EJECTION CHARGE CALCULATOR

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. Now determine the amount of BP to produce the desired pressure. An equation is shown below.

C * D * D * L = grams of BP

Where:

C - one of the values listed below
  • 0.002 = 5 psi
  • 0.004 = 10 psi
  • 0.006 = 15 psi
  • 0.0072 = 18 psi
  • 0.008 = 20 psi
D = airframe diameter, in inches
L = length of the cavity to be pressurized, in inches

Example: 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

AIRFRAME DIAMETER & DESIRED FORCE
AIRFRAME DIAMETER
100lbs
150lbs
200lbs
250lbs
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

Enter the size of the tube to be pressurized and either the size of your charge or the desired pressure. Pressing the appropriate button next to the data entered will give you the missing piece of data. The maximum number of #2 and #4 nylon screw shear pins that can be used will also be determined for either the calculated charge or pressure.

Ejection Charge Calculator