Motor Ejection Delay Issue

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Swany

Swany

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Over the weekend I launched my Madcow Avalanche with a Cessaroni J240-16 drilled to a 14 second delay. After reviewing the altimeter data, the actual ejection occurred at 23 seconds and resulted in a shredded parachute. Fortunately, this is one tough rocket and there was enough drag provided by the shredded chute (final descent rate was 32 fps) so that there was no permanent damage. Any thoughts on why the delay was so far out of bed ? I know there is a tolerance band around the delay timing, but this seems to be way outside that. Only thing I can think of is that delay grain did not light initially and miraculously did so on the plummet back to earth ?

Mike
 
Last edited:
Don't suppose you emptied the delay powder or checked the touch hole? Very rarely, a touch hole could be blocked by epoxy or potting agent.
You drilled it, so I assume there was no grease on the delay end
 
Swany said:
Over the weekend I launched my Madcow Avalanche with a Cessaroni J240-16 drilled to a 14 second delay. After reviewing the altimeter data, the actual ejection occurred at 23 seconds and resulted in a shredded parachute. Fortunately, this is one tough rocket and there was enough drag provided by the shredded chute (final descent rate was 32 fps) so that there was no permanent damage. Any thoughts on why the delay was so far out of bed ? I know there is a tolerance band around the delay timing, but this seems to be way outside that. Only thing I can think of is that delay grain did not light initially and miraculously did so on the plummet back to earth ?

Mike
Click to expand...

At T+23 seconds? Or 23 seconds after motor burnout?

The J240 burns for 3.4 seconds, so ejection at ignition plus 23 seconds is a motor delay of ~19.5 seconds. The cert allows +/- 20% (from the stock or supplied delay (16 seconds). So +/- 3.2 seconds. That still puts the delay a few seconds late of the tolerance. Alas, atmospheric conditions, altitude above sea level, imperfections in nozzle throats (unlikely, but possible with molded plastic), etc. may have played a role.

Regardless...motor ejection works well enough most of the time. But for improved accuracy, electronic ejection is definitely the more reliable method.
 
Thanks for the quick replies. I did check for grease/contamination on the delay grain when I drilled, but did not check the touch hole for blockage. The ejection event was 23 seconds after motor burnout or T+26 seconds.
 
very hard to say, and please do not take any offense to this, but are you sure you drilled it correctly? I personally saw a cert 1 go south when the flyer incorrectly drilled the delay (as confirmed by an inspection of the drilling tool after the flight).

we have all had a brain fart here and there, and sadly sometimes it results in things breaking.

all that said, it is possible that, due to any number of reasons, the element just didn't burn as expected. its happened before
 
Theory said:
very hard to say, and please do not take any offense to this, but are you sure you drilled it correctly? I personally saw a cert 1 go south when the flyer incorrectly drilled the delay (as confirmed by an inspection of the drilling tool after the flight).

we have all had a brain fart here and there, and sadly sometimes it results in things breaking.

all that said, it is possible that, due to any number of reasons, the element just didn't burn as expected. its happened before
Click to expand...
No offense taken. Believe me, I’ve made plenty of dumb mistakes so I’m not ruling anything out. Having said that, I’m m not sure how I would have been able to mis-drill the delay and end up adding 7-9 seconds to the delay.
 
Steve Shannon said:
What might have happened is igniter not inserted all the way, resulting in late ignition of the delay.
Click to expand...
It might be useful to elaborate a bit more on this - with a late ignition (say, during the last second of motor burn) you gain delay time not just from the fact that it starts burning 2-3 seconds later, but ALSO from the fact that those 2-3 seconds of burn are no longer during the motor burn of the rocket (ie are not at high pressure, where the burn rate will be higher).
 
'Bonus Delay' used to be very common, when delays on HP RMS motors were short, medium & long. To the point where, if it's big enough to carry electronics, I use them.
After a few shredded parachutes, and zippered tubes, it became apparent that even a simple timer is more consistent & reliable.

Just my .02
-Mike
 
Swany said:
No offense taken. Believe me, I’ve made plenty of dumb mistakes so I’m not ruling anything out. Having said that, I’m m not sure how I would have been able to mis-drill the delay and end up adding 7-9 seconds to the delay.
Click to expand...

True. Poor reading on my part
 

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