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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
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100%
Low pressure Pneumatic
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Total votes : 3
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Small-Bore Combustions

Unread postAuthor: Lockednloaded » Mon Oct 25, 2010 9:39 pm

This forum is full of medium sized pneumatics, most ranging from 1"-1/2" Bore, primarily made out of PVC, and used at compressor pressures. Most of these use inefficient valves such as sprinkler valves. Don't get me wrong, low pressure launchers are loads of fun, and I'm not trying to insult those who build them (because I build then quite often). Well today when going through my parts, I realized I had everything I needed for an advanced combustion: syringe, fan, chamber/barrel, and ignitor. Seeing these parts got me thinking, why are LP pneumatics so common, yet a small bore combustion could theoretically achieve better performance for a lower price?

Unfortunately, I don't have any real numbers atm to back up my thoughts, but when you take into account the pros and cons of each, it seems so much better to build an advanced combustion. In many common low pressuredesigns, the valve restricts flow, has some sort of opening time, and chamber pressure is wasted. However, in an advanced combustion, pressures can be generated exceeding 100psi, there are no flow restrictions, no valve opening time, and the propellent gasses are heated. So what gives these low pressure pneumatics the upper hand? Am I missing something?

If no one else has done so, I will test two launchers of similar proportions, one a SV pneumatic at 100 psi, and another, an advanced combustion with fan, metered propane, and multiple spark gaps.

Keep in mind that if I do this, I have no chrony or highspeed camera, so the two will be judged on damage, hangtime, and possibly range

Which do you think will win?
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Unread postAuthor: jackssmirkingrevenge » Mon Oct 25, 2010 11:00 pm

Building a pneumatic is probably simpler than an advanced combustion, and getting it to work well (no spark gaps/fuelling issues) is easier for most people.

Also, there's something reassuring about using air, the "mechanical" nature of opening a valve somehow feels more reliable than clicking an ignitor, as well as a natural inclination to think burning stuff is more dangerous whereas air... well we breathe the stuff :)

I don't know which launcher in your test would get better performance, but I don't think that's the issue at hand here in terms of preference and popularity.
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Unread postAuthor: Gaderelguitarist » Mon Oct 25, 2010 11:23 pm

I agree with JSR on this one.

There are more factors that affect a combustion's power than we normally see with pneumatics. Air pressure is just numbers, valves and tight seals. Combustion is ratios, fuel mixing, valves, tight seals, spark gap, ignition circuit blah blah...

It would be reassuring to know which performs better though.
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Unread postAuthor: POLAND_SPUD » Mon Oct 25, 2010 11:43 pm

I will test two launchers of similar proportions, one a SV pneumatic at 100 psi, and another, an advanced combustion with fan, metered propane, and multiple spark gaps
the question is - why do you compare an average pneumatic with an advanced combustion ? :wink:
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Unread postAuthor: jackssmirkingrevenge » Tue Oct 26, 2010 12:23 am

The man makes a good point, someone with the inclination to add a fan, metered propane, and multiple spark gaps etc. to a combustion can surely put together a piston valved pneumatic as opposed to a sprinkler valved one.

I think to be fair you should compare an unmodded sprinkler valve cannon to a spray'n'pray combustion.
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Unread postAuthor: Technician1002 » Tue Oct 26, 2010 12:25 am

I have informally compared youtube videos of the Sureshot tennis ball hang time and my QDV hang time. Feel free to compare yourself.

[youtube]http://www.youtube.com/watch?v=KVgMN5lU5nM[/youtube]
Sureshot 10 second hang time.

[youtube]http://www.youtube.com/watch?v=s0l5t7D0FDM[/youtube]
2 inch QDV ~11 sec hang time
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Unread postAuthor: Lockednloaded » Tue Oct 26, 2010 1:12 pm

The problem with testing a spray and pray is inconsistency, so the test would be useless.

As tech has shown, advanced combustions can keep up with advanced pneumatics
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Unread postAuthor: Technician1002 » Tue Oct 26, 2010 1:46 pm

I did have the chance to shoot side by side next to a propane spray and pray. No meter or chamber fan was used.

