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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 150-200 pounds. The next step is to determine the amount of pressure (pounds per square inch - psi) that will produce the desired amount of force.

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 D² x L

Where:

• BP= The amount of black powder (FFFFG) in grams
• C= on of the following values
  • 0.002 = 5 psi
  • 0.004 = 10 psi
  • 0.006 = 15 psi
  • 0.0072 = 18 psi
  • 0.008 = 20 psi

           (Note: C = (psi * 0.0004)

• D= The airframe diameter in inches
• L= The length of the cavity to be pressurized in inches

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 3-4 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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