Black Powder Charge Calculations Help

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mandbn

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I have a formula for the amount of black powder that should be used to reject my nosecone, but the result I am getting is oddly low (0.3 grams) and it is because the amount of pressure (psi) I calculated to eject the nosecone is coming out to about .5 psi, which is definitely wrong. It is a 4 inch diameter tube, and I know the area calculations are all correct. However, when testing the force needed to pull off the nosecone with a spring scale, it is coming out to about 2500 grams, and when I flip it upside down and shake it, the nosecone slowly loosens up. If I shake it violently enough, it will fully fall out. Is the spring scale wrong, or do I need to fix my psi calculations? What am I missing, possibly? I am dividing the force needed to pull off the nosecone by the area of the circle made by the inside of my airframe. Please help. I checked and all the units are correct.
 
A 4" tube has a relatively large cross-section surface area for the pressure to act on. If you don't have a long parachute bay for a 4" airframe, your minimum calculations would show very small amounts of powder required. 10psi (69 kPa) on a 4" bulkhead/nose shoulder would give you ~126 lbs of ejection force (~560 N or ~57 kg on your springscale [which is displaying in units of mass])

To get 10 psi in a 4" diam by 6" payload bay volume, the calculator I use says you'd only need .38 grams. For practical purposes, I'd probably up this to .5 gr with a backup of .75 to 1 gram.

Consideration: Look at the ejection charge sizes of commercial 38mm motors. They have around 1.3 to 1.6 grams of blackpowder and generally produce a very forceful ejection in that size bay volume.
 
Don't forget that the burn rate for BP is strongly pressure dependent, so the charge energy varies quite a bit according to your preferred packing / containment method. When I switched from glove fingers to canisters packed with dogbarf + tape, I found I needed 30-40% less BP. So ground test is really essential. My experience has been that calculators tend to suggest numbers that are somewhat too low to have enough margin for sticky nose cones, oversized vent holes, etc.. If your target altitude is quite high (20kft+), you need even more confinement to get a complete burn.
 
I've found this one to be the most accurate BP calculator I've found.

http://www.rockethead.net/black_powder_calculator.htm
First calculate the appropriate PSI required using the lower section. You be given two recommended pressures in PSI, the lower value for a loose-fitting chute and a higher one for a tight-fitting chute.

Then plug the appropriate recommended PSI into the upper calculator along with the length and diameter of the recovery compartment.

The link on the page to the underlying formulae are dead, but the results I've gotten from it have always proven dead-on accurate.

But always do a ground test to verify your solution.
 
Does anyone know how much force it should take to pull off the nose cone? Because the calculator said I should use 12.6 PSI, but that equates to 150 pounds of force, which I don't think I need, since it currently only takes about 5.5 pounds of force to pull off the nosecone, according to my spring scale. How tight should my nose cone be? Is it bad that when I shake it upside down, it starts loosening out slowly?
 
To some small degree, the ejection charge will expand the airframe and loosen the fit of the nosecone while pushing it out. This will depend on materials, of course. Cardboard and Blue Tube-constructed air frames will likely expand under pressure more than phenolic, fiberglass, or carbon fiber tubes.

Unless you're using a piston, the nose cone/payload section will also need to blow off with sufficient force and inertia to pull out the recovery gear, too. That's why I'm a strong advocate for doing a ground test before your new rocket is flown.

If the shake test takes a few tries to release the nosecone or payload section, it's probably got about the right balance of tension and releasability, in my opinion. Be aware that if the rocket range temperature or sunlight intensity changes significantly between prep and flight, the fit may be quite a bit looser, or tighter, than it was when tested, due to differing material expansion rates.

Good skies,
GlueckAuf
 
GlueckAuf said:
To some small degree, the ejection charge will expand the airframe and loosen the fit of the nosecone while pushing it out. This will depend on materials, of course. Cardboard and Blue Tube-constructed air frames will likely expand under pressure more than phenolic, fiberglass, or carbon fiber tubes.

Unless you're using a piston, the nose cone/payload section will also need to blow off with sufficient force and inertia to pull out the recovery gear, too. That's why I'm a strong advocate for doing a ground test before your new rocket is flown.

If the shake test takes a few tries to release the nosecone or payload section, it's probably got about the right balance of tension and releasability, in my opinion. Be aware that if the rocket range temperature or sunlight intensity changes significantly between prep and flight, the fit may be quite a bit looser, or tighter, than it was when tested, due to differing material expansion rates.

Good skies,
GlueckAuf
Click to expand...
Should I use shearpins? If so how many, or how do I figure out how many? What kind? Is it possible that the nosecone will pop out too early while the rocket is going up or falling, before it is supposed to, because this is where my main chute is housed, so it will fall quite a ways under the drogue before deploying this one?
 
I can't opine on shear pins, because my max 75mm rockets work fine with only a friction fit. I do know it takes some careful construction using threaded brass sheet "blades" or the like to do the shearing, otherwise they become "rip-out" pins and may damage the tapped holes they mount in with every flight.

