Magnelite & E&F motor failures

Hi Guys,
Just had an interesting e-mail sent to me. This person had a rocket motor
F21-4w cato right at ignition. He was using an ML-12 that had been dipped with
very little pyrogen so as to fit inside the core. He reported that it appeared
as if the propellant had not even had a chance to burn. The bottom 1/3 of the
casing had blown apart.
Has anyone else had this happen in conjuction with our products? What is an
acceptable amount of clearence around the nozzle with the igniter installed so
that overpressurization does not occur? All comments and suggestions are
welcomed. Thanks for your input. Based on what I find out I may make a change
to the recommendation for motor application of the ML-12.
Greg D.
Rocketflite/Magnelite
Reply to
Rocketflt
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I use your product. I make sure that the wires will easily slide back out the nozzle. That means that when I use my own wires (lead wires and nichrome wrapped or twisted at the top) I insert it and make sure it slides in easily in both directions, but pay extra attention to the "out" direction. I avoid bending wires in a direction that forms a "hook" like a fish hook that will catch on anything on the way out.
I remember using "insta-jets" - regular enerjet igniters (which were folded over sure-shot wick) on a long lead inserted all the way up the motor and then you filled the divergent (outside) end of the nozzle with modeling clay. Instant on. I suspect that modern motors with their casings much closer to the zero margin of safety would blow at ignition.....
BTW, you forgot to plug your website....
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-Fred Shecter NAR 20117
-- ""Remove "zorch" from address (2 places) to reply.
Reply to
Fred Shecter
It is not necessarily the igniter.
Aerotech SU motors are designed to operate at the ragged edge of the pressure capability of the casing.
If the igniter passes through the nozzle at the same moment as the ignition spike typical of errortech erosive c-slots, then it can over pressure and pop the case.
The solution is to make igniters soo poor in ignition characteristics the motors barerly ignite :)
Or use really thin wires and really flexible pyrogen material.
We know the solution is NOT for errortech to learn to design motors.
Jerry
I find with your excellent igniters, and phenolic case SU motors, the minimum throat to be about 9-10/64" depending on the thrust curve shape. In the case of errortech erosive(-sm) geometries, you might suggest a minimum of 11/64" THROAT.
Recommend them in U.S. Rockets motors and only RMS errortech motors. Let people waste hundreds of first fires trying to ignite motors. Oh, and first fire igniter wires are waaayyy too thick.
Your dip kit igniters seem thinner.
Magnelite does indeed rock.
Jerry
Reply to
Jerry Irvine
Exactly.
Point.
An actual tech post. Nice Fred.
Reply to
Jerry Irvine
Jerry But if that occurred wouldn't the fuel core show some sign of starting the burn instead of original condition. Ken
Reply to
Kenneth Jarosch
I had a similar failure with a G125 - the forward bulkhead/delay assembly simply cracked away from the case at ignition and the propellant went out. Post-firing examination of the motor remains revealed no apparent burning - there was a slight widening of the front of the core passage in the otherwise- intact propellant: apparently the gas from that amount of propellant, vaporized and ignited in one moment by a small hot "Igniterman" igniter, was enough to pressure-spike the motor and crack open the casing... as soon as the case failed and the pressure was released, the hot gases must have been cooled and dispersed to the point where there was not enough heat concentrated on the propellant surface to maintain combustion.
-dave w
Reply to
David Weinshenker
It could be visually clean but "smoother than an unignited grain", to lightly charred.
If it pops "on ignition" it does indeed burn sufficient propellant to pop it but, not a significant portion of the propellant mass to perhaps even detect it. Safe, eh? Safe enough for even a general consumer.
Just Jerry
Reply to
Jerry Irvine
You just described visual evidence of erosive burning. Take a high res close-up photo of that grain and I will put it on the internet.
I bet it was a C-grain :)
Jerry
Reply to
Jerry Irvine
Unfortunately (for photographic purposes), I traded the defective motor in for replacement to one of the dealers at our launches a long time ago...
Nope, it was a small-diameter central circular core.
-dave w
Reply to
David Weinshenker
Jerry I see it happens to motors other than the F21. From all the comments I hear a common thread. It suggest a pressure spike from small amount of fuel , maybe dust from the slot process, and a case that doesn't handle this spike.
The loose dust would probably have a higher or faster flash point. Couple this with the restricted nozzle by the igniter. KA-boom.
Ken
Reply to
Kenneth Jarosch
I had nearly the exact same thing happen to me, but with an Econojet F20-7W motor and a Jim Turner igniter. I had trouble with the Copperhead igniter and used a Jim Turner igniter that I still had. There was a loud 'pop' at ignition and the lower 1/3 of the case had split and blown apart. It was a typical 'twisted wire' dipped igniter, although I believe Jim used his own pyrogen formula.
I do think that the C-slot geometry does result in smaller initial throat openings - you have to slide the igniter in at an angle into the slot. When I examined the slug, there was no evidence of ignition, but I did not look at it under a magnifying glass.
It is possible that the thicker wire leads cause the case to over-pressurize and blow. Since then, I've gotten better at using the Copperhead igniters, and have also found some extremely fine gage wire to use when I do try and make my own.
Tony Huet.
Reply to
Tony Huet
I've seen error-tech 29mm single use motors cato on copperheads, ignitorman, firestars...it seems to be the nature of the beast.
Reply to
Kurt Kesler
The motors are erosive. It's NOT the igniter.
Reply to
Jerry Irvine
Point.
Reply to
Jerry Irvine
Snip On>>>>
I've gotten better at using the
Reply to
Mark A Palmer
I'm also thinking one element may be a fast pyrogen igniter in close contact with the propellant so that it "torches off a bigger hit" of propellant, in its initial flash, than is required to produce enough gas to bring the small initial core volume up to normal pressure. (This could cause a similar effect even without any machining dust or significant nozzle restriction...)
-dave w
Reply to
David Weinshenker
That would be an even worse case of motor design than I am accusing gary of.
Reply to
Jerry Irvine
Dave W Then we are back to what Jerry stated earlier. Use or make igniters that barely work. Weak less aggressive igniters. Boy I just tried to get away from those. That's why I went to ML's to get a sure fire.
Ken
Reply to
Kenneth Jarosch
Or my #1 suggestion. No retarded motor designs.
Jerry
Reply to
Jerry Irvine

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