Black Powder charge

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Joined
Jan 16, 2015
Messages
271
Reaction score
67
I have been searching the forum and reading up on how to calculate a proper BP charge. There is a lot of discussion on the 'proper' formula. So how do I figure out the proper amount of BP to use? Also the discussions on size, I have FFFFg which is very small.

Thanks !
 
4F is what you want so you are good there. There are a lot of online calculators which will get you in the ball park but ground testing will really determine your actual needs.
 
As stated, there are online calculators that will give you a starting point, but ground testing is always your best bet.

We found that online calculators gave us a much larger charge than necessary. Our rocket is quite large - 12.75" in diameter - which is part of he reason.

In the end it boils down to a few variables:
Pressure produced by a given amount of BP in a known volume
Surface area that said pressure acts upon (that of NC bulkhead)
Amount of force needed to separate NC (calculated from shear pin size/number)


Sent from my iPhone using Rocketry Forum
 
Invest in a remote altimeter like a Quantum or a wireless switch. It makes ground testing so much easier. An alternative is use an empty motor casing and thread the wires through the empty motor from the charge well and put a charge holder there to test the sustainer charge with your launch system.
For the main charge, as long as you have a static port that's big enough to put shooter's wire (or any 22 or 24 ga. wire through) you can twist the leads to
your main charge terminal and run the wire out the static port for the ground test.

Being able to put your flight electronics directly in the rocket really makes it easy to ground test if it has a wireless remote activation. The Quantum, TRS and others have this option and some remote switches can be temporarily mounted for a test. Kurt
 
I have been wondering about this as well since I'm building my first dual deploy rocket. One other thing I was curious about is how does one accurately measure out the BP? I have a scale that I use for weighing parts and completed rockets but it isn't quite accurate enough to weigh out a single gram of BP.
 
1. Use FFFFg powder. The smallest grain stuff gives the best results.
2. Look up any powder scales from Amazon. Try to get one with a weigh boat. That makes it handy. Here's a good, affordable example: https://www.amazon.com/dp/B002BDOHNA/?tag=skimlinks_replacement-20
3. A popular charge calculator is: https://www.rimworld.com/nassarocketry/tools/chargecalc/index.html
4. GROUND TEST! It will give you the confidence that whatever you measure out will be the right amount of BP.
5. If possible, have a backup charge, and give it +20% BP more than what you determined works best in ground testing. It's your, "Oh !@#$" charge.
 
Last edited by a moderator:
I use a set of Lee Powder Dippers https://www.midwayusa.com/product/101284828/lee-improved-powder-measure-kit
Which measure volumetrically, however when used with a scale the .3cc measure is pretty close to .3 grams of BP and the others are pretty close as well, at least close enough that I no longer use the scale ( dipping technique does play a factor in charge weight, and I always card the mound of powder off the top to be level with the top of the dipper).

As others have said GROUND TEST, GROUND TEST, GROUND TEST!
 
I just use the tubes that come in the aerotech reloads. Can measure up to 2 grams at a time and if you always use the same powder it will keep the results fairly accurate. If you have an android phone I just release an update to my app that has a calculator for black powder changes. But as memtioned before an calculation is just a starting point and you should always ground test before putting a bird in the air. You can get the app at https://play.google.com/store/apps/details?id=com.redrobotit.rocketcalculator
 
Second on Lee Powder Spoons. I never weight anything. 1cc = 1g to within a very small tolerance.

As others have said, no calculator can know everything there is to know about your rocket. Run a calc., ground test multiple times until you hone it in. For a drogue, you just need separation and to get the laundry out. For the main, you will want slightly more separation to leave room for the chute to expand; but you don't want to stress the harness and cause the NC to snap back into the airframe. There's also shear pin breakage to consider.
 
For containment, I generally use my 1.5 ml Eppendorf tubes. They hold about 2 g of BP. Access to labs = limitless free tubes. :D

People have all sorts of different setups, but here's what I do:

Deployment Cannister.jpg

1. drill a hole into the bottom of the Eppendorf tube
2. pull your igniter through it until the head is at the bottom
3. apply a drop of hot glue to the outside bottom to seal up the hole and hold the igniter wire in place
4. add BP
5. add enough cellulose insulation to pack down the BP tight and prevent it from moving away from the igniter head
6. some like to tape the lid shut, but I find the tubes slap closed plenty tight for me
 
One thing to remember, the method you use to pack your charges will affect the performance of your charge. If you put the powder loosely into a glove fingertip, it will not be anywhere near as powerful as packing it tightly into a charge well and taping it tightly shut.

