Moonburner malady

[jrkennedy2]

Well, my June 1st didn't go so well. I was at a launch attempting to fly a 4” Mania on a CTI Pro75 7521M840-P, moonburner. Simed out to about 20,000'. Morning started windy but settled down just about when the window opened (1400 hours). Dual deployment computers came online fine. GPS got lock fine. Mobius camera was rolling. Igniter installed and aft closure re-tightened. Countdown 5,4,3,2,1, launch... At about .25 seconds after initial ignition, BANG! The motor seems to have blocked the exhaust nozzle and over-pressured. It blew the rear motor case ring out, nearly cutting a “coin” out of the exhaust deflector (pretty cool to see, actually). The Aero Pac retainer split cleanly at the pressure area (rim). The recoil of the case letting go shot the 6gr case through the top of the rocket, rendering to dust the: 2 RRC2s (the entire av-bay), 1 Eggfinder GPS, and 1 mobius camera as well as mauling the harnesses badly. I was unable to locate the rear case ring and nozzle. They probably bounced to the next county. I found 5 of the grains in pretty good shape and the sixth one fractured and in pieces with some bits burned-up. Each grain showed tearing of the paper casing indicating that at least some resistance was felt to the grain movement. I utilized Gorilla Glue to fix the grains in the liner. I used a dowel to ensure the alignment of the off-center bores were straight. I verified the igniter was all the way to the top by measuring the length on the cardboard tube (included) against the length of the 6 grains. Check, check, check...

My only guess is the bottom (closest to the nozzle) grain (G6 for now) came loose as the pressure started to rise and subsequently blocked the nozzle causing over-pressure. During assembly, there was a small space, about 1”, between the G6 and the throat of the nozzle, preserved by the liner. By this I mean the liner met the nozzle and kept that space from G6. I assume that space is the combustion stabilization area since the bores are offset and need a little room to get the gasses moving a bit better toward the nozzle. Also, it would have been VERY hard to get the igniter dowel (or cardboard tube) to make any steeper a turn without that space. Any thoughts what went wrong (what I did wrong)? I have some picts to help illustrate.

I pretty sure it was assembler's error (me) not CTI.

 

[mikec]

I utilized Gorilla Glue to fix the grains in the liner.

Does it say to do that? You did bond the grains together as described in https://www.pro38.com/pdfs/Moonburner_supplement_segmented_V1.2.pdf I assume?

 

[jrkennedy2]

Their adhesive is known to get too thick as the shelf life goes on. GG was suggested as a better alternative. But, maybe others will chime-in on the adhesives.

Otherwise, yes I followed the instructions.

Have you had good or bad experience with moonburners? Anything that you recall as particularly challenging?

 

[Steve Shannon]

Don’t assume that you made a mistake. File a MESS Report (even if you’re 100% sure the mistake was yours) and contact your dealer about it.
As long as the grains were glued together with the end surfaces of each grain inhibited so the only burning surface is the core, you’re probably okay. Sometimes the bottom grain will have a larger core than the other grains but without reading the CTI instructions I don’t know if that’s the case here.

 

[mikec]

Have you had good or bad experience with moonburners? Anything that you recall as particularly challenging?

I've flown several AT moonburners and getting the grains bonded together is a pain. The CTI instructions seem even more painful than the AT ones -- "Make sure to complete all steps 2-10 within 10 minutes after mixing of the adhesive" -- yeah right, good luck with that. I can see why you wanted to use a different glue.

It seems to me more likely that this was just an overpressure CATO, maybe from a void in one of the grains. Definitely file a MESS and CTI should do a warranty replacement of the case and motor.

 

[jrkennedy2]

Thank you for the advice! I will file the "MESS" since I made a mess. :wink:

I installed the grains in the order they were marked but I didn't notice any different bore diameter. But that just means I didn't notice. I did not measure the bore diameter, just noticed that all the diameters were pretty close on the dowel as well as snug on the liner.

I was thinking about the gluing technique and I recalled that I thought that it was, well I don't want to say ineffective but, seems like most of the adhesive would be scrapped away due to the snug fit of the grains being slid down the liner, like a plunger on a syringe. Should the liner be pre-treated (goobered-up, very technical term) with like a bore mop from a cannon, dipped in adhesive ? I but then again, maybe the default process makes sense 'cause the G1 would leave a little adhesive behind for G2, etc. By the time G6 got there, it'd be nice and covered? Can you tell I second-guess myself a lot?

I have to admit, I kinda like getting my butt kicked by this motor. Means I have yet another thing to master (or at least figure out)!

What is the smallest (least expensive) moonburner out there? I may have to build a few and static fire them. Be financially painful to do a few of these size that way.

