Ratio conundrums!!??

joe blogs · Wed May 13, 2009 6:19 am

I have been wanting to make the bl-520 rear loaded cannon for some time now and that lead me to chamber:barrel ratio's. Below are the calculations for a
2 inch barrel 60 inch long
5 inch chamber unknown length (i assumed that the length would satisfied the minimum,maximum and ideal ratios 0.5,1.2,0.7 according to burnt lake http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio) the reason is that i couldnt find the spec for chamber length.
------------------------------------------------------------------------------
minimum=0.5

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =0.5 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =30pi

6.25Lpi - Lpi =30pi

L(6.25pi-pi) =30pi

L = 30pi
---------
(6.25pi-pi)

Length of chamber= 5.71 inches
-----------------------------------------------------------------------------------
maximum=1.2

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =1.2 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =72pi

6.25Lpi - Lpi =72pi

L(6.25pi-pi) =72pi
72pi
L = ---------
(6.25pi-pi)

Length of chamber= 13.7 inches
----------------------------------------------------------------------------------

ideal=0.7

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =0.7 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =42pi

6.25Lpi - Lpi =42pi

L(6.25pi-pi) =42pi

L = 42pi
---------
(6.25pi-pi)

Length of chamber= 8 inches
-----------------------------------------------------------------------------------

In the picture of the Bl-520 the chamber looks at least 20 inches. yet this is way too big for the ideal volume. Is it the metered injection, blocked off part of the chamber???????????????. The tests show that to big of a chamber will have a drastic reduction in velocity of the spud. Whats the deal???

Technician1002 · Wed May 13, 2009 7:18 am

joe blogs wrote:I have been wanting to make the bl-520 rear loaded cannon for some time now and that lead me to chamber:barrel ratio's. Below are the calculations for a
2 inch barrel 60 inch long
5 inch chamber unknown length (i assumed that the length would satisfied the minimum,maximum and ideal ratios 0.5,1.2,0.7 according to burnt lake http://www.spudfiles.com/spud_wiki/inde ... rrel_ratio) the reason is that i couldnt find the spec for chamber length.
------------------------------------------------------------------------------
minimum=0.5

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =0.5 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =30pi

6.25Lpi - Lpi =30pi

L(6.25pi-pi) =30pi

L = 30pi
---------
(6.25pi-pi)

Length of chamber= 5.71 inches
-----------------------------------------------------------------------------------
maximum=1.2

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =1.2 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =72pi

6.25Lpi - Lpi =72pi

L(6.25pi-pi) =72pi
72pi
L = ---------
(6.25pi-pi)

Length of chamber= 13.7 inches
----------------------------------------------------------------------------------

ideal=0.7

L x pi x 2.5^2 - L x pi x 1^2
---------------------------------- =0.7 L=length of chamber
60 x pi x 1^2

L x pi x 2.5^2 - L x pi x 1^2 =42pi

6.25Lpi - Lpi =42pi

L(6.25pi-pi) =42pi

L = 42pi
---------
(6.25pi-pi)

Length of chamber= 8 inches
-----------------------------------------------------------------------------------

In the picture of the Bl-520 the chamber looks at least 20 inches. yet this is way too big for the ideal volume. Is it the metered injection, blocked off part of the chamber???????????????. The tests show that to big of a chamber will have a drastic reduction in velocity of the spud. Whats the deal???

This is a COAX so part of the chamber is occupied with the barrel.

inonickname · Wed May 13, 2009 7:31 am

Yes, and seen as the barrel is two inches in diameter the volume is significant.

And anyway, if you look through the threads concerning this gun you would realize that more went into it than going to a hardware store and gluing some PVC pipe together.

Technician1002 · Wed May 13, 2009 12:19 pm

inonickname wrote:Yes, and seen as the barrel is two inches in diameter the volume is significant.

And anyway, if you look through the threads concerning this gun you would realize that more went into it than going to a hardware store and gluing some PVC pipe together.

