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Alright, I'll tip my hand a bit...

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: jimmy101 » Sat Mar 29, 2008 6:49 pm

D_Hall wrote:Also, I'm seeing optimal C:B ratios that depend upon projectile mass. This makes sense in that heavier projectiles spend more time in the barrel and as such lose more energy to their surroundings. HOWEVER, I suspect my dependence is stronger than it should be.

I've always wondered about that. And if mass affects CB then so should the static and dynamic friction. And perhaps the number of sparks and the presence/absence of a fan.

We've always assumed that optimal CB was a relatively constant parameter of a combustion gun but perhaps it isn't.

This is one of the real advantages of having a decent mathematical model. Even if it can't predict muzzle velocities with high accuracy often the trends are still useful.
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Unread postAuthor: D_Hall » Sat Mar 29, 2008 8:11 pm

jimmy101 wrote:I've always wondered about that. And if mass affects CB then so should the static and dynamic friction. And perhaps the number of sparks and the presence/absence of a fan.

I would expect all of those things to affect the optimal ratio as they all affect residence time and/or energy loss.

We've always assumed that optimal CB was a relatively constant parameter of a combustion gun but perhaps it isn't.

Enquiring minds.... :D
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Unread postAuthor: D_Hall » Sat Mar 29, 2008 10:28 pm

clide wrote:Setting the burst disc to 0 with a reasonable gun gives me
ERR: Proj never cleared muzzle
WRN: Barrel unnecessarily long


Fixed.

Problem was too large of an iteration interval. Nothing wrong with the physics per se. Had to go with 1 uS to get things smoothed out. This worked but made the sim run dog slow so I wrote a little routine to do dynamic iteration interval calculations.

In any event, a new file has been uploaded.

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Unread postAuthor: starman » Sat Mar 29, 2008 11:30 pm

I notice the warning "Combustion Incomplete" on no or small burst disk configurations. Can I take this as a guide to finding the optimum or at least the preferred minimum disk based on my other paramenters?
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Unread postAuthor: D_Hall » Sun Mar 30, 2008 9:41 am

That's the idea....

....but honestly I just don't know at this point. It may be accurate for some configurations and not so accurate for others.

Alternatively, a longer barrel will also give time for complete combustion.
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Unread postAuthor: D_Hall » Sun Mar 30, 2008 8:40 pm

By the way... links have changed and such.

HGDT webpage is still under construction but functional enough that I'll use it from here on out and just make verbal reference to it.


http://www.thehalls-in-bfe.com/HGDT




edit: In other news, I've started to compare HGDT results to Latke's data. For long barrel stuff, the results are very good. But the shorter the barrel, the further off HGDT is. However, the results start to diverge pretty quickly when the "Combustion Incomplete" warning shows it's head and yes, HGDT is significantly slower that Latke. Thus, it's fair to assume that the problem are nothing more than my in-barrel burn rates being too slow. Very easy to believe as I just kinda made some conservative guesses regarding in-barrel burn rates.
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Unread postAuthor: D_Hall » Mon Mar 31, 2008 2:20 am

jimmy101 wrote:
D_Hall wrote:Also, I'm seeing optimal C:B ratios that depend upon projectile mass. This makes sense in that heavier projectiles spend more time in the barrel and as such lose more energy to their surroundings. HOWEVER, I suspect my dependence is stronger than it should be.

I've always wondered about that. And if mass affects CB then so should the static and dynamic friction. And perhaps the number of sparks and the presence/absence of a fan.

We've always assumed that optimal CB was a relatively constant parameter of a combustion gun but perhaps it isn't.

I think I just stated above that I've been looking at Latke's data. First time, actually as combustions have never been my thing (still aren't, but I'll use whatever data I can get my hands on). In any event, everybody quotes his stuff and decrees... 0.8 is optimal!

But have they actually LOOKED at his stuff?

He uses three different types of ammo and gets three different numbers (0.6, 0.7, 0.8 all make appearances)! Yeah, 0.8 shows up more than any of the other numbers, but it only shows up for that one type of ammo.

I'm starting to think that mass and friction play large parts in all this.
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Unread postAuthor: psycix » Mon Mar 31, 2008 4:12 am

A high-static friction projectile acts as a burst disk for a little bit.
It has the same effect of "not moving yet" while the combustion is active.
Though the dynamic friction will probably also be high giving some other effects.

