RMS 24/40 Ejection Charge Delay

[JasonT]

Good morning.

I am a newbie to this hobby and I have just progressed to RMS motors.

I built an Aerotech Initiator and successfully flew it on it's maiden launch.

On the next launch, the engine ignited as per usual and sent the rocket skyward. At the apogee, the ejection charge did not fire and the rocket came down as a lawn dart. When it was 10 feet or so from the ground, I heard the ejection charge go off.

The nose cone was damaged and the rocket was irrepairably damaged. I figure that this is part of the learning experience but I am not sure what happened...as I built the motor using the same instructions for both flights.

Any ideas or suggestions for next time?

Thanks in advance!

[Doug Sams]

At the apogee, the ejection charge did not fire and the rocket came down as a lawn dart. When it was 10 feet or so from the ground, I heard the ejection charge go off.

<snip>

Any ideas or suggestions for next time?There are a couple of possibilities. Which 24/40 motor was it? Delay? It could be that the delay started burning late. Supposedly, this could be due to improper ignitor placement, but I've always been skeptical of that explanation.

My first impression on this was you got some grease on the delay grain - either o-ring lube or merely skin oil. It was enough to slow the burn but not prevent it altogether.

Another possibility is that the wrong delay was supplied with the kit, but I suspect that's one of those notions that's popular 'cause it puts the blame back on the manufacturer http://forums.rocketshoppe.com/images/smilies/smile.gif

FWIW, when I assemble a motor, I get out a couple wipes to clean my hands with. I use the kind that don't have any lotion in them. After lubing the rings, closure threads and the sides of the delay well, I set them aside and thoroughly clean my hands before handling any of the other bits.

HTH.

Doug

.

[GregGleason]

I agree with Doug.

The AeroTech delay grain is sensitive to some types of foreign matter. It is designed to burn a particular rate and petroleum products, silicone grease, skin oil, sweat, etc. will impede the burn rate. The rate of the burn is to some degree related to the amount of heat the delay element is exposed to. To quote AeroTech, "The delay burns about twice as fast during the motor burn as it does after the motor burns out". I found this out from correspondence I had with AeroTech after the event below.

I had a slow delay on an F40-7 with my AeroTech Initiator's 2nd flight. I glued a piece of paper onto the propellant grain so that the igniter would stop at the top of the grain. It turned the F40-7 into an F40-11 (according to the altimeter data the ejection occurred 12.9 seconds after launch). The Initiator did a very good impression of a V2 after apogee and when it seemed that all hope was lost the ejection fired. The only reason it didn't get a zipper is that the main chute had a metal spring to reduce opening shock (I no longer do that). The spring was uncoiled and pulled (violently yanked) off, leaving a drogue-only recovery. I learned a lot from that flight.

Now I use a small piece of masking tape (not blue paint tape, it is not sticky enough on the propellant), about 3/8" wide, and place it over the propellant grain slot to make sure I have a good igniter-stop. During motor assembly, I am careful not to touch the delay element facings.

Greg

[ghrocketman]

If one placed the delay spacer in the reload at the wrong end of the delay you could get a "BONUS !" delay as well. The bonus is two-fold....an extra "bonus" of delay seconds as well as a "bonus" you need a new rocket !

[Bill]

I had a slow delay on an F40-7 with my AeroTech Initiator's 2nd flight. I glued a piece of paper onto the propellant grain so that the igniter would stop at the top of the grain. It turned the F40-7 into an F40-11 (according to the altimeter data the ejection occurred 12.9 seconds after launch). The Initiator did a very good impression of a V2 after apogee and when it seemed that all hope was lost the ejection fired. The only reason it didn't get a zipper is that the main chute had a metal spring to reduce opening shock (I no longer do that). The spring was uncoiled and pulled (violently yanked) off, leaving a drogue-only recovery. I learned a lot from that flight.



Hmm...if this can become a manufacturer recommended way to extend a delay...


Bill

[yankee42]

Good morning.

I am a newbie to this hobby and I have just progressed to RMS motors.

I built an Aerotech Initiator and successfully flew it on it's maiden launch.

On the next launch, the engine ignited as per usual and sent the rocket skyward. At the apogee, the ejection charge did not fire and the rocket came down as a lawn dart. When it was 10 feet or so from the ground, I heard the ejection charge go off.

The nose cone was damaged and the rocket was irrepairably damaged. I figure that this is part of the learning experience but I am not sure what happened...as I built the motor using the same instructions for both flights.

Any ideas or suggestions for next time?

Thanks in advance!
This happened to my initator on its 2nd launch, the 1st of my 29/40-120 reload. Kinda sucks, but its an excuse to get a new rocket :D

[5x7]

The rate of the burn is to some degree related to the amount of heat the delay element is exposed to. To quote AeroTech, "The delay burns about twice as fast during the motor burn as it does after the motor burns out".

Greg

It's the increased pressure in the case during the burn that causes the delay to burn twice as fast as after the burn, when the pressure drops to close to ambient. The bulk of the delay is insulated from the heat of the burn by the... delay insulator.

If you put a slug of any APCP on the ground and burn it, you will see how slowly it burns compared to when it's inside the case.

[GregGleason]

That makes sense. Heat is not the only variable. I am pretty sure the delta for pressure during and after motor burn is higher than the thermal delta.

It reminds me of the Apollo 1 fire. Not only was the 100% O2 a problem, it was also under pressure. It didn't help that some of the materials in the CM were highly combustible when in a higher pressure/100% O2 environment, whereas under ambient air pressure and normal O2 ratios it was not.

Greg