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Dimensions of 75mm, 6g, CTI M 840 case liner?

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jrkennedy2

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Does anyone happen to have the dimensions of 75mm, 6g, CTI M 840 phenolic case liner handy? My "gluing session" didn't go well and I may be having to try again. :facepalm: Have not head back from vendor about getting a replacement liner so I may have to go it on my own.
 

Titan II

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32-7/16 length
2.565 ID
2.731 OD
 
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jrkennedy2

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Well, I did a couple dry-fit runs to make sure everything fit "ok". All was well. I then mixed the 2-part adhesive provided. I then applied liberally (with gloves on) to the grains one at a time as they were inserted. The mix was the consistency of thick maple syrup. I pressed/slid the first grain in to the front insulator/bulkhead. I then did grain #2. It slid down about 80% of the way then seized, leaving about 3" gap between grain #1 and #2. No amount of pole-dancing or other reasonable pressing on the wooden rod (pole) could get the grain to complete the journey. With that, the assembly was a failure. Later on I trimmed the liner to approximate length of the glued grains and then cut the liner laterally and split the liner in 2 pieces off the grain. I then sanded off the glue residue and am ready to try again. I hear Gorilla glue is the stuff to use now-a-days... I think the motor kit was older and the included adhesive was past its' expiration date resulting in a more rapid set-up (much less that the 10 minute described in the instructions). If I had an hydraulic log-splitter and could have possibly used the 10-ton ram to force the grains together but it might have fractured the grains in doing so. Regardless, it should not require that much pressure to assemble I would think.

This motor is a moon-burner configuration, has an offset-channel/bore and I think the glue is to make sure the grains don't twist, blocking the channel and causing an subsequent cato.
 

3stoogesrocketry

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Well, I did a couple dry-fit runs to make sure everything fit "ok". All was well. I then mixed the 2-part adhesive provided. I then applied liberally (with gloves on) to the grains one at a time as they were inserted. The mix was the consistency of thick maple syrup. I pressed/slid the first grain in to the front insulator/bulkhead. I then did grain #2. It slid down about 80% of the way then seized, leaving about 3" gap between grain #1 and #2. No amount of pole-dancing or other reasonable pressing on the wooden rod (pole) could get the grain to complete the journey. With that, the assembly was a failure. Later on I trimmed the liner to approximate length of the glued grains and then cut the liner laterally and split the liner in 2 pieces off the grain. I then sanded off the glue residue and am ready to try again. I hear Gorilla glue is the stuff to use now-a-days... I think the motor kit was older and the included adhesive was past its' expiration date resulting in a more rapid set-up (much less that the 10 minute described in the instructions). If I had an hydraulic log-splitter and could have possibly used the 10-ton ram to force the grains together but it might have fractured the grains in doing so. Regardless, it should not require that much pressure to assemble I would think.

This motor is a moon-burner configuration, has an offset-channel/bore and I think the glue is to make sure the grains don't twist, blocking the channel and causing an subsequent cato.
Correct me if im wrong , I have never built a CTI moonburner . You are supposed to glue all the grains together first with the cores lined up to make the motor a single grain . Then after it cures , and the alingment rod has been removed , then you glue the single grain into the liner . If you simply glue the grains into the liner without glueing the faces together , inhibiting then , you will over pressurize and blow your motor.

Eric
 

mikec

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Read the first sentence on grain assembly pdf you linked. Read it VERY carefully.
I suggest you read the whole document very carefully. You will find that (unlike the equivalent Aerotech instructions), CTI is telling you to both glue the grains together and to glue them to the liner, and to do this all in one operation. (See step #5, "Using the brush, apply adhesive on outside and top of the propellant grain.")

I've never built a CTI moonburner either, but I've built the AT M685, and there they tell you to glue each pair of grains together outside the liner and then glue each pair to each other inside the liner, and not to glue the grains to the liner at all. And that was hard enough, especially considering I had to peel all of the grains to get them to fit into the liner.

Having read the CTI instructions, I have complete sympathy for Randy. If I was going to try this, at a minimum I would do a lot of dry fitting before attempting it, and even then I wonder if it would go smoothly. It sounds to me like either the grains or the liner or both were out of tolerance.
 

3stoogesrocketry

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I will acknowledge my understanding was incorrect. I see that CTI has there own way described in there instructions. I'm used to bonding all the grains together at once , then glueing the stack of cured grains into the liner. This is why I ask for clarification all the time. Even after 20+ years of flying HPR , I still learn new stuff everyday.

Eric
 

jrkennedy2

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The mandrel aligns the cores, yep. The instructions I read seemed to indicate the grains are inserted individually:

From the pdf doc:

Pro75/98® Moonburners
M520-P, M795-P, M840-P, N1100-P, N1560-P
Special instructions for loading (supplement standard Pro75/98® instruction sheets)

located at https://www.pro38.com/pdfs/Moonburner_supplement_segmented_V1.2.pdf

.
.
.
Step 4: Install mandrel in grain #1
Slide mandrel in first grain (grain #1) (see image on the right). Note the
orientation of the grain as indicated by the arrow on the grain inhibitor.

