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Why I believe coaxials are opposite of what is best fps etc.

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Unread postAuthor: boyntonstu » Fri Apr 09, 2010 4:07 pm

velocity3x wrote:
boyntonstu wrote:Why I believe coaxials are opposite of what is best for fps.


A few post later you said:

The diameter of the barrel is the key to power, not how the compressed air feeds into it.


That statement conflicts with your original post. Essentially you're saying that it's a gun design issue and not a coax vs. piston issue. :?


Again, the diameter of the barrel is the key to power, not how the compressed air feeds into it.

Coaxials are opposite of what is best for fps if you are shooting the same WEIGHT.
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Unread postAuthor: Moonbogg » Fri Apr 09, 2010 4:32 pm

I must be missing something here. Design parameters are everything. If you want to shoot 2" ammo, you can make an inline, over under, or coaxial all to be comparable in power. Its all about making sure the C:B ratio is ideal with minimal obstruction of expanding gasses. Why can't a coaxial be just as powerful as anything else if designed well? I don't see how having the barrel in the chamber affects this so long as the C:B ratio is ideal.
You can either use a fatter or longer chamber to compensate, or not compensate and go hybrid with it or use more pneumatic pressure to compensate. Theres more than one way to skin a cat and even more ways to build a spud flicker.
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Unread postAuthor: JDP12 » Fri Apr 09, 2010 4:52 pm

Im gonna ave to agree with moon and ohers on this. A coaxial is a piston valve, just configured differently. Like moon said, design is everything. There is no definition of a coaxial that says it instantly has less potential than another sort.

Really, i doubt theyd be as popular as they are if performance wasnt that great. IMHO, its about how you the user designs and constructs it. If you construct a $hitty piston, obviously it will suck.
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Unread postAuthor: jhalek90 » Fri Apr 09, 2010 4:53 pm

I believe that a co-ax... with the same chamber VOLUME as a T piston valve, both using the same barrel, and projectile, as well as pressure, would produce and insignificantly higher FPS.




now, i might be wrong.... but im pretty sure im not.

Same pressure, same chamber size, same projectile, same barrel, same valve flow coefficient, would seem to me that both designs would have very similar muzzle velocity AND energy.

But my bet is on the co-ax actually being better.
By a tiny amount. possibly in the 1-5% range, simply do to the way the air flows in a co-ax.
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Unread postAuthor: boyntonstu » Fri Apr 09, 2010 5:03 pm

Get specific: What diameter chamber and what diameter barrel?

We are shooting 100 grains.

Forget C:B ratio for a second.
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Unread postAuthor: Moonbogg » Fri Apr 09, 2010 5:18 pm

I will answer off the top of my head, but just so you know, I myself would never bother with such a light weight projectile. I would guess a 1" barrel and 2.5" chamber would do alright.
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Unread postAuthor: Mr.Sandman » Fri Apr 09, 2010 5:20 pm

By the way i could point out a plethora of coaxials built by jsr that were cheaper and easier to make than your video.
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Unread postAuthor: c11man » Fri Apr 09, 2010 5:39 pm

were compairing cannons made with the same materials, or with the same chamber volume and barrel size?
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Unread postAuthor: boyntonstu » Fri Apr 09, 2010 5:46 pm

Moonbogg wrote:I will answer off the top of my head, but just so you know, I myself would never bother with such a light weight projectile. I would guess a 1" barrel and 2.5" chamber would do alright.


OK Fine. (100 grains is for illustration)

Now switch the barrel to 2.5" and the chamber to 1".

The areas are 4.9 and 0.785 square inches.

Assume 100 psi.

If you use a 1" diameter barrel, the projectile will feel 78.5 pounds of force.

Using the 2.5" diameter barrel, the projectile will feel 490 pounds of force.


Would you rather launch a projectile with 78.5 pounds or 490 pounds of force?

That is a very large amount of wasted potential energy.


Why would you throw away 411.5 pounds of force by choosing a coax design?

OOPS! I c&p the incorrect numbers.

