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Which source of ignition do you prefer? Let's face it, having a menacing spark is half the fun of making a serious combustion cannon. Here are my experiences with a few of them:
Flint wheel-Tried this on my first cannon. Of course I used hairspray as a propellant so I found getting the wheel getting wet a pain and replacing the flint made things seem almost primitive. NOT a fan, but it works.
Piezoelectric BBQ sparker- This does the trick, gets the job done, no wiring or batteries. It is the base option though and makes a gun look sort of cheap and amateur. Of course, it really is cheap. These only cost around $10 so you get what you pay for. Also, it isn't consistent enough and DEFINITELY is the epitome of the only works when no-one is looking rule.
Stun Gun- When properly installed, I honestly think these are the best. I didn't know much about wiring so someone getting zapped while shooting it was the demise of a nice cannon I had. Only a small amount of modification is needed to really make a nice efficient spark. They are kind of a drag though considering the 9 volt batteries really add up after a while on top of a $30 price tag.
Camera flash-I am currently experimenting with this on my new project, the spark looks pretty hot and honestly, I think it is the best bang for your buck. It may not have a steady spark like the stun gun but it seems a little bit safer that way. I like the price, I like the size, I like that it only takes a AA battery. This takes some soldering skills and electrical background to modify it correctly.
None of the above....
Modified automotive ignition system with honest to goodness spark plugs.
As you've already stated; a piezoelectric BBQ sparker costs $10 (sometimes more) and isn't the most reliable. The spark is weak, you have to run screws, wires AND figure out how to mount the unit, while keeping the finished product looking like it wasn't slapped together by a 6 year-old.
Stunguns have an awesome looking spark, but were never intended to be discharged into the open air. You still have to run wires and mount screws or bolts through the chamber, while also mounting the assembly to the side of the launcher.
Camera flash ignition requires that you have to wait for the charge to build, along with the hassle of mounting the assembly to the side of the launcher, as with the aforementioned ignition assemblies.
Spark plugs and a modified automotive igniton system? That may be a neat setup for launchers as big a "VERA", but come on?... Would you really want spark plug(s) , <--- (plural) and a modified automotive igniton system strapped to the side of your 3-4 foot long potato gun? - (Respectful teasing) Besides, if you want to mount the spark plugs in your PVC components, you need to buy a tap and a hard-to-find drill.
A flint-type lantern sparker is the ideal way to go. One hole, large ball of sparks and little to no effort making an internal shield, if you're planning on using an aerosol fuel... No batteries, no wires, no project box, no screws... and they cost less than 5 bucks.
You cannot have excessive ignition, flint lantern igniters allow some rather hot gases to escape and are more inconvenient to trigger.
I agree with you about almost all of the above, a modified car ignition system could be overkill for a spray & pray, but not for advanced combustion or hybrids, I have a reasonable knowledge of electronics (hope that doesn't sound like I'm a wanker, it's another of my hobbies) and a well made small igniter circuit could be made within a day if you are willing to learn a bit about electronics.
It can be put inside the chamber along with the battery, you only need two small wires exiting the chamber near the switch to trigger it.
I'm not trying to flame, just present an alternative view.
I don't do PVC. Period.
(And there's nothing difficult to find about the proper drill/tap set. Virtually any tool catalog will have it.)
If flint lantern lighters allowed "some rather hot gases to escape", then I guess I'd have burned my fingers, hundreds of times?
This forum and this particular discussion was not about advanced combustion cannons, nor hybrids... It 's all about an individuals opinion / preference. I just so happen to like flint lantern lighters, because they are the easiest / least expensive / most reliable type of ignition that I've used on all of my combustion cannons - Even the advanced ones.
A small igniter circuit inside of the chamber takes up space and is more difficult to mount, than a flint-type igniter. I've done spark strips, stun guns and multiple ignition points. One ignition point, slightly aft of the middle of the chamber, always seemed to yield the best performance.
As was I.
I love PVC and have never had a problem with it. Not everyone has access to a welder, nor tools/machines to custom-fabricate parts.
Try and find a 14mm x 1.25 tap in your local Lowe's or Home Depot. You CAN find one at an automotive parts store, such as O'Reilly or Autozone, but again, not all will have one. They're about $12 and most spark plugs are at least $3 - $4.
A 33/64" Drill bit is required for this tap, but you CAN use a 1/2" Drill bit.
They'll range from $5 to $15 and for that, I can buy 1-4 lantern lighters.
For a simple-to-advanced launcher, a spark plug (with it's .035" - .060" gap) is a weaker and more expensive option.
Note: I followed the topics guidelines and was very specific as to why I did / did not like certain types of ignition. If someone can prove as to why a $3-$5 flint lantern lighter is inferior to any other type of ignition, then please feel free to refute my choice.
Your local Lowe's or Home Depot isn't a tool catalog... last I checked...
PVC is cool, but I don't like the idea of working with something that will fail under near-ideal conditions at the pressure I can generate with my floor pump. That and that if I drop a PVC gun, there's a good chance it'll break or crack, then shoot shards of plastic into me next time I fire it.
I don't know if many people put their igniter circuit inside the chamber of a combustion cannon. Seems like the heat would damage it pretty quick. If one ignition point is giving you better performance than multiple, there's another more significant variable in your experimentation.
You've never passed your hand through a candle flame before? There's a big difference between a few mL of combustion products (which have very low specific heat anyway) being in contact with your fingers for a few milliseconds and transferring any significant amount of heat to your fingers.
