Black Powder Calculation Help

Alby

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I know there are several websites for calculating the amount of BP you need to deploy a chute (eg: https://rocketrycalculator.com/rocketry-calculator/bp-estimator/) along with factoring in shear pins and such. My question is really based on the Volume inside the parachute bay. Currently the empty volume of my bay is 10.75" x 3.9" I.D.. But when I stuff my parachute (inside a parachute bag) along with the tether lines and such, that volume goes to 0.00". Pretty much my parachute bag is now sitting on the charge cup with the BP.

So my question is do my calculations need to be adjusted for the fact that my filled tube volume is pretty much zero when everything is packed and ready for launch?


Thanks,
Alby
 

Crazyrocket

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I start with what the online calculators state and then ground test. Based on the results, you may need to adjust the amount of BP. Always test in a flight ready conditions, i.e., parachute packed with shock cord, etc.

Good luck!
 

Alby

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I start with what the online calculators state and then ground test. Based on the results, you may need to adjust the amount of BP. Always test in a flight ready conditions, i.e., parachute packed with shock cord, etc.

Good luck!


So I saw a document that said to estimate using the full length of the parachute tube vs. just the section the parachute sits in. So that increased my length to 15" vs. 10.75". With a recalculation, I got 1.37g of BP. As you can see from the results, the nose cone popped, shearing the shear pins, but the parachute bag didn't fully exit the body tube.

As such.... More BP for my next test.

20210708_103927.jpg
 

Alby

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This is where CJ's byline is perfect:





I use these:




I just used a scale designed for measuring grams and a popsicle stick. With a nice little plastic tray that lets me pour the final results into a centrifuge tube that I seal up.


 

Dan Griffing

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I just used a scale designed for measuring grams and a popsicle stick. With a nice little plastic tray that lets me pour the final results into a centrifuge tube that I seal up.


I’m a former rifle handloader and black powder muzzleloader shooter. For the most part, the users of black powder measure it by volume because its easier than weighing it and generally as precise as they need for BP shooting.

The two-gram polyethylene vials that BP comes in for Aerotech motors for motor ejection has “calibration” lines that are probably as precise as you need for BP measurement.
 

KilroySmith

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Success. I increased the BP by 30% to 1.78g and that blew out the parachute bag and started pulling out the parachute. 😁👍

I'd certainly round that charge up, before calling it a day. You really don't want a charge that barely pulls out the parachute, because at the field things are "different" - the humidity or temperature may change, making your nose cone tighter. You may not pack the chute as small, so there's more friction when it tries to leave the body tube. Any one of a number of things can change, and leave you with a rocket coming back with only the nose cone detached and flailing, and the chute still firmly wedged. Getting the laundry out is one of those #1 priority kinds of things.
 

Alby

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I'd certainly round that charge up, before calling it a day. You really don't want a charge that barely pulls out the parachute, because at the field things are "different" - the humidity or temperature may change, making your nose cone tighter. You may not pack the chute as small, so there's more friction when it tries to leave the body tube. Any one of a number of things can change, and leave you with a rocket coming back with only the nose cone detached and flailing, and the chute still firmly wedged. Getting the laundry out is one of those #1 priority kinds of things.



You mean a round up to 2.00/grams?
 

KilroySmith

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You mean a round up to 2.00/grams?

There are lots of opinions on what "round up" should be in this context, so I chose not to say. I think most would agree that using the minimum amount that worked in ground testing is probably too little for reliable ejection; how much to round up, however, is a subject that can create intense debate.

For me personally, I'd probably round up to 2.5-3g. IMHO, you should have enough to get the recovery harness tight (both nose cone and parachute bundle), without having so much that the nose cone rebounds back towards the rocket. Having too much (within reason) is not nearly as much of a problem as not having enough.

There are a lot of people with a lot of different heuristic answers, ranging all the way up to "Put in enough to rupture the body tube if the chute doesn't come out" (referred to coloquially as "Blow it (the parachute) out or blow it (the rocket) up"). I understand this approach; if something goes wrong and the chute doesn't come out, the rocket is likely to come in ballistic and destroy itself when it hits the ground, so blowing it up in the air in this failure scenario is safer because it won't come down as fast.
 

Alby

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There are lots of opinions on what "round up" should be in this context, so I chose not to say. I think most would agree that using the minimum amount that worked in ground testing is probably too little for reliable ejection; how much to round up, however, is a subject that can create intense debate.

For me personally, I'd probably round up to 2.5-3g. IMHO, you should have enough to get the recovery harness tight (both nose cone and parachute bundle), without having so much that the nose cone rebounds back towards the rocket. Having too much (within reason) is not nearly as much of a problem as not having enough.

There are a lot of people with a lot of different heuristic answers, ranging all the way up to "Put in enough to rupture the body tube if the chute doesn't come out" (referred to coloquially as "Blow it (the parachute) out or blow it (the rocket) up"). I understand this approach; if something goes wrong and the chute doesn't come out, the rocket is likely to come in ballistic and destroy itself when it hits the ground, so blowing it up in the air in this failure scenario is safer because it won't come down as fast.



Oh My. I like your school of thought and now that you've explained it and I've pondered the idea, it does make a lot of sense.
Going to 2.5g increases the PSI from about 20 to 28. Or about an extra 100/lbs on the bulkheads. Not a major increase vs.
the potential savings of insurance. After all a ton of $$$$ money and time is going down the drain if that chute doesn't blow. lol
 
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