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Built a combustion cannon? Then post it here! This section is for completed, finished cannons that you have built. Please include pictures and information.
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Unread postAuthor: Radiation » Thu Dec 04, 2008 12:05 pm

We know nothing about his placement of said spark gaps, he could have put them both on the same side of the chamber, he could have spread them out. Who knows. I for one can attest to the absolute advantage of multiple gaps.
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Unread postAuthor: SpudBlaster15 » Fri Dec 12, 2008 12:48 pm

Spark gaps are a far more complex issue than most people realize. Chamber volume relative to barrel volume, barrel diameter, chamber shape (Length vs width), placement of the gaps, etc. This launcher had a fairly central gap, and installing 2 additional gaps did not have a noticeable effect on performance, despite the long chamber and relatively high C:B ratio.
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Unread postAuthor: Moonbogg » Sat Dec 13, 2008 6:17 am

Punt gun FTW..LOL! Oh, what do you have to do to get in this contest? If I added a longer barrel to my newbster combustion could I have a shot at beating this guy?
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Unread postAuthor: Ragnarok » Sat Dec 13, 2008 1:46 pm

Moonbogg wrote:Punt gun FTW..LOL! Oh, what do you have to do to get in this contest? If I added a longer barrel to my newbster combustion could I have a shot at beating this guy?

Yeah, sure, but you'll have to hop back in time and post it somewhere over a year and a half ago, destroying the continuity of time, etc, etc.

Contest is long since over.
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Unread postAuthor: starman » Sat Dec 13, 2008 2:13 pm

Ragnarok wrote:
Moonbogg wrote:Punt gun FTW..LOL! Oh, what do you have to do to get in this contest? If I added a longer barrel to my newbster combustion could I have a shot at beating this guy?

Yeah, sure, but you'll have to hop back in time and post it somewhere over a year and a half ago, destroying the continuity of time, etc, etc.

Contest is long since over.


There was talk and interest in a new contest back in the summer...PCGUY was going to manage the thing...never got off the ground. I know of a few cases where people were/are holding back designs and cannons so it/they can be entered as new in the contest...

PCGUY any new word on the contest?
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Unread postAuthor: Xxplosive42o » Sun Dec 14, 2008 9:34 pm

A++ Work!!!

Looks very clean and user friendly.

Now let me give you some suggestions! Build a spark strip with 3-4 gaps for that monster; one single gap is NOT enough. Then, switchout that propane hose for some stainless steel braided hose. Finally build a propane tank holder and use hose clamps covered with rubber hosing to keep in place.

There just polite suggestions that I think would make your cannon out of this world. But in any event, your cannon is still beautiful!

Cheers! -Xxplosive42o 8)
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Unread postAuthor: SpudBlaster15 » Sun Dec 14, 2008 9:52 pm

1) I tried multiple gaps and it had no effect on performance.

2) The hose worked fine.

3) I did build a propane tank holder a few weeks after I built the cannon, which happens to be 1.5 years ago.

4) This cannon doesn't exist anymore.
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Unread postAuthor: Ragnarok » Sun Dec 14, 2008 10:24 pm

Oh dear - I've been sigged. :roll:

SpudBlaster15 wrote:4) This cannon doesn't exist anymore.

All too common in the world of spuds really. Money and space constraints frequently outweigh nostalgia and performance. Or in some cases, cannons just fall foul of damage.

If you don't mind me asking, what befell this one?
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Unread postAuthor: Xxplosive42o » Sun Dec 14, 2008 10:24 pm

I was merely commenting since the thread came back up. I wanted to address topics that I believe needed improvement. Hopefully it would help future builders who checked the thread.

NOTE: Multiple spark gaps DO improve efficiency of the burn. This has been proven in tests done by Burnt Latke and other member's of this forum. It actually improved the muzzle velocity! The test was conducted with multiple shots all of which showed the same improvement. The overall performance gain might be minimal, but for the people who want to squeeze all they can out of their cannon it is definitely an option.

The hose might work fine, but it's aesthetic quality lacks.