Pick a method and stick with it as long as it works. :)
 
Thanks! Couple more questions now. As I use the BP calculator on NASSA, it asks for length, I have NC with some space in it to the bulkhead, measured that, and in the BT do I measure to the centering ring or top of the MMT? as there is a few inches, but I figure that area around the engine will be packed with shock cord. There is about 4" in length between top of MMT and centering ring. Do I just figure it as is, empty, or with all the shock cord, and parachute and Nomex?

Thanks again for the help!
 
Ignore the shock cord and chute, basically you are looking at theempty space. I would use top of CR to next solid bulkhead as length, ignoring any motor tube sticking through the CR.
 
Okay. So I have 5.5" diameter by 17" length and say 7psi for approximately 200 pounds of force = 1.46 grams of BP and use 4x 2-56 nylon screws. So a beginning spot for me to work with. Well time to load it and see if I can get it to work. :)
 
I use a set of Lee Powder Dippers https://www.midwayusa.com/product/101284828/lee-improved-powder-measure-kit
Which measure volumetrically, however when used with a scale the .3cc measure is pretty close to .3 grams of BP and the others are pretty close as well, at least close enough that I no longer use the scale ( dipping technique does play a factor in charge weight, and I always card the mound of powder off the top to be level with the top of the dipper).

As others have said GROUND TEST, GROUND TEST, GROUND TEST!

I use these as well.


Sent from my iPhone using Rocketry Forum
 
Ignore the shock cord and chute, basically you are looking at theempty space. I would use top of CR to next solid bulkhead as length, ignoring any motor tube sticking through the CR.

+1

Also to consider is the location of the eMatch. If it is under the BP it can cause the unburnt BP to disperse before igniting to some extent. I usually put the eMatch above the BP in the well.
 
Keep in mind that an Aerotech 29mm or 38mm reload comes with about 1.4g. 24mm reload includes about 0.75g. If you have past experience with motor ejections, then those numbers are good reference points for your ground testing.

I have a little red cap (looks like an Aerotech hobby reload red cap) with a coffee stirrer handle that came with a set of flashbulb igniters from back in the day. This holds one gram of powder and is all I ever used. I just eyeball 0.25 gram increments in the cap. No need to be super precise. Also, reloads now come with powder in a graduated centrifuge vial. 1cc = 1g is a good enough assumption.
 
+1

Also to consider is the location of the eMatch. If it is under the BP it can cause the unburnt BP to disperse before igniting to some extent. I usually put the eMatch above the BP in the well.

Interesting... so would that mean you would recommend that if one was using the Eppendorf tube method as mccordmw was suggesting, that the e-match would be at the top of the BP if you used such a method? Or were you referring to a more cylindrical charge well?

I'm paying attention as I can foresee a time in the next few months where my thinking and planning will be progressing from the MPR and entry level HPR builds I'm currently doing towards the beginnings of DD entry level HPR.. (long way to go, but I'm hoovering up learnings as I progress...)
 
I use a blackpowder measure. Remember, the measure is in grains and not grams if you use one of these. You need to do a conversion. I was shown a really simple way to make a charge holder. Simply roll paper up into a tube of the appropriate size. Feed in an igniter. Fold the bottom over the wire and tape it up with masking tape. Pour the BP into the tube. Fold it over and use masking tape over it all. Very fast and effective. It worked consistently for me at a launch this summer.
bpmeasure.jpg
 
Last edited:
I use a blackpowder measure. Remember, the measure is in grains and not grams if you use one of these. You need to do a conversion. I was shown a really simple way to make a charge holder. Simply roll paper up into a tube of the appropriate size. Feed in an igniter. Fold the bottom over the wire and tape it up with masking tape. Pour the BP into the tube. Fold it over and use masking tape over it all. Very fast and effective. It worked consistently for me at a launch this summer.

I used a similar method years back when I was using xmas tree bulbs for lighting the BP. I twisted two bulbs together for redundancy and hot glued them into a paper tube. Added powder, folded over and taped. It does work very well.

