35 pm

http://www.spudfiles.com/forums/valvele ... rt,30.html

I am going to test it this summer (I hope)... Even if it looks complicated the idea behind it is simple: fill the chamber with propane to ~20-30 psi, at that moment the valve switches and the chamebr is filled with air... with the glow plug for ignition the gun should fire as soon as the right mix is achieved
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Unread postAuthor: Technician1002 » Thu Jul 08, 2010 7:40 pm

The problem with the design (don't shoot me, I'm just the messenger ) is a rich propane mixture will burn. It will burn much like a rich alcohol mixture as commonly seen in the flame in a bottle seen on youtube. A slow beautiful flame does not make for a great hybrid shot. Using mixture to decide on the ignition point will result in ignition with less than the best mix. It will fire as it starts to get close to the right mix, but not at the right mix.
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Unread postAuthor: POLAND_SPUD » Fri Jul 09, 2010 4:42 am

yeah I am aware of it... but it still seems promising
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Unread postAuthor: jrrdw » Fri Jul 09, 2010 6:36 am

BigBang J wrote:Sorry, Yeah I just realised I posted in the wrong spot.
I am finding my way around the site now
.

No worries, we give newbies a break, once! :P

If you click on the 'forums' button on top of the web page it will link you to the 'forum index' page. There you can find discriptions of the sections under the 'section titles'.

Welcome to Spudfiles.
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Unread postAuthor: BigBang J » Fri Jul 09, 2010 1:15 pm

jrrdw wrote:
BigBang J wrote:Sorry, Yeah I just realised I posted in the wrong spot.
I am finding my way around the site now
.

No worries, we give newbies a break, once! :P

If you click on the 'forums' button on top of the web page it will link you to the 'forum index' page. There you can find discriptions of the sections under the 'section titles'.

Welcome to Spudfiles.


Thanks, I hope no more mistakes :)
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