Using AT Seal Disk with CTI Reload

[thatotherguy501]

Hello TRF,

Last year I flew a CTI N5800 reload and had part of the casing bulge out after firing, right around the forward insulating plate. To help mitigate this happening in the future, I was told an Aerotech forward seal disk with the appropriate O-ring can be used in place of the CTI-provided insulating plate. I have a CTI L2375 I'm flying soon, and I'd love to use a 75mm AT fwd seal disk here. When dry-fitting the grains with the grain spacer O-rings, it looks as though the liner isn't quite long enough to accommodate the thick AT forward seal disk. Has anyone done this before/experienced this? Do you have any suggestions on how to gain the extra 1/8" of liner length needed? Or, for using the CTI insulating plate, are there any measures to be taken to help avoid bulging a casing in the future (grease perhaps)? Thanks so much for your help!

Finally, I found this thread on the topic, but it didn't address the liner being too short to use the seal disk: https://www.rocketryforum.com/threads/cesaroni-vs-forward-seal-disk.137651/

CTI 6G-XL casing bulge:
75mm AT fwd seal disk on top of grains (unable to insert further into the liner):

 

[Cory]

Leaving aside everything but the question of how to make the seal disc work in the 75... you could take that much evenly off of the grains with very little change in the thrust profile. I would use 120 grit sandpaper. This probably makes it an EX motor, but so does the use of the seal disc anyway.

 

[jd2cylman]

Any CTI 75mm motor I've flown, I've flown in AT compatible cases. I always use the AT forward seal disc. Every time, I've just left out enough grain spacer o-rings to make the seal disc work. On some CTI loads, the grain spacers were 3/32" thick, so I used thinner o-rings like AT uses. On some, I had to leave them out completely. Always worked for me. YMMV.

Edit: I guess I should add, Any CTI 75mm motor... I've even had trouble getting the FSD in AT 75 and 98 loads, and had to leave out grain spacers to make those work too.

 

[thatotherguy501]

Thanks so much for the replies. I'm looking into thinner O-rings now, and I'll see about sanding off just enough to get the grains to fit that way too. Much appreciated!

 

[richP]

I had the same problem as well. I do believe that using thinner grain spacers, or removing 1/8" from a grain would work equally well. Might even make sense to do both, since last I tried, 3/16" would have probably been ideal.
My fix (as a newbie) was to tighten down and compress the o-rings as much as possible using a bench vice and gloved hands. I was able to get the gap closed quite a bit, but still had some blow-by. Nevermind the difficulty in disassembly afterwards.
For those that mention removing grain spacer o-rings, does it make a difference (or is one preferable) in omitting them at the fwd or aft end of the motor?

 

[jd2cylman]

Nevermind the difficulty in disassembly afterwards.
For those that mention removing grain spacer o-rings, does it make a difference (or is one preferable) in omitting them at the fwd or aft end of the motor?

I bought a couple of rubber coated strap filter wrenches for just such an occasion.
As far as which end to leave out spacers; on most I left out the ones closest to the nozzle. On a two (or three, don't remember) grain I left them all out.

 

[mikec]

Leakage past the CTI insulating disk may be caused by the liner not being cut evenly or perhaps cut to the wrong length, permitting a gap, and seems to happen more often on the hotter-burning CTI propellants (C-star particularly, maybe White Thunder).

 

[Cory]

The purpose of the grain o-rings is to encourage flame propagation between grains - shouldn’t matter much unless you get a bunch of grease or glue on a grain face it will burn. I like the idea of just ditching O-rings starting from the nozzle side until the fit is good.

 

[mikec]

Are people just putting the seal disk in the liner without an O-ring? That seems like it won't seal that well.

As for the grain spacers, I'm a little leery of just omitting parts from the instructions, although I acknowledge it may not matter.

For the handful of times I've flown 75mm CTI reloads in AT hardware, I've just followed the instructions and used the supplied phenolic disk. No problems yet but obviously some people have had trouble.

The cross-compatibility thing always leave the warranty situation in the event of a problem pretty muddy. If you follow the AT recommendation and use the seal disk with a CTI reload and your hardware gets damaged, you have to get any replacement from CTI, as AT explicitly says there's no warranty from them in this use case. https://www.aerotech-rocketry.com/news.aspx?y=2016

 

[JimJarvis50]

I have flown the 75mm CTI reloads in AMW hardware with the AT seal disk and a hub quite a few times. I sand down a grain until the disk fits flush to the top of the liner. I don't use a spacer on the top grain, but as it turns out, the spacers are a perfect fit for the o-ring on the disk.

Sanding down a grain this way takes a while as quite a bit of material needs to be removed. I use a flat board with 60 grit sandpaper attached with double-sided tape. Then, just sand in circles until enough is removed. Takes about 30 minutes or so. When doing this, it is real important not to get sanding dust on your skin. So, take PPE measures to prevent this and to not accumulate too much of the dust at a time.

Jim

 

[jd2cylman]

Are people just putting the seal disk in the liner without an O-ring? That seems like it won't seal that well.

Nope, use an o-ring. As has been said, CTI's grain spacers are the same size as AT's seal disc o-rings. AT's grain spacer o-rings are the next diameter smaller. I ordered extra o-rings for seal discs from McMaster Carr and keep a bunch on hand for crossloads.

 

[FredA]

t is real important not to get sanding dust on your skin.

What's your concern Jim???
I wouldn't breath the stuff, but I really don't see the problem with skin contact.

 

[JimJarvis50]

What's your concern Jim???
I wouldn't breath the stuff, but I really don't see the problem with skin contact.

Not too long ago, there was a fellow that was cutting grains and burning scraps (I think - I don't recall the details). He got his arms covered in dust and managed to ignite it. He was burned pretty badly. I don't know if these are identical situations, particularly since the sanding I mentioned doesn't raise all that much dust, but I think if you're raising APCP dust of any sort, you should be careful.

Jim

 

[thatotherguy501]

Update here: I used a combination of 036 O-rings (1/16" thick instead of 3/32" thick), and gently sanded the face of one grain with 120 grit sandpaper. Not the fastest process, but the seal disk did fit thereafter. Upon assembling the motors, there was a slight gap between the aft closure and casing, but not more than 1/8" (partly because the seal disk is thicker than the provided insulating plate). Thank you for the advice; I will probably do this from here on out with 75 and 98mm CTI motors when possible.

 

[dhbarr]

Update here: I used a combination of 036 O-rings (1/16" thick instead of 3/32" thick), and gently sanded the face of one grain with 120 grit sandpaper. Not the fastest process, but the seal disk did fit thereafter. Upon assembling the motors, there was a slight gap between the aft closure and casing, but not more than 1/8" (partly because the seal disk is thicker than the provided insulating plate). Thank you for the advice; I will probably do this from here on out with 75 and 98mm CTI motors when possible.

Not that it matters, but wouldn't we retain ever-so-slightly more propellant by only sanding the bottom of the bottom grain; but in a conical fashion such that the center domed down into the convergent portion of the nozzle ?