Does method of BP deployment charge containment/construction matter?

For clarification, what I am asking is if the way you make or configure your BP deployment charges matters in terms of the separation force or pressure created? So, if I use the same amount of BP in a charge cup, glove finger tip, micro-vial, surgical tubing, etc, should the result be the same? Assumptions for the sake of this discussion would include a constant for the volume of airframe to pressurize, the charges are done correctly so all the BP burns, and this is for low altitude flights.

Well, from what I've read the theories abound.

This is my take. I come down to Newton's Third Law, for every action there is an equal and opposite reaction. Therefore I make charge cups that face the direction I want it to go. I want the mass/gas products going in a certain direction, namely down the direction of the airframe.



:2:

Greg

Use a cup or well or the charge might blow out the side of a thin walled rocket. Sometimes a screw on the bulkhead with zip ties for the disposable
plastic canister works. Kurt

BP works by combusting (quickly) into a hot, high-pressure gas. The pressure is what causes the rocket to separate. The nice thing about pressure is that it acts with the same magnitude in all directions. Sure, there may initially be a component of dynamic pressure, which is unidirectional, but the omnidirectional static pressure is what will separate the rocket. It's not really a useful exercise in my opinion to try to direct the BP. It works the same in a cup as it does in a glove. On the other hand, I do like that cups mount directly into bulkheads and can be easily reused with less preparation.

Somewhat tangental to the question being asked, but last high power launch I saw a BP well act like a shaped charge -- it blew backwards into the av-bay and wrecked the altimeter.

Zeroignite said:

Somewhat tangental to the question being asked, but last high power launch I saw a BP well act like a shaped charge -- it blew backwards into the av-bay and wrecked the altimeter.

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(Assuming nose cone. Substitute "payload tube" if necessary.)

I could only see that happening if the nose cone were jammed in so tightly that it couldn't be expelled. Like electricity, the explosion will always take the "easiest way out," so to speak. I don't know the physics terms, but the only way I could see the charge blowing back into the AV bay is if the AV bay was applying less force against the explosion that the nose cone was.

Think of a bullet. The only reason the bullet comes out of the shell is because it is applying less force against the explosion than the barrel and the firing pin.

Someone who can explain this correctly please help me out!

And put the drogue charge down below the parachute.Top charge mounted somewhere near top of AV bay

kjkcolorado said:

what I am asking is if the way you make or configure your BP deployment charges matters in terms of the separation force or pressure .

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What i think matters the most is the amount of BP that ignites. If the charge is packed loosely, only part of BP may ignite and result in less pressure than expected.



-Alex

azzie said:

What i think matters the most is the amount of BP that ignites. If the charge is packed loosely, only part of BP may ignite and result in less pressure than expected.



-Alex

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I've never found unburnt BP in any of my rockets. I have found that containing the BP in a cup with dog barf and masking tape instead of just folding duct tape over the powder and match cause a stronger ejection force when using the same amount of BP.

Your charge container is just a gas generator. You want to generate as much gas as quickly as possible with out cooking your chute. Directing a standard charge cup is about like directing a Roman candle, its just a question of what your going to burn. If instead you compress that powder and hold on to it a bit to build pressure, it is a whole different story. The attached picture shows my charge cups that I use. In one of these cups I have to cut the BP with slow burn powder to keep from blowing up the rocket. I use 1/2 gram of BP to separate the nosecone on a 5" fiberglass rocket. Any method for holding on to the powder and building a little pressure will suffice. BEWARE, hold on too tight and the charge will simply explode.

The charge cup shown below is simply a cup with a hole in the side for an ematch, the top of the cup is basically a spring loaded check valve.





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azzie said:

What i think matters the most is the amount of BP that ignites. If the charge is packed loosely, only part of BP may ignite and result in less pressure than expected.



-Alex

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I am far from an expert, but from what I've heard I agree with Alex. If the powder is loose it will burn more slowly than if it is tightly packed. Just lay some BP out on the ground (no containment) and ignite it. It will just burn. Now tightly contain it and ignite it - BOOM

My thoughts are that the method of containment will matter. Say you use the finger from a thin latex glove, I would think there is a chance the glove material could rupture as the initial powder is igniting causing the powder to start dispersing and reduce the force. Some of the powder becomes loose and just burns.
Where if you use tubing which is thicker and stronger it is less likely to rupture prematurely and give you more force - more BOOM

Just my 2 cents worth....

There are lots of differant ways to make an ejection charge, and some may take more or less powder to do the same job. Use the way that pleases you, ground test, and stay consistent with the method you choose.

les said:

My thoughts are that the method of containment will matter. Say you use the finger from a thin latex glove, I would think there is a chance the glove material could rupture as the initial powder is igniting causing the powder to start dispersing and reduce the force. Some of the powder becomes loose and just burns.
Where if you use tubing which is thicker and stronger it is less likely to rupture prematurely and give you more force - more BOOM

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I've not done any tests with the packing of BP, but here is a thought experiment I just ran through in my head:
-Assume the black powder is tightly packed (ie - no space between grains). The combustion of the charge begins from the outside and burns away at the charge in infinitesimal shells, kind of like an end-burner motor except there are no inhibited surfaces. The combustible surface area of this charge is equal to the surface area of the global charge shape.
-Assume the black powder is loosely packed (ie - finite space between some or all of grains). In this case, the combustible area is somewhere between the global charge shape surface area and the sum of the surface areas of each grain. In any case, we can safely claim that the surface area here is larger, presumably much larger.

I would imagine that the increased burn rate of case 2 gives you a shorter, but possibly more powerful punch. But maybe I'm wrong on this one.

Thank you for all the feedback. I'm just getting ready to start making my own charges and am still undecided as to what method. I think the reply by rcktnut/Jeff is very helpful to me (not that I didn't learn from all the discussion). I need to decide on how I want to build my charges, do it consistently the same way, and test adequately. Right?

kjkcolorado said:

Thank you for all the feedback. I'm just getting ready to start making my own charges and am still undecided as to what method. I think the reply by rcktnut/Jeff is very helpful to me (not that I didn't learn from all the discussion). I need to decide on how I want to build my charges, do it consistently the same way, and test adequately. Right?

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Exactly. The discussion has been very good and illustrates that there are many valid ways to go about preparing charges. I've even found that for different airframe geometries, I prefer different methods. In ground tests we trust.

kjkcolorado said:

Thank you for all the feedback. I'm just getting ready to start making my own charges and am still undecided as to what method. I think the reply by rcktnut/Jeff is very helpful to me (not that I didn't learn from all the discussion). I need to decide on how I want to build my charges, do it consistently the same way, and test adequately. Right?

Click to expand...

Ground test and then ground test some more...

Just make sure what ever method you choose, consider the orientation of the charge. You may ground test horizontal and then find that your charge lets the powder stray from the igniter when inverted or in some other orientation. In general, if you have enough BP it's going to separate. But if the powder has moved away from the igniter and the igniter isn't powerful enough to make up the gap you may have an issue. Good luck.


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Short answer: as long as it's tightly sealed so it can't lose contact with the e-match, it doesn't much matter.

Long answer: it still doesn't matter too much. Specialized cases where all the BP needs to burn near simultaneously (hi-alt) can be an issue, but if you just enlarge the charge the pressure difference will still blow the top.

I did my L3 in a 11.5"x3' tube, and most of it was filled with dog barf wadding. Put the charges (surgical glove fingers) at the base of the bay, covered in a foot of DB, and the nosecone sheared through the pins and popped perfectly.