Ejection charge test – failure.

[scsager]

This is a weird sort of failure. The 2-56 nylon screw sheared perfectly. Shock cord extended, laundry deployed. Everything worked just as expected ... except...

There are steel coupling nuts epoxied inside the fiberglass nose cone - and machine screws to hold the bulkhead in place. There's a GPS in the nose behind the bulkhead.

The epoxy failed between the coupling nuts and the nose cone.

I think a better (air-tight) seal between the ply bulkhead and the nose cone is needed.

 

[cerving]

Whenever I do one of these types of nose sleds I epoxy allthread and nuts into the inside of the NC coupler (the nuts act as spacers) and have about 3/4" of allthread sticking out the end of the NC coupler. You drill the bulkplate to match, and thread two nuts onto each protruding allthread. Since there's typically about 4" of allthread epoxied into the NC, it would take a whole lot of force to make the glue joints come loose.

 

[djs]

I've had this happen before (not ejection charge testing, but in flight). I use JB weld to connect the couplers (or metal tubes in my case) to the AV bay sled. My theory is that the jb weld doesn't really hold to metal very well. It's not super common, but I usually have to fix a few of them per season. When you had the ejection charge testing, did it extend the shock cord fully and snap the nose cone back? I'm guessing it's just a force thing that broke it loose.

 

[markkoelsch]

I am not a fan of that method. I tried it long ago, and discarded it. I have had much better luck using a single piece of aluminum allthread epoxied into the forward portion of the cone.

This is after I rough up the inside of the cone. Then the allthread has a washer and two nuts on the epoxy embedded end. The epoxy is then mixed with a large amount of Kevlar pulp- about 1/3 by volume. I pour that in, sink the rod and nut/washer end into the epoxy, and run the rod through a nosecone coupled centering ring.




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[Handeman]

This is a weird sort of failure. The 2-56 nylon screw sheared perfectly. Shock cord extended, laundry deployed. Everything worked just as expected ... except...

There are steel coupling nuts epoxied inside the fiberglass nose cone - and machine screws to hold the bulkhead in place. There's a GPS in the nose behind the bulkhead.

The epoxy failed between the coupling nuts and the nose cone.

I think a better (air-tight) seal between the ply bulkhead and the nose cone is needed.

I don't understand why you say you need a better "air-tight" seal. I don't see how that would have made any difference.

Did the glue fail by pulling off the nose cone, or from the coupling nuts? Did you use 80 grit sand paper to rough up the nose cone? Did you use a course file, grinder, Dremal, or some other method to roughen the surface of the coupler nut? Did you mix any fibers in the epoxy to strengthen it?

I'm glad you posted this. I intend to assemble a similar nose cone, except I am using a 54mm tube inside the nose cone. I was going to epoxy the couplers between the nose cone and the tube. I'm also putting a centering ring for the tube in the nose cone that will be butted against the coupler tube used as a shoulder. I think I'll add threaded rod from the coupler to the centering ring. That should never let go.

 

[mpitfield]

I use a specifically cut CR with some wings that line up with the bulkhead, where I epoxy self clenching nuts (commonly called PEM nuts) in place. Had many hard deployments, never had a failure of this kind; currently looking for some wood to knock on! Ideally these can be slide by the shoulder of the nosecone and epoxied in place. With plastic nosecones that is simple just squeeze the nosecone, with FG you just make two sides of the CR flat to clear the coupler then flip it over on the shoulder and epoxy in place.

Here is a 3" nosecone poly-carbonate nosecone for dual trackers, GPS and Com-Spec AT-2B



54mm FW/FG nosecone removable coupler, for a dual trackers, GPS and Com-Spec AT-2B



3" plastic nosecone, for a retrofit altimeter bay and tracker.



An assembly on it's side that was being mocked up for a 4" plastic nosecone, showing the CR attached to the bulkhead. This was a bit complicated becasue this nosecone had a CO2 system, altimeter and tracker in the nosecone with an ARRD on the bulkhead and a custom chute tube.

 

[tim cubbedge]

I do something similar. I cut out a coupler bulkplate & epoxy some T-nuts to backside & then epoxy that to the NC coupler.

 

[jd2cylman]

Scott,
If that bulkhead inside the nose is 1/2" thick or even 3/8", you could use counter sunk flat head screws in to the edge to hold the bulkhead in.

 

[Keisling]

What kind of epoxy did you use and how did you prep the surface before bonding, particularly those of the metal pieces? Almost every metal part with threads will have residue from cutting lubricant on it and needs be cleaned. It also looks like the epoxy around your eye bolt was applied over masking tape and failed.