Why not glue propellant slugs?

Loki and CTI have certain reloads, in 75 and larger, that recommend gluing the grains together. Why not glue most of them anyway? Is it meant to minimize burn through?

I believe on some of the longer, higher impulse loads the reason for this is to prevent the grains from being spit out of the motor which could lead to a cato. Im sure someone will chime in with further information.

Dave A said:

Loki and CTI have certain reloads, in 75 and larger, that recommend gluing the grains together. Why not glue most of them anyway? Is it meant to minimize burn through?

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Dave A said:

Loki and CTI have certain reloads, in 75 and larger, that recommend gluing the grains together. Why not glue most of them anyway? Is it meant to minimize burn through?

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Because they are developed, tested and certified in the manner that the manufacturer specifies.

The reasoning had to come from testing and then usage in the field. Just wanted to know why we shouldn't do others as well.

You misunderstand. You are not gluing the grains together. The grains still need to burn from each end as well as from the core. You are gluing the grains into the liners. This prevents the casting tubes the grains are cast in from spitting out the nozzle, possibly causing a Cato. When this happens you are not "spitting grains" like you hear most often but spitting pieces of casting tube. Very seldom do grains actually come apart and get ejected from a nozzle. This usually ends in a Cato.

Some moonburners have the grains glued together to create a monolithic grain that burns from the core out and prevent the grain ends from burning. As Jim said the glueing of grains is usually the casting tube to the liner to prevent casting tube from being "spit" out the nozzle.

Greg

Yes, I should have clarified, would you recommend this in all 3" or larger motors?

Not if the manufacturer doesn't!

Greg

L/D ratio, Length to Diameter ratio matters more than just diameter alone.
On longer motors: the nozzle end grain will burnout quicker than the one at top end of the motor.


JD

Dave A said:

Yes, I should have clarified, would you recommend this in all 3" or larger motors?

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JDcluster said:

L/D ratio, Length to Diameter ratio matters more than just diameter alone.
On longer motors: the nozzle end grain will burnout quicker than the one at top end of the motor.


JD

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I thought there was more of an issue with longer motors.
It makes sense on a long motor. Casting tube pieces could have more time to fall out and clog the nozzle.
I plan to burn a 75mm AT M-1850, it's fairly long and thought why not glue in the grains?

I thought that the force of the grain stack from the take off stressed the bottom grain and could lead to a cato if the grains were not glued. In addition to spitting out the casting tubes. Not 100% sure if this is true. It would vary by the structural strength of the propellant grain.

gluing is also a major pain in the butt, if not needed, I skip it. Glued my two K1127's, skipped it on the J1026. J510W's dont get glued in, but they're plenty long. Just depends on the design. The bottoms burnout quicker/more for sure....

If you glue the faces together, you'd seriously affect the burn profile. Some motors need that, some it would lead to bad things.


Now, I definitely think aerotech should be cutting and gluing the slugs on F reloads to dodge hazmat....

Your question of why not. If it's a commercial load and you alter the instructions, it will void the warranty.

Why are some grains glued in? To hold them in place so that they don’t compress or collapse the grains below them. This could be do to high G flights, thin casting tubes or soft propellant. It will also maintain the space between grains to promote flame spread to the ends of the grains. It also minimizes spitting of the casting tubes.

Why are some grains glued together? To maintain core alignment and or to inhibit the burning of the grain faces.

Tony

Dave A said:

I thought there was more of an issue with longer motors.
It makes sense on a long motor. Casting tube pieces could have more time to fall out and clog the nozzle.
I plan to burn a 75mm AT M-1850, it's fairly long and thought why not glue in the grains?

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I flew an M1850W earlier this spring and was surprised that the assembly instructions didn't state anything about gluing the grains in. I did as I was "told" and sure enough, the motor performed perfectly...

AlnessW said:

I flew an M1850W earlier this spring and was surprised that the assembly instructions didn't state anything about gluing the grains in. I did as I was "told" and sure enough, the motor performed perfectly...

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That's because the motor operates at a pretty low pressure, and as such isn't likely to spit any casting tubes on shutdown. Commercial loads are designed for conservatism, ease of use and assembly, and to be operated by folks that have no idea how they work. So, the less responsibility they place in the hands of the end user, the lower the likelihood of user error.

Warranties and commercial business practice aside, it's always good practice to glue your grains in.

In EX I quite usually run pressures 2-3x what many manufacturers use. I even glue as small as 38mm. One I didn't glue this year in a static test spit all but one casting tube and a chunk of a GOOD liner. But then again, it operated at over 2K psi. Not gluing that one was stupid. The case became single use though it did not cato or bulge. It just distorted the snap ring grooves a fair bit. Burn rate of 3/4"/sec can do that... It was a fast propellant. Usually fast burning formulas turn the cases into single use, if run at pressure. Snap ring grooves distort.

Gluing does not have to be difficult. Scott of Loki showed one good way to do it. Tape the grains together first at the joints, spaced how you want them. Then glue the whole stack into the liner at one time. Glue of choice cannot get to the grain faces that way. Look up Scott's directions. I think he posted a video.

That said, follow the manufacturer's assembly procedures if you are not flying EX. If they tell you to glue it, do it the way they say to do so, with the glue they say to use. If not, then don't.

Gerald

Had a 38mm CTI reload try and spit out the grain 2 seconds into the burn.




Quite neatly folded itself in half

Mind you the acceleration figures showed the burn was abnormal

prophecy said:

That's because the motor operates at a pretty low pressure, and as such isn't likely to spit any casting tubes on shutdown. Commercial loads are designed for conservatism, ease of use and assembly, and to be operated by folks that have no idea how they work. So, the less responsibility they place in the hands of the end user, the lower the likelihood of user error.

Warranties and commercial business practice aside, it's always good practice to glue your grains in.

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Thanks Steve for the explanation!