Nozzle question

Look at the igniter plugs, they say what size motor they fit. O.k., I just looked at some. A8 & B4, B6 & C6, B8 & C5.

Yes. For Estes:

B6 and C6 are identical and take purple plugs.

The fascinating things occur in the 24mm diameter motors. Some have identical nozzles but the centerbore of the propellant can be slightly different depth. So two motors may use the same plugs but they have different peak thrusts and slightly different sustaining thrust.

C11 and E9 use black plugs.

D11 and D12 use White plugs.

Compare the thrust curves. Look on the NAR website.

-Fred Shecter NAR 20117

don't forget that as far as BP motors are concerned, they also have "molded" (actually pressure pressed during assembly) combustion chambers above the nozzles...... the igniter plugs are slightly oversize and they squeeze the igniter up against and into this small combustion cavity......It is this small combustion cavity that results in the characteristic "intial spike" of thrust......pretty spiffy huh?

shockie B)

Nope.

The spike is a motor design feature. The plug does not add appreciably to the spike.

Compare Estes C6 and D12 thrust curves from NAR from before and after the plug era.

The plugs are really great BTW.

Jerry

jerry, He said "combustion cavity", not "plug".

Maybe you should work on getting that new brain as fast as possible.

Or, you could actually *read* a post first, before responding to them.

I did not say the igniter plug had anything to do with the spike. I said that the igniter plug held the igniter in place not only in the nozzle but also up in that small combustion cavity... And yes the spike is a motor design feature, but that's what I said.. and this combustion cavity does result in that characteristic spike as after the combustion cavity is consumed you have a neutral burning end burning motor....

shockie B)

and for those of you who would like more information about the design and function of combustion cavities see: Volume #5, Summer 1997 issue of Journal of Pyrotechnuics and the report entitled, " Model Rocket Motors, Theory and Design " heres the abstract:

"A semi-empirical theory is presented for the design of model rocket motors that use Black Powder for fuel. By choosing the values of a few adjustable parameters, a hobbyist can construct motors that perform satisfactorily without extensive or dangerous trial and error. Formulas are given for calcualting the nozzle diameter,the combustion cavity height,and such performance descriptors as specific impulse and average thrust for any size of model rocket motor"

I might add that Ed Brown, the Estes BP motor guru reviewed the report ........

It is the roughly hemispherical grain face formed when the initial cavity has regressed to the case walls that causes the spike, which then tapers off again into end burn as the walls burn out. But yes as I think you are saying, the geometry of that part of the burn is dependent on the initial geometry of course. Luckily it all happens quickly due to the rapid low pressure burn rate and combustion stability of BP, otherwise they'd wheeze off the pad then kick in full bore far too late.

Did you know that the old B14 cores were drilled, after the regular pressing process? That was apparently too hazardous (no, really?) and labor intensive an operation for continued mass production, thus the demise of the motor. If there were any other reasons I don't know what they were, this is what I was told. There were a couple of guys in the Toronto crowd back in the 70's that used to core out C's and D's for fun, and seemed to have working parameters for doing so.

Mike D.

Gone wrong, that could give a whole new definition to "fun".

-Kevin

I always wondered about that - looking at the motors, I couldn't see how else you would make such a thin core...

-dave w

thanks for that info Mike.....

if they use some form of mandrel to get the small combustion cavities, why not use one for the cored motors as well instead of dangerous drilling and the associated labor costs...would that be possible? I mean if you look at amateur rocketry books, they use this technique.

shockie B)

Essentially that's what they did with the B8 and C5. the mandrel was just a bit deeper than the other ones.

The B14 drilling probably took place right after pressing, while the propellant was still a bit damp, which probably made it safe enough. But the labor costs were probably getting to be prohibitive. (how many times can I use "probably" in this msg?)

From what I know about pressing BP motors (mostly picked up from reading posts here, on Arocket, and in David Sleeter's book), as the mandrel size gets smaller, the odds of successfully pulling it out of the core get worse. The very thin core in the B14 motors might be too small to be able to reliably extract a mandrel without damaging the motors.

Estes doesn't just have to worry about making motors; they have to worry about making motors *consistently*, and with a high confidence in them working as designed. "Iffy" production steps, like removing tiny mandrels from motors, just don't make economic sense.

- Rick "On a one-off basis, however..." Dickinson

The way to tell is if they use the same color plug. THe A8 and B4 do. The B8 has been out of production for a while, but probably matches as well.

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!

Did they have plugs back when they made A8-0's?

Yes, I agree, but I do have an old A8-0 which has a decidedly smaller nozzle than the current A8's/B4's. The A8-0 is definitely a thin wall, metric, modern motor, but it's obvious they changed the nozzles at some later time.

Bob,

You're slipping. The B8 is a C5 sibling, and uses the blue plug. Do you have the flu? It's not like you miss this :)

Doug

cavities, why

drilling and

Yes, I believe that is why they went from the B14 to the B8 - to use a die to form the core instead of drilling - I'm sure I'll be corrected if that's wrong. But even the shorter, thicker core of the B8 gave them production problems.

Dale Greene

Why doesn't somebody just call Vern and ask him?

His office phone (address 225 Main St, Canon City, CO) is in the phone book.

MJ

And the B8 was never worth a tinker's damn as a load lifter, either. I could never tell the difference between a B6 and a B8.

the flu? It's not like you miss this :)

Aaaaakkkk!

Bob Kaplow NAR # 18L TRA # "Impeach the TRA BoD" >>> To reply, remove the TRABoD!