5 Newbie Questions: Liquid Fuel, Buttstocks, and Operation

[Jellydonut]

Howdy!

Complete newbie here, and I just had some questions about fuels and operation before I start building. I had a Howitzer-style cannon several years ago, but it got dismantled due to disuse and inconvenience. We ran that one on Consort hairspray, and it worked well!

Anyway, lately I've rediscovered my fascination with spud guns, and this time, I was hoping to make something easy to handle that I could shoulder, with metal construction, and no moving parts. Essentially, just a tube with a stock and hand grip. I was hoping to use iron pipe from the hardware store for the barrel (I have a hard time trusting PVC).

If you want a picture of what I'm thinking, just imagine one of these, without the break barrel:


Here are my main concerns:
1) What should I fuel it with? I was thinking of using a liquid fuel such as isopropyl alcohol, a starter fluid, or Consort. I don't want to have to deal with gas tanks and stuff. Does alcohol really provide enough oomph to lob a spud effectively as something like hairspray? I'd love to use it since it's cheap and non-toxic, but I'd rather have range.

2) How should I fuel it? I'd really rather not have a cap to unscrew, or an injection port that could blow out (unless anyone knows a good way to make one of these). I was thinking of just squirting a given amount down to the bottom of the barrel (possibly with an atomizer), with no extra holes in the gun.

3) If I decided to go with a fuel like starter fluid or consort, I would end up adding the fuel, THEN the potato. Would the pressurization from the potato-loading likely cause explosion with any of these?

4) Is the idea of having a buttstock on the gun a big no-no? I don't really see anyone really using them.

5) Has anyone tried cyclohexane as a propellant? I've heard it lets out a really good bang.

[jackssmirkingrevenge]

Look into syringe fueling:

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A simple way to meter the exact amount of fuel needed to ensure efficient and consistent power.

[jimmy101]

I would use a disposable butane lighter. You should get several dozen shots out'a one and they only cost a couple bucks. You could try skipping the syringe and directly plumbing the lighter into the chamber. You could use the one-potatoe two-potatoe approach to counting how long to fuel from the lighter. The correct timing would have to come from trial and error.
http://www.inpharmix.com/jps/Hacking_lighter.html
just glue the gas hose from the hacked long-handled lighter into the chamber. The area is so small that it'll be under fairly little pressure and shouldn't be hard to hold in place. If you hack two lighters you could use the mechanism of one for the ignition system and the mechanism of the other for the fueling system.

You could also build a syringe and the butane lighter and valving into the stock to get spot on fueling every time.

Liquid fuels like alcohol or hexane will work but the volume of fuel will be tiny, like less than one drop, which is very hard to do reproducibly. A gaseous fuel is easier to work with and can be much more consistent. The energy content of the various fuels are all basically the same (within 5% or 10% of each other) so the fuel choice doesn't much affect the performance. The amount of fuel, you have to use the correct amount, is much more important than the exact choice of fuel.

You won't be able to fuel then load. Your design looks like it'll be a mono-gun (barrel and chamber are the same diameter) and the position of the ammo is what defines the volume of the chamber. Wild-ass guess that the chamber and barrel volumes will be the same which means when you ram the ammo home you'll pressurize the chamber to twice atmospheric pressure. That pressure will likely just pop the ammo back out'a the barrel. So, you'll need a vent in the chamber so that air can escape as you load the ammo. The vent can be pretty small, like 1/16", and you can just cover it with tape when firing.

The gun is pretty small and has a short barrel. Get a copy of Dave Hall's HGDT and fiddle with it to estimate what kind of performance you'll get. Given the overall size, it might not be very impressive but then again that particular type of gun isn't meant to have all that great a range.

There is nothing wrong with a butt stock. It isn't done very often in combustion just because combustions tend to be too big to comfortably shoulder. Stocks are common on smaller pneumatics.

For consistent performance you'll need a way to ram the ammo to the same position in the barrel every time. Either used a measured ramrod or put a stop screw (or two) in the barrel at the proper location.

[jimmy101]

Here is a quick calc using HGDT. I assumed an 8" barrel and 8" chamber both at 1.5" diameter and a 1.4 oz round (about 40grams). The estimated muzzle velocity is 167 feet per second (~110 MPH). Not all that great but more than twice as fast as you could throw a spud.