RMS-24/40 Motor Aft Closure

[JasonT]

Good afternoon.

I am just getting started using RMS motors. My first attempt resulted in one perfect maiden flight, and the following flight a disaster for the rocket.

I purchased the same kit, Aerotech Initiator and I had basically the same exact results :(

This time, the aft closer on my motor flew/blew off of the motor 20 feet into the flight. The propellant grain, o-rings, nozzle, etc, all blew out of motor and the rocket crashed to the ground 10 feet from the pad. Luckily the rocket is intact, save for one of the 3 fins.

Here are some pics:
Picture 1 (http://www.flickr.com/photos/40384190@N04/3713586440/)

Picture 2 (http://www.flickr.com/photos/40384190@N04/3713586414/)

Picture 3 (http://www.flickr.com/photos/40384190@N04/3712776821/)

Has anyone had this experience? I am 100% sure that the aft closure was screwed in to the stopping point, so I am at a loss on how to resolve this for my 5 attempt at RMS.

Thanks in advance!

[Shreadvector]

If therer was a void (bubble) inside the proellant grain, when the flame front hit it, there would have been an increase in burning surface and increase in pressure. Since the 24/40 casing has very fine threads and operates right on the edge of it's limits, a slight overpressure can easily blow the aft closure off. Also, if any debris clogs the nozzle during burn it can lead to an overpressure. How can you determine if this might have been the case? Simple: look at the propellnat grain and delay grain. They will both stop burning when there is a sudden and total loss of pressure. If there is a cavity in the propellant grain, then that may have been a bubble. The delay should be only semi-burned from the lower end with the top immaculate.

If the delay grain is burned up, then it somehow burned too fast and the ejection charge went off early. This would not have produced the failure mode you described in words and pictures - it would have blown the recovery system and since the propellant was still buring you would have had flame out bothe the nozzle and forward closure of the motor.

[GregGleason]

When I had a problem with an RMS motor, I wrote AeroTech about it and described the situation to them and they gave me some direction. It can't hurt to ask the vendor. They have always been helpful to me and it is in their interest to give you assistance.

Greg

[BRS Hobbies]

Some of the reload kits require that you put a piece of tape on one end of the propellant grain (as stated in the instructions). The tape is to make sure that the igniter isn't inserted to far into the motor which could result in the ejection charge functioning at the time of ignition. This sounds like what could be happening to your rocket.

Best regards,
Brian

[Shreadvector]

Some of the reload kits require that you put a piece of tape on one end of the propellant grain (as stated in the instructions). The tape is to make sure that the igniter isn't inserted to far into the motor which could result in the ejection charge functioning at the time of ignition. This sounds like what could be happening to your rocket.

Best regards,
Brian

That is impossible. The tape is there to simply keep the ingiter down in the propellant grain as it will not ignite if the igniter is not there.

If you inseted the igniter beyond the propellant, you might start to ignite the bottom of the delay grain, but this would burn at a VERY slow rate without the high pressure generated when the propellant is burning. The delay would most likely never ignite or try to ignite and then snuff itself. The delay burns from one end to the other, so it is absolutely impossible to instantly ignite the ejection charge unless you drill a hole directly through the delay grain (or if you leave out an o-ring or seal).

[JasonT]

That is impossible. The tape is there to simply keep the ingiter down in the propellant grain as it will not ignite if the igniter is not there.

If you inseted the igniter beyond the propellant, you might start to ignite the bottom of the delay grain, but this would burn at a VERY slow rate without the high pressure generated when the propellant is burning. The delay would most likely never ignite or try to ignite and then snuff itself. The delay burns from one end to the other, so it is absolutely impossible to instantly ignite the ejection charge unless you drill a hole directly through the delay grain (or if you leave out an o-ring or seal).

Good morning.

In this case, I did not leave off the tape on top of the propellant grain. It was present and the grain actually was lit for a several seconds. The delay charge was ejected from the motor un-ignited.

If you look at Pic 1, you can see that the grain started to burn but I guess the burn was stopped after the blowout of the aft closure.

Thanks!

[BRS Hobbies]

Fred,

It says in the Aerotech reload directions - "WARNING: FAILURE TO INSTALL THE DELAY ASSEMBLY CORRECTLY AS SHOWN MAY RESULT IN THE EJECTION CHARGE FUNCTIONING AT THE TIME OF IGNITION OF THE MOTOR, POSSIBLY DAMAGING YOUR MOTOR AND
ROCKET."

Best regards,
Brian

[JasonT]

Fred,

It says in the Aerotech reload directions - "WARNING: FAILURE TO INSTALL THE DELAY ASSEMBLY CORRECTLY AS SHOWN MAY RESULT IN THE EJECTION CHARGE FUNCTIONING AT THE TIME OF IGNITION OF THE MOTOR, POSSIBLY DAMAGING YOUR MOTOR AND
ROCKET."

Best regards,
Brian

The ejection charge was not ignited and was blown out of the motor undamaged.

Thanks!

[Shreadvector]

Fred,

It says in the Aerotech reload directions - "WARNING: FAILURE TO INSTALL THE DELAY ASSEMBLY CORRECTLY AS SHOWN MAY RESULT IN THE EJECTION CHARGE FUNCTIONING AT THE TIME OF IGNITION OF THE MOTOR, POSSIBLY DAMAGING YOUR MOTOR AND
ROCKET."

Best regards,
Brian

Correct. Exactly as I stated. I will re-state/re-word it again. The only way for an ejection charge to go off at ignition is if you do not install the seals/o-rings. the tape on the top of a non-full-length propellant grain has nothing to do with the ejection charge. The tape is simply there to keep the igniter at the top of the short grain.

[Rocketcrab]

Good morning.

In this case, I did not leave off the tape on top of the propellant grain. It was present and the grain actually was lit for a several seconds. The delay charge was ejected from the motor un-ignited.

If you look at Pic 1, you can see that the grain started to burn but I guess the burn was stopped after the blowout of the aft closure.

Thanks!

For what it's worth, I had the exact same thing happen to me about 2weeks ago. I was using a rather old [7+ years] F24 reload. The rocket lifted off fine, but the motor shut down at about 10 feet altitude. We found all of the RMS casing save for the aft closure, which given that everything else was intact, I have to think was the failure point. The ejection charge and forward closure were all intact and not burned. The propellant grain had started to burn but had apparently overpressurized and put itself out. I'm not an expert by any means, but I have been using RMS motors since 1992 and this was a first for me. I have no explanation for what happened, although I suspect the age of the the reload may have played a part, as does the fact that F reloads really "fill up" the 24mm RMS casing. If that aft closure wasn't on tight enough [although I can't imagine that it wasn't] the results we seem to have shared might be the result. Maybe it was "just one of those things". :rolleyes: