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C:B Research

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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C:B Research

Unread postAuthor: benstern » Mon Apr 16, 2007 7:00 pm

We have not done C:B testing on hybrids and frankly i think it needs to be done. If we have proper and reliable data we can maximize the hybrid cannon's potential. We need someone with a 4x mix capable hybrid who also owns or has access to a reliable accurate chronograph. We will use golfballs as ammo for consistency (unlike the latke tests)

We need separate testing done for 2x 3x and 4x mixes.

I will outline the process.

*obtain 20 foot golfball barrel.
*shoot and record velocity of 5 rounds shot out of cannon at 2x mix with full length barrel. Repeat at 3x and 4x mixes
*cut a foot off of barrel
*shoot and record velocity of 5 rounds shot out of cannon at 2x mix. Repeat at 3x and 4x mixes
*cut a foot off of barrel

repeat the process until you get a 4 foot barrel and your done.

I will donate $5 to help with thuis research's expenses
Anyone who wants to help advance spudgun's power can also donate some resources.
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Unread postAuthor: Fnord » Mon Apr 16, 2007 7:58 pm

Wasnt noname the one who said they wanted to do a bunch of tests like this?
There was another thread titled "what kind of tests?" which Im too lazy to link to/find.
If you can find a chronograph for $5 Ill do the tests :) Im making a small handheld steel hybrid that might be entered in the contest. Doubt it will win anything but I had fun building so far (its almost done).
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Unread postAuthor: noname » Mon Apr 16, 2007 9:58 pm

I could try it, but my parents are just now letting me try an extremely basic hairspray combustion. A full size hybrid will probably take a year or so.
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Unread postAuthor: jimmy101 » Tue Apr 17, 2007 1:18 pm

Didn't boilingleadbath post a week or so ago about a much easier way to get to C:B data without going through the slow process of chopping the barrel? I though his approach (if it was his) was a reasonable way to get the entire C:B profile from a single shot of the gun.

Not only saves having to chop up the the barrel but it cuts down the number of firings from say 5x(number of barrel lengths) to just 5.

His approach requires a laptop or PC in place of the chrony. Any old laptop or PC should work, even a 10 year old one that someone wants to get rid of.

When Latke did his combustion C:B tests it was an incredible amount of work. IIRC, it was something like two 8+ hour days for all the firings. (I think he was doing 9 shots per barrel length.)

One problem with using a golfball barrel is that golfballs aren't the most common ammo. And, I think they are much lower friction than a typical spud. Friction is an important factor in 1x combustion guns and optimal friction is not zero.

If you want to reduce the shot to shot variation of spuds the easiest way to do it is to just weigh the individual spuds. Then, instead of reporting muzzle velocity as the final measured value, report the kinetic energy instead. (It is easy to get the spud's mass without having to recover the spud after firing. 1. weigh the spud. 2. Load into barrel equiped with muzzle knife. 3. Collect and weigh the shavings. 4. Pojectile mass is original spud weight minus the weight of the shavings.)
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Unread postAuthor: benstern » Tue Apr 17, 2007 5:27 pm

Hybrids differ from normal combustions in that they don't need the friction from the ammo to hold the gases in until the proper pressure. They have a burst disk that does that.

golf balls are actually becoming a quite common ammo.
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Unread postAuthor: jimmy101 » Wed Apr 18, 2007 10:42 am

Benstein: Yes, I know hybrids don't respond to friction the same way a standard combustion does. The point is that friction is different between a spud and a golfball. The optimal CB for a hybrid firing a spud is not necisarily the same as for a golfball (which has much lower friction).

With a spud you have to have some friction just to ensure that the spud actually seals the barrel reasonably well.
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Unread postAuthor: benstern » Wed Apr 18, 2007 5:01 pm

So what you are saying is that Latke's C:B ratio tests only pertain to potato projectiles? If so, there is way more testing to be done.
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Unread postAuthor: jimmy101 » Thu Apr 19, 2007 10:54 am

Actually, Latke's CB tests only pertain to taters and his custom gasket slug, and his choice of chamber diameter and volume, barrel diameter, fuel, number of spark gaps, ...

Yes, there is a lot more testing to do. Heck, Latke's data is just about the only reliable data available on combustion guns.
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Unread postAuthor: AmYisroelChai » Thu Apr 19, 2007 12:39 pm

I am in for $5 as well. The money will be transferred upon the full funds being raised.

Is there any reliable C:B ratio for Pneumatics? Or is it bassically 1.2:1?

Thanks

PM me if the research is going to happen I can Pay Pal from US onyl. Maybe ask PC Guy to Sticky a seperate thread to raise funds/materials.
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Unread postAuthor: Scope » Thu Apr 19, 2007 12:42 pm

For pneumatics people use 3:1 or 2:1...
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Unread postAuthor: jimmy101 » Thu Apr 19, 2007 1:31 pm

GGDT will calculate the optimum C:B for a particular chamber and valve. Usually the optimal C:B is around 0.25 or so (a compressed air gun will be most efficient with a barrel ~3x longer than for a similar sized combustion gun).

But, efficiency is not a real important factor in a compressed air gun, hence people frequently use chambers much larger than the one that is most efficient.

With a compressed air gun you can trade efficiency for performance by using a chamber that is much larger than what is optimal for efficiency. (I don't think that works for combustion guns.)

This is why you see C:B ratios of 2:1 or 3:1 for compressed air guns. The gun will propel the spud to a higher velocity but make very poor use of the total energy in the chamber. But, chamber energy is cheap, it just takes a little more pipe and a little longer to pressurize.

GGDT is your friend. It'll give you a very good estimate of how the gun will perform if you know the characteristics of your valve. If you don't know the exact characteristics of your value (which is a common occurence) then GGDT will still give you reliable relative numbers that will give you an idea about how the gun's performance will change based on the chamber volume or barrel length.
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Unread postAuthor: AmYisroelChai » Thu Apr 19, 2007 7:12 pm

Ok my mind is on finals so i cant give a in depth analytical follow up... But thank you all very much for the help...
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