C:B help for a newbie (please)

[spencetron]

Hello, this is my post here, and Id like to introduce myself. I have recently got into spudding, and literally a day after making my first "spray and pray" cannon I knew I wanted to make something more advanced. I will be making a over-under cannon with a golfball barrel, a 4" chamber, metered MAPP gas, stun gun ignition with a spark strip, a minigun-style grip, and a chamber fan. I will post pics and stuff when its all finished, but I am having a awfully hard time with C:B ratios. I have done countless hours of research, but I want to check my values, because I dont want to screw this up, I want this launcher to be perfect. Now, I know I want a 48" barrel (1.5" SDR-21), which according to my math, has a 2154.5ml volume. So to get that .8:1 ratio, I would need a chamber with a volume of 1723.6ml's, correct? It just seems strange to me that the volume of the chamber is supposed to be less than the volume of the barrel. Please correct me if I am wrong here (and yes, I have used many of the online C:B calculators, but I <shamefully> fail to understand them). Thanks for taking your time reading this, and be nice to me, Im new.

[xpitxbullx]

You are actually understanding it very well. Nothing to correct. I have a cannon with a .57/1 ratio and it shoots softballs close to 900ft.

You're looking at about an 8" in length chamber of 4" tubing. If you are going over under with a propane meter and tank, you may want to try a 3" chamber with a length of about 15". Might be easier to hook up to.

Jeff

[starman]

You'll be fine with what you've got going. The mistake most people make is to build a huge 5 or 6 liter chamber then stick a 3' GB barrel on it. Anything between .5 and 1.5 to 1 is considered the high performance window. Don't worry about nailing it exactly.

Re your SDR 21 barrel...you should consider sleeving it in 2" sched 80 PVC pipe. It fits perfectly in there. The sdr 21 tends to bow some, especially longer sections of it. The Sched 80 will stiffen it up. It will look nicer and beefer too.

Also, since you're going all out on this, consider installing a female cam lock adapter on your chamber breech and the male adapter on the barrel ends. This allows you to quickly swap barrel sizes and styles.

No need to be intimidated...you presented your query well and you have obviously been studying the subject. We want more like you!!

Welcome to Spudfiles!..

[spencetron]

Thanks guys. Pitbull: I chose to use a 4" chamber because I figured since the optimum chamber shape for flame propagation would be a sphere, a 4" chamber would fit the bill better than a 3". The differences most definitely will be minuscule, but I want to squeeze as much juice out of this thing as possible. Ive also already bought the 4" tubing, and 4"-2" fittings, so there is no turning back.


Starman: I was planning on sleeving the SDR-21 into 2" SCH-80 anyways, not only to strengthen the overall structure of the cannon, but to prevent any chance of the thin walled SDR-21 breaking or causing bodily harm to me. I was thinking about using cams, but I decided to use 2" female/ male screw on fittings (forgot the exact name of the parts). So I still have the benefit of interchangeable barrels, at a much lower cost.

[SpudFarm]

was it you i talked to on MSN?

anyway you have a nice amount of clue on what you are doing.

you know more than most people on here about things!

welcome!

[TwitchTheAussie]

I think he's pretty much got it. hell he's pretty smart. Good luck. All I can say is dont fret over anything too much mate.

[spencetron]

hey pitbull, I just ran the math by, and a 8"X4" chamber will have a volume of 2553.48ml. which is larger than the volume of the 1.5"X48" barrel (2154.5ml). So that would give me a C:B ratio of 1:0.8, if Im not mistaken, is the opposite of what I need. I was under the impression that the volume of the barrel is greater than the volume of the chamber. Can someone please clarify, which one is bigger; chamber, or barrel?

[xpitxbullx]

1.5" x 48" = 84.78 cubic inches = 1389.29ml

pi x r x r x length = cubic inches

You barrel does not have as much volume as you think.

Barrel is bigger, chamber is smaller

2154.5ml = 131.475 cubic inches for barrel
80% of that is 105.18 cubic inches
Divided by 12.54 gives you length in inches for a 4" pipe.
8.38 inches of 4" pipe for your chamber.

4" barrel size
chamber square inches = 3.14 x r x r = 12.54

Multiply that by the length of the chamber 8.38" = 105.18 cu inches

105.18 cubic inches = 1723.59 ml

Yep. that what I came up with.

Recalculated with the proper barrel volume of 1389.29ml , your chamber should now be 6.75"

Jeff

[starman]

Starman: I was planning on sleeving the SDR-21 into 2" SCH-80 anyways, not only to strengthen the overall structure of the cannon, but to prevent any chance of the thin walled SDR-21 breaking or causing bodily harm to me. I was thinking about using cams, but I decided to use 2" female/ male screw on fittings (forgot the exact name of the parts). So I still have the benefit of interchangeable barrels, at a much lower cost.

That's cool, I started out with screw-ons and later moved to cam locks. They make it easy to implement a burst disk...another thing that is a big performance boost, even for 1X combustions.

[spencetron]

I got my cubic inches to ml conversion waaay wrong, my bad. Thanks pitbull.

[xpitxbullx]

No problem. Good luck with your gun and let us know how it turns out.

[jimmy101]

Spencetron, welcome to the forums.

I wouldn't worry too much about the chamber volume. Pick your barrel length then put the largest volume chamber on that is consistent with the overall size of the gun and how you want to use it.

A CB of 0.8 maximizes the efficiency of the chamber. To maximize the performance of the barrel you generally need a chamber larger than CB 0.8. So, if your design is based on the barrel length and diameter, and the ammo, then the optimal CB is not 0.8.

For high sectional density ammo (like a golfball) the maximum muzzle velocity will probably be with a CB of perhaps 2 to 3 (that's just a WAG). That means that for a particular barrel a chamber larger than CB 0.8 will increase the muzzle velocity.

HGDT says the CB should be very large, ~7:1, to get the most out of the barrel and ammo combination. That CB gives a chamber that is probably impractical. Looks like a CB of 2 or 3 will outperform the CB 0.8 by enough to make it worth considering while still giving a gun with a reasonable overall size, weight etc.

(Interestingly, at very large CB ratios HGDT predicts a "mine-sized" chamber affect, the muzzle velocity eventually starts to drop as the chamber volume gets very large.)

EDIT: I had to guestimate the friction for the HGDT analysis. The inlayed sensitivity analysis graph shows the affect of different chamber lengths (which is proportional to chamber volume) on the muzzle velocity. The CB 0.8 gun has a chamber length of about 7". A CB of 3 would need a 26" long chamber. Everything was calulated based on a 1.7" ID barrel and a 4" ID chamber.

[Canuck]

Whats that program called? I googled it and nothing came up.

[SpudFarm]

look in "D_Hall's" signature.

[jimmy101]

Canuck

Sorry, I usually include the links to programs;
Hybrid Gun Design Tool
http://thehalls-in-bfe.com/HGDT/

(A generic combustion gun is a hybrid gun with burst disk that fails at 0 psig and is fueled to 1X.)

HGDT is only a month or two old but it is in Google already (search HGDT hybrid gun).