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C:B Ratio Decision

Unread postPosted: Tue Jun 16, 2009 5:13 am
Author: Mitch21
Hi fellow spudders

I'm new to SpudFiles here, and I have a question. (I did search)
My friend and I are planning to build a combustion cannon, and I am yet to work out the PVC dimensions according to an ideal C:B ratio. I have looked on the spud files wiki, and I read that "a C:B ratio of about 0.6-0.8:1 is the most efficient for a given chamber using potatoes as projectiles." I will be shooting paintballs, and ammo that will fit into that diameter barrel. Im not sure if I should have the chamber volume smaller than the barrel volume, or the chamber volume larger than the barrel volume. To me it seems obvious there will be more power and a build up of pressure within the barrel if the chamber volume is larger than the barrel volume. Any advice or suggestions?

And one more thing, what size PVC pipe could I use for a barrel that paintballs will fit into?

Thanks
Mitch21

Unread postPosted: Tue Jun 16, 2009 5:24 am
Author: Technician1002
I think HGDT (Hybrid Gun Design Tool) will let you play with various combinations including projectile mass to find the best ratio. It will model the expected performance of all your variables.

http://thehalls-in-bfe.com/HGDT/

Unread postPosted: Tue Jun 16, 2009 6:45 am
Author: niglch
Having a larger C:B ratio (>0.8:1) does not negatively affect performance, but it does mean that you will have to use more fuel to less benifit. The point of the 0.8:1 thing is to have the projectile exit the barrel as soon as pressure is no longer generated in the chamber and the projectile stops accelerating. Thus, there is a minimum of "wasted" energy. However, if fuel economy isn't a top prority, you could get some extra power by using a larger C:B ratio. Personally, I wouldn't go higher than 1.5:1 since there is little added power benifit after this point.

Unread postPosted: Tue Jun 16, 2009 8:06 am
Author: Mitch21
I downloaded the HGDT and it doesn't work. I get an error reading "Runtime error 339 Component MSCHRT20.OCX or one of its dependancies not correctly registered: a file is missing or invalid."

I downloaded it twice and it still doesnt work.

Unread postPosted: Tue Jun 16, 2009 9:26 am
Author: MountainousDew
Mitch21 wrote:I downloaded the HGDT and it doesn't work. I get an error reading "Runtime error 339 Component MSCHRT20.OCX or one of its dependancies not correctly registered: a file is missing or invalid."

I downloaded it twice and it still doesnt work.

If you are running Vista, I think I can help...
I got that same message at first, what I have to do is:
-Click "Start"
-Type "HGDT" in the search bar
-Click the file titled, "HGDT"
-When the file opens, click on the executable file "HGDT"

If you don't understand, there is a FAQ section on his site, or someone may translate my gibberish...

As for your question for the paintball barrel, 3/4 sch 120 PVC pipe, JeepKahn (a user on this site) can sell you some in 4' lengths...
Plug!

Unread postPosted: Tue Jun 16, 2009 10:23 am
Author: daberno123
Another option if you want to buy local is 3/4" CPVC.

I've also heard good things about aluminum tubing tubing that McMaster sells with an ID of .68.

Unread postPosted: Tue Jun 16, 2009 12:10 pm
Author: Mitch21
Ok thanks, I was planning on using SCH 80 piping, or should i rather get SCH 120? For the barrel I plan on using copper pipe so it does not sag or bend.

Unread postPosted: Tue Jun 16, 2009 1:12 pm
Author: jimmy101
There is a common misconception as to what the "optimal CB is 0.8" means.

CB 0.8 is the most efficient ratio (for the tested ammo and barrels). If you think about it for a while you'll realize that efficiency is really pretty irrelevant. Do you really give a rat's ass if you use $0.02 or $0.03 worth of fuel per shot?

What 99.9% of spudders want is the best performance given certain design constraints such as the overall size of the gun and the ammo. The fuel cost per shot is generally way down on the list of important characteristics of the gun.

A CB of 1 or 2 will outperform a CB 0.8 gun if the barrel and ammo is held constant. So, assuming you want to get the most "ooomph" out'a the gun you'll want to use a CB greater than 0.8.

Unread postPosted: Tue Jun 16, 2009 1:27 pm
Author: Mitch21
Yes I realised that already, thats why I was asking if I should have a smaller or larger chamber to barrel ratio in my cannon to get the most "ooooomph" out of it.:roll:

I hope to start building it soon, it is going to be based on a MG42 machine gun with a unique magazine bolt action loading system! I am just finalising plans! :D

Unread postPosted: Tue Jun 16, 2009 2:42 pm
Author: jimmy101
Mitch21 wrote:Yes I realised that already, thats why I was asking if I should have a smaller or larger chamber to barrel ratio in my cannon to get the most "ooooomph" out of it.:roll:

It comes down to;

1. Pick your ammo, barrel diameter and barrel length. The longer the barrel the better the perfomance. But how long do you want to gun to be? YOU have to make a choice somewhere since there is not such thing as an "optimal" design. If you are going to handhold the gun while firing then the total length probably needs to be 6 feet or less. A 10 foot gun will perform better but you won't be able to hand hold it. YOU have to make that decision.

2. Pick the largest diameter pipe you can get for the chamber. You're limited here to what the local hardware store sells. (Assuming you don't want to buy larger diameter pipe online.)

3. Scale the length of the chamber so that the CB is say 1.5. That'll get pretty near the best possible performance out'a your barrel. A bigger chamber will boost performance but you are into the "diminishing returns" range.

The gun will be LOUD, which may be good or bad depending on what you want. The gun will out perform (in terms of muzzle velocity) the same barrel + ammo combination on an 0.8 CB chamber. Typically higher muzzle velocity is considered a good thing. Typically you don't care about efficiency.

If you want to put some hard numbers to the gun design then download HGDT and fiddle with it.

Unread postPosted: Tue Jun 16, 2009 8:48 pm
Author: TurboSuper
It's weird though, according to Latke's tests, a 0.8:1 ratio actually outperformed a higher ratio:

http://www.burntlatke.com/jpg600/15cb-graph.gif

Unread postPosted: Tue Jun 16, 2009 9:23 pm
Author: Gaderelguitarist
More chamber volume = more fuel per shot.

Also you have to consider that a larger chamber would take longer for all of the fuel to diffuse.

So with a larger chamber comes more fuel consumption and timing constraints.

Unread postPosted: Tue Jun 16, 2009 9:29 pm
Author: Ragnarok
TurboSuper wrote:It's weird though, according to Latke's tests, a 0.8:1 ratio actually outperformed a higher ratio.

That was with a constant chamber size, not a constant barrel size. Big difference.

If you're using a chamber of a fixed size, C:B should be about 0.8:1 for best performance.

If you're using a barrel of a fixed size, a higher C:B (within reason) will improve performance. I should note that although increased cost of fuel might be pretty irrelevant here, the cannons will however get noisier, so if noise levels need to be kept controlled, large C:B ratios should be avoided.

But whatever is being held constant, less than 0.6-0.8:1 C:B shouldn't be used (except in very exceptional circumstances).

Unread postPosted: Wed Jun 17, 2009 9:57 am
Author: Mitch21
Thanks for all the useful info guys! :wink:

Unread postPosted: Wed Jun 17, 2009 3:39 pm
Author: jimmy101
TurboSuper wrote:It's weird though, according to Latke's tests, a 0.8:1 ratio actually outperformed a higher ratio:

http://www.burntlatke.com/jpg600/15cb-graph.gif

Like Rag and I have already said, Latke got higher CB ratios by shortening the barrel! That makes a huge difference in how the guns behaved.