The piggybanks all look worried when I walk by now...

 

[Steve Shannon]

Thank you for the advice! I will file the "MESS" since I made a mess. :wink:

I installed the grains in the order they were marked but I didn't notice any different bore diameter. But that just means I didn't notice. I did not measure the bore diameter, just noticed that all the diameters were pretty close on the dowel as well as snug on the liner.

I was thinking about the gluing technique and I recalled that I thought that it was, well I don't want to say ineffective but, seems like most of the adhesive would be scrapped away due to the snug fit of the grains being slid down the liner, like a plunger on a syringe. Should the liner be pre-treated (goobered-up, very technical term) with like a bore mop from a cannon, dipped in adhesive ? I but then again, maybe the default process makes sense 'cause the G1 would leave a little adhesive behind for G2, etc. By the time G6 got there, it'd be nice and covered? Can you tell I second-guess myself a lot?

I have to admit, I kinda like getting my butt kicked by this motor. Means I have yet another thing to master (or at least figure out)! [emoji3]

What is the smallest (least expensive) moonburner out there? I may have to build a few and static fire them. Be financially painful to do a few of these size that way.

The piggybanks all look worried when I walk by now...

You’re talking about the glue on the outside surfaces of the cylindrical grains but the most important glue used in the moon burn motors is that applied directly to the flat grain ends, the tops and bottoms, which bond the separate grains into one integrated grain. Those surfaces, unlike a Bates grain motor, do not burn. Only the off-center core burns which results in the distinct crescent [emoji287] cross-section while burning.

 

[ericm541]

If you fly areotech there's a few 54MM moonburner motors, J135,K185, K270. All require the forward extended closure, but no grain bonding, why static fire a commercial motor?

 

[djs]

What is the smallest (least expensive) moonburner out there?

CTI has a 29mm G54 red moonburn motor. It's lots of fun (and no grain face gluing).

 

[jsdemar]

You’re talking about the glue on the outside surfaces of the cylindrical grains but the most important glue used in the moon burn motors is that allied directly to the flat grain ends, the tops and bottoms, which bond the separate grains into one integrated grain. Those surfaces, unlike a Bates grain motor, do not burn. Only the off-center core burns which results in the distinct crescent [emoji287] cross-section while burning.

I agree with Steve. It is far more important to inhibit the end faces of the grains for this motor then to have them glued into the liner. I don't think gorilla glue is best for the end-face gluing. It should be an inhibitor based on HTPB. The instruction say there is part A and part B to the adhesive. I expect this is the same HTPB+curative that comes with the O3400 which I've flown.
https://www.pro38.com/pdfs/Moonburner_supplement_segmented_V1.2.pdf

The thrust curve for the M840 shows a high initial spike already. Anything that would expose more propellant surface would give the cato you had.

 

[jrkennedy2]

If you fly areotech there's a few 54MM moonburner motors, J135,K185, K270. All require the forward extended closure, but no grain bonding, why static fire a commercial motor?

Well, I shredded a lot of rocket and electronics with my last catastrophe! Figure I’d better get it right before I shred another one/set. [emoji12]

 

[JimJarvis50]

You’re talking about the glue on the outside surfaces of the cylindrical grains but the most important glue used in the moon burn motors is that allied directly to the flat grain ends, the tops and bottoms, which bond the separate grains into one integrated grain. Those surfaces, unlike a Bates grain motor, do not burn. Only the off-center core burns which results in the distinct crescent [emoji287] cross-section while burning.

And following up on this, I don't think Gorilla Glue (the foaming type typically used) would be the right glue to use on the faces of the grains. Needs to be the HTPB type.

Jim

Edit - Didn't see John's post

 

[jrkennedy2]

You’re talking about the glue on the outside surfaces of the cylindrical grains but the most important glue used in the moon burn motors is that allied directly to the flat grain ends, the tops and bottoms, which bond the separate grains into one integrated grain. Those surfaces, unlike a Bates grain motor, do not burn. Only the off-center core burns which results in the distinct crescent [emoji287] cross-section while burning.

Ah! Mongo-grain...

Impossible to ship and even harder to pour I bet.

 

[mikec]

It should be an inhibitor based on HTPB.

Aerotech just tells you to use regular hobby epoxy for their moonburners, so I'm skeptical that this makes that much difference, although I don't know if Gorilla Glue would work.

 

[jrkennedy2]

OK, now I’m learning something! HTPB is sort-of APCP in non-cured form to which you add isocyanates to make binder AND fuel. The initial trouble I had was that the adhesive CTI supplied, HTPB, was super viscous and even though I rehearsed several times, the assembly, when I mixed the adhesive, applied to the first grain, and inserted into the liner, it seized about 3/4 the way to the bottom, never to move again! Is there a proper “solvent” that will temporarily reduce the viscosity while not damaging the HTPB?

