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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 » Sat Apr 10, 2010 7:24 pm

POLAND_SPUD wrote:this topic is a bit pointless for most of us but it's a good source of info for new members...
which I am taking as the chamber volume being identical, the pressure being identical and the barrel being identical, then you've got a discussion that's worth having
true


Assume that they are identical in performance.

What else would you consider before choosing?
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Unread postAuthor: MrCrowley » Sat Apr 10, 2010 7:30 pm

What else would you consider before choosing?

Lots of things, all of which are irrelevant to this discussion. The other things you would choose depend on the design and design alone. A breechloader isn't necessary if you're shooting arrows so a co-axial will be fine, perhaps you need it to be compact, a coaxial would be the way to go. Perhaps you want it to resemble a bazooka or cannon, coaxial would be the way to go. Perhaps you want to save money, a coaxial is the way to go (depending on design).

The discussion is pointless, if power and power alone was all that mattered, no one would be using blowguns to pilot sprinkler/piston valves. The fact of the matter is that performance isn't everything, design and function can also be equally important.
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Unread postAuthor: tghhs » Sat Apr 10, 2010 7:36 pm

Hmmm how about some on topic discussion?

hahaha, well Rag, To enhance the flow in my coaxial valves i flare the end.

Doing this i hope to (just slightly) decrease the ratio of chamber to barrel, creating that 'snap' open when the chamber pressure hits the barrels surface area on the piston.

I also believe that this would create a more laminar flow for the air. Still who knows? what do you think boyntonstu?
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Unread postAuthor: boyntonstu » Sat Apr 10, 2010 7:51 pm

MrCrowley wrote:
What else would you consider before choosing?

Lots of things, all of which are irrelevant to this discussion. The other things you would choose depend on the design and design alone. A breechloader isn't necessary if you're shooting arrows so a co-axial will be fine, perhaps you need it to be compact, a coaxial would be the way to go. Perhaps you want it to resemble a bazooka or cannon, coaxial would be the way to go. Perhaps you want to save money, a coaxial is the way to go (depending on design).

The discussion is pointless, if power and power alone was all that mattered, no one would be using blowguns to pilot sprinkler/piston valves. The fact of the matter is that performance isn't everything, design and function can also be equally important.


Save money with a coaxial?

Please explain.

With a 3/4" T valve breech loader, only 3/4" tubing and fittings are needed.

I believe that a barrel sealing piston is smaller, lighter, and easier to make than a coaxial piston.

My piston weighs 198 grains.

What makes a coax inexpensive?

Safety: Until I place the barrel into the piston T of the Trom-Boyn when it is pointing down range, the projectile cannot be discharged.

In addition, I would prefer to be behind the breech than in front of the muzzle.

I never point the muzzle of any gun in the direction of anyone/anything that could be damaged or injured. My #1 safety rule!
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Unread postAuthor: MrCrowley » Sat Apr 10, 2010 7:58 pm

Please explain.

With a 3/4" T valve breech loader, only 3/4" tubing and fittings are needed.

I believe that a barrel sealing piston is smaller, lighter, and easier to make than a coaxial piston.

My piston weighs 198 grains.

What makes a coax inexpensive?

Like I said, it depends on the design. But if you want to make a tee piston valved cannon with a 4" chamber and 4" tee, a coaxial with a 4" chamber will be cheaper. Way cheaper. Just need two couplers, the 4" pipe, two 4" bushings and that's more or less it. With a tee piston valve you need a 4" end cap, 4" elbow, 4" tee, two 4" bushings (depending on design) and the 4" pipe. In NZ, it would work out a lot cheaper.

smaller, lighter, and easier to make than a coaxial piston.

Huh? It would depend entirely on design, yet again. Why would the piston be lighter and smaller in a tee piston valve? Or easier to make? I really do not understand.

Safety: Until I place the barrel into the piston T of the Trom-Boyn when it is pointing down range, the projectile cannot be discharged.

Put a safety ball valve on the pilot valve in the coaxial. What the hell are you doing Stu? You're completely ignoring the discussion and bringing up matters which depend entirely on design and have nothing at all to do with the topic at hand.

In addition, I would prefer to be behind the breech than in front of the muzzle.

