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Hybrid C:B

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

Unread postAuthor: rna_duelers » Thu Jun 21, 2007 5:48 am

The name says it all.But What would be a good ratio for a hybrid at say ohh 4X,I saw a figure flying around that .3:1 is good but I would require a 14meter barrel for that,which isn't really feasible or practical(not like hybrids are practical in the first place).

Some help with would be great as I am in the process of making a new Hybrid to make up for that poor excuse of a hybrid I had before.
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Unread postAuthor: Velocity » Thu Jun 21, 2007 10:24 am

Ratios for...

1x mix = 0.7:1
2x mix = 0.4:1
4x mix = 0.2:1

EDIT: Thanks to Jimmy for predicting such ratios
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Unread postAuthor: dongfang » Thu Jun 21, 2007 12:27 pm

Hi,

What´s the reasoning behind those figures?

As far as I can see, and under various assumptions and simplifications, you will get about the same pressure in the barrel with all the compression ratios. But maybe one wants to increase chamber charge pressure in order to increase barrel pressure, too.

OK, if using the numbers "in reverse", then is makes sense. "I have a chamber of x liters and a barrel of y liters. How much pressure should I use to be sure the chamber isn´t too small?"

If I were playing w hybrids, I´d try 0.7:1 throughout. And, if it wasn´t a time when wasting fuel is a sin, I´d just go for 1:1 or something like that, and waste fuel. After all, the "ideal" C:B ratio never was supposed to be the one that gave you the best performance - but simply the ratio where reducing the chamber size starts to hurt performance-wise (ok I´m sure everybody knows that....)

Just my thoughts. I´m not really experienced with it.

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Soren
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Unread postAuthor: jimmy101 » Thu Jun 21, 2007 12:43 pm

Dongfang: " I´d just go for 1:1 or something like that, and waste fuel. After all, the "ideal" C:B ratio never was supposed to be the one that gave you the best performance"


That isn't true for a combustion gun. The optimal C:B ratio maximizes the muzzle velocity of the gun. On both sides of the optimal C:B the muzzle velocity drops off.

The "ideal" C:B ratio is supposed to be the one that gives you the best performance.[/i]
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Unread postAuthor: dongfang » Thu Jun 21, 2007 4:55 pm

Hi, Jimmy101

Well we might agree, really ...

If I had a 1:1 gun and I modified it to 0.7:1 by extending the barrel, then sure it should perform better, if it otherwise behaves as burntlatke etc.´s guns.

Would you expect it to also perform better if modifying it to 0.7:1 by replacing the chamber by a smaller one?

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Soren
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Unread postAuthor: rna_duelers » Thu Jun 21, 2007 5:47 pm

An ratio of .7:1 is more practicle and affordable as PVC and fittings are not to cheap in Australia and I plan this sucker to get Supersonic with a Golfball.But the machining proccess is going to take a while,as the chamber needs to be lathed,a custom union,barrel support and various other.So I have plenty of time to figure out and play around with C:B raitos.

Dogfang.I see what you mean by the ratios.And waste of fuel is not a issue as I can get hold of cans of MAPP gas for $3.80 from my uncle.
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Unread postAuthor: dongfang » Fri Jun 22, 2007 8:50 am

Ok, for a sonic GB at reasonable pressure you might as well plan with a long, long barrel from the start... I´d say around 5-6 meters minimum.

If you oversize the chamber from the beginning you can always extend the barrel if you can´t the the speed you want.

PS: I had good experience with long, thin chambers. You can use a thinner-walled pipe then, and there are some possibilities to play around w the number and position(s) of sparker(s). I had this sonic plastic ball gun last year that was quite sensitive to this.

If PVC is expensive, 50mm OD 3.5mm wall aluminum is perfect for golf, and it might cost only moderaetly more.

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Unread postAuthor: Velocity » Fri Jun 22, 2007 12:51 pm

I don't think you need a barrel that long to shoot a golfball supersonic... 10 feet (about 3 meters I think) should be suitable enough. And you don't really want an oversize chamber on a hybrid, because the fuel will not combustion completely by the time that the projectile is out of the barrel. This will cause a loss in power.
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Unread postAuthor: dongfang » Fri Jun 22, 2007 1:56 pm

Let´s see .. ta da da -- 340 m/s with a 3m barrel...

Assume constant acceleration.

v = 340 m/s = at
s = 3m = .5 at^2

t = 2 (.5at^2)/(at) = 2s/v = 6m/(340m/s) = 17.6 milliseconds.

a = v/t = 19267 m/(s^2). (almost 2000 g)

A GB weighs 0.046 kg and has a cross section area of A=14.3 cm^2.

F = ma = pA = 886.3N

p = 62 N/(cm^2) =6.31 bar. This is a lower limit of the pressure that will do the job. In my experience you need something like double the computed value to meet the target.

Yeah OK maybe ...

