Grain Geometry 101

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dpower

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So, I happened to be studying the thrust curves in the Aerotech catalog, and became curious about the different thrust curves of SU vs Hobbyline RMS G motors. I'm sure most of you know this, but it took me a bit of head scratching, until it became clear that this is due to the grain geometries. The SU motors, at least the G77/78/79 would seem to have a circular bore (though AT doesn't specify this anywhere I could find), compared to the C-Slot of the Hobbyline RMS (obvious, since I've assembled those). Then again, the "high-power style" RMS G motors, such as the G77 & G79, must also use a circular bore, though I haven't used those.

Anyone know why they choose to use different geometries? Is one cheaper to produce? Or just to give us choices? Is the regressive curve intentional for certain designs, to get sufficient velocity off the rod? One downside I've had to the C-Slot, some igniters are a very tight fit, and they're a bit more of a pain to assemble, needing to align the grains (for the newer 2 grain 24/40-120 reloads).

Anyhow, I'm just looking for some discussion on this. This is a nice summary for AP rookies like me:

http://en.wikipedia.org/wiki/Solid-fuel_rocket#Grain_geometry
 

r1dermon

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im sure someone a lot more knowledgeable will chime in shortly, but the round "core" of most reloads is split into grains (in most reloads). this is called a bates grain design. when the motor pressurizes, the hot gasses will burn in between the grains as well as down the core. this grain design provides more surface area initially for a higher average thrust.

generally speaking, i think the grain geometry is due to several things, including ease of manufacturing, and specific motor design.

as i say, im sure someone more knowledgeable will chime in, but this is my basic understanding of the designs.
 

MarkH

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Just a guess : The C slot may be more accomodating to the copperhead ignitor. Flat ignitor, and flat slot are a better fit for the ignitor, and less volume to pressurize since Copperheads have less pyrogen than First Fires.
 

troj

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Grain geometry affects thrust profile.

What you refer to as a "circular bore" is typically a BATES grain, which is designed to keep the thrust profile neutral, by keeping the surface area constant as the grain burns.

As an example, the cores in the SRBs on the Space Shuttle are star grains, which provides a higher initial thrust (more surface area), which then reduces as the star's "points" burn away, resulting in less surface area.

-Kevin
 

WillMarchant

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Grain geometry affects thrust profile.

What you refer to as a "circular bore" is typically a BATES grain, which is designed to keep the thrust profile neutral, by keeping the surface area constant as the grain burns.
The Bates grain is a clever way to generate a neutral thrust profile. Strict core burners produce more thrust as they burn because the central core gets larger which exposes more burning surface area. Because the Bates grains burn on their ends as well, the length of the grain decreases which balances the increase in surface area in the core.
 

shreadvector

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Grain geometry affects thrust profile.

What you refer to as a "circular bore" is typically a BATES grain, which is designed to keep the thrust profile neutral, by keeping the surface area constant as the grain burns.

As an example, the cores in the SRBs on the Space Shuttle are star grains, which provides a higher initial thrust (more surface area), which then reduces as the star's "points" burn away, resulting in less surface area.

-Kevin
Only the top segment of the Shuttle SRM is a star grain. The rest are Bates. And is that all caps (an acronym) or someone's name (only the first letter capitalized)?

http://www.nakka-rocketry.net/th_grain.html

Aerotech makes several types, Bates and slotted. It's easier to make slotted as there is no tooling for the round core of the Bates - they simply cast the cylinder and then cut a slot after curing.

You know, poking around the web for ten minutes makes me think that the term BATES was an abbrevia-acronym for BAllistic TESt motor and that the grains used were those we now call "BATES" grains. Of course therre are other papers published for Ballistic Test Motors that use a slab of propellant, so I wonder if this is simlar to "cato"??

:y:
 

dpower

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<snip>Aerotech makes several types, Bates and slotted. It's easier to make slotted as there is no tooling for the round core of the Bates - they simply cast the cylinder and then cut a slot after curing.<snip>
If it is simply a matter of cost, this make sense. And I can see where they *might* not want to use this lower cost method on SU motors, since it would make it a bit more difficult to insert the igniter. While the SU G motors seem to be Bates, the thrust curves would seem to indicate a slot for SU F motors.
 

dpower

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The Bates grain is a clever way to generate a neutral thrust profile. Strict core burners produce more thrust as they burn because the central core gets larger which exposes more burning surface area. Because the Bates grains burn on their ends as well, the length of the grain decreases which balances the increase in surface area in the core.
I had read that, and found it an interesting combination; in addition to the more level profile, this makes it really convenient for manufacturing, transporting, and assembling motors; could this invention have been an unintentional, though beneficial, side effect?

