Arrow gun: Barrel length vs fps question.

[Alanstone]

Greetings spuddy buddies!

I have been designing a pneumatic arrow launcher and have a question I am hoping someone will be willing to answer. The design is not complete but I am getting there.

Firstly, the arrow gun uses a QDV. Credit for inspiration must go to Technician 1002 and Auxiliary.

Here is my question. When determining fps and muzzle energy is there a benefit to the fact that the arrow fits outside the barrel? When using Ggdt (thanks to D. Hall) to determine the mentioned figures, does the arrow shaft, as the arrow moves along the barrel, act as an extension to the barrel length? So if I have a 30" barrel and a 31" arrow, will the max fps and ft*lb be determined by a 30" barrel or by a 61" barrel.

While researching, I read in two places, that the shaft acts as a barrel extension until it leaves the tip of the barrel but I can't get my head around that.

[farcticox1]

very nice design

[jimmy101]

Your barrel is 30" and the length of the arrow shaft has no affect on that.

[Alanstone]

Thank for the answer Jimmy101. Thanks for the compliment farcticox1.

[farcticox1]

That would be sweet as a pellet or BB gun as well, put a scope on top and I would be really excited

[Alanstone]

Do you mean something like this?

[Alanstone]

I went to a paintball shop yesterday and found out about remote lines and pneumatic fittings. So the above is meant to be run off a remote line. At the same time I would like to see if I can fit a 9oz bottle ahead of the trigger and have a microline running to the valve. I would be ok with having it tethered but I think many would prefer a CO2 bottle on the laucher instead.

[farcticox1]

Do you mean something like this?

NICE :bounce:

[mark.f]

The shaft doesn't really act as an "extension". The acceleration still occurs for the length of the barrel since when the base clears the "muzzle" it unseals and pressure drops.

Design looks good, although some labeling could make things a little clearer. Now just to get pictures instead of ray tracings.

[mobile chernobyl]

I would think actually that the shaft just creates a larger 'dead space' area - as the entire barrel is empty in the beginning - and the volume only increases to approximately 2x that dead space up to the point of arrow leaving the barrel. This would create a softer acceleration curve - which may be beneficial in closer replicating that of a bow's effect with the string (thinking more along the lines of recurve, compound bows probably have funky acceleration curves but I'm sure companies like Mathews/hoyt/etc. have those curves precisely calculated!). Only reason I mention that is I know harmonics can play an effect on an arrow - based on my armchair engineering experience of watching bow's launch arrows in slow-mo on youtube

So back to your GGDT question for modeling - use the barrels volume as a 'dead space' value - and you can stick with the barrel length as the actual barrel length - diameter would be the inside diameter of the arrow shaft, not the barrel itself. That's my understanding of the problem - others may have a better suggestion.

Awesome layout and render BTW! I like the bullpup design execution!

[Alanstone]

Now just to get pictures instead of ray tracings - mark.f

Much water needs to flow under the bridge before we get to photographs! I have use of a 3D printer in December (perhaps a little sooner) with which I will do as much prototyping as I can. In the meantime I need to get from the 3D design to shop drawings so that I can get the metal bits manufactured.

The shaft doesn't really act as an "extension" - mark.f

As I said in my first post, I didn't think it did but had read that it did. I was hoping I had missed some obscure principle since I could have done with the additional fps.

use the barrels volume as a 'dead space' value - mobile chernobyl

I am designing a collar to hold the arrow back until the pressure develops in the barrel. I don't know how well my plan will work but I have to get rid of the dead space. With the dead space and I am getting 230ft/s and 57 ft*lb on a 479 grain arrow.
That's not going to be enough though. If I can nullify the dead space I get 280 ft/s and 82ft*lb and that's more workable.

Only reason I mention that is I know harmonics can play an effect on an arrow - mobile chernobyl

I think there is quite a difference between the arrow been driven from the rear by a string and it being pushed from the front by the barrel. I don't envision any flexing taking place.

diameter would be the inside diameter of the arrow shaft, not the barrel itself - mobile chernobyl

I agree and thank heaven's for that because it makes a difference to the fps. I expected more from 850 psi. It's clear why manufacurers of arrow lanchers are turning to HPA. Don't get me wrong, CO2 is workable, marginally enough to take a grizzly or buffalo at 50 yards but here is the problem....

280 ft/s and 82ft*lb becomes 261ft/s and 73ft*lb after the arrow has travelled 50 yards. More importantly it's dropped 61inches! At 400 ft/s muzzle velosity the arrow is doing 381ft/s after 50 yards and the drop is 29 inches. To get 400fp/s you'll need 2000 psi.

So I need to hold the arrow back until the dead space is nullified if I want this launcher to have full hunting capability.

The barrel is 7075-T6 with an od of 0.331" and a wall thickness of 0.014". That seemed too thin to me when I first looked at it. Yet if my calculations are correct, it's got a burst pressure of over 6000 psi and a working pressure of over 4000psi. Those numbers feel far to high, so if any of you guys know how to work it out and would be prepared to check that for me I would be very grateful.

Thanks for taking time to give input gents I really appreciate it.

[D_Hall]

So back to your GGDT question for modeling - use the barrels volume as a 'dead space' value - and you can stick with the barrel length as the actual barrel length - diameter would be the inside diameter of the arrow shaft, not the barrel itself. That's my understanding of the problem - others may have a better suggestion.

This would work.

Another approach would be to model the barrel as the combined length of the barrel and arrow, but then have the length of the arrow or barrel (whichever is longer) as the projectile's initial position. In other words, if you have a 30" barrel and a 31" arrow, model the barrel as 61" but have the initial position of the projectile listed as 31". While mathematically rigorous, this approach is no doubt confusing to those who don't understand it and I suspect the source of the "2X barrel length" statements that started this thread.

[Alanstone]

Thanks for your input D_Hall. If I held the arrow back until the pressure reached 850 psi in the barrel, the dead space would be zero right?

Auxiliary, is your launcher still operational? Can you remember what the diameter of the piston is? I am assuming it's around 12.6 mm.

How is the spring and buffering you used holding up? You mentioned you are getting semi auto. I am wondering if you are getting valve bounce or are you limiting the CO2 entering the chamber by means of a small hole or something. I ask because I would have thought the pressure would keep the piston from closing again.

My piston is 7mm and if I am working it out correctly then there is 27 kg force on the piston at 850 psi. I used an online calculator to design the spring but I am worried it's either going to be too much or too little. Did you put a piece of rubber hose under the spring the help buffer the piston?

Sorry about the 20 questions…

I managed to incorporate a 9oz CO2 bottle into the gun which I am really stoked about. If anyone is interested, I will attempt a rendering that will show the internals of the launcher.

[sharpshooter]

Still trying to make heads or tails of that first drawing... Where is the chamber? Is it that little area behind the arrow? Or does it not technically have a chamber, and instead the valve is essentially dumping just the air in the line between the tank and valve? And then your 'barrel' actually fits inside the arrow? (the tube around it is essentially just for show?)

To the point: what volume are you working with as the chamber?

edit: I think I've got it - the small area behind the barrel is the chamber, released via a manual valve (push rod from the trigger). The barrel is actually inside the arrow, while the tube around it is just looks/safety.

[Alanstone]

You have the workings right sharpshooter. The shroud is there to protect the barrel and it looks neater too,in my opinion. There is 2.8 "cu co2 in the valve but it looks like I may have to increase that. Although it's a work in progress, here is a rendering of the launcher further along in its design.