The Mighty M840 bonding grains

The Rocketry Forum

Help Support The Rocketry Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

Wayco

Desert Rat Rocketeer
TRF Supporter
Joined
Mar 23, 2011
Messages
4,328
Reaction score
2,089
Location
Goodyear, AZ
I'm about to bond the grains in the M840, my first try at this technique. Has anyone here done this before?
I could use any tips on this if you have.
I have read the supplemental instructions included, and opened the adhesive kit. Since this is a 75mm Moonburner, I have to supply the mandrel to align the grains on, any suggestions on what I should use?
My main concern is assembling all six grains onto the mandrel in less than 10 minutes, before the adhesive cures.

So if you have done this job, let me know how you went about it.
 
I just built and flew the M650 Moonburner and I used a 3/4 in dowel and followed the instructions.

Although I did read them atleast 15 times before I started the build.

I would recommend a longer cure epoxy, less stress on being fast and getting it all aligned.
 
I have done 3 of them. Like said, use a mandrel. I used a wooden dowel. Remove the dowel before the glue sets up. Make good coverage with all surfaces. Pretty straight forward if you follow the included directions.
 
I just built and flew the M650 Moonburner and I used a 3/4 in dowel and followed the instructions.

Although I did read them atleast 15 times before I started the build.

I would recommend a longer cure epoxy, less stress on being fast and getting it all aligned.

The CTI motor comes with the bonding glue. It has a nice long pot life.
 
Yeah, I have read through it twice so far, and will probably read it again in the morning. I like to do these kind of things early in the day, while I'm fresh.
I will probably use the supplied epoxy though. Do a couple of dry runs so I have everything lined up, and go for it. Sounds like it might be messy, did you do anything special to clean it up?
 
Give me a call in the morning before you go live with the glue. I will give you a few pointers that may help you out.

630-670-4608

-Jason
 
Last edited:
Yeah, I have read through it twice so far, and will probably read it again in the morning. I like to do these kind of things early in the day, while I'm fresh.
I will probably use the supplied epoxy though. Do a couple of dry runs so I have everything lined up, and go for it. Sounds like it might be messy, did you do anything special to clean it up?

I assembled one using a 3/4 copper pipe that I had on hand. Assembly is straight forward and the adhesive supplied gives plenty of assembly time. Read the instructions a couple times and proceed, it really isn't as daunting as it first appears.

Greg
 
Give me a call in the morning before you go live with the glue. I will give you a few pointers that may help you out.

640-670-4608

-Jason

Thanks Jason, I will do that. Are you on Central time? We also need to discuss ejection charges on the UDS...

I assembled one using a 3/4 copper pipe that I had on hand. Assembly is straight forward and the adhesive supplied gives plenty of assembly time. Read the instructions a couple times and proceed, it really isn't as daunting as it first appears.

Greg

Thanks Greg, my main concern was the 10 minute cure time they mentioned in the instructions. Did you find it to be longer?
 
Thanks Greg, my main concern was the 10 minute cure time they mentioned in the instructions. Did you find it to be longer?[/QUOTE]

I had everything laid out and if I remember correctly, I believe it only took me 4-5 minutes to assemble and then waited for a good while before the adhesive turned "rubbery"
I definitely remember being really nervous and concerned about the assembly and after I was done, thinking what was all the fuss was about. :)
Greg
 
Thanks Jason, I will do that. Are you on Central time? We also need to discuss ejection charges on the UDS...

Yup,

Chicago time. I'm usually up by 8am or so. Feel free to reach out.
 
Grains.jpgCore alignment is not super critical. I assembled an M685 (6 grain) at Bruno's without the aide of a dowel, and got it pretty close. Just eyeballed the core as I slide the grains in the liner. Only problem I had with the alignment not being perfect was the igniter caught an edge when inserting and it took a few tries to get it all the way to the top. At ignition the motor catoed in spectacular fashion. Kidding :).

I asked Karl at AT about the alignment and he said AT has test fired moonburners with purposely misaligned cores and they performed nominally.

Also the AT instructions have you bond the grain faces in pairs outside the liner, letting them cure and then putting them in the liner and bonding the pairs together.

Might want to check the fit of the grains in the liner too before beginning.
 
Last edited:
I have seen a nice video of someone bonding and building a CTI M2250

Will look for it :

https://youtu.be/JYYYqeniVwE

Kenny

Thanks Kenny, I found that video too. Once I determined it was for a different type of motor I stopped watching it. It's like 20 minutes long.

I also want to thank everyone who contributed to this thread, I feel a lot more confident I can do this right the first time. Now I'm gonna clear a space in the epoxy room, lay everything out and fit it all together with a dry run.
 
Hi Wayne,
I have built several M840's and last week did a N1560 moonburner.
If you can build it where it is cooler (A/C) you will have no problems with the pot life.
If you build it in the heat you will need several more hands and need to work fast.
Don't bother messing around trying to "paint" the adhesive on to the grain. Put on nitrile gloves, pour the adhesive into you hands and apply it on to the grains. And yes, the grain ends also get adhesive.
I use PVC tube for a mandrel. Pull it before the adhesive cures.
Put painters tape and paper on the motor liner. The adhesive oozes over the top as you drop in the grains. Once built, wipe off the adhesive from the liner and remove the tape. Easy clean-up.

