Does my math check out for calculating BP deployment charge?

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Well-Known Member
Dec 9, 2015
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Does this math sound correct? I'm trying to calculate the amount of BP required for my charge, but I want to know exactly *why* I need that amount. I will be ground testing this before launching of course, but want to know if I'm doing this math correct, and if I'm coming up with a good number to start testing with.

This is for a 38mm MD rocket, dual deploy. Both the drogue and main parachute compartments are 7" in length. The calculations I've don'e don't include removing the volume of the parachute from the equation, since I would rather estimate high. The rocket is fiberglass, and I don't think extra pressure will be an issue for the airframe to handle. On the other hand, I don't want to shoot the nose off like a bullet....

I'll be using a single 2-56 nylon shear pin on each section. Everything that I have read online says that I need 35 pounds of force to reliably shear the pin each time.

With the inner diameter ≈ 1.5", surface area of the bulk head would be ≈ 1.77 inches^2.

Multiple online calculators show that I will need ≈ 20 PSI of pressure to create the 35 lb force required to shear the pin and separate the rocket, based on an area of 1.77 inch^2.

The NASSA rocketry calculator says that for my tube dimensions, to achieve ≈ 20 PSI I would need ≈ 0.13g of FFFF black powder.

Rounding this up to .2g gives me a pressure of ≈ 31 PSI, which works out to about 55 lb of force.

Can anyone tell me if my thought process on how I got to this number sounds correct? Does 55 lbs of force sound like a reasonable number to shear the pin and pull my parachute out? It isn't packed in the tube super tight, but isn't very loose either.

Comments would be appreciated, thanks!
I have not used those calculators personally, but the result is similar to what I would use.

I use the formula from "Modern High Power Rocketry" as a starting point. Diameter (inches) x diameter (inches) x length (inches) x 0.006 = BP quantity (grams)
For your example it works out to 0.1g

Given it is a f/g rocket I would upsize a little as a starting point (cardboard is less forgiving). I would ground test at 0.15g as a starting point. So you are getting about the same as I would be working with.

Main thing is to ground test.
That is an exceedingly small charge. I like to use to more than one shear pin- usually two in a rocket of this size.

I would likely be looking at more like .3 or .4. Granted, that is my bias- I want to make sure stuff is coming out. You are doing it right though with ground testing. I do recommend that you do the testing full up i.e. With chutes, protectors, and harnesses.
A single 2-56 nylon screw has a shear strength of 31 - 46 lbs. You might be cutting it really close with such a small charge. I've had trouble getting consistent results with BP amounts under 0.5 g.

+1 to everyone who says to ground test.
I'm using only one (1) 2-56 shear pin on my Blackhawk 38mm MD and ended up using 0.7 grams BP for both main & drogue.
+1 to .5g as a min amount. anything less and I've had trouble making sure it all lights off in a consistent manner. containment is really key with any charge, even moreso with small amounts.

I calculate for an empty bay. then you don't have to worry about leaks (i vent my bays so they don't pop early) or how much chutes compress.