What's the primary reason for cutting propellant grains?

What's the primary reason for cutting propellant grains into segments? Is it to prevent cracking during curing, make them easier to handle
and/or modify the core geometry, add scalability/modularity to engines, or something else altogether?
Thanks in advance! Dave Harper
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David Harper wrote:

The last two, mainly... if you build a core-burning motor with one long grain then the thrust curve is very progressive, as the increase in diameter of the core increases the burning surface area.
The short segments (known as "BATES" grains) burn on both ends as well as the sides, which reduces the length of each one as the core diameter increases, and thereby produces a thrust level that is more consistent throughout the burn time.
It also does allow one manufactured grain to be used in several different motor sizes, as in many of the Aerotech "RMS High Power" types.
-dave w
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Thanks for the info! As a followup question, couldn't you cast a single, long segment with a high surface area, star core configuration that burns/erodes away towards a circular burning core, thus providing a more stable thrust curve?
Also, with segments, what's to keep them stationary when they burn (not smacking each other around)? Are they generally adhered to the motor case somehow?
Thanks again! Dave
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David Harper wrote:

There's more core geometries than nose cone shapes. Different core geometries help tailor different thrust profiles for different propellants. I'm not sure what you mean by a "stable" thrust curve, other than rapid motor pressurization to prevent chuffing. It can be done, but complex core shapes are harder to cast/machine and thin propellant sections are more susceptible to mechanical damage. Good ignition methods can bring a motor with simple geometey up to pressure just as rapidly
All the Bates grain motors I've built are free-standing, not case bonded. I've used grain spacers before, but they were meant to ensure rapid grain face ignition, not physical support during burn. Case bonding grains brings up issues with propellant mechanical properties and radial pressure differentials.
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Gary Bolles
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Gary wrote:

I've not done any high power stuff, though I've certainly 'kibitzed' here often enough. This is piqued my interest, though.
So, what is apparently being said (and re-affirmed by the animation), is that the grains are initially held in place at their faces, but once burning commences, they are then held in place by the gas pressure of the burning grains? Is this a correct summation?
David Erbas-White
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Yes.
Equilaterally.
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Jerry Irvine, Box 1242, Claremont, California 91711 USA
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David Erbas-White wrote:

I don't think "held in place by the gas pressure" is the correct interpretation. I'm certain there is grain movement within a burning motor due to acceleration, the longitudinal (fore - aft) pressure gradient within the motor case, and flow friction. The evolving gases at the burning grain surfaces may prevent direct contact, but I don't think segmented, free-standing grains are "stationary" within a burning and/or accelerating motor. They would certainly tend to move towards the nozzle end, I would think. I haven't really thought about it too much. My AR experiments are based upon other's already successful motor designs.
I'm sure one of the manufacturers can make a better comment on segmented grain motion in an operating motor.
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Gary Bolles
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Gary wrote:

Held in place by physical expansion, caused by gas pressure.
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Yeah, but in order for them to be in static equilibrium, the pressures on both sides of the segment would have to be exactly equal. There's significant delta-P's in the chamber (although small in relation to the overall chamber pressure), and if one side of the segment were to burn a "tiny bit more" propellant over a given time, it would experience a pressure differential and thus a force.
Regardless, even IF the segment had the exact same pressure of both sides, they'd cancel each other out and you'd still be left with acceleration, vibration, and shear forces from the gas flow that could cause segments to move.
Dave
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David Harper wrote:

Seems like the most probable behavior (if the segments were free to slide in the liner) is that they would stack at the rear of their possible travel against the aft closure assembly (carried there by the acceleration, and the lengthwise pressure difference in the chamber), with a space between them maintained by the "air bearing" effect of the gas generated from the end surfaces. Whether this latter effect would be dominant would depend on factors including the length of the motor and the acceleration of the rocket... I've heard that where both are extreme (i.e., a J570 machbuster), there can be problems with acceleration squeezing the stack of grains backward so hard that the propellant is distorted, constricting the core... epoxying the grains into the liner to keep them in fixed positions has reportedly improved reliability in such applications.
-dave w
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snipped-for-privacy@gmail.com (David Harper) wrote in

How about the residue and slag deposits keeping the slugs in place?
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It's possible, but I doubt the slag and/or residue would form bonds/barriers in the chamber to prevent grain movement, since the solids are most likely molten until exiting the chamber. Even solids that did form on the chamber walls would have poor mechanical properties at that elevated temperature.
Dave
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snipped-for-privacy@gmail.com (David Harper) wrote in

But the fuel grains are usually a fairly close fit,any deposits would prevent movement.Aren't you supposed to wrap tape around them if they are too loose?
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Nothing can prevent movement of grains after ignition short of gluing them to the liner.
Pressure and massflow are powerful forces.
Powerful enough to send stuff to space with a device smaller than 2 cars.
Jerry
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Jerry Irvine wrote:

Does it actually matter whether the grains remain in their original position or slide aft? It might change the center of gravity a bit... what else changes?
(Has anyone ever done side-by-side firings with loose vs. glued grains?
-dave w
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Only if they slide suddenly. Propellant is not all that shock resistant from physical breakage.

The AT 29mm G240 in any rocket works fine. Stick fins on it and about 1/2 of them fail due to excessive G's dislodging and cracking the grains.
Been there, done that too :)
Jerry

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Jerry Irvine wrote:

When did AT make a G240? (It seems they have discontinued a lot of types - especally single use, and smaller ones in general - over the years, based on such things as motor lists included in some simulation programs...)
-dave w
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When Jerry (AT's first dealer) bought them.

Jerry stopped buying them.

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Jerry Irvine, Box 1242, Claremont, California 91711 USA
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David Weinshenker wrote:

I've never case bonded KNSU BATES grains in PVC, but experience with standard core grains leads me to believe it is not a good idea. My early PVC sugar motor experiments were pretty disappointing with case bonded cast grains; routine overpressure CATOs. I believe my problems were caused by differing modulus of elasticity between the (brittle) KNSU and the more elastic PVC; the chamber (core) pressre would split the propellant grain since the PVC allowed expansion. The increased burning area would overpressurize the case and FOOOMP! Free standing grains improved reliabilty greatly as the PVC could expand while the pressure was equalized around the grain and prevented cracking/splits.
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Or..... We could just look into a spent motor and see if we can see anything that would tell us what happened. Naw, I think the discussion of theories is much more fun! :-)
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Drake "Doc" Damerau
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