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A question regarding ratios...

Post questions and info about combustion (flammable vapor) powered cannons here. This includes discussion about fuels, ratios, ignition systems, safety, and anything else relevant.
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A question regarding ratios...

Unread postAuthor: Velocity » Fri May 25, 2007 11:34 pm

OK, I understand the the chamber to barrel ratio regarded by most people to be the best is 0.7:1 or 0.8:1. However, I was thinking about this yesterday, and I guess I do not really know what it means. If someone could just answer all four of the following questions with either a "Better" or "Worse" response, it would help me tremendously.

Combustion A has an "ideal" ratio, and Combustion B does not

1) If the barrels of two combustions are at a set length, and the chamber volume of Combustion B is increased, how would Combustion B perform with respect to Combustion A?
2) If the barrels of two combustions are at a set length, and the chamber volume of Combustion B is decreased, how would Combustion B perform with respect to Combustion A?
3) If the chambers of two combustions are at a set volume, and the barrel length of Combustion B is increased, how would Combustion B perform with respect to Combustion A?
4) If the chambers of two combustions are at a set volume, and the barrel length of Combustion B is decreased, how would Combustion B perform with respect to Combustion A?

I personally think that the answers are:
1) Better
2) Worse
3) Worse
4) Worse

I am not really confident in these answers though (#1 especially), so thats why I posted this question. I used the Burnt Latke test data to get some of my answers

Some of these questions may seem blatantly obvious to many of you. However, I just want to insure that I have all of my facts straight, and all in one place. Ultimately much of this information regarding ratios should help me out as I plan the dimensions of my hybrid.

Thanks
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Unread postAuthor: CS » Fri May 25, 2007 11:42 pm

1 - Better
2 - Worse
3 - Worse
4 - Worse

*edit*
Ooops, I didn't spot the clause of launcher A holding a ideal ratio.
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Unread postAuthor: cerberus » Fri May 25, 2007 11:51 pm

1) increasing chamber size never hurts, but may not help depending on the size of the barrel and chamber initially.
2) shrinking the chamber will most likely hurt performance, especially if you're decreasing it below ideal C:B ratio.
3) if the barrel is increased too much it will hurt performance because projectile will start slowing down inside the barrel.
4) if the barrel is shortened too much it will hurt performance as the projectile will leave the barrel before it gets up to maximum speed.

So in short you were basically correct in your initial assumptions. You should note though that in a hybrid the ideal dimensions will change, with more gas being produced in the explosion you will probably want a longer barrel.
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Unread postAuthor: mark.f » Sat May 26, 2007 8:31 am

1) increasing chamber size never hurts, but may not help depending on the size of the barrel and chamber initially.


1) If the barrels of two combustions are at a set length, and the chamber volume of Combustion B is increased, how would Combustion B perform with respect to Combustion A?
Rmich
1.) Better
pimpmann22
1.) Better


I tend to disagree there. While it may help or not make any significant difference up to a point, it will start decreasing performance after that point. Jimmy will probably jump in and give the exact reason in a few moments, but I'm just looking at this in general right now, from what I think I know.

EDIT: my favwite approach to this dilemma is to build the cannon starting with the barrel, like with my friend's golfball cannon. We built a 10' golfball barrel, then built the 6" chamber to have roughly a 1:1 CB ratio.
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Unread postAuthor: Velocity » Sat May 26, 2007 10:15 am

Thanks for your replies. The reason I asked was because for the upcoming hybrid which I was planning, it would be difficult to have a low C:B ratio. I don't like having really short chambers followed by a really long barrel in an over under style, because it gets really annoying to hold because of BBS (Bendy Barrel Syndrome) and it takes away from the aesthetics as well.

If I could build my hybrid with any ratio I wanted, disregarding aesthetics and BBS, I would have a 0.3:1 ratio. From what I have heard, hybrids are more efficient and get more power out of lower ratios due to their use of higher mixes... can anyone verify this?

(I know that this topic is not really a combustion topic, but its dealing more with combustion ratio's now than hybrids, so I guess the combustion section is an OK place for it to be)
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Unread postAuthor: SpudBlaster15 » Sat May 26, 2007 10:23 am

markh11q wrote:I tend to disagree there. While it may help or not make any significant difference up to a point, it will start decreasing performance after that point. Jimmy will probably jump in and give the exact reason in a few moments, but I'm just looking at this in general right now, from what I think I know.