Sorry if this comes off like I'm criticizing your "past" cannon and your posts. I am just trying to put down my ideas, which is something this forum was designed for.

Cheers! Xxplosive42o
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Unread postAuthor: SpudBlaster15 » Sun Dec 14, 2008 10:51 pm

All too common in the world of spuds really. Money and space constraints frequently outweigh nostalgia and performance. Or in some cases, cannons just fall foul of damage.


So very true. Many of my launchers have met the same fate. My hybrid was by far my favorite cannon of 2007, but it is sadly now disassembled in a box. Someday I will revive it, but that will be a day when it is warmer outside.

If you don't mind me asking, what befell this one?


Chamber cracked. I rebuilt it to be much stronger last winter, but I have not fired it since May. Just not too interested in combustion launchers anymore.

NOTE: Multiple spark gaps DO improve efficiency of the burn. This has been proven in tests done by Burnt Latke and other member's of this forum. It actually improved the muzzle velocity! The test was conducted with multiple shots all of which showed the same improvement. The overall performance gain might be minimal, but for the people who want to squeeze all they can out of their cannon it is definitely an option.


Burnt Latke tested 2 gaps against 3 and found no statistically significant difference between the results for each setup. Please provide a link to controlled testing done by "other members".

HGDT will give you a decent approximation for what you can expect by adding multiple gaps. It will depend on many factors, and there are no set rules that apply to all cannons.

The hose might work fine, but it's aesthetic quality lacks.


I'm not going to go through the trouble and expense of using a braided stainless hose if a standard $1.19/ft PVC reinforced vinyl hose works just as I intend it to.
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Unread postAuthor: Ragnarok » Sun Dec 14, 2008 11:33 pm

Xxplosive42o wrote:NOTE: Multiple spark gaps DO improve efficiency of the burn. This has been proven in tests done by Burnt Latke and other member's of this forum. It actually improved the muzzle velocity!

Go back to Burnt Latke's results, and look not at what they tell you, but what they don't.

First important thing missing. Spud mass, it's variable enough that any results without recording the mass of the particular spud slug are instantly less use.
There are also two changed variables (the first being the number of sparks). The two spark method sparks near the axis of the chamber, the three spark near the wall.
Lastly, it does apply to a 1.3:1 C:B cannon, which means the faster burn is necessary.

Not that I'm slagging Latke, the information provided on the site is of great value - but it could have been a lot better with the spud masses, even to the nearest gram or two.

However, we shall persevere. In any true scientific test, it's wise to discard the data points that fall abnormally far from the mean, as they likely are a result of an error in the test's conditions.
To be fair, we will discard the two most abnormal points from each set to give us 10 results for each - #9 and #11 from the 3 spark test, #7 and #9 from the 2 spark test. This is particularly important in light of the lack of spud mass information.

Eliminating these results, the averages are 369 fps with an SD of 35.7 (3 spark) and 345 fps with an SD of 55.8 (2 spark).
To the untrained eye, those results might look fairly conclusive.

It's not that clear cut. The 24 fps difference, if you consider the implications of the three sigma rule for a normal probability distribution, is pretty minor.
It's an improvement, but it's going to be lost within the natural variation in spud velocity. However, it does seem to tighten up consistency a little.

I'd be very interested to see where these other tests are if you have them. I have no memory of such a thing, and being the nerd I am, I'd be most keen to see this extra information.

SpudBlaster15 wrote:Chamber cracked.

Of course. I knew it had happened to one of your launchers. Couldn't remember if it was this one.
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Unread postAuthor: Xxplosive42o » Mon Dec 15, 2008 1:25 am

SpudBlaster15: Burnt Latke tested 2 gaps against 3 and found no statistically significant difference between the results for each setup


The average for 3 gaps was 391.4 fps vs. 364.9 fps for 2 gaps. Nuff said.

You need to re-read what I wrote, "The overall performance gain might be minimal, but for the people who want to squeeze all they can out of their cannon it is definitely an option. "

It is minimal like I stated but it did make an improvement. Look for example, there isn't even a shot below 300 fps for the 3 gap; while, there was 3 cases of high 200's for the 2 gap setup.

