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Small-Bore Combustions

A place to ask general spud cannon related questions.
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Which will perform better

Advanced Combustion
3
100%
Low pressure Pneumatic
0
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Total votes : 3
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 27, 2010 12:03 pm

Technician1002 wrote:Can you think of any scientific variable we are missing on the two hang times that would make the hang time results invalid? Maybe the temperature in Hawaii was warmer giving them a less dense fuel air concentration, but I think that would be negligible.


Fair enough, but it still isn't as good as a chrony reading, over such a large trajectory air temperature has a significant effect on air density for example.

A lot of small variables can add up to skew the result, which is why muzzle velocity for equivalent projectiles would give much better results - and a good chrony can be purchased for well under $100 so it's not asking the impossible ;)
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Unread postAuthor: Technician1002 » Wed Oct 27, 2010 12:17 pm

True, but then you need both parties to post chrony results for comparison. So far neither party has posted results with a tennis ball.

I do have results for t shirts, apples, oranges, and a few other select projectiles. :D

Medium apple.. 882 FPS.

I also have some high speed video. T shirt at 500 frames/sec over about 5 feet.. Composite shot for both speed measurement and projectile tumble analysis. Speed is just under 500 FPS. Chrono is on the left. Video confirmed chrono reading with this oversize projectile. Measure stick behind the shirt is 5 feet long.
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Unread postAuthor: Ragnarok » Wed Oct 27, 2010 12:33 pm

Can you think of any scientific variable we are missing on the two hang times that would make the hang time results invalid?

Firstly, both were only roughly timed.

With a Tennis ball, muzzle energy nearly doubles for the difference between a 10 second and an 11 second hang time.
Given how much difference a single second makes, "accuracy" to only the nearest second is not good enough for comparison here.

For all we know, Sureshot actually got 10.4 seconds and you got 10.6 seconds, or you got 11.4s and them 9.6s. That makes a huge difference.

As small differences in hang time mean big differences in velocity, I'd say that hang time measurements with tennis balls need to be at least good to 0.1 seconds for there to be any meaningful comparison.
And given there are two people measuring with their own reaction times and timing methods, that seems an unlikely prospect.

It's also heavily affected by high altitude wind, exact drag and mass of the tennis ball brand, etc, etc, etc... Really, approximate hang time measurements to the second is useless for much more than saying they're within a factor of 2 of each other on muzzle velocity.

Chronographs are really the only way that you'll get a useful comparison between these two.
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Unread postAuthor: Technician1002 » Wed Oct 27, 2010 12:44 pm

The meaningful comparison here is the air cannon at less then 100 PSI has a longer hang time than an advanced combustion of about the same size. The precise lead of one will take finer instrumentation to find the actual lead. This imprecise measurement does provide useful ball park results.

There are many more air cannons on youtube with tennis ball upshots. Most are much less than 10 seconds hang time. The Sureshot clearly outperforms any of the other air cannons in tennis ball hang time.

None of these cannons are near the same class in power.
[youtube]http://www.youtube.com/watch?v=AGPzbDKgxZQ[/youtube]

[youtube]http://www.youtube.com/watch?v=6Y_gStxSPa8[/youtube]

[youtube]http://www.youtube.com/watch?v=_WWkg3eAEyM[/youtube]
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Unread postAuthor: qwerty » Wed Oct 27, 2010 12:53 pm

I play alot of tennis and find that the new balls are hairy (please don't make a joke out of that :roll: ) and the older ones are much less hairy therefore reducing the drag on the ball.
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Unread postAuthor: Ragnarok » Wed Oct 27, 2010 2:14 pm

Technician1002 wrote:The meaningful comparison here is the air cannon at less then 100 PSI has a longer hang time than an advanced combustion of about the same size.

One second difference when measured very roughly to the nearest second? You'll have to excuse me not putting much faith in that.
For all I know, they measured 10.5 and rounded down, and you measured 10.5 and rounded up.

The other thing is that I think the Sureshot results are probably out. They would relate to a much lower velocity than they get from their heavier compound rounds, so I think they've probably underestimated for some reason - be it a bad shot (can happen, even with a metered combustion), a bad ball, bad conditions or just mistaken timing.
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Unread postAuthor: MrCrowley » Wed Oct 27, 2010 2:50 pm

One thing I have learnt is that the common 1.5" porting piston valve run at normal operating pressures (80-120PSI) can easily be out-performed by a fairly easy to build combustion. A chamber fan, dual spark gap, MAPP metered, decent barrel length (at least 5') and an optimized chamber size (probably around 4"x12" or so).

Whereas if I subsituted that with my V.A.L. 1.5" porting piston cannon for example, the combustion would outperform it by about 50m/s or so with a golf ball last I recall. If I ever wanted a mid-powered cannon like that again, I probably would go combustion. Although it would be quite a bit more expensive for me to build, it would be portable.
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Unread postAuthor: Ragnarok » Wed Oct 27, 2010 5:08 pm

Quite easily, yes - the valveless nature and low density gasses of combustions can give them a lot more oomph at higher velocities than they get credit for.

My reckoning is that unless you're going pretty high pressure in a mid size cannon, then combustions are at least competitive, if not better. (We are talking a straight competition between pneumatic and combustion here, hybrids are a different matter)

Usually, after you get past a launcher size where 120+ psi (and ideally 150 psi or more) isn't practical any more, I'd recommend combustion - unless you have to use pneumatic for some reason, of course. I'd guess that often means 1.5-2" upwards.
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