Moonburner?

[prfesser]

PS... I'd bet that gluing the grains together is more about keeping the off-center core hole aligned on a moonburner than it is about "sealing off" the ends of the grain to prevent them from burning off between the grains in flight... if the holes aren't aligned and don't stay aligned, the motor is going to be hard or impossible to install an ignitor into, get lit correctly, or burn right during the thrust phase...

Gluing the grains together does keep the core aligned, but it also inhibits the grain faces so the propellant only burns from the inside outward, and not from the ends. Faiure to glue the grains together will certainly result in failure.

Most likely you're right, although I think a multi-grain moonburner would work by first placing spacers (o-rings) between grains, then sliding a snug-fitting mandrel down the cores to align them, and finally gluing the grains into the liner. Essentially it would be like a Bates grain motor except that the cores are offset. Be interesting to model that with a motor program. Hmmm...time to load my GRAINS2000 program...

Best -- Terry

 

[Bamraam]

Most likely you're right, although I think a multi-grain moonburner would work by first placing spacers (o-rings) between grains, then sliding a snug-fitting mandrel down the cores to align them, and finally gluing the grains into the liner. Essentially it would be like a Bates grain motor except that the cores are offset. Be interesting to model that with a motor program. Hmmm...time to load my GRAINS2000 program...

Best -- Terry

The biggest feature of a MB is the long burn time. If you purchase a MB and don't glue the grains together, you will get a vastly different burn profile, and unless you modify the nozzle to accomodate the extra propellant burning it will definately fail.

 

[heada]

https://www.lokiresearch.com/images/Documents/54mm_K350_L1400_instructions.pdf
The Loki instructions have neither the gluing of the grains nor the grain spacing o-rings so neither are required for a moonburner to work. Each manufacturer has their own process, design and reason for the way a motor works for them.

 

[neil_w]

I appreciate this thread necro, I've been wondering about moonburners for a while.

 

[watheyak]

https://www.lokiresearch.com/images/Documents/54mm_K350_L1400_instructions.pdf
The Loki instructions have neither the gluing of the grains nor the grain spacing o-rings so neither are required for a moonburner to work. Each manufacturer has their own process, design and reason for the way a motor works for them.

Based on the packaging I think the Loki moonburners are one long grain.

I have two K350s waiting to be flown.

 

[heada]

Based on the packaging I think the Loki moonburners are one long grain.

I have two K350s waiting to be flown.

Based on the instructions I assumed multiple grains. I checked my I110 and it's a single grain. That'd make gluing them together moot.

 

[prfesser]

The biggest feature of a MB is the long burn time. If you purchase a MB and don't glue the grains together, you will get a vastly different burn profile, and unless you modify the nozzle to accomodate the extra propellant burning it will definately fail.

Not "vastly different". If a moonburner was made of two grains, spaced apart so that all four ends of the grains burn, there would be a slight increase in Kn but not an enormous increase. It would depend rather a lot on the length of the single grain and the separate grains. I've run some sims; the thrust profile of a single monolithic grain vs two grains, each half the length of the single grain and spaced to permit burning on all ends, do not differ all that much.

If you have simulations that suggest otherwise, I'd like to see them, or at least give the dimensions and other specifics.

FWIW some workers define a "moonburner" as a core that is tangent to the edge of the grain; an "offset core" is not centered but is not touching the casting tube. The thrust profile can be varied significantly---from fully regressive to progressive/regressive--- by simply changing the amount of offset.

Best -- Terry

 

[msjohnso]

Yes, AeroTech was the first company to manufacture and sell them commercially, in the early ‘80s. We currently produce them in 29, 38, 54, 75 and 98mm diameters.

Bill Wood was the inventor of the grain geometry. Yours truly coined the term “moonburner” for Bill’s design. Bill never filed for a patent on it, but I believe he presented a paper describing them at a propulsion conference. I used to have a copy and would post it if I could find it.

Update: Here it is.

Fascinating paper...the idea of an off-center star core is intriguing. Was any further research done on *that* grain design? (which I admit probably isn't practical for a hobby motor - fabrication costs would put it out of reach for most of us cheap rocketeers).

 

[Bamraam]

We've never tried firing a MB without gluing the grains together. And you're probably right that a 2 grain MB won't make much of a difference but I would think more grains would make a larger difference. Motor will certainly produce more thrust at startup.

 

[Blast it Tom!]

Yes, AeroTech was the first company to manufacture and sell them commercially, in the early ‘80s. We currently produce them in 29, 38, 54, 75 and 98mm diameters.

Bill Wood was the inventor of the grain geometry. Yours truly coined the term “moonburner” for Bill’s design. Bill never filed for a patent on it, but I believe he presented a paper describing them at a propulsion conference. I used to have a copy and would post it if I could find it.

Update: Here it is.