A larger chamber will increase performance due to more energy being available, but there comes a point when a larger chamber will decrease performance because of the fuel taking too long to burn. This will generally not happen, unless of course you are filling empty salt mines with air/fuel mixture. (BLB's analogy)

If you are building a chamber around a barrel, and don't mind the extra noise output, go with something around 1.5:1. If you are building a barrel around a chamber, go with something around 0.6 - 0.8:1.
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Unread postAuthor: mark.f » Sat May 26, 2007 11:16 am

Sorry but I'm having trouble communicating this morning. Are you saying that having a larger chamber won't hurt performance, or that it won't hurt performance significantly?

The latke tests lacked, in my opinion, in that they varied barrel length. It would be more complicated, but I think we need a test that holds barrel volume at a constant, and varies the chamber volume. It'd be one hell of a rig, but I think it could be done, rather inexpensively, with some sort of telescoping chamber.

<img src="http://www.markfh11q.net/images/telescopingcombustion.png">

The screw on the bottom is the set screw. The black thing is a 2" SCH-80 coupling. The outer pipe is 3" SCH-40, and the inner pipe is 2" SCH-40. In the all-forward position, the pipe seals against a gasket on the inside of the 3x2" bushing, which eventually goes down to a 3/4" female thread for a 3/4" test barrel.

EDIT: I did not mean to say track. There will be a series of tapped holes on the bottom, which can be plugged by a machine screw when not in use, and in the effective chamber volume.
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Unread postAuthor: jimmy101 » Mon May 28, 2007 3:20 pm

Combustion A has an "ideal" ratio, and Combustion B does not

1) If the barrels of two combustions are at a set length, and the chamber volume of Combustion B is increased, how would Combustion B perform with respect to Combustion A?


Combustion B's performance might increase, decrease or stay the same.

If Combustion B had a too small chamber then increasing it to the optimal C:B (whatever that might be) will increase performance.

If Combustions B started off with a slightly too large chamber then small increases in chamber volume might give very small increases in performance.

Large increases in chamber volume will start to decrease performance, particularly when the C"B ratio starts to get above perhaps 2:1.

Combustion A has an "ideal" ratio, and Combustion B does not

2) If the barrels of two combustions are at a set length, and the chamber volume of Combustion B is decreased, how would Combustion B perform with respect to Combustion A?


Again, what will happens depends a lot on whether Combustion B had a chamber that was too large or too small to start with. If it was too large then decreasing the chamber volume towards the optimal would tend to increase performance. If the original C:B wasn't too far off the optimal (within perhaps a factor of 2) then the change in performance would be fairly small. As the chamber is decreased below the optimal the performance will start to drop. Though modeling suggests that the drop off in performance is fairly minor until the C:B gets well below 0.8. For example, 0.6 would perform pretty well, 0.4 would start to be significantly less powerful.

For questions three and four there is no prediction possible since again, we don't know if combustion B starts off too big or too small. So, the answers are;
3. better or worse
4. worse or better

And of course, there is always the question of how much of a change does there need to be to be considered measurable or worthwhile. In my experience C:B ratio is a pretty soft number. Within a range of perhaps as much as a factor of two the performance of the gun changes fairly little. So, C:Bs in the range of perhaps 0.6 to 1.5 are frequently indistinguishable.

Has anyone ever measured an optimal C:B for a hybrid? I wouldn't be surprised if things weren't a bit different than a conventional combustion gun. Different optimal C:B, different sensitivity to non-optimal C:B ratio ...
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Unread postAuthor: boilingleadbath » Mon May 28, 2007 5:12 pm

If we presume that launcher Bravo initially has a higher C:B ratio than 'optimum', than I'd expect (assuming we avoid salt mines):

1) Better - less pressure drop over cycle; higher average force
2) Worse - more pressure drop over cycle; lower average force
3) Better - more distance to accelerate; higher efficiency (presuming final C:B ratio > .8-.6)
4) Worse - less distance to accelerate; lower efficiency

(heh... woops, I compared 'new Bravo' to 'old Bravo' instead of 'new Bravo' to 'Alpha'. It's a more meaningful comparison anyways...)

That is, the combustion acts a bit like a pneumatic.

This is working from the 'fast combustion' viewpoint, in which combustion occurs over a fairly insignificant amount of projectile travel, and thus there isn't much ramp-up time 'penalty' for a larger chamber.
At least with latke's launchers, which used multiple spark gaps, this seems to be a fairly good approximation.

I won't explain why I feel it's a good approximation, as I don't want to start off-topic discussions.

From the 'slow combustion' point-of-view, everything gets fuzzy, as performance becomes heavily dependent upon chamber geometry, characteristics of the projectile, so-on-and-so-forth.

********************
For a hybrid, I'd just make the chamber over sized... you have burstdisks, which create the perfect scenario for the application of the 'fast combustion' model.
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