This makes me wonder how the change in 29/40-120 G64 reload, from a single grain to two grains (for shipping purposes) affected the thrust curve? Seems like it should have made it even more regressive, as the two grains shrink lengthwise. Doing a little digging, I see that the NAR cert doc hasn't changed since 1994, yet the 2009 catalog shows a distinctly different shape for the G64 from the 2005 catalog (and the cert doc).
 

WillMarchant

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I had read that, and found it an interesting combination; in addition to the more level profile, this makes it really convenient for manufacturing, transporting, and assembling motors; could this invention have been an unintentional, though beneficial, side effect?
I don't know. Fred posted some stuff earlier about the development of this grain type. Sounds like somebody should do some digging and then write an article for Rockets Magazine. Hint hint... ;)
 

shreadvector

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If it is simply a matter of cost, this make sense. And I can see where they *might* not want to use this lower cost method on SU motors, since it would make it a bit more difficult to insert the igniter. While the SU G motors seem to be Bates, the thrust curves would seem to indicate a slot for SU F motors.
Virtually every single use Aerotech motor and reload kit for the RMS are the slotted grains.

I do know that the newer insanely-high-specific-impulse motors use the round core Bates grains. The instructions on the Aerotech website should show these. The new G80 is rumored to use two different grains inside to keep the average thrust under 80N.

http://www.aerotech-rocketry.com/cu...MR-SU_Instructions/g80t_new_su_in_21003-1.pdf

http://www.aerotech-rocketry.com/cu...ons/MR-SU_Instructions/g78g_su_in_21003-2.pdf

"standard" F single use with slots:
http://www.aerotech-rocketry.com/cu...ions/MR-SU_Instructions/f_std_su_in_21004.pdf

Non-standard with either slot, Bates or moon:
http://www.aerotech-rocketry.com/cu.../MR-SU_Instructions/g_non-std_su_in_21001.pdf
 

troj

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Virtually every single use Aerotech motor and reload kit for the RMS are the slotted grains.
In the hobby cases, that's an accurate statement. In HPR cases, Bates grains predominate.

-Kevin
 

troj

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This makes me wonder how the change in 29/40-120 G64 reload, from a single grain to two grains (for shipping purposes) affected the thrust curve? Seems like it should have made it even more regressive, as the two grains shrink lengthwise. Doing a little digging, I see that the NAR cert doc hasn't changed since 1994, yet the 2009 catalog shows a distinctly different shape for the G64 from the 2005 catalog (and the cert doc).
The old was probably a bit progressive, while the new is likely more neutral.

In a Bates grain, as the core enlarges, exposing more surface area, the grains also get shorter, reducing the surface area. Thus the fairly neutral profile.

-Kevin
 

cjl

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The old was probably a bit progressive, while the new is likely more neutral.

In a Bates grain, as the core enlarges, exposing more surface area, the grains also get shorter, reducing the surface area. Thus the fairly neutral profile.

-Kevin
The G64 is a C-slot motor though, so it was already a somewhat regressive burn even with the single grain. You would be right if it were a coreburning motor, but as it is, I think it went from regressive to more regressive.
 

daveyfire

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The old was probably a bit progressive, while the new is likely more neutral.
C-slot geometries with the slot against the wall tend to have a small spike in area at the beginning (as the burning surface area increases briefly while the slot widens), then a continuous decrease in area as the propellant burns across the web thickness. Since burning area is proportional to chamber pressure, and chamber pressure is proportional to thrust, we end up with a thrust trace that follows the same profile. AT motors use phenolic nozzles, which tend to erode during the burn, further compounding this effect. (This makes flying these loads in heavy/draggy rockets very entertaining, since the last few tenths of a second of burn aren't doing much to accelerate the rocket; you can see the effect very clearly.)

Edit: Chris got it while I was typing ;)

C-slots provide the benefit of longer burn times. White lightning in the 29/40-120 case (in a c-slot configuration) is a G64, while in the 29/120 case (in a BATES configuration) is a G79. For hobbyists graduating from Estes-style construction, the longer burn time (and lower average thrust) can mean the difference between a successful flight and a shred. Depending on the tooling setup, they can also be easier to mass produce in small sizes than a core burning motor.

