Spark location? - Hair spray spud cannon

[SpudMcJim]

Has there been testing on the best location for a plain hair spray combustion spud cannon? Let's stay simple and a straight combustion chamber inline with the barrel. Should the spark be closer to the potato or the back end (capped)?

My first build has a 4" combustion chamber with 17" cut length of pipe and the end fittings (see avatar) for an effective length around 19".
I have two barrels so far (screw on), both are 2" dia, 49" and 73" long with the screw fitting. My spark location was not thought out, and it happens to be mounted 13" from the end cap.

[Gun Freak]

Have a look at this.

[SpudMcJim]

Thanks for the link.

They changed the chamber volume by altering the position of the tennis ball, to show the effect of having ignition at the rear, centre and front of the chamber. The igniter was in the same position, but the tennis ball was moved.

The author said, the "rear" ignition appeared more powerful, but then again it's a bigger chamber volume (injected 8.8mL of butane as opposed to 6.6mL in the central ignition test) so you can't really make a direct comparison.
You'd need a proper setup with adjustable ignition and measure shots on a chrony in order to adequately evaluate the effects of gap placement on performance, really I made this just to have some cool footage

[jackssmirkingrevenge]

If you're worrying about performance, then ignition placement isn't really the place to start. I would focus on the proper metering of pure fuel as opposed to using hairspray, as well as a chamber fan to ensure good mixture.

For ignition, you ideally would have multiple ignition points. Have a look here: http://www.spudfiles.com/spud_wiki/inde ... park_strip

[SpudMcJim]

In that series of posts, a link to test that "Insomniac" did with a clear soda bottle shows the flame propagation of actually moving the igniter.

He did say, It's important to note that not all the varience in flames was caused by spark location, as the mixture was varying significantly from shot-to-shot.

The only conclusion I could see was the rear spark provided the fastest flame propagation. And that implies more push, for a spray and prey, without a fan.

[jimmy101]

The only conclusion I could see was the rear spark provided the fastest flame propagation. And that implies more push, for a spray and prey, without a fan.

I would take that conclusion with a grain of salt.

Firstly, there isn't any back pressure and no barrel so the relevance to a typical gun is questionable. In a gun with a barrel and ammo a fair amount of combustion occurs before the ammo starts to move. You then probably get some turbulence as the ammo starts to move and the gases, burned and unburned, start to move.

Secondly, the rear spark only gives a single flame front. For that to be actually faster combustion that speed has to be greater then the sum of the flame fronts' speeds for a central spark.

[SpudMcJim]

Today (Friday 7/12/2013) my friends and I had a chance to do some test firing!

With the 4 foot barrels we consistantly got 198-210 yards (600 ft) with potatoes. Then we put a little bit more hair spray in the chamber and we got 200-250 yards. The longest was with a 2.5" barrel (48") and a longer shot of hair spray. It went almost 300 yards.

While the 2.5" barrel was on the launcher, I put an old tennis ball in and it went about 300 yards but we didn't have a confirmed landing spot. That was the only tennis ball we had, so one data point.

I also tried a few shots with a 2" barrel 73" long (6 ft). There was some variation due to wind and tumbling, but it seemed to be good for 20-25 more yards than the 2" dia, 49" barrel.

Lots of fun!