Field Notes
How to Size Black Powder Ejection Charges (and Prove Them on the Ground)
The D²×L and volume formulas for FFFFg ejection charges, worked examples for 3- and 4-inch bays, and a ground-test procedure.
By Burn Wire
Most lost high-power rockets are not lost because the motor was wrong. They are lost because the recovery system never came out. The charge was light, the bay was sealed, the pins did not shear, or nobody ground-tested the packed rocket.
This guide is the charge-math half of that problem. It sits next to the free dual-deployment walkthrough and the recovery hardware guide. Those tell you what to buy. This one tells you how much FFFFg to weigh, why the popular 15 psi rule breaks on fat airframes, and how to prove the number on the ground before a cert official is watching.
The only universally correct charge is the one that separates your airframe, with your pins, packed the way you will fly it. Every formula below is a starting guess.
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Behind the perforation
- What the charge is actually doing
- Powder grade
- The 15 psi rule of thumb
- Ideal-gas form (so you can see where 0.006 comes from)
- Worked example: 3-inch bay, 18 inches long
- Worked example: 4-inch bay, 12 inches long
- When 15 psi is too much
- What length to measure
- Charge wells, wadding, and e-matches
- Ground-test procedure that is worth doing
- Motor ejection vs electronic charges
- Field checklist
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