BP Estimator

Warning: The use of black powder is dangerous and should be done with proper precautions. Some cities, counties, states, and countries have restrictions on its use so please check your local laws and regulations. The purchase of black powder is prohibited to those under the age of 18. I suggest you find an experienced mentor prior to your first use. MOST IMPORTANT: Always ground test black powder charges before a flight and do so at a safe distance. Ground testing should never when someone is holding the rocket.

A black powder charge is the most common and reliable method of ejecting a parachute from your rocket. Sure, you can resort to much more exotic means such as carbon dioxide canisters but they are much less reliable. In your rocket, the motor’s ejection or the dual deployment charges will ignite. This ignition will generate hot gases that pressurize the rocket’s airframe. The expanding gases will exert a force on the bulk plate of the nose cone. This created force will eject the nose cone, shock cord, and parachute out of the rocket airframe.

Properly sized charges increase safety and make the hobby more enjoyable. Three very bad things can happen to rockets that use dual deployment or altimeters as recovery devices: no ejection (ballistic recovery), too powerful of an ejection (blow it out or blow it up), or an early deployment such as deploying the main at apogee.


Formulas:

Formula 1: Pressure.
Formula 2: Volume of an airframe – Diameter in the insider diameter and length is the unfilled length.
Formula 3: Grams of black powder.

This calculator is designed to give you an approximation or estimation as to how much black powder to use for an ejection charge for rocketry. For Standard high powered rockets around 3-4 inches in diameter, 10 to 15 PSI is typical. If you use a piston ejection systems or another specialized system, you will require less. Please see the instructions for these ejection systems. Tight fitting parachutes require more pressure to eject. Larger diameter rockets usually require much less power than the calculator estimates.

The key is ground testing. Ground testing can reveal the minimum amount needed under ideal conditions. None of this information applies to BP substitutes like Pyrodex or smokeless powers, which generally don’t work with rocket recovery systems.

Warning!!!! I am going to reiterate: Always ground test to ensure proper deployment of parachute laundry prior to your first launch. This calculator is meant to give you a starting point for ground testing. A safe distance is essential or injury may occur.


Charts:

Airframe Diameter and Desired Force 

Below is a list of airframe diameters and the pressure placed on the bulkhead (lbs) by a chosen pressure (psi). You can quickly see that the large the bulkhead, the more pressure placed on it by a charge. The extremes, smaller and larger tubes, will result in less reliable results.

AIRFRAME DIAMETER 50 lbs75 lbs100lbs 150lbs 200lbs 250lbs 
1.5″29 psi43 psi57 psi85 psi114 psi142 psi
2.1″15 psi22 psi29 psi44 psi58 psi73 psi
2.6″10 psi15 psi19 psi 28 psi 38 psi 47 psi 
3.9″4.5 psi6.5 psi8.5 psi 12.5 psi 16.5 psi 20.5 psi 
6.0″2 psi3 psi3.5 psi 5.3 psi 7.0 psi 8.8 psi 
7.5″1 psi1.7 psi2.3 psi 3.4 psi 4.5 psi 5.7 psi 
8.0″1.0 psi1.5 psi2.0 psi3.0 psi4.0 psi5.0 psi
10.0″0.7 psi1.0 psi1.4 psi2.0 psi2.5 psi3.2 psi

Simple Charge Estimates

The below calculation will be off for high altitude flights and both smaller and larger body tubes. For example, I would not use 7.5 grams for my first ground test of a 10-inch body tube. This number is likely hyperinflated and would damage your bulkhead.

Body Tube Inside DiameterEstimated Ejection Charge Size
1.53 inch 0.25 gram per 18 inches of length
2.15 inch0.50 gram per 18 inches of length
2.56 inch0.50 gram per 12 inches of length
3.00 inch0.65 gram per 12 inches of length
3.90 inch1.1 grams per 12 inches of length
5.38 inch2 grams per  12 inches of length
6.00 inch2.59 grams per  12 inches of length
7.51 inch4 grams per  12 inches of length
8.00 inch4.6 grams per  12 inches of length
10.00 inch7.2 grams per 12 inches of length

Shear Strength (LBS) of Plastic Screws

The calculator above uses the minimum number to calculate the max number of screws you can shear. Ground testing will ensure the pins shear during launch.

Screw SizeMinMax
M22739
2-563146
4-405076
M36791
6-3275114

References:

Below is a list of links and calculators that I have used to make this site and during my years in rocketry.

  1. Insane Rocketry
  2. Rimworld
  3. Rocket head
  4. HARA Rocketry
  5. MCFisher
  6. VerK