Keep up the good work DH!
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Unread postAuthor: jimmy101 » Mon Mar 31, 2008 10:38 am

D_Hall wrote:I think I just stated above that I've been looking at Latke's data. First time, actually as combustions have never been my thing (still aren't, but I'll use whatever data I can get my hands on). In any event, everybody quotes his stuff and decrees... 0.8 is optimal!

But have they actually LOOKED at his stuff?

He uses three different types of ammo and gets three different numbers (0.6, 0.7, 0.8 all make appearances)! Yeah, 0.8 shows up more than any of the other numbers, but it only shows up for that one type of ammo.

I'm starting to think that mass and friction play large parts in all this.

If you look at Latke's data you'll also notice that even though 0.8 might be optimal for a particular gun+ammo combination there is really no difference in performance in the range of CBs from about 0.5 to 1.0. The incremental change in velocity on going from say 0.6 to 0.7 CB is not statistically significant. If it weren't for the fact that Latke collected so much data it would be impossible to say that there is any difference at all. So, 0.8 (or whatever) may well be optimal but the drop in performance in going from a CB of of 0.8 to 1.0 (shortening the lenght of the barrel by 25%) is insignificant. "Insignificant" in both a statistical and practical sense.

Another caveat to Latke's data is that he never actually measured the velocity versus projectile position. He measured the velocity versus barrel length. Others then took that data and recreated velocity versus position graphs. It is possible that the rather broad peak in the CB versus velocity graph is actually a fairly sharp CB versus velocity relationship that is muddied by the shot to shot variation in the velocity versus position, and hence optimal CB, relationship. In other words, even though he was trying to keep things constant for each shot, each shot has a unique optimal CB. The graphs just average that affect out.

I don't think there is much doubt that mass and friction (and chamber diameter vs. barrel diamter and number of sparks and a fan and ...) all have an affect on the optimal CB. The question is how much of an affect.
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Unread postAuthor: jimmy101 » Mon Mar 31, 2008 10:54 am

Here is a graph of CB versus velocity for Latke's 1.5" barrel shooting spuds.

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As you can see, there is really no difference in velocity in the CB range of about 0.5 to 0.9. Even up to a CB of 1.2 the difference in velocity is only weakly statistically significant.
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Unread postAuthor: D_Hall » Mon Mar 31, 2008 1:38 pm

Thanks for pointing all that out, Jimmy.

Another I noted... I don't see anywhere where he weighed his projectiles (or at least, reported the weights).
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Unread postAuthor: starman » Mon Mar 31, 2008 2:05 pm

It's almost like there's some harmonic resonance component between chamber and barrel, just like in bass reflex speaker systems' cabinet/port relationship. There definitely some sort ot resonate efficiency peak going on here.

Speaker mass/compliance would equal spud size/mass/barrel friction etc., maybe.....
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Unread postAuthor: jimmy101 » Mon Mar 31, 2008 5:28 pm

D_Hall wrote:I don't see anywhere where he weighed his projectiles (or at least, reported the weights).

Ya, that has always bugged me about Latke's studies, no masses or friction values. I sent him an email once asking about it but he never answered. (It is possible that he gets so much junk email that he never saw mine.)
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Unread postAuthor: starman » Mon Mar 31, 2008 6:22 pm

jimmy101 wrote:
D_Hall wrote:I don't see anywhere where he weighed his projectiles (or at least, reported the weights).

Ya, that has always bugged me about Latke's studies, no masses or friction values. I sent him an email once asking about it but he never answered. (It is possible that he gets so much junk email that he never saw mine.)


He did the 3 tests, tennis ball, spud and 3/4" slug, all having different masses and friction coefficients and came out with the .8/1 thing (pretty much) across them all.

While friction and weight does have an effect on absolute performance, relative performance still seems to cling to the .8/1 "law" somehow. David, have you found the mathmatical model that explains this phemonenon? What in your calculations accounts for it?
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Unread postAuthor: D_Hall » Mon Mar 31, 2008 8:44 pm

Hmmm.... Been playing around with turbulence levels. I think I may be on to something here. Looks like the Latke launcher may have extreme turbulence up front that decays or something.... I'll have to play around with this a bit!
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