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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 7:38 pm

Ragnarok wrote:
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.



Thanks for correcting my math error. Appreciate that.

GGDT accurately predicts the chrono performance of the Trom-Boyn with the following :

Image


Design a coax with GGDT and let's compare.
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Unread postAuthor: Ragnarok » Fri Apr 09, 2010 8:57 pm

boyntonstu wrote:Design a coax with GGDT and let's compare.

Here ya go. Same pressure, nearly the same chamber volume. For convenience, I used the dimensions of Type L 1.25" copper - and 600 psi is within the ~750 psi working pressure of drawn Type L 1.25" tubing. (If you're fussed, Type K tubing will add more strength - just add an inch to the chamber to compensate for the volume lost.)

Image

The coaxial offers an increase of 45 fps and 28.8 fpe. Like I say, that sub-calibre valve on the Trom-Boyn is not optimal.

Before you start talking about "sacrificed" C:B ratio, bear in mind that coaxial can still manage a 36" chamber, where the numbers are 832 fps and 326 fpe. The volume taken by the barrel is easily compensated for by a longer chamber.

Basically, valve performance is pretty important to a pneumatic, and valve diameter is integral to that.

~~~~~

On the note of pneumatic efficiency, eventually I hope I'll get Project 3vo fully up-and-running. And that is an exercise in trying to wring out every last metre per second out of the launcher.

It'll have the same ~20mm calibre (UK 22mm copper and US Type L 3/4" copper are pretty close in dimensions). So, if/when I'm done with it, we'll see what it can pull off with a 212 grain projectile.
Simulations for that come out in the 260mps/870 fps range at 35 bar/~500 psi with the 39" barrel. (Although most of the time, it'll be running a 47" barrel.)
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Unread postAuthor: boyntonstu » Fri Apr 09, 2010 9:22 pm

Ragnarok wrote:
boyntonstu wrote:Design a coax with GGDT and let's compare.

Here ya go. Same pressure, nearly the same chamber volume. For convenience, I used the dimensions of Type L 1.25" copper - and 600 psi is within the ~750 psi working pressure of drawn Type L 1.25" tubing. (If you're fussed, Type K tubing will add more strength - just add an inch to the chamber to compensate for the volume lost.)

Image

The coaxial offers an increase of 45 fps and 28.8 fpe. Like I say, that sub-calibre valve on the Trom-Boyn is not optimal.

Before you start talking about "sacrificed" C:B ratio, bear in mind that coaxial can still manage a 36" chamber, where the numbers are 832 fps and 326 fpe. The volume taken by the barrel is easily compensated for by a longer chamber.

Basically, valve performance is pretty important to a pneumatic, and valve diameter is integral to that.

~~~~~

On the note of pneumatic efficiency, eventually I hope I'll get Project 3vo fully up-and-running. And that is an exercise in trying to wring out every last metre per second out of the launcher.

It'll have the same ~20mm calibre (UK 22mm copper and US Type L 3/4" copper are pretty close in dimensions). So, if/when I'm done with it, we'll see what it can pull off with a 212 grain projectile.
Simulations for that come out in the 260mps/870 fps range at 35 bar/~500 psi with the 39" barrel. (Although most of the time, it'll be running a 47" barrel.)



Look what happens when you use a larger barrel.

Using 3/4" L at 600 psi is a breeze.

All it takes is a larger T and piston.

The 17" chamber beneath is tiny in comparison to the barrel.

Image
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Unread postAuthor: c11man » Fri Apr 09, 2010 9:26 pm

i fail to see how that even relates to the discussion? or is you that fails
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Unread postAuthor: Crna Legija » Fri Apr 09, 2010 9:40 pm

i think that 60% flow coef is a bit much
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Unread postAuthor: Mr.Sandman » Fri Apr 09, 2010 9:48 pm

-_- wrote:i think that 60% flow coef is a bit much
Agreed and a 212 gram projectile? That seems very heavy. Im looking at some of this ggdt info and it seems to be entered wrong.
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Unread postAuthor: Ragnarok » Fri Apr 09, 2010 9:59 pm

boyntonstu wrote:Look what happens when you use a larger barrel.