Wow, neat! Another Westinghouse-Sunnyvale guy! Thanks! (my boss , out of Westinghouse- East Pittsburgh worked with Sunnyvale folks on a bunch of projects, more related to electric motors...) Saved for posterity!

 

[prfesser]

I just finished a quick-and-dirty sim for a moonburning motor 1.8" dia, 0.75" core, 0.325" nozzle throat. Results are for one 8" long grain vs. two 4" long grains. Ends of the grains are uninhibited.

		I-total	Peak P	Avg P	Thrust, N
Grains	Class	Ns	psi	psi	At 0 s	At 1 s	At 2 s	At 3 s	At 4 s	Burn time
1--8"	J150	931​	628​	340​	300​	240​	225​	190​	150​	6.15 s
2--4"	J152	933​	797​	344​	390​	300​	225​	170​	120​	6.09 s

Both peak (i.e., initial) pressure and peak thrust are considerably higher for the two-grain motor, because there's more area burning. Also, thrust for the two-grain motor dropped off more rapidly; the motor would be somewhat more regressive. But class, total impulse, average pressure, and burn time are all virtually unchanged. The thrust curves' shapes are very similar apart from the initial spike.

Actual results would depend on the propellant properties (a and n). So if one wanted the long burn time of a moonburner and a higher initial thrust, and was willing to accept a faster drop in thrust, a multi-grain motor might be worth investigating. But if erosive burning occurred, giving an even higher initial pressure...boom.

Of course this is all theoretical. YMMV.

Best -- Terry
"In theory there is no difference between theory and practice, but in practice...there is."

 

[AeroTech]

Terry,

Where this all goes south is when you start using multiple (4-6) grains for the longer moonburners like M and N size. The core Kn and initial pressure goes way up and the motor gets more regressive. One of the advantages of a moonburner over BATES is that you can have a longer grain and keep the core Kn at a more reasonable level.

I just finished a quick-and-dirty sim for a moonburning motor 1.8" dia, 0.75" core, 0.325" nozzle throat. Results are for one 8" long grain vs. two 4" long grains. Ends of the grains are uninhibited.

		I-total	Peak P	Avg P	Thrust, N
Grains	Class	Ns	psi	psi	At 0 s	At 1 s	At 2 s	At 3 s	At 4 s	Burn time
1--8"	J150	931​	628​	340​	300​	240​	225​	190​	150​	6.15 s
2--4"	J152	933​	797​	344​	390​	300​	225​	170​	120​	6.09 s

Both peak (i.e., initial) pressure and peak thrust are considerably higher for the two-grain motor, because there's more area burning. Also, thrust for the two-grain motor dropped off more rapidly; the motor would be somewhat more regressive. But class, total impulse, average pressure, and burn time are all virtually unchanged. The thrust curves' shapes are very similar apart from the initial spike.

Actual results would depend on the propellant properties (a and n). So if one wanted the long burn time of a moonburner and a higher initial thrust, and was willing to accept a faster drop in thrust, a multi-grain motor might be worth investigating. But if erosive burning occurred, giving an even higher initial pressure...boom.

Of course this is all theoretical. YMMV.

Best -- Terry
"In theory there is no difference between theory and practice, but in practice...there is."

 

[Bamraam]

I'll run some simulations later with our P98 4GMB, I suspect the results will be similar to yours. The total and average Ns will be similar due to the same amount of propellant in both sims and roughly the same configuration (MB). In your sims segmenting the grains results in 30% more initial thrust, to me that's a significant change in thrust profile. I did expect the chamber pressure to be higher than your sim shows, I'll sim some larger motors too and see if there's much difference.

 

[Bamraam]

We can see here that more grains seem to have more of an impact on thrust profile from a single 24" grain to 4 - 6" grains, almost doubles the initial thrust. I'm tempted to see what the difference is on a 6G...

 

[o1d_dude]

I’m going to call up OP Captain Low and Slow and tell him his thread from 2013 has made comeback and is going strong.

 

[Ez2cDave]

Terry,

Where this all goes south is when you start using multiple (4-6) grains for the longer moonburners like M and N size. The core Kn and initial pressure goes way up and the motor gets more regressive. One of the advantages of a moonburner over BATES is that you can have a longer grain and keep the core Kn at a more reasonable level.

AEROTECH M650 "Moonburner" ( My eventual Level 3 Cert Motor ) . . . Rocket is 7.5" dia., 134" Length . . . A nice, gentle, "slow ride" to Level 3 Cert !

Dave F.











30 lb. - DRY WT. ( All-Up, No Motor )



35 lb. - DRY WT. ( All-Up, No Motor )



38.5 lb. - DRY WT. ( All-Up, No Motor )

 

[dhbarr]

I’m going to call up OP Captain Low and Slow and tell him his thread from 2013 has made comeback and is going strong.

Progressive burn thread :-D