I never point the muzzle of any gun in the direction of anyone/anything that could be damaged or injured. My #1 safety rule!

What stops you from doing the same in a coaxial?

I really have no idea what you are going on about now. You are definitely not comparing apples and apples, more like apples and ways to eat apples.
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Unread postAuthor: tghhs » Sat Apr 10, 2010 7:58 pm

@boyntonstu
Not sure if we were posting at the same time (pretty sure we didn't) but please don't ignore my relevant post and change subjects to safety.

I DON'T CARE HOW YOU LOAD YOUR GUN.

I want to know if modifying a coaxial barrel with a flare can give better flow than a equivalent T-valve.

EDIT: what do you think MrCrowley? is it worth my while flaring the end?
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Unread postAuthor: POLAND_SPUD » Sat Apr 10, 2010 8:09 pm

personally I am not a big fan of coaxials as in most situations it simply isn't the most practical design.. but I am aware that it might be better in some situations... this is mostly due to the fact that they can't be breech loaded

(well yeah I know that theoretically you can breech load them but you know what effect it has on barrel length and overall length)

performance wise there is so little difference between the two and it depends on so many variables that it is almost impossible to point out which one is better

@tghhs
sigh, geez this is a stupid ass topic!! GRR!!


'I curse you!! you've been a very bad topic!!' :D

lol relax

the reason why no one responded is probably because most of us are not experts in fluid dynamics... In a typical piston valve this would help but in coaxials air has to do a 180 deg turn to get into the barrel and the other surface of the flare might have some negative effect on it... but as I said it's not something that I am good at so don't blame me if I am wrong
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Unread postAuthor: tghhs » Sat Apr 10, 2010 8:40 pm

POLAND_SPUD wrote:@tghhs
'I curse you!! you've been a very bad topic!!' :D

lol relax

the reason why no one responded is probably because most of us are not experts in fluid dynamics... In a typical piston valve this would help but in coaxials air has to do a 180 deg turn to get into the barrel and the other surface of the flare might have some negative effect on it... but as I said it's not something that I am good at so don't blame me if I am wrong


Yeh i agree. Just wanted some actull input.
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Unread postAuthor: Gippeto » Sat Apr 10, 2010 9:33 pm

tghh...Adding the "flare" DOES provide an increase in velocity. You need to stretch it out some though. I use 11 degrees as the internal angle, and keep the od transitions smooth to prevent flow problems in the chamber.


Stu...Get back on your medication.


Gun Freak...you should pay closet attention to what you read. Your question was answered. Stu did a tee piston and Rag did a co-ax with equal properties....review to find the answer to your question.

I'm done with this thread. What a friggin waste of time. :evil:
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Unread postAuthor: Ragnarok » Sun Apr 11, 2010 10:23 am

boyntonstu wrote:Assume that they are identical in performance.

But, that's the question. Are they?

What else would you consider before choosing?

Let's be honest - I wouldn't normally choose to build a coaxial.

I do quite like the ability to swap barrel lengths practically and don't need the space saving of a coaxial. Still, NONE of that is related to actual ballistic performance.

I believe that a barrel sealing piston is smaller, lighter, and easier to make than a coaxial piston.

...um, no. A barrel sealing needs the extra length to extend over the chamber port to the sealing face, and a coaxial piston doesn't.
A coaxial piston can inherently be lighter and smaller than a barrel sealing piston.

In addition, I would prefer to be behind the breech than in front of the muzzle.

While a reasonable point about muzzle loading, if you're working on a load before pressurisation sequence, there's no extra danger to the user.

Given the not unknown possibility for sealing faces to detach from the piston*, pressurizing before loading is a potential danger even with a barrel-sealing breech-loader.
*Not to mention that compressed air at the kind of velocity it leaves a valve is not exactly safe either!

Anyway, coaxials CAN be breech-loaded, they just aren't normally built that way.
I've got a set of plans for a piston valve coaxial which can be breech-loaded. haven't really much drive to build it, but it would certainly work if I did.

Gippeto wrote:Stu did a tee piston and Rag did a co-ax with equal properties....review to find the answer to your question.

Bear in mind, those are simulated with rough parameters. I don't have the potential to generate flow coefficients in a heartbeat.

