75mm grains to big for liner

My M1101 75mm 4 grain reload, the grains are .008 bigger than the liner and wont slide in. I talked to Brian at CTI about the numbers and he suggested I sand down all 4 grains to fit. That could take a while sanding about .010 off each one. Liner must have absorbed some moisture over time and shrunk a little. They only use 2 ply on 75mm reloads so he doesn't like peeling outside layer of paper off the grain.(I think I have done this before) He is going to get back with me on the exact tolerances of the liner and the 75mm grains. Liner is about 2.750", and the grains are 2.758". I hate sanding for this by hand! Maybe get a new liner. Anybody else have this problem?

How much space between the liner and the case? How about expanding the liner with a hairdryer and see if the grains will slide in? Or chilling the grains in the freezer?

Any commercial mfgr, worth it's salt should make this right. To the point of authorizing a vendor to replace the motor.

Failing that...

Make sure there isn't a burr on the liner lip giving you the problem. Use a knife or sand paper to chamfer the inside edge. That might help.

I don't really see the difference between sanding and peeling the outside of the grains. If there is enough to sand I would think there would be enough to peel. My inclination would be to peel and then glue the grains into the liner.

Good luck and YMMV.

--Lance.

jderimig said:

How much space between the liner and the case? How about expanding the liner with a hairdryer and see if the grains will slide in? Or chilling the grains in the freezer?

Click to expand...

I will mic the ID of case and OD of liner when I get home from work tonight to see how much room I have. Can you freeze grains to get them to shrink a bit? Never tied this with propellant before. I will get back with you tomorrow. Thank You for the advise!

llickteig1 said:

Any commercial mfgr, worth it's salt should make this right. To the point of authorizing a vendor to replace the motor.

Failing that...

Make sure there isn't a burr on the liner lip giving you the problem. Use a knife or sand paper to chamfer the inside edge. That might help.

I don't really see the difference between sanding and peeling the outside of the grains. If there is enough to sand I would think there would be enough to peel. My inclination would be to peel and then glue the grains into the liner.

Good luck and YMMV.

--Lance.

Click to expand...

I have touched up the end of liner with sandpaper. I have even sanded one grain with 180 grit for a few minutes on outside to see how fast it comes off. Not fast enough for me! Then there is 4. Your right, sanding or peeling, whats the difference? Peeling is faster! Have not contacted the vender about this yet. I went right to CTI and Brian. Waiting for exact tolerance numbers from Brian.

On some AT motors of same size, I have had to peel off the glassine layer to get them to fit.

I had a 75mm L1395 in which the liner was too long, like almost 2", vendor told me to cut it down gave me the exact length. It was still a little long,ended up with a gap on the aft closure, nothing bad happened .

I've had to grind the inhibitors on Kosdon grains to get them to fit into the liner. Then peel the liner to get it to fit into the casing.

I'm still going to be sad when I burn the last of my Kosdon loads.

If a vendor told me to I had to cut 2” off a liner I would tell him to go to (somewhere) and bad mouth him for the rest of my life.
The manufacturer should replace defective parts including shipping both ways.

M

MClark said:

If a vendor told me to I had to cut 2” off a liner I would tell him to go to (somewhere) and bad mouth him for the rest of my life.
The manufacturer should replace defective parts including shipping both ways.

M

Click to expand...

I respect the all the vendors too much to do something like that, the limited vendors that travel and attend launches don't need that. Not going to bad mouth on the forum either,vendors just sell the motors, the vendor helped me over the phone both times I called.

If the vendor knowingly sells you a wrong part (saying you have to cut liner) that is totally unacceptable.
The motor was not certified with the requirement of end user cutting liner so they are selling uncertified motors

timbucktoo said:

On some AT motors of same size, I have had to peel off the glassine layer to get them to fit.

Click to expand...

Now I remember! I had to peel glassine layer off of AT M1297 reload (almost same as CTI M1101) to get it to fit in my CTI case. Crossloads I think they call them. Motor burned just fine. The rest of the flight, well not so much!

I see what you mean, He didn't know, I bought the motor at a regional launch intending to fly it at my local club later. 2 months later I checked the parts and fit a week before the launch and found it long, he contacted CTI about it too.

I also remember it being harder to clean up afterwards. (after peeling the glassine off of AT M1297 reload) Not saying that peeling the layer off had anything to do with it, but it might.

HyperSonic said:

My M1101 75mm 4 grain reload, the grains are .008 bigger than the liner and wont slide in. I talked to Brian at CTI about the numbers and he suggested I sand down all 4 grains to fit. That could take a while sanding about .010 off each one. Liner must have absorbed some moisture over time and shrunk a little. They only use 2 ply on 75mm reloads so he doesn't like peeling outside layer of paper off the grain.(I think I have done this before) He is going to get back with me on the exact tolerances of the liner and the 75mm grains. Liner is about 2.750", and the grains are 2.758". I hate sanding for this by hand! Maybe get a new liner. Anybody else have this problem?

Click to expand...

At Red Glard I helped some respectable people who know what they are doing sand down a similar liner with an electric sander and old 80 grit sand paper. It took like 15 min and 6 or so passes even with the powered sander.

I'd request a new liner once you know the ID of it is off (as opposed to swollen grains).

Sanding vs. peeling would be to remove less material than peeling. With sanding, you may sand unevenly or too far into some spots.

I like the idea stated above of peeling and gluing the grains in. You would need to be sure the flame can't get between the liner and the remaining casting tube in case the casting tube is not thick enough to prevent a burn from the outside of the grain inward.

I had to sand old AT phenolic liners to get them into the case before - probably back in the 90s with a 38mm reload. That is a long process by hand.

Chill out, folks. It happens, and it is not limited to one mfg. I had a 76mm Loki motor with the same problem. I peeled off the outer layer of glasine; then they were a little too small. So I touched them up with masking tape. Motor flew like a bird.

Edit: Scott from Loki was extremely helpful, as was my vendor, OneBadHawk.

Minor problems may be unavoidable, but customers should never have to make large adjustments. If you get a motor that has significant problems that you don’t feel comfortable fixing, turn in a MESS report please. The certification organizations need to know when quality control suffers.

Just peel to fit. I doit all the time. I don't sand, I peel off the paper. Sometimes as far as the brown paper.

Yep. Peel the casting tube, glue the grains in. Done.

But yes, QC is a concern. Just stumbled across a sealed reload last night that was missing the top grain. Huh.

After talking with Brian at CTI, the plan is what alot of you have suggested. Peel and glue. The problem is half liner, and half over sized grains. The liner should be 2.559 with .004 tolerance plus or minus. My liner is 2.550. My grains are about .008 to .010 bigger than the grains Brian mic of same motor White propellant 75mm still at CTI. He likes sanding over peeling because sometimes when you peel it comes off thin in one area and thick in another. This unevenness could cause trouble with flame on back side. It would take too much sanding to take off .010 by hand. So the game plan is peel, sand a little to make sure it slides in liner nicely, then glue them in place. The glue will fill any voids on back side if it did not peel evenly. He doesn't recommend freezing the grains because of different expand contract ratios between the casting tube and the propellant which might cause unwanted stress on grain and small cracks develope. I know others have problems with CTI and the G78BS. In my experience with Brian and CTI, he called me back two days in a row and talked for 45 min each time. He answered every question I had and then some. He was very professional and I feel he did a great job telling me my options and what to do. When they matched what you guys have already said, I knew I was on the right track. Its not that I don't believe you guys, because I very much trust your expertise in these matters, I just wanted to here it from Brian and CTI. If I didn't respect you guys advise, I wouldn't be here!