If your rocket's air frame is well-sealed, you'll be wise to vent the air frame with a tiny hole or two to prevent a pop-off of the nosecone/payload section during ascent due to the rocket's higher, ground-level internal pressure vs the lowering external air pressure, especially with high altitude flights, say 2000 feet or more.

Sorry, bud, I don't understand the last half of your last sentence. Can you clarify? Thanks.

Good skies,
GlueckAuf
 
Many of the ejection charge calculators will include the required number of shear pins, if used. Typically, I would use 2-56 (~2mm) or 4-40 (~3mm) nylon screws, but it depends on your specific requirements. The only reliable way to find out is to perform deployment tests.
 
mandbn said:
Should I use shearpins? If so how many, or how do I figure out how many? What kind? Is it possible that the nosecone will pop out too early while the rocket is going up or falling, before it is supposed to, because this is where my main chute is housed, so it will fall quite a ways under the drogue before deploying this one?
Click to expand...

Or tape.
 
Is it for single or dual deploy? If single you don't really need shear pins, but using them will give you a much more consistent ejection force requirement. If dual you need them for the main but not really for the drogue.

The most important thing, though, as other people have said: ground test. I find I typically need quite a bit more black powder than the calculators say.
 
JoePfeiffer said:
Is it for single or dual deploy? If single you don't really need shear pins, but using them will give you a much more consistent ejection force requirement. If dual you need them for the main but not really for the drogue.

The most important thing, though, as other people have said: ground test. I find I typically need quite a bit more black powder than the calculators say.
Click to expand...

Good point. Since he was calculating, I assumed it was dual deploy.
 
Okay, is it bad that my nosecone comes off with just 5.5 pounds of force? I feel like it should be a much higher number given that the inner diameter is 3.9 inches.
cwbullet said:
Good point. Since he was calculating, I assumed it was dual deploy.
Click to expand...
It is dual deploy. First, the rocket will split in half at apogee, 5280 feet, deploying the drogue, then at 700 feet it will shoot out the nosecone, deploying the main parachute. How much force should it require to pop out the nosecone? From that, I can calculate PSI. I just want to know how much force would be safe enough to avoid a premature deployment. Also, would there be any concern that the drogue might deploy early, or would that not be a risk since it is going up and there shouldn't be any forces that might pull it apart?
 
How much force should it require to pop out the nosecone? From that, I can calculate PSI. The BP calculators many here have recommended will tell you the recommended PSI as well as the amount of 4Fg required to achieve that pressure. No one has ever recommended to me that "pull-out force" like you'd do to manually remove a nose cone has any relevance to DD, only PSI.

I just want to know how much force would be safe enough to avoid a premature deployment. The altimeter will take care of that. The DROGUE will fire at apogee, and the MAIN will fire at whatever altitude you select. If you're worried that the excessive shock of the DROGUE event might cause the MAIN's parachute to yank out as well, that is a risk if the nosecone is too a loose fit. Do a shake test to confirm sufficient friction, and you should have no worries in this regard.

Also, would there be any concern that the drogue might deploy early, or would that not be a risk since it is going up and there shouldn't be any forces that might pull it apart? Drag separation at burnout is a risk if the DROGUE section's friction fit is too loose. What happens is that at burnout, the fin section experiences higher drag than the sleeker upper section(s), and if they're too loose, they might separate before the altimeter fires the charges. Once again, a shake test will help confirm adequate friction, and MOST IMPORTANTLY, the ground test will confirm, as closely as one can short of an actual flight, just how properly all the components do their respective jobs.

I hope this answers your questions. Best of luck!

Good skies,

GlueckAuf
 
GlueckAuf said:
How much force should it require to pop out the nosecone? From that, I can calculate PSI. The BP calculators many here have recommended will tell you the recommended PSI as well as the amount of 4Fg required to achieve that pressure. No one has ever recommended to me that "pull-out force" like you'd do to manually remove a nose cone has any relevance to DD, only PSI.

I just want to know how much force would be safe enough to avoid a premature deployment. The altimeter will take care of that. The DROGUE will fire at apogee, and the MAIN will fire at whatever altitude you select. If you're worried that the excessive shock of the DROGUE event might cause the MAIN's parachute to yank out as well, that is a risk if the nosecone is too a loose fit. Do a shake test to confirm sufficient friction, and you should have no worries in this regard.

Also, would there be any concern that the drogue might deploy early, or would that not be a risk since it is going up and there shouldn't be any forces that might pull it apart? Drag separation at burnout is a risk if the DROGUE section's friction fit is too loose. What happens is that at burnout, the fin section experiences higher drag than the sleeker upper section(s), and if they're too loose, they might separate before the altimeter fires the charges. Once again, a shake test will help confirm adequate friction, and MOST IMPORTANTLY, the ground test will confirm, as closely as one can short of an actual flight, just how properly all the components do their respective jobs.

I hope this answers your questions. Best of luck!

Good skies,

GlueckAuf
Click to expand...

Okay, so do you think we will need shear pins for the drogue chute as well, just to be safe?
 
mandbn said:
Okay, so do you think we will need shear pins for the drogue chute as well, just to be safe?
Click to expand...
Pins for the initial apogee separation is rarely necessary to prevent drag separation unless you have a ULTRA low tube engagement surface (most recommend 1 caliber), or ULTRA low friction at this joint, or you have a very violent burn out.

