CTI Space(r) Race

It used to be that one of my least favorite things to do was to glue together CTI 98mm motors. The process is to apply Gorilla Glue to a grain and insert it into the end of the liner. Then, you add one of the spacer o-rings, apply glue to the next grain and push it in, and then repeat until all of the grains are inserted. Towards the end of the process, you have to push all of the grains into their final position at the end of the liner. It's a race to get that done before the glue sets up. The directions used to say to dampen the grains to help the glue set up. If you did that, you'd lose the race (ask me how I know that). The directions were changed a few years ago at my suggestion to omit the dampening step, but it is still an exciting process when you get to the last few grains.

In my recent assemblies, I have modified the procedure to make the process less stressful. In a nutshell, I glue the little spacer rings to the grains in advance. Then, when the glue is applied to the grains, they can be pushed all of the way to the bottom of the liner. Thus, you don't have to worry about the grains getting stuck before they are all inserted. I've done this on a half dozen or so CTI motors, as have others, and the process has worked well as have all of the motors assembled in this manner. You're welcome to try my technique, and as Stu Barrett would say, your mileage may vary.

To glue on the spacer, I first clean off the spacer with alcohol and then scuff it up with some sandpaper. Then, I apply a bead of epoxy around the edge of the grain, and then I wrap the top of the grain with a piece of mylar. Finally, I put the o-ring inside the mylar and tap it down into the glue. Once the glue cures, you can assemble the motor.

Jim

Jim,

This is immensely helpful. I have avoided flying CTI motors that required bonding out of fear that they would seize up before I was done. It sounds like this approach might make the difference between success and frustration.

Worsaer said:

Jim,

This is immensely helpful. I have avoided flying CTI motors that required bonding out of fear that they would seize up before I was done. It sounds like this approach might make the difference between success and frustration.

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Yep, this process makes the gluing easy.

The other issue I've had with these motors is getting the XL style nozzle into the case. The o-ring seems just a bit too large. I've had to pull these in using a long piece of all-thread.

Jim

This is definitely easier when loading the motor alone. Fortunately, I have had help with the 4 I've flown. Assembly-line fashion, with the right supplies lined up ahead, and the scar tissue to motivate our timeliness!

I have done something similar with my EX motors. I use silicone RTV because it is more flexible after curing than epoxy.You don't want to have a crack or gap for hot gases to creep through, especially under acceleration.

JimJarvis50 said:

The other issue I've had with these motors is getting the XL style nozzle into the case. The o-ring seems just a bit too large. I've had to pull these in using a long piece of all-thread.

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What I've done is cover the threads inside the case with a layer of clear packing tape, with excess tape sticking out. Then grease the surface of the tape. Feed the nozzle into tape first evenly. Then push the nozzle into the case, keeping it square with the case. It may require a block of wood and tapping with a mallet to get it to start. Then remove the tape. This is the way I've gotten many nozzle into larger EX motors, too.

jsdemar said:

This is definitely easier when loading the motor alone. Fortunately, I have had help with the 4 I've flown. Assembly-line fashion, with the right supplies lined up ahead, and the scar tissue to motivate our timeliness!

I have done something similar with my EX motors. I use silicone RTV because it is more flexible after curing than epoxy.You don't want to have a crack or gap for hot gases to creep through, especially under acceleration.

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I don't think these spacers are intended as a gas seal? Ivey and I just about lost an N5800 at Balls a few years back. We were leaning with all we had.

Jim

JimJarvis50 said:

I don't think these spacers are intended as a gas seal? Ivey and I just about lost an N5800 at Balls a few years back. We were leaning with all we had.

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Not a gas seal. Primarily as a spacer to allow the grain faces to start burning at ignition. However, it also has to fill well enough not to have a major hot spot on the liner. And not filled with something that flames easily, like epoxy.

JimJarvis50 said:

I don't think these spacers are intended as a gas seal?

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It's my understanding Pat G. uses some dots of RTV silicone (not a continuous bead) as grain spacers in his research motors. No seal intended there, obviously. The smaller CTI motors use a concave surface on the forward end instead of o-rings.

Regarding the original technique, I was wondering if it couldn't be made slightly easier by using a wrap of masking tape instead of the mylar and just tacking the o-ring on with some drops of superglue. However, I have no experience how good the bond needs to be for this.

Reinhard

Question... While I have only glued one Loki motor, and I did it as per instructions. Why not use 60 minute epoxy, thus eliminating the frenzy? I ask because I seriously have no idea why GG is supposed to be used exclusively.

Reinhard said:

It's my understanding Pat G. uses some dots of RTV silicone (not a continuous bead) as grain spacers in his research motors. No seal intended there, obviously. The smaller CTI motors use a concave surface on the forward end instead of o-rings.

Regarding the original technique, I was wondering if it couldn't be made slightly easier by using a wrap of masking tape instead of the mylar and just tacking the o-ring on with some drops of superglue. However, I have no experience how good the bond needs to be for this.

