Causes for ejection failure?

[billdz]

On Saturday my Jart 5 flew nicely about a mile high on an Aerotech K400, but the chute did not eject and it lawn darted 3' into the ground, had to go buy a shovel to dig it out (tough rocket, remarkably little damage).

During the flight, one observer on the ground said he thought he heard an event. I did not hear anything, nor did other observers. After recovery, inspection revealed that the ejection charge did fire.

So why didn't the chute eject? Several theories have been put forward, but none seems like a great fit:
* nose cone was on too tight - don't think so, I remember it slipped in nice and smooth
* parachute was jammed in too tightly - don't think so, this is a 5" rocket, plenty of room
* not enough powder in the charge - possible, the instructions do say "additional ejection charge may be required for rockets exceeding 4 inches in diameter," but I've flown this rocket several times previously without additional powder.
* charge "smothered" by shock cord - The K400 protruded from the motor mount by an inch or so, and the chamber was quite full with the shock cord, chute, nomex blanket, and JL Chute Release.

Any other thoughts or theories?
Thanks,
Bill

 

[Bat-mite]

Was this motor deploy or altimeter deploy?

NC too loose and ejection gasses blew by? Any new holes in the NC that could have leaked pressure? If motor deploy, shorter motor made for larger volume to fill?

 

[MikeyDSlagle]

Stupid question but was the motor still in the rocket when you retrieved it?
Weak or insuffient ejection charge would be my guess. Maybe some of the gases blew out through the spent motor.
Charges should work just as well if not better in a cluttered payload, it is the pressure that moves the nose cone, not the blast itself...or else baffles wouldn't work.
I had a Wildman Sport lawndart and it went about a foot into the ground, luckily the ground was soft enough I could dig it out with my pocket knife. I have an E-tool in the Xterra now. No damage to rocket other than clear coat removed from nose cone. Wildman glass is tough stuff.

 

[Steve Shannon]

Was there loose unburned powder in the wreckage?
I have seen CTI motors with insufficient powder for large diameter or long bay rockets. I have added powder as described.

 

[billdz]

Thanks for the replies!
*Was this motor deploy or altimeter deploy? motor
*NC too loose and ejection gasses blew by? Don't think so, seemed a good fit, not too tight, not too loose.
*Any new holes in the NC that could have leaked pressure? None seen.
*If motor deploy, shorter motor made for larger volume to fill? This is the longest motor I have used with this rocket.
*Stupid question but was the motor still in the rocket when you retrieved it? Yes, motor still in.
*Was there loose unburned powder in the wreckage? None seen.

"Wildman glass is tough stuff." Agreed!

Any other thoughts?

 

[jnobels]

It is possible the charge burned after “landing” too and just vented out the back because the nose was subterranean.

Somebody mentioned the trick of making sure there was no space forward of the ejection charge so that the powder can’t “fall away” from the delay grain when/if the rocket arcs over. I’ve had 2 aerotech deployment failures I’m blaming on this.

 

[billdz]

It is possible the charge burned after “landing” too and just vented out the back because the nose was subterranean.

Somebody mentioned the trick of making sure there was no space forward of the ejection charge so that the powder can’t “fall away” from the delay grain when/if the rocket arcs over. I’ve had 2 aerotech deployment failures I’m blaming on this.

Yes, the charge burning out the back while the rocket was in the ground would explain why no one heard a pop while the rocket was in the air. But what would ignite the charge, just pure force of impact?

And yes, I can see why it is important to leave no space forward of the charge.

 

[Bat-mite]

Altimeter did not detect apogee until it hit the ground? Loose wire that touched on impact? Is it a downloadable altimeter?

 

[DaveW6DPS]

How did you calculate how much powder to use?

Was it normal ffffg black powder?

 

[billdz]

*Altimeter did not detect apogee until it hit the ground? Loose wire that touched on impact? Is it a downloadable altimeter? This was motor ejection, no altimeter. Although I think after this I'm going with redundant ejection.
* How did you calculate how much powder to use? Was it normal ffffg black powder? I used the entire vial of powder supplied by Aerotech.

 

[MikeyDSlagle]

My lawn dart was caused by electronics failure. The rocket took less damage then on a previous with too short a delay...Go figure. That is the only mishap I have had with electronic apogee events. I have had to repair two rockets for improper delay timing and rebuild my Callisto when the delay went early. The sight of that rocket burning on the pad was the nail in the coffin. Where feasible I will use electronics and redundancy where possible.

 

[Uncrichie]

"inspection revealed that the ejection charge did fire."
Just curious as to what made you believe this?

 

[billdz]

"inspection revealed that the ejection charge did fire."
Just curious as to what made you believe this?

Good point, I suppose it is possible the red cap popped off at impact and the powder just spilled out. But see the attached pic, looks to me like burn residue inside the charge holder. What do you think?

 

[tfish]

Did you put the "little flat washer" in the FC before you added the BP?

Tony

 

[MikeyDSlagle]

I've never flown a DMS but pretty sure there will be residue in the charge cavity regardless whether or not the charge blew. The delay is pretty much an end burning grain of slow propellant and it will flame up into the cavity to ignite the charge. Your charge may have dumped prior to launch or even at nose over.
The observer who says he heard an event...well sometimes folks say things just to be saying things or to try to add their 2 cents or to try to be helpful or they just HAVE to be part of EVERYTHING. We all know folks like that.

 

[billdz]

Did you put the "little flat washer" in the FC before you added the BP?

Yes, definitely.

"Your charge may have dumped prior to launch or even at nose over." I don't think it spilled prior to launch but, since most people heard no event, I'm now starting to doubt that the charge did ignite. Guess we'll never know for sure.

 

[Steve Shannon]

I've never flown a DMS but pretty sure there will be residue in the charge cavity regardless whether or not the charge blew. The delay is pretty much an end burning grain of slow propellant and it will flame up into the cavity to ignite the charge. Your charge may have dumped prior to launch or even at nose over.
The observer who says he heard an event...well sometimes folks say things just to be saying things or to try to add their 2 cents or to try to be helpful or they just HAVE to be part of EVERYTHING. We all know folks like that.

The washer retards the flow of gas from the ejection charge side to the motor case, which keeps the pressure of the ejection charge from flowing out through the nozzle instead of blowing out the nosecone. It’s necessary. I’ve seen L2 candidates fail because the washer was left out and the ejection charge failed to pop the nosecone as a result.

 

[MikeyDSlagle]

The washer retards the flow of gas from the ejection charge side to the motor case, which keeps the pressure of the ejection charge from flowing out through the nozzle instead of blowing out the nosecone. It’s necessary. I’ve seen L2 candidates fail because the washer was left out and the ejection charge failed to pop the nosecone as a result.

Right right. But if the charge is not there....Would there still be a bit of residue from the delay grain burning through the touch hole?

 

[billdz]

Also, if the charge fires at 5k' apogee but the NC does not pop off, would the sound be audible on the ground? Or will the sound be muffled inside the rocket?

 

[Bat-mite]

5000 feet? I doubt you'd hear it. You might see the little ball of white smoke, but pretty far away to hear it.

 

[Steve Shannon]

Right right. But if the charge is not there....Would there still be a bit of residue from the delay grain burning through the touch hole?

I don’t know. There’s little to no pressure from the motor side to the ejection charge side to deposit residue. I would not think that there would be much residue without a charge.

 

[Steve Shannon]

Also, if the charge fires at 5k' apogee but the NC does not pop off, would the sound be audible on the ground? Or will the sound be muffled inside the rocket?

Very difficult to hear if the nosecone doesn’t come off, even when close to the ground.