And don't you need an unsealed hole in the gun if you're going to use a flint igniter?
Stun gun is my favorite. I buy 150k volt stun guns for about $20 and they are very reliable and easy to use multiple sparks with, even with hybrid mixes. They last a long time as well as do the batteries. Hundreds of shots with no issues. They are small and can fit nicely in an enclosure. Plus, it just sounds cool to tell someone you have a stun gun in your cannon that is making it fire. They work fine in all metal cannons with no shocks to the user.
Obviously, you missed the point - As did D_Hall; Do most spudgunners have a tool catalog lying around? Are taps that have the same thread pitch as a spark plug readily available? You CAN order most anything online, but have to pay for the part, the shipping & handling and then wait for it to get to you. I can go to almost any Wal Mart or local hardware store and pick-up a flint-type lantern lighter.
Then, by all means, build yourself a launcher that isn't PVC-based.
I was responding to a comment made by Zeus, which talked about mounting the circuit inside the launcher. Multiple ignition points burn too much of the fuel, too fast - fine for larger C:B ratios, such as anything larger than 1:1. - However, I've often found that getting your fuel to deflagrate, rather than closer to detonate- is the best for launchers with C:B ratios under 1:1
Twisting a flint-type lantern lighter, your fingers leave the knob before
there's any chance of getting burned. Worst I've ever had happen was for the igniter to pop outwards, which resulted in no damage to it- or injury to myself.
?? The flint igniter is mounted in a pre-drilled hole... the hole is sealed by the component.
omniscient, so your favorite ignition is a flint? Does it make you feel like Flint Eastwood?
Moonbogg, may I remind you that you aren't JSR, thus horrific puns are not acceptable.
Fair enough with the flint, if it's your favourite, who am I to doubt your taste, I prefer electronic ignition, triggering can be better and when applied properly, will result in faster ignition.
PVC when used around 100PSI is near it's failure point, it's been used far beyond it's rating without failure, but it can fail and when it does, it's failure mode can result in horrific injuries, not to mention PVC cannot be detected by x-ray scans, thus each entry wound has to be probed to find the shrapnel.
Just be cautious of what material you trust, seamed steel fails in a "good" way, splitting along the seam without producing shrapnel.
Same with ABS, it only splits and is fairly light, though somewhat more expensive than PVC.
A good rule with pressure ratings, 3:1 is near enough failsafe for our sort of launching, 2:1 be wary as a failure can occur if there is defects in the material, 1.5:1 quite simple, Run!
Thanks to DYI for posting something like this some time ago thus inspiring me.
If you fuel was detonating, you'd know. A 1x detonation would turn any PVC into a cloud of shrapnel.
I'm pretty sure out of McMaster's 3,264 different taps, you can find one that matches the threads on a spark plug. If not, epoxy it. While this may seem incredibly inconvenient to you to order something over the internet as opposed to driving to Wal-Mart, I do it all the time and am not bothered by the two-day wait and the minute shipping cost (which for a part you can't find anywhere else might as well be zero). Guess that's just a personal preference, since I've had experiences with flint and am frankly a little unnerved by it.
I prefer electronic, though I do not mean to say that flint ignition is ineffective. Both work, but electronic seems more reliable and safe to me.
Flint igniters: perfect for a basic combustion cannon. I've used them on small and large ones and they light the correct fuel mix every time with more leniency than a small piezoelectric igniter.
Piezoelectric igniter: Never had a problem with these either. When using metered propane (or even starting fluid after I've had a chance to play around with how much to use), I've never had a misfire that wasn't due to a fueling error. To those having reliability issues, blame your wiring and spark gap before you blame the igniter. All connections should be solid and well insulated, and your spark gap should be no more than 1/4" wide, and more like 1/8".
Stunguns: Only played around with one, but it seems to work reliably. I bought the type where the circuit is potted completely and is pretty small after taken out of the stungun casing. These work good on 12-18 volts, and it hasn't failed me yet (although I've just played around with it.
Camera flash: Performance-wise, they are great, but unless the circuit is assembled with care and thought, you will have reliability issues. The basic camera flash -> ignition coil setup is the best IMO and can be accomplished only using the flash tube as a switch.
Those are just my experiences. Feel free to disagree with them.
I guess it's all in what you define as easy and convenient. To me, if I don't have to move my butt off this chair and I can find the part online in 10 minutes or less, it's easy and convenient (Note: You *DO* have a tool catalog lying around...it's called your computer.). Going to WalMart is easy, but it's a lot less convenient.
As for the costs of shipping/handling... So what? Typical cost of driving a car is $0.30/mile. It's five miles round trip to WM. That's $1.50. Subtract that off the $5 shipping/handling for something so small and I'm left here thinking, "Are you REALLY sitting there arguing over a couple bucks?" And heck, if I can buy it from Amazon, it's FREE shipping(*).
I just checked, I *can* buy it on Amazon.
As for the rush, with McMaster I'll have the tap tomorrow. With Amazon, the day after tomorrow. Unless you're REALLY in a hurry, that should be fast enough (and if you are, I submit you need to work on time management skills). It's certainly faster than I'm likely to find myself in WalMart for anything other than a dedicated trip.
(*) Sort of. Amazon Prime. Suffice to say I buy enough stuff from them that I'm making a killing on shipping/handling costs.
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