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Rich Holmes 01-10-2014 10:59 AM

BP motor ejection pressure
There probably are others, but here is an ejection charge calculator; you can use it to calculate how much powder you need to get a given pressure, or conversely how much pressure you get from a given amount of powder, depending on the volume of the airframe needing to be pressurized.

My question is, how can you do the latter for BP motor ejection charges? Presumably it would be similar, but I don't know how much of what kind of charge is used in the various BP motors; also, I don't know how differences in construction between BP and composite motors would come into play.

Anyone have information on this? I'm interested in working out some of the physics behind ejection and its implications for shock cords.

Jerry Irvine 01-10-2014 11:13 AM

Our 24mm and 29mm SU motors used a bit over 1/4g of 3F BP and reliably ejected 4" x 18" sections.


Rich Holmes 01-13-2014 03:41 PM

So no one knows?

Jerry Irvine 01-13-2014 05:46 PM

You could make a fixture to test it with a pressure vs time trace with a known exit port size. Some of the HPR electronics accept a serial sensor so you could repurpose an otherwise rocket object to perform the test.

Then publish it in SR.

Model rocketry has been around a long time, but not every question has been answered.


"Model rocketry is too diverse for the Estes catalog." - Jerry Irvine

shockwaveriderz 01-13-2014 09:49 PM

Are you asking what size of black powder is used in Estes Black Powder rocket motors versus the BP used in Composite Ejection charges? Or are you also asking the amount used by the various BP motors, depending on their size, 13/18/24/29 millimeter.

From my taking Estes BP motors apart it looks to be 3F in size. I could be wrong. Most composite BP ejection charge powder I believe is 4F. The only difference in burn characteristics between 3F and 4F, or for that matter 2F etc is burn rate. Smaller particle BP(4F) has a faster burn rate than 3F. Pressed BP versus powder BP also burns at slower rates.

The gas constant of 4F BP versus any other size BP is independent of grain size.

Hope this helps.

Terry Dean

Jerry Irvine 01-13-2014 09:50 PM

Errortech uses 4F.

BP has a flat burning rate vs pressure curve! 4F has more surface but the burning rate is the same.

It may pressurize a vessel differently. That was the original question.

Tech Jerry

Rich Holmes 01-13-2014 10:34 PM

What I'd like to know basically is: How much pressure is exerted on a nose cone given the volume of the rocket (or rather the portion that gets pressurized) and the type of BP motor? Ideally for a variety of motor sizes. Knowing how that pressure varies with time would be even better, though that probably gets too complicated.

So for example, for HPR, I can go to and put in 4" diameter, 10" length, 1 g 4F, hit "Calculate pressure", and it tells me 15.42 PSI.

(That page makes things a little too complicated with its table of values of C; really the relation is just

P = M / (0.0004 * D^2 * L)

so here M = 1 g, D = 4, L = 10 and you can just plug those in to get P.)

I'd like to be able to do the same for BP motors, but doing that requires knowing the quantity of charge in a given motor, and whether the constant 0.0004 is appropriate for BP motor charges too, or should be something different. (Though I'm not looking for any great precision, so maybe assuming I can use 0.0004 is good enough; then all I need is the sizes of the charges for various motors.)

Jerry Irvine 01-14-2014 06:04 AM

NAR S&T publishes motor propellant masses for all certified motors.

Rich Holmes 01-14-2014 06:32 AM

Originally Posted by Jerry Irvine
NAR S&T publishes motor propellant masses for all certified motors.

Yes. But not as far as I can see ejection charge masses.

Jerry Irvine 01-14-2014 07:11 AM

Originally Posted by Rich Holmes
Yes. But not as far as I can see ejection charge masses.
The initial mass minus the burnout mass minus the propellant mass = the delay and ejection mass. But you are correct. I would estimate most BP ejections to be under 1/4 gram. That is actually too big for most smaller diameter model rockets and results in recovery system failures.

If mini motor ejections are over 1/8 gram they are too large. They are also of low accuracy. I feel from personal experience they can vary in mass 30% or so.

USR used 1/4g 3F BP for C-F motors and that was suitable for rockets 1" - 2.7" diameter and certainly lengths up to 18".


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