What are they equivalent to in terms of black powder? I have a 4" and 6" project on the drawing board and CO2 will have to be a major part of both recovery systems on both projects.

Warren

Warren, that's not so easy to answer, however, if you check out the link below you may find something useful:

http://rouse-tech.com/pdfs/CD3_Manual06.pdf

On page 7 there is a chart that shows the recommended size CO2 cartridge for the diameter of the rocket and the length of the compartment.

HTH,

John

it looks like a 5 to 1 but this is what I found:

Method 1: Rough guidelines by compartment size

This method is based solely on typical charge sizes for typical rockets. It’s a rough starting point for your ground testing. Simply use the table
below to locate the recommended CO2 cartridge size for the diameter and length of the recovery compartment.
Recommended Cartridge size by recovery compartment length and diameter (typical)
Length (in)
Diam____(in)__6"_10"_14"__18"_22"_26"__30"_34"_38"_42"__46"_50"
_________2" 12 g 12 g 12 g 12 g 12 g 12 g 12 g 12 g 12 g 16 g 16 g 16 g
_________3" 12 g 12 g 12 g 12 g 12 g 12 g 12 g 12 g 16 g 16 g 16 g 16 g
_________4" 12 g 12 g 12 g 12 g 12 g 16 g 16 g 16 g 25 g 25 g 25 g 25 g
_________5" 12 g 12 g 12 g 12 g 12 g 16 g 16 g 25 g 25 g 25 g 25 g 25 g
_________6" 12 g 12 g 12 g 12 g 16 g 16 g 25 g 25 g 25 g 25 g 38 g 38 g
_________8" 12 g 12 g 16 g 16 g 25 g 25 g 38 g 38 g 38 g 38 g NR NR
________10" 12 g 12 g 16 g 25 g 25 g 38 g 38 g 38 g NR NR NR NR
________12" 12 g 16 g 25 g 25 g 38 g 38 g NR NR NR NR NR NR
NR = Not recommended

_______

Method 2: Sizing by Black Powder Equivalence

This method might be preferred for those experienced flyers who are comfortable with their own or published black powder “ejection charge
calculators” or techniques. This method is also suitable for those retrofitting previously flown rockets with a successful base of experience.
Step 1: Determine the black powder charge (in grams) using the method you prefer. A widely used black powder charge calculator is
available on the web at Rocketry Online’s INFOcentral web page. Rocketry Online’s URL is www.rocketryonline.com. You can find the
SIZING GUIDE
calculator by going to the INFOcentral page and finding the “recovery” section. The “recovery” section’s index has a “black powder use” page
where the calculator resides. Note: There is no need to add in “extra” black powder charge for flights expected to exceed 20,000 feet.
Step 2: Multiply the black powder charge size (grams) by 5.0 to determine the amount of CO2 (also in grams) required to achieve the same
compartment pressure.
Step 3: Round up to the nearest sized CO2 cartridge. Use this as the starting point for your ground testing.
Example: My previous rocket has flown and successfully deployed at 12,000 feet three times using a 2.5 gram black powder charge. What
size CO2 cartridge should I use to replace the black powder charge system?
2.5 x 5.0 = 12.5 grams CO2. Round up to the nearest sized CO2 cartridge: 16.0 grams.
Use a 16 gram CO2 cartridge as the starting point for my ground testing.
Here’s a brief description of the underpinnings of this “conversion factor” method (the black powder pressure calculation is borrowed from Ted
Apke’s “Ejection Charge Calculator” page on ROL’s INFOcentral web page):
Quoting from Ted:
The ejection charge equation is:
Wp = dP*V/R*T
Where:
• dP is the ejection charge pressure in psi.
• R is the combustion gas constant, 22.16 (ft-lbf/lbm R) for FFFF black powder. (Multiply by 12 in/ft to get in terms of inches)
• T is the combustion gas temperature, 3307 degrees R for black powder
• V is the free volume in cubic inches. Volume of a cylinder is cross section area times length L, or from diameter D,
V=L*p*D2/4
• Wp is the charge weight (mass, actually) in pounds. (Multiply by 454 g/lb to get grams.)
Here’s an example calculation. Suppose you want to generate 15 psi inside a 4” diameter rocket in a parachute
compartment 18” long. That makes a volume of 226 in3. The amount of powder you need will be:
Wp = 15*226(454)/12(22.16) 3307
Wp = 1.75 grams
Continuing with the example provided by Ted Apke above, here’s how to solve the same problem using CO2 instead of black powder:
Start with the ideal gas law: n= PV/RT
n= number of moles CO2 required.
P= ejection pressure desired in atmospheres (1 atm = 14.7 psi)
V= volume of parachute compartment in liters (1 liter = 61 in3)
R= universal gas constant (=0.08206)
T= Temperature of expelled CO2 gas at deployment in degrees Kelvin (273K)
In this example:
P= 15psi = 1.02 atm
V= 226 in3 = 3.70 liters
T= 273K
Number of moles CO2 = (1.02*3.70)/(0.08206*273) = 0.168 moles.
Lastly, one mole of CO2 = 44 grams, so 0.168 moles * 44g/mole = 7.41 grams CO2.
Thus, we have the result that 7.41 grams CO2 produces the same ejection pressure as 1.75 grams Black powder.
Dividing these results gives a ratio of 7.41 g CO2/1.75 g BP = 4.24. We recommend adding a safety factor of approximately 20%, resulting in
a “conversion factor” of 5.0 grams CO2 per gram of black powder.