SpudFiles

Hello Im new here.

For the last year me and my friend Jeff have been intermittently working on a simple combustion cannon made from black ABS (I think) drain pipe. Since I only found this place recently, we learned a lot of stuff by trial and error. As it is now it has a 4" dia. chamber thats about 2.5 feet long, and about 5 feet of 2" barrel (we did no calculations beforehand, and the dimensions have changed over time).

The fuel is propane, and we were fraught with problems. It basically fired about 5% of the time. Many ignition systems were tried, starting with a blowhole, and evolving through various steel-wool and camera-flash devices. Part of the problem is that we had no idea how much propane we were using, or how much we should be using. After researching the issue here, we have a better idea now.

So today we added a chamber fan, and chose a new ignition system. After failing at trying to wind my own ignition coil (lol ), we salvaged about 4" of nichrome wire from an old toaster, and hooked it up to the 12v power supply we were using for the fan. It heated up to an intense yellow color, hotter than your average toaster. It lit a butane lighter and a propane torch with ease.

Encouraged by these results, I twisted some the wire around the screws that were previously supposed to be a spark gap, and hooked it up. Well the wire was shorter now so it immediately heated up to white and exploded apart .

Anyways, we got it working using a longer piece of nichrome, and between that and the fan, we are actually getting about 3/5 shots off now. I came here to look for extra info on glow-plug like igniters, but there was nothing so I made this post instead.

The system is quite reliable, it seems like the wire would last indefinitely as long as you kept it from getting too hot. The length can be adjusted to fit any particular power source. Our 4" (approximate) length draws 4.5A @ 12V. The main advantage to us is that it requires absolutely no circuitry, runs at standard (car) battery voltage, and it allows us to set off the cannon from as far away as we want, although right now the tether is only about 4 feet.

The cons are the amount of current you need to run it, you cant run this off AA batteries. Also there is a 4 second lag upon connecting it, but that is not a real problem if you aren't trying to hit a moving target :.

Cool idea, might try it
But have you heard of a propane meter, if you make one you will get a decent bit more power and very consistant results(If made well about 99/100 shots will work)

Yes, I found out about that yesterday. However we have reached the end of our budget, until I get a job.

Have you tried an actual glow plug? I know they have lower voltage glow plugs for r/c cars, but I'm not sure if they would work.

a short lenghth of wire from hairdryers work with a 9v.

I've actually used Christmas lights. Just bust the glass and throw some voltage through it.

finally, a new member that reads the rules before he posts.. (sigh of relief)

[quote="timmeh87"]

The cons are the amount of current you need to run it, you cant run this off AA batteries. quote]

Actually, many rc vehicles have used rechargeable AA size batteries at more than double your current draw. (10amps - speed 400 motors for example)

Might be some unaccounted variables here, but;

2500 mah NiMh should last about 33 minutes @ 4.5amp continuous, @ 4 seconds per shot, that's ~ 500 shots.

Sounds interesting. I've never read about anybody using nichrome to ignite a fuel mixture in spudguns. I've seen multiple instances where one used steel wool connected across a voltage to ignite both spudguns and "other" things, but not nichrome wire.

If you wire some batteries both in parallel and in series, you may be able to gain a good enough current supply, but that'd be a lot of batteries. Probably would be best to use a rechargeable cell like Gippeto suggested, maybe a gel-cell sealed lead acid battery (although all mine is D-E-A-D now because of lack of charging time. Even my old truck battery is killed as well).

I disagree that you can't run it off standard alkaline batteries. Have you tried one or two 6v lantern batteries? These have an enormous capacity, although I'm not sure of their internal resistance. The general trend I've seen is that internal resistance goes down as battery physical dimensions go up, so you might be able to get one to work. If it's still not enough, you can trade off voltage for max. supply current by wiring them in parallel (not sure about how cheap 6v's are so I'm assuming one would only buy two).

timmeh87, welcome to the forum and thanks for a lucid first post.

You might consider 4 9V batteries. Kind of pricey, but 2 pairs in series then in parrallel should give ~18V at about 4~5 amps. A bit smaller than a car type battery. As others suggest, many RC battery packs will supply the needed power. BTW, it's the power that controls how hot, and how fast, the wire heats up. Power is voltage*current, so you don't actually need 12V. A 1.5V AA will source about 8 amps (when new), ~12 watts power. You said 12V at 4.5A, which is 54 watts. So, roughly 5 AA batteries will supply the same power. (Of course, they won't last very long so you might want to use rechargeables, bu they are usually only 1.2V each).

For fueling, you might consider a <a href="http://www.inpharmix.com/jps/Sluka_Spud ... syringe</a> or even a <a href="http://www.inpharmix.com/jps/Fuel%20Met ... ml">butane lighter</a>. The syringe setup is very accurate, and if you have a farm supply store nearby, it'll cost perhaps $4 total.

Butane lighters can be used with a syringe, or just plumbled directly into the chamber. A lighter has the advantage over a spary can in that it supplies fuel much slower. That makes it a lot easier to reproducably fuel your gun. Only problem with butane lighters is that their internal pressure changes a fair amount with temperature.

Thanks for the responses.

The AA battery thing was based on prior experience of not being able to power a Nintendo 64, which only requires about 2 amps, off of them. (By the way Im from the benheck.com forums). However I guess a piece of wire is a little less sensitive than a computer , and yeah it would probably work ok, especially with a shorter piece of wire. Im doubtful that a 9v batter could crank out 4A for very long, and at like $3 each Im not about to start getting any.

We were thinking of running this out of a car, since we have to drive out of my neighborhood to shoot anything heavier than paper towels out of it anyways. Also, I have a 10Ah, 16v NiCd battery from like 1970. It doesn't hold its charge for very long, but it can crank out massive amounts of power. I was playing around with it and almost burned out our element . We'll build a 'control panel' for it and then maybe post pics.

Also, we did not try an actual glow plug, Ive never even seen one in real life before. Im assuming it would work.

Sounds good.

Only drawback I'm seeing over spark ignition is the delay for the wire to heat up to ignition temperature.

Could be a bit of a niggle in handheld cannons that the trigger needs to be held down for a few seconds but with large, slow reloading ones I doubt anyone could care less.

Very simple and I'd expect it to be pretty reliable with a well mixed fuel.

After all, what exactly is going to go wrong in a wire loop between two battery terminals and a switch

Just need a good switch that can take a fair few amps bunged through it.

this sounds pretty elementary but if you use just a regular glow plug you could wrap it in flash cotton or flash paper

my uncle tried it like that and it worked pretty well

This has been discussed before.
http://www.spudfiles.com/spudtech_archi ... f=6&t=7122
http://www.spudfiles.com/spudtech_archi ... f=4&t=5431

Mmm but not here here, just there here