I did look at the grain ends and there was evidence of tearing at the bind site but the depth of the irregular tears are quite shallow, .001” estimated. Perhaps the HTPB would have become the grain and fractures would have been as likely to occur mid-grain since the binder was essentially the same stuff as the grain?

Much thanks!!!!

 

[mikec]

the adhesive CTI supplied, HTPB

People are just speculating about that, I have no idea what the CTI adhesive is -- CTI doesn't use HTPB in their propellant AFAIK (it's thermoplastic) so I don't know why they would use it for adhesive. As I've said, AT just tells you to use hobby epoxy on their moonburners; if you use 30-minute you have more time. Further, they don't tell you to glue the grains to the liner, so you can assemble them outside the liner or at least not worry about having them seize.

 

[markkoelsch]

Aerotech just tells you to use regular hobby epoxy for their moonburners, so I'm skeptical that this makes that much difference, although I don't know if Gorilla Glue would work.

You might well be wrong. The gorilla glue might fail exposing the grain ends. I would suggest that you use the recommended glue/inhibitor.

 

[JimJarvis50]

People are just speculating about that, I have no idea what the CTI adhesive is

No, not speculating...

Jim

 

[djs]

when I mixed the adhesive, applied to the first grain, and inserted into the liner, it seized about 3/4 the way to the bottom, never to move again!

I'm confused by this- how did you fly the motor then?

 

[jrkennedy2]

https://en.m.wikipedia.org/wiki/Hydroxyl-terminated_polybutadiene

Does not indicate any ready solvents. But, second semester organic chemistry basically kicked my butt and ended my hopes for pre-med. Got a C... So I could be missing it.

I think my next try I will sand the grains so the nearly fall to the bottom of the liner under their own weight. The one that got stuck required about 20# of pressing to get it to move during rehearsal without any adhesive! The adhesive only made it worse. All the grains were this tight.

Bottom line, I think the mystery is solved. GG in this initial high thrust motor, 2000 Newtons, will not do. I need to find out the exact fit required of the grain in the liner. And I need to know the expected viscosity of the HTPB.

Any volunteers? CTI?

 

[djs]

Bottom line, I think the mystery is solved. GG in this initial high thrust motor, 2000 Newtons, will not do. I need to find out the exact fit required of the grain in the liner.

I'm not sure you are getting the point here.

There are two types of grain gluing

1. Gluing the grains into the liner, so that they cannot move vertically. This is what you did
2. Gluing the grains on their ends to each other. This is what really needs to be done.

#2 is required because a moonburner type propellant grain cannot allow burning to occur in the gaps between the grains. Otherwise it will CATO.

 

[jsdemar]

People are just speculating about that, I have no idea what the CTI adhesive is -- CTI doesn't use HTPB in their propellant AFAIK (it's thermoplastic) so I don't know why they would use it for adhesive. As I've said, AT just tells you to use hobby epoxy on their moonburners; if you use 30-minute you have more time. Further, they don't tell you to glue the grains to the liner, so you can assemble them outside the liner or at least not worry about having them seize.

No, it is HTPB+curative for the adhesive. The binder in some CTI propellants is CTPB, and HTPB in others. Technically, it isn't a "thermoplastic", but it can be extruded.

Also, just because AT tells you to use hobby epoxy, it doesn't mean it is the best adhesive to use. Epoxy cracks and burns easily, allowing the flame front to get under the epoxy. I gave up on using epoxy inside a motor 15 years ago.

 

[mikec]

No, it is HTPB+curative for the adhesive.

I stand corrected, the MSDS for the Pro98 (and I assume Pro75) adhesive on the CTI website https://www.pro38.com/MSDS/SDS--Pro98_Adhesives--ver1.00-EN.pdf says it's HTPB.

All I know about CTI propellant formulation is from https://www.freepatentsonline.com/6740180.html but I expect that some propellant types use this and some don't.

 

[jrkennedy2]

I'm not sure you are getting the point here.

There are two types of grain gluing

1. Gluing the grains into the liner, so that they cannot move vertically. This is what you did
2. Gluing the grains on their ends to each other. This is what really needs to be done.

#2 is required because a moonburner type propellant grain cannot allow burning to occur in the gaps between the grains. Otherwise it will CATO.

I followed both steps as described. Difference is that I used GG instead of HTPB and curative since latter got the first grain stuck part-way down the liner on my first try. There is no visible evidence of burning between grains except that grain 6 is very damaged so it is hard to tell on that one. What is seen between grains is small roughness left behind as the GG failed, like when you tear apart a fiberous material bonded by glue. I think my primary failure was adhesive choice.