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Unread postAuthor: jimmy101 » Fri Jun 22, 2007 4:11 pm

dongfang wrote:Would you expect it to also perform better if modifying it to 0.7:1 by replacing the chamber by a smaller one?


Actually, yes (sort of). You might think it is a no brainer that a smaller chamber will preform less well. But, a smaller chamber burns faster and the pressure rises faster. The faster it burns the higher the pressure is as the spud is "consuming" the barrel as it moves. So it is possible to increase performance by using a smaller chamber. But, like the barrel length affects, there is an optimal chamber for a given barrel, too big decreases performance as does too small.

In the extreme domain people have taken to call this the "mine sized chamber affect". A grossly large chamber will significantly underperform the correctly sized chamber for a given barrel. The very large chamber burns very slowly. The grossly large chamber burns so slow that the entire barrel is used as the spud accelerates at a pressure just sufficient to overcome static friction. During the spud's movement the chamber pressure is essentially constant, it neither rises significantly because of continued burning nor falls because of the increased volume added by the barrel.

Where you get into some really ugly details is when you consider small changes in CB, near the optimal, for a particular barrel. If you start with Latke's gun that he did the 1.5" data with, the optimal CB was 0.6 (IIRC). If you slightly increase the chamber volume it is possible that you could slightly increase the velocity. By "slightly" I mean the CB goes from 0.6 to 0.7 resulting in an increase in velocity of less than 10%. If you continue to increase the chamber volume then you quickly reach the point where the muzzle velocity starts to drop. The $5 question is where that occurs. Is it after increasing CB by 10% or is at 100%? (100% would be a CB around 1.2 if 0.6 is optimal and 1.6 if 0.8 is optimal.)
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Unread postAuthor: dongfang » Fri Jun 22, 2007 6:52 pm

Hi Jimmy,

I guess we´ll have to settle this with a good old gun duel, :twisted: :lol: 8)

OK we do not have the same "imagination" on how the combustion happens. Fortunately, physics don´t care about imagination, just reality.

And I think I can understand what you are saying: One can imagine that an over-large chamber just adds dead space, and the dead space doesn´t really contribute until after the projectile has left the gun. But that being said, I still think it is strange to optimize for zero acceleration at the muzzle :o

I guess there is some experimentation to be done.

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Soren
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Unread postAuthor: rna_duelers » Fri Jun 22, 2007 7:41 pm

Battle of mathematics going on right here!Who will prevail?

Now If I was to use a Barrel that was 5m in length for golfballs then it would have a C:B of .95:1(or near enough to).The chamber is a Liquid oxygen tank that is dis used and has a burst pressure of around 11,000psi/748 Bar.It is going to be a burst disc type.And the 6.31Bar pressure needed(Did I read It correct?)can be attained easily at many more atmosphere shots,higher pressure could be achieved even more easily.

@Rmich-A 3meter barrel would have a C:B of 1.9:1,a lot of energy would go to waste on that C:B.
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Unread postAuthor: Velocity » Fri Jun 22, 2007 7:46 pm

RNA Duelers: When I said you would need a 10 foot (3 meter) barrel to break the speed of sound, I did not say how big the chamber would be. The chamber should be around 20% to 40% of the volume of the barrel.
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Unread postAuthor: rna_duelers » Sat Jun 23, 2007 12:09 am

Rmich,I was just stating that a 3meter barrel would be that C:B.

I figured a barrel of ohh 14meters would have a ratio near enough to .3:1 so haha yeh I think I'm am just going to have to stick with a 4-5meter barrel,but I expect great velocities from that at a 4X mix..

It's coming into winter and my birthday so I will get some $$ and a lot of free time so This seems like the perfect time to start my project.

Below is a rough picture,the union is going to be bolted together.Hey who ever said hybrids had to be quick to arm?The grey is a sparkplug and the blue is PVC and the black around it is a sleave of Aluminium for strength and so I can change the barrels easily.
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Unread postAuthor: dongfang » Sun Jun 24, 2007 5:29 am

rmich:

OK, if you have such a good chamber, by all means try with a high pressure and a short barrel. And of course, when you don´t build or select your chamber but take what you can find, it makes sense to match a barrel to the chamber, not the other way around.

6.3something bar is a lower bound for the pressure it will take to get a GB sonic @ 3m. Lower bound means, unless I screwed up the calculation, it is certain that less than 6.3 bar will NOT get you sonic speed.
You will need more pressure, to overcome friction and drag, to accelerate the mass of the gas itself along with the ball, to pay for the fact that pressure will not be constant ... etc etc. The question is, how much more.

The last time I designed a high performance gun (the all-combustion no-hybrid sonic gun), as far as I remember I did this lower limit computation, and another one of how many grams of gas that would provide the energy necessary. I then took the higher of the lower bounds, multiplied it by 2 or 3 (just a guess) and went on from there...

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