Step 5: Apply adhesive on grain #1
Using the brush, apply adhesive on outside and top of the propellant
grain.

Step 6: Install grain #1
Slide grain into the case insulator using the mandrel to push it all the way
down.

Step 7: Apply adhesive on following grain
Apply adhesive on the next grain. The adhesive should be brushed on
the outside as well as the two propellant faces.

Step 8: Install following grain
Noting the orientation of the grain as indicated by the arrow, slide the
grain over the mandrel and push it all the way down. Make sure the grain
is properly seated on the previous grain by pushing down on it using a
piece of plastic tubing, airframe or yard tool handle.

Step 9: Repeat
Repeat steps 6 and 7 for all other remaining grains. The last grain does
not require application of adhesive on the top surface.

Step 10: Clean up case liner
Wipe of any excess adhesive using paper towel along the top edge of the case liner to ensure that the nozzle will fit
inside the case liner once the adhesive is cured. Also any spilled adhesive on the outside of the liner should be wiped
off.

Step 11: Remove the mandrel
While pushing down on the top grain remove the mandrel. Ensure that the grains do not move during mandrel removal.

Step 12: Adhesive curing
Set grain aside for a minimum of 2 hours before handling it and keep it upright at all times.
.
.
.

However, the thought of gluing them together OUTSIDE the liner is interesting! Hummmm...

Speaking of hummmm, thoughts about the Gorilla glue? Or should, I (I can't help myself...) not monkey around with that?!?
 
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rms

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I have assembled the 75 mm 840 moon burner and the OP was correct in gluing and inserting each individual grain into the liner using a mandrel to line up the offset core, as the supplementary Pro 75 & 98 instructions indicate. I think the faulty adhesive hypothesis may have merit, in my experience I did not encounter any rapid setting of the adhesive and was easily able to clean up excess adhesive from the rear and outside of the liner after assembly was complete. Approximately 6-7 minutes later after first starting assembly. It is a couple years ago but I believe temp was around mid seventies during assembly.
I will admit that I probably didn't mix the two part adhesive for a full three minutes though. Three minutes mixing glue would certainly seem like a long time to me anyway. The glue left in the cup hardened just fine.

Greg
 

3stoogesrocketry

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If you look up Loki's grain assembly on YouTube , yes they are not moonburners , he simply tapes the grains together , then covers the whole stack with glue and pushes it into the liner . It is far superior and easier in my own opinion only.

Eric
 

mikec

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I think the CTI process would be straightforward (though messy) if the grains fit well into the liner with the glue on them and the glue took a while to cure. Given your seizing experience, I suspect that either the glue was faulty or the grains were tight in the liner, or both. I'm not really sure why CTI wants you to glue the grains to the liner, it's not usually required for low-thrust motors.

I'd use 30-minute epoxy before I used Gorilla Glue. I found with the AT motors that you can't reliably glue the grains together outside the liner because the liner holds the grains in alignment; you can do a couple that way, but not a stack of six. I might try a method like the AT M685 where you glue pairs of grains together outside and then glue those together into the liner later.
 

3stoogesrocketry

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I think the CTI process would be straightforward (though messy) if the grains fit well into the liner with the glue on them and the glue took a while to cure. Given your seizing experience, I suspect that either the glue was faulty or the grains were tight in the liner, or both. I'm not really sure why CTI wants you to glue the grains to the liner, it's not usually required for low-thrust motors.

I'd use 30-minute epoxy before I used Gorilla Glue. I found with the AT motors that you can't reliably glue the grains together outside the liner because the liner holds the grains in alignment; you can do a couple that way, but not a stack of six. I might try a method like the AT M685 where you glue pairs of grains together outside and then glue those together into the liner later.
I use a 4 foot section of 2 inch aluminum 90 degree angle sprayed gently with mold release . To hold the grains straight , use rubber bands around the grains to the aluminum angle. Before gluing the grains , rubber band the first grain to the angle and stand the angle vertical . Then one at a time , glue , stack , rubber band . I have done this meathod with up to a 5 grain 75 moonburner .

Eric
 

OverTheTop

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I had the glue go off on me when gluing an N1100 at Thunda back in 2015. I mixed the glue and even working quickly only managed to get three grains home (with extreme pushing for the last one!) before the process was deemed impossible to complete. Luckily all of the three were home, so I could put the remaining three in another day. The temperature was too hot and set the glue off in about three minutes flat. Managed to clean it up, get some more glue and complete the assembly a few months later. Flight was my successful L3 cert.

So, things learnt:
If it is hot the glue goes off really quick
If it is hot, prechill the grains
Assemble in an airconditioned room if possible :)
Mix the glue in two half-batches. This keeps the exothermic reaction heat to a lower level and set the glue slower.
Have assistants helping from the start
Don't stuff around using a brush and delicately apply the glue. That takes valuable time. Just wear gloves and slap it on ASAP!
Motors are much more relaxing to assemble in cool weather.
I also just inserted the grains then (and since also) without a mandrel, but with careful alignment when pushing them home. This saves a fraction of time, but does increase risk a little.

I hope you can save your motor. I just burnt one of them the other day. Love the long burn :)
 
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