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Hybrid C:B ratios

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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Hybrid C:B ratios

Unread postAuthor: SpudBlaster15 » Thu Jun 28, 2007 6:08 pm

As many are already aware, the optimal chamber to barrel ratio for a standard propane combustion launcher is between 0.5:1 and 1:1. Based on this, it is assumed that the optimal ratio (no energy wasted) for a 4x hybrid is around 0.2:1 - 0.3:1. We know that an over sized chamber decreases the performance of a combustion launcher due to the slower rate of combustion, but, would this also be the case for a hybrid launcher, which uses a burst disk to contain the combustion pressure until a certain level is achieved? Post your thoughts.
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Unread postAuthor: jimmy101 » Fri Jun 29, 2007 1:07 pm

Well, a gasoline engine is basically an 8x hybrid with a 1:8 chamber to barrel volume (actually, I think it works out as a 1:7 chamber:barrel).

A 1:7 chamber to barrel ratio is a CB of 0.14.

This would be the CB that maximizes efficiency.

My Quad, 8X, full-auto hybrid "spud gun";
a 1990 Toyota Celica.
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Unread postAuthor: sharpshooter » Fri Jun 29, 2007 2:24 pm

This is just a guess, but here it goes

It would make sense to me if at a 2x mix, half of the optimal ratio of a normal 1x would be prime in the hybrid. That being said, i would also assume that the extra pressure should increase burn rate, therefore altering the results on that somewhat.

And then so forth... 3x=1/3 of 1x, 4x=1/4 of 1x

just my $0.02 worth
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Unread postAuthor: jimmy101 » Fri Jun 29, 2007 3:50 pm

BTW, continuing on the automotive analogy,

The peak power generally occurs at the peak efficiency, or very close to it.
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Unread postAuthor: boilingleadbath » Fri Jun 29, 2007 4:35 pm

An interesting alternative strategy using the burnt latke data is to add in the energy lost due to atmospheric pressure, multiply by the mix, and then subtract the atmospheric pressure out again...

M(Ev+Ep)-Ep

Of course, I'm suggesting this because it's really easy for me to do the comparison, as EVBEC 1.4 and later already do this calculation (to allow modeling of hybrid mixes).

Such indicates that:
1x -> 1.43/1.66/1.25
1.5x -> 2/2/1.96
2x -> 2/2/2.4
3x -> 2/2/4.7

These are B:C ratios because I'm too lazy to invert them.

(no inter-data-point guesses; these are the fastest of the data points I could do the calculation for [only those that latke tested) - order is 2.5", 1.5", and .75" data.)

We note that the 2.5" data and the 1.5" data indicate that a C:B ratio of .5:1 is best until really high mixes. I don't think that's a valid result.
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Unread postAuthor: Jolly Roger » Mon Jul 02, 2007 2:39 am

Sharpshooters right, all depends on the mixes it is at. If you have a chamber say 100cc on a 2x mix, and you want to achieve the same results from a 4x mix, you would technically make the chamber 50cc. Then you have the addition of the higher original chamber pressure which will also indirectly add to the performance. As boiling said, burnt lake will be able to calculate it for you, as almost everyones circumstances will be different. And an over-sized chamber shouldn't decrease performance, just efficiency.
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Unread postAuthor: SpudBlaster15 » Mon Jul 02, 2007 2:43 pm

Jolly Roger wrote:Sharpshooters right, all depends on the mixes it is at. If you have a chamber say 100cc on a 2x mix, and you want to achieve the same results from a 4x mix, you would technically make the chamber 50cc. Then you have the addition of the higher original chamber pressure which will also indirectly add to the performance. As boiling said, burnt lake will be able to calculate it for you, as almost everyones circumstances will be different.


This is common knowledge, and really doesn't have much to do with the original topic.

Jolly Roger wrote:And an over-sized chamber shouldn't decrease performance, just efficiency.


What is your reasoning behind this?
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Unread postAuthor: boilingleadbath » Tue Jul 03, 2007 12:20 pm

Excuse me... but I'm fairly confident that a 2x (50cc) outperforms a 1x mix (100cc).

Anyways, jolly roger must be working from the fast combustion (displacement definition) standpoint. (which, assuming a high-bursting burstdisk, is essentially the case)
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Unread postAuthor: SpudBlaster15 » Tue Jul 03, 2007 1:31 pm

I believe he was speaking in terms of efficiency. Should a 100cc chamber containing a 1x mix have a barrel that is optimized for this volume, a 50cc chamber containing a 2x mix should theoretically yield the same efficiency (negating the initial mixture pressure).

However, I agree that the 50cc/2x mix should outperform the 100cc/1x mix due to factors other than the potential chamber energy (faster combustion, less heat loss to the chamber walls, greater pressure within the system meaning greater acceleration).
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