And at what point was that what I said? As you put it earlier: "First, compare apple with apples."

As you don't actually use that barrel on the Trom-Boyn, to bring it into the discussion is to not "compare apples with apples".

What you use on the Trom-Boyn is a Type L 3/4" barrel, 38" long. I have demonstrated that it is perfectly viable for a co-axial to support the same barrel and pressure as the Trom-Boyn, and actually offer a better muzzle velocity.

Yes, coaxials are usually limited as far as barrel diameter, but you're not using a barrel diameter outside of those limits!
Besides, that wasn't your discussion point. The point at hand was velocity.

We're disregarding energy here for one good reason - some people just don't want a bigger barrel. They want the energy they can get out of THIS barrel size, not just as much energy as they can get.

If their chosen barrel diameter is one that is too big for them to build a coaxial cannon around it, then they'll have to build something else.
If the barrel diameter is small enough, then a coaxial can offer as respectable a velocity as a barrel sealing launcher.

To me, it sounds like you're trying to turn this discussion into something it wasn't. If you don't think coaxial cannons can support a big enough barrel and enough energy, then don't build them. But as far as the available velocity in a given barrel size, they are not lacking.

~~~~~

Also, could you please stop quoting my entire posts? It really isn't necessary, and just takes up a lot of space (given that I often write rather long posts). If you're quoting, you really need to trim it down to just the bare essentials you need to quote, or alternatively...

If you think that there's going to be any confusion as to who you're addressing, starting a section with @Ragnarok or @MrCrowley or whatever will work better than quoting posts of several hundred words.

~~~~~

-_- wrote:i think that 60% flow coef is a bit much

Not completely impossible - but on the unlikely side, I admit.

Mr.Sandman wrote:... and a 212 gram projectile?

Gr in GGDT does not represent "gram", it's short for "grain" (of which there are about 15.4 to a gram). 212 grains is about 13.7 grams.
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Unread postAuthor: Mr.Sandman » Fri Apr 09, 2010 10:10 pm

Edit, Im still doubting the 60 percent coefeccient for obvious reasons. Im still thinking based on previous points made that there is only a slight performance increase with non coaxial piston valves.

You said that they were limited to barrel size, but say you have a 1 inch coaxial chamber and a 1/2 inch barrel @ 450 psi, You will have a certain fps, but you could just as easily add a 6mm barrel and double your fps if you use the same weight projectile.
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Unread postAuthor: Ragnarok » Fri Apr 09, 2010 10:19 pm

Mr.Sandman wrote:Im still doubting the 60 percent coefeccient for obvious reasons.

A reasonably well designed valve can manage 60% flow-coefficient quite easily. Although how well designed the Trom-Boyn's valve may be is a question that may need to be asked, BoyntonStu does have a chronograph, and presumably, he's worked out the valve's Cv from that.

My guess however is that he's underestimated valve diameter (I think it seals against a female end, not a male end, so presumably, it's a 0.625" orifice, not a .569" one), and adjusted the flow co-efficient up to compensate.
Compensating for that possibility, you get a 50% coefficient, which seems a more likely number.
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Unread postAuthor: boyntonstu » Sat Apr 10, 2010 7:31 am

Ragnarok wrote:
Mr.Sandman wrote:Im still doubting the 60 percent coefeccient for obvious reasons.

A reasonably well designed valve can manage 60% flow-coefficient quite easily. Although how well designed the Trom-Boyn's valve may be is a question that may need to be asked, BoyntonStu does have a chronograph, and presumably, he's worked out the valve's Cv from that.

