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The logic of a very tight projectile

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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The logic of a very tight projectile

Unread postAuthor: boyntonstu » Thu Apr 29, 2010 1:55 pm

My experience with tight fitting cork based projectiles got me to thinking.

I noticed that a very tight projectile seemed more powerful than a looser fitting one.

My explanation is that a looser fitting projectile will move down the barrel before the chamber is completely open and there will be blowby.

A tight fitting projectile will act like a burst disk in that until a threshold pressure has been built reached, the projectile will remain at rest.

Ideally, a high friction breech area followed by a greased barrel would make the best gun.

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Unread postAuthor: deathbyDWV » Thu Apr 29, 2010 1:57 pm

Although I've never thought about that, it makes perfect sense...
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Unread postAuthor: Hubb » Thu Apr 29, 2010 2:01 pm

'Tis true. In fact, this was just being discussed (in a thread highjack) not too long ago. JSR has posted the concept and there have been a couple of launchers that use the concept.

He's better at remembing where threads are, so I'm sure he will pop in here in a minute to give about 13 links. :lol:

Now, the ones I'm thinking of actually use the projectile as a type of valve. But I understand what you are saying. It would certainly boost performance of a launcher that has a slow opening valve.
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Unread postAuthor: matti » Thu Apr 29, 2010 2:21 pm

spring airguns is good example.. spring compresses the air to highpressure before the projectile starts to move. if you put loose ammo to spring airgun you can see that the difference is huge.
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Re: The logic of a very tight projectile

Unread postAuthor: velocity3x » Thu Apr 29, 2010 2:52 pm

boyntonstu wrote:I noticed that a very tight projectile seemed more powerful than a looser fitting one.


a high friction breech area followed by a greased barrel would make the best gun.


You are correct. I'm currently shooting a cannon with a barrel bore that is .001" large than the projectile. It's not as tight as you suggest with the use of grease but, it's tight enough to make a huge increase in power (and accuracy).
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Unread postAuthor: bobgengeskahn » Thu Apr 29, 2010 2:56 pm

matti wrote:spring airguns is good example.. spring compresses the air to highpressure before the projectile starts to move. if you put loose ammo to spring airgun you can see that the difference is huge.


another thing to cosider with an airsoft platform is that there is the hop up unit that is considerably smaller than the 6.00 +/- 0.05 bb (if youre using high quality bbs). this idea is a huge debate in the airsoft sniper community in the hop up v. non-hop up discussions.


EDIT: Matti: I reread your post and realized that you could have been talking about actual BB guns or pellet rifles. just dont want to confuse anyone, but i guess ya'll know whats on my mind :-p
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Last edited by bobgengeskahn on Thu Apr 29, 2010 3:09 pm, edited 1 time in total.

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Unread postAuthor: jackssmirkingrevenge » Thu Apr 29, 2010 3:07 pm

Hubb wrote:He's better at remembing where threads are, so I'm sure he will pop in here in a minute to give about 13 links. :lol:


One is enough ;)

Now, the ones I'm thinking of actually use the projectile as a type of valve.


It's the ultimate extension of the logic and though fiddly to put together, it's the best possible type of "valve" - maximum flow and zero opening time.

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Unread postAuthor: psycix » Thu Apr 29, 2010 3:28 pm

The forces involved are so high the friction of the projectile is neglectable.
The "burst disk effect" and the blowby reduction simply make up for it and increase power.
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Unread postAuthor: Technician1002 » Thu Apr 29, 2010 3:51 pm

jackssmirkingrevenge wrote:It's the ultimate extension of the logic and though fiddly to put together, it's the best possible type of "valve" - maximum flow and zero opening time.


I like the way you think. Your style is slightly faster than mine, but not by much. Maximum flow and minimum opening time work well for me.
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Unread postAuthor: POLAND_SPUD » Thu Apr 29, 2010 5:02 pm

@technician
yeah but your has to be actuated while JSR's idea does not... it might not seem like that much of a difference but it does offer some advantages

I am wondering whether there is any increase in muzzle velocity due to shock heating?
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Re: The logic of a very tight projectile

Unread postAuthor: Ragnarok » Thu Apr 29, 2010 8:09 pm

boyntonstu wrote:A tight fitting projectile will act like a burst disk in that until a threshold pressure has been built reached, the projectile will remain at rest.

Yeah, but don't forget that while the pressure is building up, the net force on the projectile is relatively low - as the projectile is starting from stationary, the distance moved by the projectile is relatively small.

Let's quantify this. If I were to make HEAL with a 10 bar "static friction" for the projectile in the breech, it would increase performance by less than increasing the barrel length by a centimetre.
What we're talking about here is a tiny difference. As I have said before, once valve opening time is below 2 milliseconds (with most barrels), any further improvement is next to irrelevant as far as muzzle velocity is concerned.

Of course, an airtight fit does have advantages, by reducing the air lost past the projectile during the shot, but if you've got a good valve (that being the caveat here), a "high friction" fit will not confer any meaningful advantage.

Ideally, a high friction breech area followed by a greased barrel would make the best gun.

Personally, I polish my barrels to a mirror shine, then apply PTFE spray. Not from a friction point of view (although it certainly helps that), but as part of the care I apply to my barrels.

I have to clean them every now and again - and a polishing essentially guarantees that any muck has been removed, and the PTFE spray reduces further muck build up.

Besides, a barrel kept in that state of affairs promotes laminar air flow, and thus better performance.
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Unread postAuthor: Moonbogg » Fri Apr 30, 2010 2:25 am

I have thought about this before as well. I ran some projectile friction numbers through HGDT and the difference, like rag said, is negligible even with high friction amounts like 20psi. Combustion vs pneumatic might be different, I don't know. But with combustion, the gasses expand and that projectile is going OUT with them, wether it wants to or not, and at the same speed as the expanding gas.

EDIT: If the barrel is too long and there is not enough combustion to fill it, then the dragging projectile could see some slowdown perhaps.
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Re: The logic of a very tight projectile

Unread postAuthor: boyntonstu » Fri Apr 30, 2010 7:58 am

Ragnarok wrote:]but if you've got a good valve[/i] (that being the caveat here), a "high friction" fit will not confer any meaningful advantage.


IOW the average non-good valve gun, (you know who you are), will do better with a high friction projectile.

Try it.

YMMV.
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Unread postAuthor: ONEWING » Fri Apr 30, 2010 8:07 am

When I was playing around with magnets here http://www.spudfiles.com/forums/viewtop ... tml#280310 I placed a ring magnet (1" OD, 1/4" ID) around a .177 barrel. using a ball valve no matter how slow it was opened the velocity achieved was always the same (penetrating 5 drink cans at 100psi).
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Unread postAuthor: Big-E » Fri Apr 30, 2010 10:06 am

You know, Old Red Ryder type BB guns (prior to the use of steel BBs) used lead BB shot, and a tight bore at the breach area of the barrel to hold the BBs in place and produce higher muzzle velocities, the lead was able to deform a little to pass through the breech on firing.

When Steel BB's became the norm, they switched to a magnet near the breech area to retain the ammo, and the bore was less restrictive, which is why the guns lost a little power. that's also why they went to a hybrid system (catapult/pneumatic).

Just some food for thought.
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