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Jack Anderson's Black Powder Calculator
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How much FFFFG Black Powder does it take the pressurize a rocket ?
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Estimating how much FFFFG Black Powder to
use
A starting point for determining the amount of Black Powder (BP) to
use is to determine the amount of desired force on the base of the nose cone. A suggested value is 150200 pounds. The
next step is to determine the amount of pressure (pounds per square inch  psi) that will produce the desired amount of
force.
PSI by Airframe Diameter and Desired Force
Aiframe
Diameter 
100 Pounds 
150 Pounds 
200 Pounds 
250 Pounds 
2.6" 
19 psi 
28 psi 
38 psi 
47 psi 
4.0" 
8 psi 
12 psi 
16 psi 
20 psi 
6.0" 
3.5 psi 
5.3 psi 
7.0 psi 
8.8 psi 
7.5" 
2.3 psi 
3.4 psi 
4.5 psi 
5.7 psi 
The table shows that smaller diameter airframes may need a higher pressure than
larger diameter airframes.
Once you've selected a desired force (150, 200 or 250 pounds) on the nose cone,
determine the psi that will produce that force. For example, 16 psi will put 200 pounds of force on the nose cone of a
4" diameter rocket.
Next determine the amount of BP to produce the desired pressure. The equation is
as follows:
BP = C x
D^{2} x L
Where:
Example:
A 6" diameter airframe, 22" long parachute
compartment, 7 psi
The tables don't have a value for 7 psi. However, a little too much is better
than any amount of too little. Therefore select 10 psi (C = 0.004).
0.004 * 6 * 6 * 22 = 3 grams BP
The next step is ground testing. Ground testing can reveal the minimum amount
needed under ideal conditions. It is usually best to not fly with the minimum amount of BP that works under ideal
conditions. Assume that you calculate that 3 grams of BP are needed. Ground testing shows that 2 grams is the minimum
amount for a reliable deployment. You will probably want to use 34 grams during flight.
Some high altitude flights have experienced recovery system ejection problems.
The cause may be that BP may not combust as well at 30,000 feet as it does at sea level, or the rockets may have used
components (nose cone, parachute) with a much greater mass than typical hobby high power rockets, and therefore the
methodology for hobby rockets may not apply to significantly more massive rockets.
None of this information applies to BP substitutes like Pyrodex or smokeless
powers, which generally don't work with rocket recovery systems.
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