To calculate the chamber, use GGDT as it has a formula for a coaxial chamber. The hole in the gas due to the space occupied by the barrel is the inner diameter. For 2 inch pipe it is 2.375 inches. Dimension found here;
http://academic.evergreen.edu/projects/ ... c/pipe.htm

joe blogs · Wed May 13, 2009 5:02 pm

<Those are the formula's for a coax>

I sort of did factor in the barrel occoping space. Though i didnt factor in the OD i did factor in the ID which wouldnt make that much difference. This is why the formula

L x pi x 2.5^2 - L x pi x 1^2

Notice i minused the barrel algebraic length ie coax formula

is done. If you read properly you will see that i know its a coax. Granted that i have my formulas right what occupies the other space?. This is the formula for a Coax with a 30 inch chamber.

30 inch long 5 inch chamber

30 x pi x 2.5^2 - 30 x pi x 1^2
---------------------------------- = 2.625 ratio
60 x pi x 1^2

This is how long some of the coax's and even over under cannons chambers look. The reason i am asking this question is that i want to build a coax with a decent chamber length so that it holds like a rocket launcher. a coax with a 8 inch chamber length (ideal) does not hold right. Are all these long chambered cannons sacrificing power for length or is there something else? Are the ratio's right?

ralphd · Wed May 13, 2009 8:46 pm

put everything into Excel. Have a cell for every part then use the function"GOAL SEEK" to fine tune it. Works every time.

joe blogs · Thu May 14, 2009 1:10 am

Does the fan assembly in the bl-520 take up much volume? I plan on making this cannon.

But the fan assembly on the bl-520 looks like this.

I hope to understand this as i dont have a clue about my ratio problem. Maybe i could internally partition off the chamber so a 20" chamber would act like a 8" chamber.

jagerbond · Thu May 14, 2009 5:19 am

Joe,
Internally our chamber runs a total of 22". With deductions of barrel and vent piping, fan, etc our chamber ends up at 298 cubic inches. Barrel length measures 62 before brake openings and equates to 195 cubic inches. So we are at 1.5 chamber to 1 barrel ratio. All of our assembled 520's come with take down barrel assembly, so longer barrels / GB barrels and lower C:B ratios are a swap away.

We have found our customers prefer this ratio due to report.

Make sure to run your vent tube to the bottom to help evacuate the deoxygenated between rounds.

Good luck, look forward to seeing your launcher.

inonickname · Thu May 14, 2009 5:33 am

As has been said before, you wont make it, but you could match the performance.

The metering syringe is a balanced spool valve that is regulated. The ignition would be a HV source such as a stungun (of course he didn't just attach a stungun to it). (Jager, as you are a sponsor of the site I can remove this info if you want for your marketing interest..)

If you need much else I'm sure people would help..

joe blogs · Thu May 14, 2009 6:48 am

I am not using the metered syringe as the injection method. if you look at my crappy paint picture you will see that im using a ball valve system to regulate my ignition gas. I am using a 2 ignition coils in anti parrallel to generate seven sparks on a spark strip. the sparks are 1 cm each.

Thanks jagerbond for answering my question fully and reading exactly what i was saying. I managed to find cheap ($35) stainless steel cam locks for 5" pipe. This is what i plan on using to open up the breach. The cam locks are pressure rated to 350 PSI.

For any Australian spudders out there the store to go to if you live in Northern NSW or South queensland is Rural Buying Service. I have tried pluming stores like Reece,Tradelink and Eagles to no avial.

jagerbond · Thu May 14, 2009 3:18 pm

The ignition would be a HV source such as a stungun (of course he didn't just attach a stungun to it). (Jager, as you are a sponsor of the site I can remove this info if you want for your marketing interest..)

No need to remove. We do offer a low cost ignition module http://ultimatespudgun.com/product_info ... cts_id=219

Mike
Sureshot / http://www.ultimatespudgun.com

Hotwired · Thu May 14, 2009 3:25 pm

Cute lil thing ^^

It looks like it works exactly like a stungun circuit with a questionmark hovering over the low voltage switching method.