The idea, of course, is to use ignited black powder to pressurize parts of the airframe so that they separate. Sufficient pressure has to be generated so as to overcome friction between the airframe and a coupler, and any additional anti-separation devices like small nylon screws often used to reduce the risk of drag separation.

The rule of thumb for ejection charge size is 0.006 x (airframe volume to be separated).

For an airframe diameter, D (inches), and a length of airframe, L (inches), this equates to:

Ejection charge size (grams) = D2 x L x 0.006

For smaller diameter airframes, this should produce approx 15psi of pressure. For larger airframes, since the square of the diameter is used, the pressure is going to get very high very quickly.

The key, after calculating the weight of powder needed, is to ground test the whole thing. The sections should separate easily - in fact it should really fly apart without any hesitation. Be sure the area is clear.

This table shows charge size by common airframe diameters:

                Airframe ID (inches)                                     Ejection charge (g)

                            1.53                                                    0.5 (per 36” length)

                            2.15                                                    1 (per 36” length)

                            2.56                                                    1 (per 25” length)

                            3.0                                                      1 (per 18” length)

                            3.90                                                    1 (per 11” length)

                            5.38                                                    1 (per 6” length)

                            6.00                                                    1.3 (per 6” length)

                            7.51                                                    2 (per 6” length)