I am not familiar with the pieces you are talking about since I am using delay-grained 54mm motors still. As I understand it though, the threads in these forward units are 1/4-20. It would seem that if you screw in a 1/4-20 threaded harness mount, whether it is all thread or a welded eye, if you can also place a 1/4-20 nut and use it as a jam nut, you should be ok. Another solution which is much more complicated is to drill out the old hole and tap it with a bigger thread such a 5/16 or you tap it with a 1/4-20 helicoil and glue in the insert. I really do not like this method though, because I am not certain of the amount of shock force a helicoil insert and thread can take in this particular direction. If we were going perpendicular to the long axis of the hole, I am pretty certain it would hold adequately, but going in parrallel with the long axis of the hole, I get more concerned. I have pulled helicoils out of their mounts before on numerous ocassions. I bet Warren could give you a good answer to this.

Heli-coils (STI thread) have greater external thread contact area to the base material than the standard thread bolt or screw has to the interior. The heli-coil, in theory, will not fail until the bolt or screw has failed.

Oh, by the way, on the original CTI forward closure post. Build the motor including the delay grain, plug the flash hole to the ejection well with grease and install no ejection charge, just stuff the well with dog barf or grease and tape it over. Same thing folks have done for years with Aerotech cases.

For a more permanent solution, sand the inside of the ejection well with 40 grit sandpaper and fill it with epoxy - for extra credit, embed a 1/4-20 forged eyebolt.

Yep, I guess epoxy comes to the rescue again, doesn't it. Maybe even at the flight line. It would be interesting to pull test an eyebolt to see how strong such a joint would be.

I guess the other option along these lines would be to tap the flash hole, thread same forged eyebolt into the forward closure, and back with a fender washer / locknut inside the closure.

Thanx for the comments!

Bob, you can't do anything on the inside surface of the closure - you wouldn't be able to install the delay grain and that is the primary thing that holds the case under pressure. Without the delay grain, you'll have hot exhaust shooting up into your bird as it also comes out the nozzle and it will burn your forward closure. I can't recall the CTI forward closure, but I believe it is much like an AT - not enough there to thread for an eyebolt. Sand it with 40 grit and fill with epoxy and a forged eyebolt and you'll be good to go. I have one AT closure like that with over 20 flights on it.

Let me poke at this a bit ..

If for example I thread a bolt through a tapped (or heli-coiled) flash hole, this seals the forward closure, so I'm not sure how exhaust would shoot up into the bird, provided that the eyebolt is indeed tight, correct?

You are right that there isn't much to bite into if one is just tapping the closure. A helicoil or threaded insert would be needed, just as is used in the longburn plugged motors that CTI sells. It will be interesting to see what the pull strength of attachment will be - I suspect epoxy will be needed if one cannot use a backing washer inside the closure.

I'm not sure how re-usable the CTI forward closure will be. Looking at the remnants of a motor I fired at LDRS, the plastic or whatever the material is does indeed show some level of burning on the interior and more worrisome, at the lip that interfaces with the liner tube. This is the seal that I would be most worried about.

On a slightly different thread, is it normal for there to be a significant difference in length between the grain and the liner? In a K300P CTI motor I have on my bench, which is a 6XL size, there must be 3/8" slop in the assembled motor, where the grain can slide back and forth in the liner. I'm kinda surprised that there isn't a shim or some such to make up any difference in length to keep everything snug, so to speak.

Hi Bob,

I have flown the CTI L935, which is also a 54mm 6GXL, in my Kestrel. What I did, and would do again, was to take a 1/4"t nut and pot it into the ejection well with epoxy, with the flat side of the t-nut seated into the bottom of the well. Of course, you have to sand the inside of the well, like Warren said, so the epoxy has some surface area to adhere to. I don't think I've ever heard of anyone using a 54mm forward closure more than once, and I know for a fact I never have. I'm pretty sure they're only meant to be used once.

Thanx Ken - the Kestrel is exactly what I am working on ...

T-nuts work well for a number of rocket things. In a minimum diameter rocket, I'm find there is just enuf clearance between airframe and motor casing to use a tiny t-nut for a rail button attachment, for example.

You can flatten the tangs of the T-nuts. I'm thinking that with the correct drill bit, for the forward closure problem, one could flip the t-nut around (like it is normally used in wood application) and get a pretty snug seal.

Out to the shop ...

There are also t-nuts without tangs - these are normally what I use. I did the closure the way I did it because, other than potting in the t-nut, no modifications to the forward closure were necessary - no drill bit required.

Has anyone posed the question to Dr. J. at CTI? It would be great if he would offer up an aluminum reusable threaded front closure for at least the 54mm motors, considering the demand. As the price of altimeters goes down and the simplicity of use improves, I think we'll see motor-based ejections be pushed down to only the smaller motors. I only use motor eject in 29mm or smaller now, and that will change once I get my Raven.

-Ken

I don't know if you guys use them, but I find TIME-SERTS much better than heli-coils for inserts. They seal better and have a very positive lock to the hole.

http://www.timesert.com/

Edward