Since I went exclusively to e-matches, I just use copper pipe caps as charge wells and that's been very good and consistent too. You don't need to over think this. You just need a single method that gives consistent results. It really doesn't matter which method you use, as long as it opens the rocket and gets your drogue or main into the air stream so they can open and pull everything into alignment on the way down.
 
Ground test went well. 1.46g of BP deployed the NC about 6-8 feet across the lawn. Did use the little lab tube design that Mccordmw picture showed, worked great. Thank you all for the help and input. Now to work on the next part.
 
Great to hear. A handy tip is also to write the amount of BP on the side of the tube. That way, if you make any up ahead of time, you won't get them mixed up with others. I usually have a primary charge and a backup at +20% more BP.
 
Great to hear. A handy tip is also to write the amount of BP on the side of the tube. That way, if you make any up ahead of time, you won't get them mixed up with others. I usually have a primary charge and a backup at +20% more BP.

Yeah, once you have 4 or 5 DD rockets, the numbers start to run together. You can write them on the bulkhead, but be careful that they don't get stained over with burnt BP.
 
I always measure the length of open tube to the top, not the area between the bulk plate and the nosecone or second bulk plate. You want to pressurize the entire empty area to make sure you have enough power to completely separate the pieces.

I also use the Rocket Calc app on the iPhone. It has a BP calculator that will tell you how many shear pins your charge will break, in either 2-56, 4-40, or 6-32 sizes. It makes it really easy to figure some margin of safety. I think most folks tend to use too much BP, myself included, because we are all used to the fact that many reloads come with 1 gram. Seeing how many screws the charge will break makes it a lot easier to dial in the amount. Of course things like nosecone weight also come into play.

For best results and especially for repeatability BP should be contained in a tube (think of a barrel). When the powder is ejected from the tube it will be directed into the flame front so the grains are much more likely to ignite. If it's in something like a glove tip or latex tubing it's likely some of the grains will not ignite as they are blown in random directions. Jim Jarvis did some excellent testing that shows that using a tube greatly improves the effectiveness of BP as an ejection charge.

Just something to think about.


Tony
 
Another question about BP charges: As I am using the NASSA calc and Rocket Calculator app on iPhone, as comparison, it asks for PSI: How do you determine what is a good PSI pressure for your rocket? Current input for charge is 4" and 38" in length, is 100 pounds of force enough or 150, I was leaning towards the 150 as I would like to ensure deployment. How much pressure can the fiberglass tube handle to failure? Though I know the sheer pins should work prior to reaching tube failure...

Thanks for the help and input!
 
We need to know the number and size of the shear pins.

Typically, people use 3. If only one is used the coupler/cone can "cock" and jam.

I believe a #2 nylon screw has about 30~35# of shear and #4 are in the 50~55# range.

So if you have 3 of the #2 screws you need at least 100# to ensure they will shear
 
Another question about BP charges: As I am using the NASSA calc and Rocket Calculator app on iPhone, as comparison, it asks for PSI: How do you determine what is a good PSI pressure for your rocket? Current input for charge is 4" and 38" in length, is 100 pounds of force enough or 150, I was leaning towards the 150 as I would like to ensure deployment. How much pressure can the fiberglass tube handle to failure? Though I know the sheer pins should work prior to reaching tube failure...

Thanks for the help and input!

Multiply the number of shear pins times the shear force required to break each pin. Then see what pressure is required in order to apply that amount of force to the bulkplate or nosecone base. Divide the force by the area of the bulkplate or base to determine the pressure. Now double that pressure (or even triple it) to provide a safety margin.
So, let’s say it requires 25 lbs. to shear one pin and you have three pins. That means the minimum total force needed must exceed 75 lbs.
Now let’s say that the area is 10 square inches. (I’m just picking easy numbers to illustrate the math; you have to use real numbers).
75 lbs. divided by 10 square inches gives 7.5 lbs.per square inch (psi). If you triple that in order to give yourself a safety factor, that would mean you need 22.5 psi. But, there’s something else to consider. That’s just the pressure needed to shear the pins. You also want to accelerate the nosecone or av-bay fast enough to pull the parachute out.
You can calculate that using Force = mass x acceleration or you can simply start with the amount of powder needed to pressurize it as above and then ground test until it looks right. The safety factor may result in plenty of acceleration.
The important thing is that you probably don’t need (or even want) 100 psi.


Steve Shannon
 
Back
Top