My Apple Cannon (ABS TEST CANNON) is the same size as the spray n pray. We were closely matched in range with apples when I ran 35-40 PSI. It was game over when I went to 70 PSI. Both are 4 inch chambers a couple feet long with 2 inch barrels about 7 feet long. Mine is coaxial so my chamber was slightly smaller with a slightly longer barrel. We were not perfectly matched, but close in launcher sizes.

The spray and pray was much louder so it seemed higher power, but that was just muzzle flash.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 27, 2010 7:06 am

Lockednloaded wrote:The problem with testing a spray and pray is inconsistency, so the test would be useless.

As tech has shown, advanced combustions can keep up with advanced pneumatics


"hang time" is hardly a scientific measurement, I'd want to see chrono readings with consistent projectiles ;)

Also, what would you consider to be an "advanced pneumatic" in this case? Combustions can only compete if you limit the pressure used in pneumatics, upping the pressure will easily compensate for poor valve flow or opening time.
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Unread postAuthor: Technician1002 » Wed Oct 27, 2010 10:03 am

I would consider advanced pneumatic as anything with better flow than a sprinkler valve and an opening time less than 4ms.
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Unread postAuthor: velocity3x » Wed Oct 27, 2010 10:30 am

Lockednloaded wrote:As tech has shown, advanced combustions can keep up with advanced pneumatics


Well, I must disagree. I think pneumatics have the potential to do much heaver work at high speed that combustions.

However, in an advanced combustion, pressures can be generated exceeding 100psi


The same and higher is true for pneumatics.
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Last edited by velocity3x on Wed Oct 27, 2010 10:36 am, edited 1 time in total.
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 27, 2010 10:35 am

velocity3x wrote:Well, I must disagree. I think pneumatics have the potential to do much heaver work at high speed that combustions.


1x combustions certainly, but when you're talking hybrids it's a very different story.

The benefits of combustions become apparent at larger bores, where the relatively low pressure generated is not as much of a disadvantage, and the fact that you essentially don't need to worry about a valve gives a massive construction benefit over pneumatics.
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Unread postAuthor: velocity3x » Wed Oct 27, 2010 10:38 am

jackssmirkingrevenge wrote:1x combustions certainly, but when you're talking hybrids it's a very different story.


My knowledge of hybrids is limited. What kind of weight payload can hybrids handle?
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Unread postAuthor: jackssmirkingrevenge » Wed Oct 27, 2010 11:35 am

velocity3x wrote:My knowledge of hybrids is limited. What kind of weight payload can hybrids handle?


In the same way that pneumatic efficiency varies with heavier projectiles (because the higher projectile inertia can compensate for a slow opening valve) a hybrid could conceivably be tuned for optimal mixture combustion by varying the burst pressure of the disk - for example if a hybrid performs optimally with a 1 gram projectile and burst pressure of 200 psi, if you want to use it with a 5 gram projectile you can have a disk that bursts at 100 psi, obviously the relationship would not be linear but with the help of HGDT it could be approximated.
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Unread postAuthor: Technician1002 » Wed Oct 27, 2010 11:55 am

"hang time" is hardly a scientific measurement, I'd want to see chrono readings with consistent projectiles


The "hang time" can be valid if some of the conditions are matched well between the launchers.

1 Elevation.. Both tests were performed near sea level. The air cannon was at 300 Feet above sea level (Topo map verified) and I presume the combustion in Hawaii is near sea level. A Google map search of the business location may confirm that.

2 Barrel, Both tests were done with a tennis ball barrel.

3 Projectile Both tests used a regulation tennis ball. The variable in weight and size is small in regulation tennis balls.

4 Time measurement, both taken from video in real time.

5 Wind, both shots were done in relatively still air.

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.

Chronograph measurements would be nice to have fore both launchers, but with the absence of this, we work with the best we have. At the moment that is video of hang time of like projectiles.
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