Ragnarok: Lastly, it does apply to a 1.3:1 C:B cannon, which means the faster burn is necessary.


I would think since SpudBlaster's chamber has a volume of 4916 cc's and a C:B ratio of over 1.5 that his chamber would be more in need of a spark strip than Burnt Latke's; while, they only had a chamber volume of 2600 cc's and a 1:3 C:B. The fact that their cannon benefited from 3 spark gaps should indicate that SpudBlaster's would as well, if not more because of his enormous chamber. But once again, thats just my take on it. I could be wrong as I am not the most experienced builder on here.

And I will search the forum for the results. I was almost certain it was somebody from here who tested their cannon comparing variations of spark gaps awhile back. If it isn't on here I will find the results again elsewhere. Either way I'll investigate the matter for sake of argument.

Anyways, I was just contributing my 2 cents. Sorry if you both took it the wrong way or something. :?

Cheers! Xxplosive42o 8) [/quote]
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Unread postAuthor: Ragnarok » Mon Dec 15, 2008 2:38 am

Xxplosive42o wrote:The average for 3 gaps was 391.4 fps vs. 364.9 fps for 2 gaps. Nuff said.

No, like I said, there is not enough said.
Aside from the fact that it is entirely dependent on spark placement and chamber geometry, those results are really missing things, and need more work done on them before you can draw any serious conclusions from them.

There is no greater testament to the fact that performance is far more complex than adding more sparks, and that it's dependent on a myriad factors of chamber geometry, than Crusader.

If you've not heard of Crusader, (then I'm very disappointed in you... I kid. Mostly) it's the fastest atmospheric combustion out there, with chrono results of Mach 1.5 repeatedly - it's debatable whether the test circumstances were affecting the result, but I'm personally pretty sure it was at least supersonic.

Crusader used only one spark gap placed at the rear of the chamber, what conventional wisdom would have as near the worst possible placement, with a lone spark about as far from the centre as possible.

There is however something that came up in the SF chat - I think I was talking with DYI that time.
The truth of the matter was that although Crusader is the only supersonic combustion I know of (excepting Dongfang's oxy-enhanced combustion), despite the fact that other launchers have used a similar chamber geometry - so I am of the opinion that another factor, one I think is fairly unique to Crusader, contributed heavily to its success.

I've yet to test my WAG (Wild Arse Guess) on that point though.
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Unread postAuthor: Xxplosive42o » Mon Dec 15, 2008 2:49 am

Well put Ragnarok, you would make a great lawyer just so you know.

You disproved my logic and I accept defeat in this case. It is still hard for me to accept after 12 shots proving one had greater speeds than the other that the results are irrelevant. But like you said I did not ask what was "missing"; therefore, it could be a plethora of reasons.

Thanks for giving me the 101!

Cheers.
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Unread postAuthor: starman » Mon Dec 15, 2008 3:11 am

In my own unscientific case, one spark gap in the center was a better performer than one spark on the edge of the chamber...and...3 gaps equally down the the center was a better performer than one gap in the center. Unmeasured results sure, but definitely detectable by human senses shooting golfball ammo.

Also, HGDT seems to echo what I've sensed...in my case. I can't make a determination on other chamber sizes and configurations.

I believe placement has everything to do with performance increase or not. For instance, 2 gaps placed within a couple of inches of each other in an 18" chamber isn't going to make any performance difference. However, if 2 different spheres of flame, equi-spaced, in the chamber simultaneously each burn 1/2 the chambers fuel air, then total chamber burn will be twice as fast. In the same way, 3 gaps simultaneously burn 3 spheres of fuel/air in the chamber offering yet some further faster chamber burn.

Obviously, diminishing returns kick in when flame spheres are smaller than the diameter of the chamber walls. HGDT predicts this as well. Size and geometery of your chamber and gap placement make all the diff....
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Last edited by starman on Mon Dec 15, 2008 3:19 am, edited 1 time in total.
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