It is far worse than before for coax's.
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Last edited by boyntonstu on Fri Apr 09, 2010 7:22 pm, edited 1 time in total.
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Unread postAuthor: Ragnarok » Fri Apr 09, 2010 5:50 pm

Gippeto wrote:The larger volume available at the valve in a co-ax should result in lower air velocities inside the chamber

... depends. The average air velocity in the chamber is equal to:
(Mass flow rate)/(Air density * Chamber cross-sectional area)

If air is flowing through the valve at the same rate, and the air is the same density (i.e same pressure & temperature), then the flow-rate is inversely proportional to the cross-sectional area of the chamber.
For the same chamber diameter, the volume the barrel takes up, will result in a lesser chamber cross-sectional area, and the air velocities in the chamber therefore be greater.

However, there are two caveats on what I've said above.

- Mass flow rate is not usually the same in a coaxial as it is in a barrel sealer. Obviously, the coaxial valve has air flowing into it from all angles, but that air has also got to make a sharper change of direction.

- Coaxials usually have a greater chamber diameter (usually a result of over-compensation for the barrel's volume), which means that the chamber cross-sectional area is usually higher.

boyntonstu wrote:Take any coax and pull out the barrel to use as the chamber. You automatically have a more powerful gun.

... yes, but the change might well be insignificant.

I've talked about C:B ratios before. Here's a nice graph:
Image
On this graph, the X-axis is C:B ratio. The Y-axis is the percentage of the theoretical maximum energy possible from an infinite C:B ratio with no velocity or valve losses.

What you're looking at here is figures for HEAL firing a 20 gram projectile.

The blue line is the theoretical energy that can be achieved from a given C:B ratio, with no velocity or valve losses.
The red line is the energy if velocity and valve losses are taken into account - courtesy of Apocalypse simulations.
The green line is the energy if losses are taken into account - but simulated with a halved valve diameter (no other changes).

You can see that the muzzle energy tends to a limit, with definite diminishing returns.
HEAL's normal C:B ratio is roughly 1.5:1, from which it gets about 80% of the muzzle velocity that an infinite C:B ratio would offer. That is, no matter how much larger I make the chamber, it cannot manage more than 25% more muzzle energy.