 

[tfish]

Is this the same motor?

https://www.rocketryforum.com/showt...6g-CTI-M-840-case-liner&p=1705881#post1705881

Tony

 

[OverTheTop]

I have flown a couple of these. I mix the glue and then apply it quickly to the grains on the required locations (outer cylinder and faces where needed) and push the grains home with a big stick. In order to apply it quickly I just use gloves (not the supplied brush) and grab the necessary amount of glue and quickly smear it (both hands get sticky) on the specified surfaces. That grain is then rammed into place. I mixed half the glue for the first three grains, and then after they were installed mixed the other half. That gets around the problem of the glue being exothermic and the generated heat setting it off quicker to some degree. Work quickly!

I have been similarly caught with an O3400 going off really quickly in the past due to high ambient temperatures. Consider chilling the grains a little, or keeping the mixed glue in the pot a bit cooler by setting in ice water. Might give you a little more breathing space :wink:

 

[Steve Shannon]

I have flown a couple of these. I mix the glue and then apply it quickly to the grains on the required locations (outer cylinder and faces where needed) and push the grains home with a big stick. In order to apply it quickly I just use glove and grab the necessary amount of glue and quickly smear it (both hands get sticky) on the specified surfaces. That grain is then rammed into place. I mixed half the glue for the first three grains, and then after they were installed mixed the other half. That gets around the problem of the glue being exothermic and the generated heat setting it off quicker to some degree.

I have been similarly caught with an O3400 going off really quickly in the past due to high ambient temperatures. Consider chilling the grains a little, or keeping the mixed glue in the pot a bit cooler by setting in ice water. Might give you a little more breathing space :wink:

To help keep the glue from kicking, as soon as you mix it, pour it into a shallow metal pan. That doesn’t allow the heat to build up.

 

[jrkennedy2]

Is this the same motor?

https://www.rocketryforum.com/showt...6g-CTI-M-840-case-liner&p=1705881#post1705881

Tony

Yep, that was my initial try...

But I'm stubborn. I'm already getting the itch to buy another 6gr case, another Mirage rocket, and yes, another M840. Or, is that I'm just slow to learn...???

I wonder if CTI will sell and extra "batch" of the adhesive so I don't get "scarcity fear", a.k.a., do I have enough to cover the grains effectively?

 

[mikec]

If the grains are as tight in the liner as you seem to be encountering, I'm doubtful that the bonding process is ever going to work out well.

 

[blackjack2564]

Peel the glassine off the grain if need be, to get an easy fit. Pre fit all grains.

Place glue in fridge for 20-30 minutes to stall "setting"

Do NOT try this procedure if temp over 80. If done at 70 ish.... you will have 30 -40 minutes work time with several hour cure.

I helped do many O-3400's over time & a few moon burn. Worst scenario was at night outside, had been over 100 during day & felt sooooo much cooler that night. BUT it actually wasn't, turned out to be 90. We mixed and began to glue. Luckily there were 2 of us, that knew what we were doing. Every thing was pre fit. In any case we had less than 10 minutes before thing started seizing up and a real rush to get last grain in.

One pours a bit of glue on sides grain while turning, [whole shebang done horizontally] while other using gloved hands, smears it all over surface of grain & insert into liner with dowel to top. Repeat till all loaded. Use fingers to smear on grain faces with care to go right to edge of hole. [a bit in core won't hurt, have paper towels at the ready if needed to wipe out core clean.] Stand vertical on nozzle while curing.
Have acetone [best] or denatured alcohol on hand with plenty of towels to wipe clean glue where it shouldn't be!!

Lessons learned...stay away from higher ambient temps when doing this & if borderline...stick glue in fridge as mentioned. Its a heck of a lot easier, with 2 people doing this. {especially if 1 has experience doing it correctly}
Every procedure has it's own unwritten little tricks, now you know a couple of them...lol

Remember these motors are cast as single grain....numbered & cut-a-part for shipping. You are trying to make it "whole" again, so it burns as one long slug of propellent. Keeping the ends of each grain sealed, so they don't ignite. Just the opposite of a normal bates grain motor, where all ends are consumed with core at same time.

Edit PS:
if building/gluing a standard bates grain, push grain in liner only 1/2in from edge, then you can put O-ring spacer,on face of of grain, pushing all into place, with next glue covered grain.[because you building flat/horizontal on table]
Not worrying about the O-ring "rolling" up and getting caught, like I have seen when folks build vertical trying to drop O-ring on top of inserted grain. Then they wonder why the last grain won't quite fit in....lol