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Post questions and info about hybrid (compressed gas with fuel) powered cannons here. This includes discussion about fuels, ratios, ignition systems, build types, safety, and anything else relevant.
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Unread postAuthor: D_Hall » Fri May 30, 2008 1:22 am

drex wrote:i to am giting some funky funky readings,

Details?
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Unread postAuthor: drex » Fri May 30, 2008 1:31 am

lol, i try to recreate the errors but now i cant, lol. its kinda like when you take your car to the shop because it has been making a funny sound but once you get there the noise stops.


off topic kinda:what do you do/what kind of education do you have? because this program seems to be a bit to cool for your average guyt o make
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Unread postAuthor: Lentamentalisk » Fri May 30, 2008 1:37 am

ok, I understand the freezing. Yes, I realize that that was majorly unideal conditions, I just though it strange.
On the graphing, would it show any better if I were to, say, change the viewing rectangle (rescale the graph) , or does it graph every 25 calculated points, and not change with the viewing rectangle?

edit: also, I am new to the whole burst disk thing, and so wondering: is it even remotely possible to get a burst disk to not burst until 1500-2000psi across a 1" diameter?
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Unread postAuthor: D_Hall » Fri May 30, 2008 10:27 am

drex wrote:off topic kinda:what do you do/what kind of education do you have? because this program seems to be a bit to cool for your average guyt o make

Follow either link in my .sig. Read the last question in the FAQ.
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Unread postAuthor: D_Hall » Fri May 30, 2008 10:29 am

Lentamentalisk wrote:On the graphing, would it show any better if I were to, say, change the viewing rectangle (rescale the graph) , or does it graph every 25 calculated points, and not change with the viewing rectangle?

Rescaling won't help in your case.

edit: also, I am new to the whole burst disk thing, and so wondering: is it even remotely possible to get a burst disk to not burst until 1500-2000psi across a 1" diameter?

It's totally possible (and done all the time in industrial circles), but you aren't going to do it with aluminum foil.
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Unread postAuthor: SpudFarm » Fri May 30, 2008 11:21 am

but you aren't going to do it with aluminum foil.


rather a flange and aluminium plate
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Unread postAuthor: Lentamentalisk » Fri May 30, 2008 4:24 pm

then how high can one get across a 1" union? I am trying to see how high of a mix is worth making. Using HGDT I noticed that you can even get better results with a low pressure burst disk by using a lower mix, so I dont want to way over do it.
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Unread postAuthor: jimmy101 » Thu Jun 05, 2008 4:00 pm

The availability of GGDT (v4.6) and HGDT(v0.5.4) makes it possible to compare the performance of combustion and pneumatic guns without actually having to build both guns. Given the usual caveats about the reliability of the models, the suitability/accuracy of the various input parameters etc...

3"x12" chamber, 2"x36" barrel, 100g shell with 10 PSIG friction. Both guns with CB ~0.8. Wild-ass guess as to the time it takes to open a 2" ball valve of 0.1 seconds.

Looks to me that HGDT/GGDT say a generic combustion will greatly out perform a same sized ball valve pneumatic charged to 120 PSIG.

Switching the pneumatic to a chamber sealing valve using the parameters D_Hall has on his design page for the Orbit Watermaster 1" sprinkler valve... The combustion still outperforms the pneumatic.

I thought that it was because of the low CB ratio, combustions are most efficient at CB~0.8 (but not necisarily most powerful), pneumatics usually have CBs of a couple. So, lengthened the chamber of both guns to 3x what it was, CB now 2.25. Combustion still greatly outperforms the pneumatic.

Somebody else want to give this a try? I've not used GGDT all that much and I might have something goofed up. The combustion results appear reasonable to me. The pneumatic results seem way to low.

The two screen shots are for the CB 2.25 and Orbit sprinkler valve comparison.
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Unread postAuthor: SpudFarm » Thu Jun 05, 2008 4:04 pm

that is becuse the pneumatic flow is chocked i belive.
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Unread postAuthor: DYI » Thu Jun 05, 2008 4:29 pm

The pneumatic results are oddly low because the flow is choked so terribly. A 1" sprinkler valve doesn't even have a quarter the flow of the 2" port on the combustion. Use a 2" burst disk valve on the pneumatic, and it should be close to the combustion, or maybe even more powerful.
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Unread postAuthor: jimmy101 » Thu Jun 05, 2008 5:17 pm

Spudfarm and DYI

Thanks, like I said, I'm not to familar with GGDT.