My guess however is that he's underestimated valve diameter (I think it seals against a female end, not a male end, so presumably, it's a 0.625" orifice, not a .569" one), and adjusted the flow co-efficient up to compensate.
Compensating for that possibility, you get a 50% coefficient, which seems a more likely number.


0.625" is EXACTLY CORRECT!!!!

The seal surface is a 3/4" > 1/2" fitting reducer.

The ID of the fitting reducer is the OD of 1/2" copper tubing which is 5/8' or .625". I incorrectly used .569 which is the ID of type M tubing that was my original sealing face. I used a ring of it to widen the sealing face area in my "Sten" design. I discovered that the narrow fitting surface does not wear, so I left it out and I forgot about it

Thanks very much for going over my design with a fine tooth comb. :oops:


In any event, I maintain that using the largest barrel for a given weight projectile is the way to design for the maximum fpe and fps.


A slight tangent, think of the impulse time given to a baseball when struck by a bat.

The force is huge, the time is minuscule.

A dart blowgun expert uses a puff, not a long blow.

Check out this 5 shot blowgun!

[youtube]http://www.youtube.com/watch?v=_xUcRx63OvA[/youtube]

A hammer valve chamber size calculates to what C:B?

How do you model a hammer valve chamber/cannon?
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Unread postAuthor: inonickname » Sat Apr 10, 2010 8:04 am

I'd rather have to make a high pressure co-ax than a tee valve. Getting a round section of pipe to hold pressure is easy. Getting a large tee (and just making it to begin with) to hold high pressure is much harder.

If you want to really prove your point, make guns with as many practical similarities as possible- same pilot volume, dead space, chamber volume, pressure, barrel length, chamber diameter etc., then run them over a chronograph multiple times.
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Unread postAuthor: boyntonstu » Sat Apr 10, 2010 8:09 am

The Trom-Boyn has a slip-in barrel design.

So I 'slipped in' a 1/2" barrel keeping everything else the same.

Down went performance.

Image
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Unread postAuthor: Ragnarok » Sat Apr 10, 2010 8:18 am

boyntonstu wrote:In any event, I maintain that using the largest barrel for a given weight projectile is the way to design for the maximum fpe and fps.

Well, yes. Given that the muzzle energy is the integral of the force with respect to displacement, a higher diameter barrel will result in a higher muzzle energy - and for a given projectile mass, that also means higher velocity.

Or, in simpler terms: (Average pressure in barrel) x (Volume of barrel) = Muzzle energy.

But the case is still that people don't just use the biggest barrel they can get their hands on. There are other goals than just the pursuit of raw energy.

If I wanted to, I could order up the parts for a 26mm barrelled cannon that could muster 2500 joules at the muzzle and have it built by the end of next week. I could take it further - a 4000 joule 33mm cannon. I could have that done quite easily as well.

But I don't want to. These are launchers that while they chase raw energy, lose the appeal of my 20mm launchers which manage 400-700 Joules.

Yes, more energy is good, but I don't want to have to change the barrel size in order to get it. More pressure, better valves, improved air flow - any of these are fine, but when I want to build a 20mm shoulder fired launcher, that's what I want to build. I don't want to build a 33mm cannon with an impractically long barrel!

If you want to chase energy in any form, then by all means feel free to build yourself a cannon with a whopping great barrel diameter.
But if you're working with a barrel size which a coaxial can be built around, then you're snubbing them unfairly. They're capable of more than respectable performance, and in terms of raw velocity, they are not lacking.

What this comes down to arguing over is that coaxials can't usually have as big a barrel as a barrel sealer - and while that's true, it's not a reason they can't be built if a smaller barrel size is desired.
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Unread postAuthor: Crna Legija » Sat Apr 10, 2010 8:41 am

also the valve flow coef whet down to.
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Unread postAuthor: Gippeto » Sat Apr 10, 2010 8:56 am

Gippeto wrote: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?


The new bit.

It's been modeled, should be proof enough I'm thinking.

Would like to get to the bottom of your reasoning Stu.
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