Reinhard

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Anything that would hold the ring in position would be fine. My experience is that superglue doesn't hold to the grains.

Jim

jimzcatz said:

Question... While I have only glued one Loki motor, and I did it as per instructions. Why not use 60 minute epoxy, thus eliminating the frenzy? I ask because I seriously have no idea why GG is supposed to be used exclusively.

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I used to think the Gorilla Glue was used because it expanded and would make a gas seal around the grains. Not so sure about that, though. I've heard Aerotech is recommending an Elmers version of Gorilla Glue that doesn't expand. I also have a CTI motor that came with epoxy that isn't Gorilla Glue. I think the glue with that motor glues two of the grain faces, so you don't want something that expands for that. But, I don't know what kind of epoxy it is.

Jim

JimJarvis50 said:

I used to think the Gorilla Glue was used because it expanded and would make a gas seal around the grains. Not so sure about that, though. I've heard Aerotech is recommending an Elmers version of Gorilla Glue that doesn't expand. I also have a CTI motor that came with epoxy that isn't Gorilla Glue. I think the glue with that motor glues two of the grain faces, so you don't want something that expands for that. But, I don't know what kind of epoxy it is.

Jim

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Jeroen has started recommending Elmers pro max glue instead of gorilla as well.

tbonerocketeer said:

Jeroen has started recommending Elmers pro max glue instead of gorilla as well.

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FYI, this recommendation was based on the gorilla glue being hard to find in Canada. I spoke to him in depth about gluing grains. As far as epoxy, its definitely not recommended and has proven to not work on N10,000's.....right, Gus?!?

GRIFFIN said:

FYI, this recommendation was based on the gorilla glue being hard to find in Canada. I spoke to him in depth about gluing grains. As far as epoxy, its definitely not recommended and has proven to not work on N10,000's.....right, Gus?!?

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I remember that being one reason, and I think the Elmers didn't require the water and expand as much which he also likes.

tbonerocketeer said:

Jeroen has started recommending Elmers pro max glue instead of gorilla as well.

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Do you mean Glue-All Max? That's a polyurethane glue, similar in formulation to regular gorilla glue. A good match to HTPB for adhesion.

Bottom line is to not use epoxy.

Please explain why not to use epoxy. If I missed the answer already, I apologize. Just point me in the right direction.

jsdemar said:

Do you mean Glue-All Max? That's a polyurethane glue, similar in formulation to regular gorilla glue. A good match to HTPB for adhesion.

Bottom line is to not use epoxy.

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Yeah, that's it. It works the same but I was told doesn't require the water part.

jimzcatz said:

Please explain why not to use epoxy. If I missed the answer already, I apologize. Just point me in the right direction.

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Kinda hidden in replies #4 and #7 in this thread. Basically, epoxy is not flexible when it cures, and it tends to burn faster than polyurethanes. It also can create a hot spot and burns along the edge of the casting tube and liner.

Some of the CTI motors come with HTPB and curative to mix together as the adhesive. Same stuff we use as binder when making propellant.

John, thanks.

Just as a curiosity, the glue is applied to the outside of the grain so that its bonded to the liner and not to the face of the grain proper correct?

grouch said:

Just as a curiosity, the glue is applied to the outside of the grain so that its bonded to the liner and not to the face of the grain proper correct?

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The application Jim mentioned starting this thread is not in CTI's instructions. It's a little glue to keep the oring spacers in place before pushing the grains in. The glue will be at the edge of the casting tube and a little bit on the face. That was the issue I wanted to mention regarding epoxy not being the best thing to use.

Per instructions, some CTI loads require gluing in the grains, either with supplied HTPB/curative (Pro98 M6400, N10,000 and O3400) or user-supplied Gorilla glue (some 75s and 98s). This is on the outside of the grains, with some care not to get it on the faces. But some does bleed around the edges.

The other application per CTI instructions is to glue grain segments together for certain reloads. This is to create inhibited faces and form a longer grain.

Epoxy is bad for all the above. Polyurethane glue (gorilla, or Elmer Glue-all Max, not the white stuff) is compatible with the grains and is more flexible than epoxy (which also can create hot spots).

jsdemar said:

What I've done is cover the threads inside the case with a layer of clear packing tape, with excess tape sticking out. Then grease the surface of the tape. Feed the nozzle into tape first evenly. Then push the nozzle into the case, keeping it square with the case. It may require a block of wood and tapping with a mallet to get it to start. Then remove the tape. This is the way I've gotten many nozzle into larger EX motors, too.

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When I took the N-5600 apart, the nozzle-end O-ring was torn. Good chance, I think, that this happened when the motor was built. I'll need to give your trick a try.

It seems pretty obvious to me that the O-rings for these 98mm motors are not ideally sized. However, I have not had a problem before with the Gen 1 cases. This was my first use of a Gen 2 case, and it looks to me like the step where the case reduces to the ID of the case might at a higher angle in the Gen 2 case.

Jim