Couple O' Questions

[Grignacke]

Hey guys!
I've built two spud guns, a spray 'n' pray and a one that takes metered mapp / propane- much fun
I'm interested in building a hybrid, it seems like the next logical step (besides a pneumatic, but charging gung ho into a hybrid seems more... fun...) Anyway, I did read the sticky about fueling, but I have a few questions on hybrids in general:
-How exactly does one meter compressed air in? I understand that you port in, say, 5 atm (or is it 4 :S) for a x5 mix, but is there a meter pipe involved, or do you just measure the pressure inside the chamber?
-Is galvanized steel a suitable chamber / barrel material?
-What mix is required for a spud (or more suitable projectile) to break the speed of sound?
-Also, what do you guys think of Mapp vs. propane as fuel?

Thanks guys! Grignacke

[Pete Zaria]

@Grignacke,

I know charging gung-ho into hybrids seems like a lot of fun, but it's really a good idea to build a few combustions and pneumatics first, and spend LOTS of time reading up on the operating principals, construction, and safety of hybrids before building one. With combustions and (most) pneumatics, if you screw up, the worst case scenario is some PVC shrapnel and a bad day. With hybrids, life-altering or life-ending injury is a very real possibility.

1. There's no one way to meter air. Yes, you can simply do it by adding X psi and watching the gauge carefully. Other hybrids use more complex systems (such as an on-board regulator to deliver air at a set pressure). Another, simple but more dangerous method, is to mix an atmospheric (unpressurized) propane/air mixture in a separate "mixing tank" and then pressurize it up to the desired pressure by piping water into the mixing tank to compress the gas. This ensures a correct mix (assuming your initial mix was ignitable) but is also dangerous, because it temporarily creates a pipe bomb in the mixing tank (as such, a burst disk or pressure release valve is a necessary precaution).

2. Yes, galv. steel is a usable chamber material. It would also work for the barrel, however, you don't really *need* steel for the barrel unless you're planning on running some pretty high mixes.

3. There's no definitive answer as to what mixture would be required to break the speed of sound - there are too many factors involved, the size of the chamber and barrel, specs of the burst disk, weight and shape of projectile, etc... an "efficient" hybrid could probably get objects supersonic at 1.5 or 2x. With a heavy projectile, bad C:B ratio, etc... it might take a lot more.

4. Mapp is a little hotter burning and therefore produces a little more pressure than propane. Having said that, the difference isn't very big (around 10-15% if I recall correctly) and in most locations, propane is a lot cheaper.

Check out a few of the nicer hybrids in the Hybrid Showcase.

Peace,
Pete Zaria.

[Moonbogg]

Spuds don't usually stay in once piece when fired from a hybrid. I think I may have had a few spuds stay together at 2x, but they usually break apart. Limes are my ammo of choice for 2-3x use.
As said before, steel is good for a chamber, but something lighter can be used for the barrel, such as aluminum. Aluminum also makes a good chamber and is easier to work with usually.
For pressurizing air, I just fill the chamber with air directly after adding the propane and use a simple bike pump with a gage to fill it. A hybrid doesn't have to be dangerous if you use it conservatively (2-3x) with strong materials such as steel or aluminum. If built poorly, it will certainly be dangerous. If over fueled, it will become deadly. Learn to use HGDT, check out the showcase, experiment with designs and ask tons of questions and read up a lot. Know exactly how you will use it and understand just how it works before building. Stay away from plastics. Some people have used plastics, but I myself wouldn't be comfortable with it.

[Grignacke]

Thanks guys! Love the Cobra Striker, btw!

So just to clarify on the mechanics of fueling, I'd want to have one opening from the chamber to my pipes that carry air / gas, then have one branch that has a propane meter set up and another that has a quick connect valve and a ball valve for air, then when I'm pumping in air I'd want to just open it up to the chamber and watch the pressure gauge on the bike pump / air compressor go up till it hits what I want?

Also, any tips on sealing it all? Does the opening for the fueling system in the chamber ever become problematic?

Thanks again for the friendly responses- and thanks for moving this to the right discussion, whoever did that

[jackssmirkingrevenge]

It depends how big you're going to make it, with my mini hybrid I added the fuel with a syringe then pumped it to the correct pressure with a shock pump.

In this case, especially for smaller chambers, it's important that you take the dead volume of the pump (any volume between the pump's check valve and where it attaches to your filling valve) as part of the chamber volume when calculating the amount of fuel to be added.

[saefroch]

Also, any tips on sealing it all? Does the opening for the fueling system in the chamber ever become problematic?

Teflon tape seals almost any threaded fitting. Spudblaster15 recently had a problem with damaging seals on ball valves with an oxy-air-propane hybrid, but that shouldn't be an issue for you... I hope.

Look through the hybrid cannon showcase, the pictures will clarify any questions you have about meter designs. There are a lot.

And you don't even really need a meter... but it's suggested.

[Grignacke]

Thanks for the help guys- I've been looking at the cannon showcase, gathering info, etc.

Last question: I've been searching on Mcmaster for decent camlocks, and have found a ton, but most steel ones aren't rated past 250 PSI- will they be able to survive the pressure created in a hybrid gun? I understand it's the barrel, but 70-100$ is alot to shell out to have them destroyed on the first shot

[saefroch]

Pressure rating does not equilibrate to failure rating. Those camlocks you're looking at will most likely survive the pressure rating, especially if you're shelling out over $70 and they're steel.

[Grignacke]

Gotcha. Thank you very much!

[Grignacke]

Alright- here's a sketch of what I have in mind. To fuel, I'd meter in the appropriate amount of propane Then, I'd port the proper amount of air, allowing the pressure gauge to find the pressure for the whole chamber. Is this correct? (also, forgive me for continously finding something to ask about- I simply do not want money and limbs wasted on this project )

[Pete Zaria]

Alright- here's a sketch of what I have in mind. To fuel, I'd meter in the appropriate amount of propane Then, I'd port the proper amount of air, allowing the pressure gauge to find the pressure for the whole chamber. Is this correct? (also, forgive me for continously finding something to ask about- I simply do not want money and limbs wasted on this project )

Where's the burst disk / valve? You need to "hold back" the pressurized propane/air mix before it's fired, and the projectile (in 99.99% of cases) won't seal tightly enough in the barrel to do it by itself - so you need a burst disk or a piston valve, basically.

Peace,
Pete Zaria.

[Lockednloaded]

Pete Zaria,
I think he plans to use a cam-lock fitting to hold the burst disk, Its depicted in the sketch

[Grignacke]

Forgive me for my lack of burst disk. This was a sketch just so that I would get the fueling right.

Now that I think about it, a shaped charge generally has a cone and a rounded end to allow the force of the explosion to be directed out, would a rounded end to a spud gun have a similar effect?

[saefroch]

Maybe, but I'd bet not, as spudguns are not powered by a gas deflagrating (burning), and not a solid detonating.

[Grignacke]

true, true. So, is that a solid fueling plan?