One note on the N1560. The core is tapered. Have to use a couple different mandrels during the build.

Good Luck,

Chris
 
Yeah, I have read through it twice so far, and will probably read it again in the morning. I like to do these kind of things early in the day, while I'm fresh.
I will probably use the supplied epoxy though. Do a couple of dry runs so I have everything lined up, and go for it. Sounds like it might be messy, did you do anything special to clean it up?

If they supplied a glue, you need to use it. It is probably a R45 curative mix to bond the grain together.
 
If they supplied a glue, you need to use it. It is probably a R45 curative mix to bond the grain together.

Dang Chris, you must have been on the same frequency as me, I was just posting up about that. I talked to Jason (Griffin) this morning before I did the deed, and he explained that the "glue" they include is actually two of the ingredients Sharon uses when she mixes up her motors. I was just checking in the "Experimental Composite Propellant" book to see if I could get the components right. Sharon has the chemistry degree, so I'm way behind on terminology when she explains this stuff, but basically, the epoxy included has what I think is PBAN and R45 curative. When it's cured, it is a yellow rubber material.
I finished the bonding this morning, and built the motor about an hour ago. Only thing I noticed was when the last grain was in place, there was about an inch of space between the last grain and the end of the liner where the nozzle goes. The nozzle slides down into that space about 3/8". I cut a ramp into the propellant from the center of the grain to the offset core to facilitate getting the igniter in, and used a boat tail rear closure on the nozzle and nozzle ring. Since I only have a 6XL casing, and this is a 6 grain motor, I also used a spacer on top of the forward closure, another first for me. There might be some anxiety associated with this launch.... Carbon fiber 75mm minimum diameter rocket that sims over 25k, and has always exceeded the sims, despite my adjustments to Cd in Thrustcurve. Open rocket has also been conservative with the altitude, but I did submit the paperwork with Kloudbusters for an over 25k flight, so if it does manage to go higher I'm covered.
Once again, I want to thank all of you that contributed to this thread. This forum is a great place to get answers to almost anything rocket related.
 
Last edited:
Glad you got it together, you wondering what the fuss was about now too? I actually will be flying the 840 next weekend in a 75 md too and mine sims to about 25,000'. The one I assemblied a few years ago got delayed at our launch the last couple years, hopefully third time is a charm.
We will have compare notes in a couple weeks on our flights. Good luck with yours!

Greg
 
Glad you got it together, you wondering what the fuss was about now too? I actually will be flying the 840 next weekend in a 75 md too and mine sims to about 25,000'. The one I assemblied a few years ago got delayed at our launch the last couple years, hopefully third time is a charm.
We will have compare notes in a couple weeks on our flights. Good luck with yours!

Greg

Not much fuss, but all the comments really helped with the anxiety. So your motor has been assembled for several years? I'm not gonna challenge you to a drag race, but both of us together with our rockets for a picture would be great. Look me up, I probably won't be flying mine until Sunday am. I will be camped next to Wildman's place. There's a picture of my campsite in the "Airfest" thread.
Good luck with your's too!
 
Hey Bob, you almost got to see this rocket, I was prepping it for Hellfire. When I did the Open Rocket sims with the M840 it exceeded the waiver, so I had to set it aside. I probably could have found a smaller motor for it, but with all the other rockets we had, I decided to leave it behind.
This has been one of my stealth builds, I started a build thread on it, but lost interest. It was a post L3 slump I went through. When I got back to it, I attacked it with a vengeance and didn't take any pictures. I just looked through my photobucket and found one shot of it in the 10 ft. tower I built for a flight on an L395 mellow:

070.jpg


It's a pretty rocket, I left the carbon fiber exposed, there are no decals. Here it is with the M840:
001.jpg


I will probably post up a launch report on Airfest, I will try to get some more shots of it there.
 
Last edited:
Not much fuss, but all the comments really helped with the anxiety. So your motor has been assembled for several years? I'm not gonna challenge you to a drag race, but both of us together with our rockets for a picture would be great. Look me up, I probably won't be flying mine until Sunday am. I will be camped next to Wildman's place. There's a picture of my campsite in the "Airfest" thread.
Good luck with your's too!

A picture would be great, but unfortunately your in Arizona and I'm in Nova Scotia, Canada. I only wish I could get to your neck of the woods. But we still can compare notes after our flights.
Greg
 
A picture would be great, but unfortunately your in Arizona and I'm in Nova Scotia, Canada. I only wish I could get to your neck of the woods. But we still can compare notes after our flights.
Greg

Hey, it's only 3,315 miles, or 5335 kilometers. :y:
I though everyone was gonna be in Argonia, Kansas for Airfest. That's only 2,324 miles. If you left now, you might make it in time.
I'm just kidding, its hard to realize how diverse the members on this forum are. I will be posting up my results when I get back from Airfest.
 
I have never heard of an XL nozzle, where would I find that? Maybe that's why it's melting the bottom edge.
Thanks for bringing that to my attention, I just contacted CTI asking about it.
 
Last edited:
If it is an XL nozzle, the bottom of the nozzle would be nearly flush with the end of the boattail. With the M840 9 second burn yeah I think you could get some melting. The M2245, m2250, and K2000 have the longer nozzle among others.
There is a note about using the boattail with XL nozzles only, on the pro 75 dimension drawing on the CTI site.
 
Back
Top