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
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.
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?
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.
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?
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.
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.
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.
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.
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 Irvine, Box 1242, Claremont, California 91711 USA
Opinion, the whole thing. <mail to: firstname.lastname@example.org>
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?
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...)
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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