# Charge Conversion Question

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#### Lowpuller

##### Well-Known Member
Please check my math, assumptions and conversion factors,

I only have a grains BP measure.

If I have done the math correctly there are 15.42 grains per gram, or roughly 15 grains per gram.

I have only been able to find 3f but it has work successfully for me in the past. If I remember correctly I add about 10%-20% to the value I get from on online BP calculator for 4f powder.

The calculator I used calculated 0.8 grams for both my drogue and main charge. I am raising this to 1.0 gram to compensate for 3f which I will measure out as 15 grains.

Ground test, ground test, ground test!

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Your math is correct. 14.78 grains to be exact with .8 grams and adding 20% over.

Another tip that will help is to tightly contain your charge.

There is only 1 conversion method I trust, and that is ground testing. Pack the charges the EXACT same way, and it really doesn't matter what you use (in my opinion) if it works successfully on the ground (just talking about low altitude flights here, I'm not sure about higher altitude flights dealing with atmospheric pressure change), then it works in the air. This is especially critical with how you pack the charges. When I was testing my 3" darkstar, 3 grams performed DRASTICALLY differently when I packed it differently. Put it in a fingertip from a latex glove, and it barely worked. When I packed it into an estes motor case, and packed tons of dog barf in there on top, it shot my nose cone almost 70 feet away. So remember to pack your charges consistenly.

There is only 1 conversion method I trust, and that is ground testing. Pack the charges the EXACT same way, and it really doesn't matter what you use (in my opinion) if it works successfully on the ground (just talking about low altitude flights here, I'm not sure about higher altitude flights dealing with atmospheric pressure change), then it works in the air. This is especially critical with how you pack the charges. When I was testing my 3" darkstar, 3 grams performed DRASTICALLY differently when I packed it differently. Put it in a fingertip from a latex glove, and it barely worked. When I packed it into an estes motor case, and packed tons of dog barf in there on top, it shot my nose cone almost 70 feet away. So remember to pack your charges consistenly.

Absolutely. Calculators and math give you a starting point for testing, but they do not take every real-world contingency into consideration. You don't want to "air test" your deployment. Test on the ground, find the right amount, and launch with confidence. Consistency is king.

Make sure you test with the rocket in ready-to-fly condition (motor in, chutes protected, harnesses secure, shear pins in (if you are using them, or tape on the NC shoulder, etc.). You can run the wires out of one of the vent holes, hook up a simple Estes launch controller, and voila! However, if you use an Estes controller, be forewarned that the continuity test will fire the e-matches. (Because of the incandescent light bulb that Estes uses for continuity, the amount of current sent through the continuity test is more than enough to light a low-current igniter/e-match.)

The OPs original question is how to convert grains to grams!

Ah, and he said "Ground test." Sigh. I am getting old. Sorry.

I always thought BP was measured as 1 gram = 1 cc of volume, or close enough. Isn't that the point of BP and measuring by volume instead of weight like the smokeless powders?

Anyway, I use the cc measurements on the container to measure my BP and of course ground test to determine the number of cc you actually need.

I just weigh the BP on a small electronic scale I got on eBay (100g full scale, 0.01g resolution. Many people use the 1ml = 1g equivalence. I have in the past also.

The scale also weighs my epoxy and hardener.

I just weigh the BP on a small electronic scale I got on eBay (100g full scale, 0.01g resolution. Many people use the 1ml = 1g equivalence. I have in the past also.

The scale also weighs my epoxy and hardener.

This is what I do too. This scale is what I use. At \$12.72 getting one is a no brainer.

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