Also, as I have already said, the Trom-Boyn is inherently hindered by its sub-calibre valve.
My point was more to demonstrate that the Trom-Boyn wasn't a sterling example of a barrel sealer and that coaxials weren't automatically hindered by their design.

What I have done doesn't prove anything of how a well design coaxial and a well designed barrel sealer would compare to one another. My guess is the difference is pretty insignificant.

tghhs wrote:I also believe that this would create a more laminar flow for the air.

Depends. While it improves valve-to-barrel flow, it will inhibit chamber-to-valve flow.

While normally, V-2-B flow is the more limited, and thus worth trading C-2-V flow for, you will need to bear in mind that there is a point where taking it too far will start to undo the gains accrued.
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Does that thing kinda look like a big cat to you?
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Unread postAuthor: boyntonstu » Sun Apr 11, 2010 11:44 am

What else would you consider before choosing?

Let's be honest - I wouldn't normally choose to build a coaxial.


What specific reasons deflect you away from coax's?

I believe that a barrel sealing piston is smaller, lighter, and easier to make than a coaxial piston.


...um, no. A barrel sealing needs the extra length to extend over the chamber port to the sealing face, and a coaxial piston doesn't.
A coaxial piston can inherently be lighter and smaller than a barrel sealing piston.

To seal .625" my .785" piston weighs 198 gr.

If it was a coax, what would the size of a typical chamber and its sealing diameter?

Please, a little explanation on how it is possible to make a light coax piston.


I have no idea of how my 198 gr oak piston stacks up in weight against other pistons. Anyone?
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Unread postAuthor: Technician1002 » Sun Apr 11, 2010 11:48 am

Not all coaxial designs have the valve at the breech of the cannon chamber. For good flow and adequate volume a large diameter chamber is useful. This is a breech loader with the breech loading immediately following the valve in the tank. This coaxial design won a competition in an engineering contest.
The shirt acceleration and muzzle velocity numbers are impressive for a launcher of this size. Here is the team I mentored in the competition with their launcher in action.

Just a note on GGDT and this cannon, we tried to find the valve coef my matching our in barrel accelerations against GGDT. It is way above 60%. We measured projectile position vs time on launch. Projectile position every foot of travel was captured on a graph.

The data is in an older thread if you want my performance graphs.
Image

Coaxial performance can be seen here;
http://www.spudfiles.com/forums/ggdt-an ... 18090.html
At 60 PSI it performed like we would have expected at 100 PSI with a normal valve.
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Unread postAuthor: boyntonstu » Sun Apr 11, 2010 12:00 pm

What is the weight of your piston?
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Unread postAuthor: c11man » Sun Apr 11, 2010 12:04 pm

how to make a light coax piston? simple, make a piston shaped like a cup. it can be made quite short like half the diameter of the piston long. attach the sealing face to the inside of the cup and this also reduces pilot volume to very minumal.

a piston weighing 198gr seems quite light seing at it is 13 grams witch to me means 13 paperclips and that is quite unbelievable.

also can you put your weights in a grams or oz as it is easyier for most people to visualize.
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Unread postAuthor: Technician1002 » Sun Apr 11, 2010 12:05 pm

Which piston? The 2 inch is about 4 oz. It is machined from UMWHDPE. The 2.5 inch one is about 6 oz, and the 1 inch marshmallow cannon piston is about .5 ounces (14 grams). The pistons are less dense than water. They float in water. The density is very close to that of water. The 2.5 inch piston has about the volume of a cup of water (1/2 pint - 1/2 lb). With the recess for the rod to retract into and the bored center for the rod to pass through, brings the weight down another 2 oz. It was designed for speed. Valve opening time at 60 PSI is under 1 ms.

Moving the valve to the muzzle end of the chamber helps the high speed flow as there is no 180 turn around from the chamber into the barrel at near supersonic air flow rates. The airflow bend is less than 90 degrees in this design from the chamber into the barrel.

For newer forum members, my avatar is an apple impacting on a bottle of water about 2 ms after contact.
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Last edited by Technician1002 on Sun Apr 11, 2010 12:22 pm, edited 2 times in total.
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