Test it like you would test the nose cone. Full up ready to fly, lift by the payload bay and shake. If it doesn't come apart, you're probably OK, as long as it comes apart easily with the deployment charge that you GROUND TEST. The tightness of the fit here, like the nose cone, is often tuned with pieces of tape and may vary with the conditions of the day due to humidity and temperature variances.

The one sure thing that will tell you more than anything else: GROUND TEST!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
 
Last edited:
I know this is kind of an older thread but I have a question. One of the calculators I've been using online for the gunpowder calculation asks what PSI I want to test at. Is that the PSI of the braking strength of the shear pin? That's values that then it's saying like 2000 g of gunpowder which I know is way too much. So what PSI is it referring to?
 
So for two shear pins of number two size says it's average is 50 lb of force and 25 lb each pin.
MarsLander said:
PSI is the pressure in the vessel. A good calculator should also tell you the force on the bulkhead.

This calculator:
https://rocketrycalculator.com/rocketry-calculator/bp-estimator/

Will give you the PSI and the force on the bulkhead. It is the force on the bulkhead you want to use with the # and size of shear pins.

This is what I use: (https://web.archive.org/web/2013102...rials.org/datastore/cord/Shear_Pins/index.php)

View attachment 635108
Click to expand...
Hmm, interesting. This is where things get a little muddled for me. I have the same chart, and if i calculate the shear stress needed to break a #2 nylon shear pin with a 0.08 inch diameter, using the appropriate shear strength of the material, i get 5000 PSI.

If i disregard the material properties, i still get 5000PSI since shear stress is T=F/A

so T = 25lbs/0.005 in^2

BUT if i add the 5000PSi to the calculator, you can imagine how much gunpowder i need hahaha

Do these calculations for amount of gunpowder take into consideration the material properties of the shear pins and the shear stress needed to break them?

Thanks for your help and information. I appreciate it : )

EDIT: I am re-reading your post and you mentioned the force on the bulkhead is what i need to look at? So if i follow correctly, the 50 lbs it takes to break 2 pins is 50 lbs on the bulk head correct?
 
Last edited:
Lt72884 said:
So for two shear pins of number two size says it's average is 50 lb of force and 25 lb each pin.

Hmm, interesting. This is where things get a little muddled for me. I have the same chart, and if i calculate the shear stress needed to break a #2 nylon shear pin with a 0.08 inch diameter, using the appropriate shear strength of the material, i get 5000 PSI.

If i disregard the material properties, i still get 5000PSI since shear stress is T=F/A

so T = 25lbs/0.005 in^2

BUT if i add the 5000PSi to the calculator, you can imagine how much gunpowder i need hahaha

Do these calculations for amount of gunpowder take into consideration the material properties of the shear pins and the shear stress needed to break them?

Thanks for your help and information. I appreciate it : )

EDIT: I am re-reading your post and you mentioned the force on the bulkhead is what i need to look at? So if i follow correctly, the 50 lbs it takes to break 2 pins is 50 lbs on the bulk head correct?
Click to expand...
Pressure in the compartment pushes on the bulkhead with some force, based on the area of the bulkhead. The bulkhead pushes on the shear pins with that force, but it's applying the force over a tiny area so the resulting pressure is huge. The pressure in the compartment is not the pressure cutting the pin.

Just like pushing lightly on a sharp knife puts a huge pressure on the edge, enough to slice right through a carrot 🥕.
 
Last edited:
Azamiryou said:
Pressure in the compartment

Pressure in the compartment pushes on the bulkhead with some force, based on the area of the bulkhead. The bulkhead pushes on the shear pins with that force, but it's applying the force over a tiny area so the resulting pressure is huge. The pressure in the compartment is not the pressure cutting the pin.

Just like pushing lightly on a sharp knife puts a huge pressure on the edge, enough to slice right through a carrot 🥕.
Click to expand...
LOL, this is exactly what i was just getting ready to type, well something close to it haha. I found the formulas i needed for the BP charges and figured out that its the pressure on the bulkhead, as stated above by the other user, that creates enough pressure to shear the pins at the 5000PSI at that point on the pin itself, not the entire airframe.

thanks for this
 
Important to note: those results are just a starting point. My experience has pretty much always been that the calculated powder charge will shear the pins, but not do much more. Perform ground testing, and keep increasing the BP until the sections separate with "authority". They don't have to pull all the way to the end of the shock cord, but you should be in no doubt that the laundry will get ejected in flight,
 
JoePfeiffer said:
Important to note: those results are just a starting point. My experience has pretty much always been that the calculated powder charge will shear the pins, but not do much more. Perform ground testing, and keep increasing the BP until the sections separate with "authority". They don't have to pull all the way to the end of the shock cord, but you should be in no doubt that the laundry will get ejected in flight,
Click to expand...
Agreed. Last year i helped with the ground testing of a rocket a rocket and thats how it worked out. Found a starting point and increased until at least 3\4ths of everything was out.
 

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