If I'm reading you correct, for this size gun, "generic" type valves (ball valves open too slowly, typical sprinkler valves are flow choked) basically suck as pneumatics? At least, compared to a generic combustion of the same size. To get the pneumatic to outperform the combustion you would have to go to a custom built valve with a much larger throat. Since the required throat is a funtion of the barrel ID, large bore combustion guns will tend to out perfom a pneumatic built with an "off the shelf" valve (same chamber volume, barrel dimensions, ammo etc.)
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Unread postAuthor: D_Hall » Thu Jun 05, 2008 9:34 pm

jimmy101 wrote:If I'm reading you correct, for this size gun, "generic" type valves (ball valves open too slowly, typical sprinkler valves are flow choked) basically suck as pneumatics? At least, compared to a generic combustion of the same size. To get the pneumatic to outperform the combustion you would have to go to a custom built valve with a much larger throat. Since the required throat is a funtion of the barrel ID, large bore combustion guns will tend to out perfom a pneumatic built with an "off the shelf" valve (same chamber volume, barrel dimensions, ammo etc.)

Depends on your definition of "generic" and "off the shelf."

A 2" sprinkler valve purchased online will outperform the combustion gun (at least, according to the models), but I wouldn't consider it "custom" by any stretch of the imagination.

That said, you are correct in that pneumatics don't scale well to large bores. They quickly become cost prohibitive, yada yada....
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Unread postAuthor: Lentamentalisk » Thu Jun 05, 2008 11:25 pm

D_Hall wrote:That said, you are correct in that pneumatics don't scale well to large bores. They quickly become cost prohibitive, yada yada....


...hence our interests in semi-auto and cartridge combustions...
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Unread postAuthor: jimmy101 » Fri Jun 06, 2008 11:19 am

D_Hall

Gotcha, makes perfect sense. Still a 2" valve is probably pretty pricey.

A combustion gun should be much less sensitive to a choke in the gas flow path. Much higher gas temp, much lower gas density --> much less pressure drop across the choke. If you model the combustion with a 1" burst disk it makes very little difference in the performance of the combustion. I suppose HGDT and GGDT model the choke a lot differently though, HGDT isn't using the "flow coeffic" parameter at all, which seems to be pretty critical to the performance of the pneumatic.

So, assuming a standard 1" sprinkler valve, reduce the choke affect by going to a 1" barrel instead of 2", drop the ammo mass (100g to 25g) and friction (10 PSI to 2.5 PSI, probably should have used 5 PSI) ... I get the pneumatic outperforms the combustion by ~13% in muzzle velocity, 28% in KE. Significant but not a huge difference.

But now the guns' CB is 3. Both combustion and pneumatics will perform better with a longer barrel. So, increase the barrel length to 108" for a CB of 1. Now the pneumatic outperforms the combustion by just 4% (8.5% in KE). I think it would be pretty difficult to actually measure a 4% change in performance.

So I'm not seeing a gross difference in performance compairing an "advance combustion" (metered fuel, chamber fan, double bevel spudcutter, single spark gap) with a "generic" pneumatic made with commonly available (and affordable) off-the-shelf components. Looks to me that to get a pneumatic to really outperform a combustion you have to go to a custom built valve.

But, if you are using a custom built valve then you should probably compare that with a burst disk combustion (the burst disk combustion would be easier to make than most custom valves for a pneuamtic?) ...

YMMV
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Unread postAuthor: D_Hall » Fri Jun 06, 2008 5:07 pm

jimmy101 wrote:A combustion gun should be much less sensitive to a choke in the gas flow path. Much higher gas temp, much lower gas density --> much less pressure drop across the choke.

Agree.

I suppose HGDT and GGDT model the choke a lot differently though, HGDT isn't using the "flow coeffic" parameter at all, which seems to be pretty critical to the performance of the pneumatic.

Actually, they handle the choking in virtually identical manners. The difference is that since combustion guns tend to have very straight forward plumbing, it's easy to make assumptions based on whether or not it's a blunt or contoured transition between the chamber and barrel (whether you know it or not, when you select one of those, you're selecting a flow coefficient). Pneumatics, by comparison, can have all sorts of crazy gas paths that make simple assumptions concerning flow coefficients impossible to make with any sort of reliability.

Looks to me that to get a pneumatic to really outperform a combustion you have to go to a custom built valve.

More or less. The beauty of pneumatics isn't so much in their performance, but in their clean and simple operation. Oh, and in their ability to deliver reliable and yet variable power from the same package.
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