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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 ?

What is the diameter of the parachute section in inches ?

What is the length of the parachute section in inches ?

How many PSI of pressure do you need ?

This calculator is meant to give you an idea of how much BP to use in your ground tests.  You should always start ground testing with the smallest amount that you feel might work.  Also, note that PML "Piston Ejection Systems" require even less pressure than traditional systems. To calculate how much PSI your rocket requires, see below.

You need Grams of FFFFG Black Powder

10 to 15 PSI is a good range for safe parachute ejection for a 4 inch diameter rocket.
How much PSI will my rocket need ?

What is the diameter of the parachute section in inches ?
Use between PSI for a lose fitting parachute
and PSI for a tighter fitting
parachute in my calculator above.

Email the author of this calculator your feedback

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.

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:

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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