One downside to c-slot/moon configurations is the increased insulation requirements -- compare the really thick liners included with hobby line reloads with those used in the high power cases. BATES (I heard it was "Ballistic Analysis Test and Evaluation System") grains, like all core burning geometries, insulate most of the casing wall with the propellant itself until the very last moment of burn. Straight core motors can be (and are) run with no thermal liner at all to provide extremely high propellant loading fractions.

Also, due to the exposed liner, c-slots tend to "pollute" colored exhaust flames with extra soot, causing the color to be washed out.

Offset port motors also have significantly off-balance propellant masses, which can cause dynamic stability problems in minimum diameter applications with narrow stability margins. They also tend to produce slightly off-center thrust; even though the core flow through the nozzle is supersonic and thus has no idea what is happening upstream of the throat, in small nozzles like we see in hobby rocket motors, the subsonic boundary layer thickness can be significant enough to "transmit" information about the offset upstream flow out into the supersonic expansion section.

C-slot/offset port geometries aren't often seen in professional motor applications due to these drawbacks. But they make an excellent candidate for a mass-produced hobby rocket motor. The F40 and G64 are two of my all-time favorites!
 

troj

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Sorry; misread the description of the new G64. Somehow, I read it as saying that it's a core-burner.

-Kevin
 

dpower

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...Virtually every single use Aerotech motor and reload kit for the RMS are the slotted grains....
Thanks for the links Fred. Upon further review, the G77, G78, and G79 LMS instructions clearly state a cored grain - presumably these are identical to the SU G77/79/79 motor grains. These are all newer motors, of course, but if cost is the main driver, it is curious that the newer motor grains would be bates, not C-Slot.
 

dpower

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The G64 is a C-slot motor though, so it was already a somewhat regressive burn even with the single grain. You would be right if it were a coreburning motor, but as it is, I think it went from regressive to more regressive.
The curves in the 2005 & 2009 catalogs are consistent with this.

G64-2005.JPG


G64-2009.JPG
 

dpower

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C-slot geometries .....
Good stuff David, thanks.

One downside to c-slot/moon configurations is the increased insulation requirements -- compare the really thick liners included with hobby line reloads with those used in the high power cases. BATES (I heard it was "Ballistic Analysis Test and Evaluation System") grains, like all core burning geometries, insulate most of the casing wall with the propellant itself until the very last moment of burn. Straight core motors can be (and are) run with no thermal liner at all to provide extremely high propellant loading fractions.
I hadn't realized the connection between hobbyline case differences and grain geometry differences, this makes sense. Is there a difference in case thickness due to the (slightly) higher initial higher thrust in hobbyline G reloads?
 

troj

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RE: BATES versus Bates.... (since some seem to want to remove egg sacks of lice)

Per Experimental Composite Propellant, it's BATES (BAllistic Test Evaluation System).

-Kevin
 

daveyfire

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Per Experimental Composite Propellant, it's BATES (BAllistic Test Evaluation System).
Yep - Terry's got it right, as usual. I had a paper e-mailed to me, too, from Bob Geisler et al. describing the original BATES motors fired at Edwards AFRPL back in the sixties. Very entertaining read, included (among other things) descriptions of the 15lb, 70lb, and 800lb "Super BATES" motors, details of the agglomeration size determination tests (flying an aircraft through the plume of a SRM immediately after shutdown to gather particles), and, um, "creative" terms for ballistic anomalies which I can't type here :D

Any paper with Bob Geisler's name on it gets promoted to the top of my reading list. This one with the history of BATES in it is wayyy up there. AIAA-1998-3981 for those interested.
 

troj

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Yep - Terry's got it right, as usual. I had a paper e-mailed to me, too, from Bob Geisler et al. describing the original BATES motors fired at Edwards AFRPL back in the sixties. Very entertaining read, included (among other things) descriptions of the 15lb, 70lb, and 800lb "Super BATES" motors, details of the agglomeration size determination tests (flying an aircraft through the plume of a SRM immediately after shutdown to gather particles), and, um, "creative" terms for ballistic anomalies which I can't type here :D

Any paper with Bob Geisler's name on it gets promoted to the top of my reading list. This one with the history of BATES in it is wayyy up there. AIAA-1998-3981 for those interested.
Considering your background, I was pretty sure you had it right. I looked in Sutton, first, but amazingly enough, couldn't find a reference -- I could find descriptions of BATES grains, but not that name.

Then I thought to look in Terry's book.

-Kevin
 
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