On a more practical (i.e. finite) scale, doubling the chamber to 3:1 would increase muzzle energy by less than 10%.

~~~~~

Now... something you probably don't want me to say.

Take into account the whole difference that arises from changing the valve diameter.
The fact that coaxials almost invariably have a full diameter valve means that any flow restriction at the valve is pretty small. The Trom-Boyn on the other hand, does not have a full diameter valve, and that means it suffers for it.

The next point is that coaxials usually have a high ratio of chamber cross-sectional area to barrel cross-sectional area. 3:1 or more is not unusual, so that means that velocity losses in the chamber are minimised. And back to the Trom-Boyn, which has the same diameter for both barrel and chamber - and again, it'll suffer for it.

You're here to talk about how the Trom-Boyn betters a coaxial cannon when the Trom-Boyn is actually limiting the performance that it could get out of its barrel. A co-axial does not limit the performance it can get out of its barrel. It might limit the performance it can get out of its chamber, but that's usually insignificant compared to limiting the barrel.

For the same barrel and pressure* as the Trom-Boyn, I'd back a co-axial over it for fps and fpe!
*Of course, the trouble with 600 psi in high diameter tubing exists, but it's not impossible to find pipe that could handle that. Some decent steel tube or some nice aluminium would take that quite easily.

However, if you were talking about a more optimised barrel-sealing cannon (HEAL being a possible example), without such limitations at the valve,, then you've got something that a co-axial would have to work at keeping up with, but the Trom-Boyn is not thus blessed.

The areas are 1.43 and 0.785 square inches.

Don't know how you've managed to get an area of 1.43 sq. in. for a 2.5" barrel. That'd be a 1.35" barrel.

The number you want is 4.9 sq. in.
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Unread postAuthor: velocity3x » Fri Apr 09, 2010 5:53 pm

boyntonstu wrote:Get specific: What diameter chamber and what diameter barrel? We are shooting 100 grains. Forget C:B ratio for a second.


Why should chamber diameter, grains of projectile or C:B ratio figure into the discussion when (in your own words) you stated......" again, the diameter of the barrel is the key to power, not how the compressed air feeds into it."

AGAIN....This is a design issue! It's not a coax vs. piston issue!
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Unread postAuthor: Moonbogg » Fri Apr 09, 2010 5:55 pm

boyntonstu wrote:
Moonbogg wrote:I will answer off the top of my head, but just so you know, I myself would never bother with such a light weight projectile. I would guess a 1" barrel and 2.5" chamber would do alright.


OK Fine. (100 grains is for illustration)

Now switch the barrel to 2.5" and the chamber to 1".

The areas are 1.43 and 0.785 square inches.

Assume 100 psi.

If you use a 1" diameter barrel, the projectile will feel 78.5 pounds of force.

Using the 2.5" diameter barrel, the projectile will feel 143 pounds of force.


Would you rather launch a projectile with 78.5 pounds or 143 pounds of force?

That is 45% of wasted potential energy.


Why would you throw away 64 pounds of force by choosing a coax design?



Thats a fine idea, but you run into major volume issues. With the larger barrel, keeping the chamber the same, it would be like slamming the projectile really hard for an instant, while the smaller barrel would be like pushing it firmly and accelerating it over a greater distance. Plus, the pressure would drop instantly, and you would not realize 100 psi in any real fashion. With the smaller barrel, the pressure would drop more gradually, allowing more "time for thrusting" thus resulting in an epic win.
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Unread postAuthor: boyntonstu » Fri Apr 09, 2010 6:49 pm

Moonbogg wrote:
boyntonstu wrote:
Moonbogg wrote:I will answer off the top of my head, but just so you know, I myself would never bother with such a light weight projectile. I would guess a 1" barrel and 2.5" chamber would do alright.


OK Fine. (100 grains is for illustration)

Now switch the barrel to 2.5" and the chamber to 1".

The areas are 1.43 and 0.785 square inches.

Assume 100 psi.

If you use a 1" diameter barrel, the projectile will feel 78.5 pounds of force.

Using the 2.5" diameter barrel, the projectile will feel 143 pounds of force.


Would you rather launch a projectile with 78.5 pounds or 143 pounds of force?

That is 45% of wasted potential energy.


Why would you throw away 64 pounds of force by choosing a coax design?



Thats a fine idea, but you run into major volume issues. With the larger barrel, keeping the chamber the same, it would be like slamming the projectile really hard for an instant, while the smaller barrel would be like pushing it firmly and accelerating it over a greater distance. Plus, the pressure would drop instantly, and you would not realize 100 psi in any real fashion. With the smaller barrel, the pressure would drop more gradually, allowing more "time for thrusting" thus resulting in an epic win.


What size was the chamber that we kept the same?

Assume a 30" coax.

You now have a 30" 1" chamber an a 30" 2.5" barrel.

Would you rather go to 600 psi with a 1: chamber or a 2.5" chamber?

When is enough chamber enough.

In my latest 3/4" Trom-Boyn, the barrel is 38" long and the chamber is 17".

Therefore the C:B ratio is 0.45.

Certainly enough chamber for excellent performance,

Heck! Why not use 1:1 and use a 2.5" chamber?

Or, just pump the smaller diameter chamber to force as much air as you would have used in the coax.

BTW In a coax, the chamber volume is very much reduced by the volume of the barrel within it.
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Unread postAuthor: Moonbogg » Fri Apr 09, 2010 7:25 pm

You are thinking of full length coaxials here. I was thinking of a coax where the barrel extends beyond the chamber, like a sureshot cannon. I think I know what you are saying. Having a full length coax and pulling the barrel out would make the cannon more powerful in some cases, but not always. Again, it all depends on the volume ratios and other design factors. If you want to yank the barrel out of your coax, let us know how it goes then.
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Unread postAuthor: Gippeto » Fri Apr 09, 2010 7:26 pm

Stu, if you want to compare the two layouts (co-ax vs tee), you must as you say...compare apples to apples. Hampering one or the other isn't a reasonable comparison.

New situation;

BOTH have the same chamber VOLUME and barrel.

BOTH use the same pressure and projectile.

Given these EVENLY MATCHED variables, why do you think the tee piston would produce higher fps and fpe?
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