(Yet Another) 100k Attempt

[FredA]

The only way I can think of to get a clean, BATES-style shutoff is by casting something inert into the space where the slivers would occupy, so the motor runs out of propellant when it is still at a reasonable Kn.

This can also be a way to balance the motor if the inert spacer is low-weight

 

[Spacedog49Krell]

A long tail-off is pretty much guaranteed with a moonburner or D grain, because of the way that the propellant burns away to a sliver. If you want to see why, you can download my openMotor software and use the "grain" tab to see the shape of the propellant grain as the burn progresses. The only way I can think of to get a clean, BATES-style shutoff is by casting something inert into the space where the slivers would occupy, so the motor runs out of propellant when it is still at a reasonable Kn. That would be a bandaid solution that reduces impulse and mass fraction, though if that sliver of propellant isn't helping you maybe you don't want it.

I downloaded your openMotor software. I've seen others using openMotor for their Moonburner development. I want to compare your regression equations to Bill Wood's original equations. I find most tail-off issues result from poor initial core L/D selection.

In my profile picture I'm next to my 6" Moonburner test motor. Tests conducted at the maximum web thickness delivered a 52 second burn time with a significant tail-off at burnout. When using an optimized grain, the burn time was reduced to 36 seconds with a very short tail-off.

 

[eggplant]

I downloaded your openMotor software. I've seen others using openMotor for their Moonburner development. I want to compare your regression equations to Bill Wood's original equations. I find most tail-off issues result from poor initial core L/D selection.

In my profile picture I'm next to my 6" Moonburner test motor. Tests conducted at the maximum web thickness delivered a 52 second burn time with a significant tail-off at burnout. When using an optimized grain, the burn time was reduced to 36 seconds with a very short tail-off.

Hmm, not sure what core L/D would do, but I can see how tweaking the core shape might also help eliminate the tail-off. Maybe instead of a simple circular port, a teardrop with the point towards the center of the grain would eliminate some of the slivers (though it would also make it a faster burn).

Fortunately, openMotor can help answer questions like this, because rather than using closed-form equations for calculating regression, it calculates the distance transform of the core geometry and then finds level-set contours on it to determine the surface area of the grain at any regression depth. This means that in addition to the preset geometries available in other software, it also allows you to load in a custom port geometry from a DXF file and simulate it just as easily.

 

[rocket_troy]

I downloaded your openMotor software. I've seen others using openMotor for their Moonburner development. I want to compare your regression equations to Bill Wood's original equations. I find most tail-off issues result from poor initial core L/D selection.

In my profile picture I'm next to my 6" Moonburner test motor. Tests conducted at the maximum web thickness delivered a 52 second burn time with a significant tail-off at burnout. When using an optimized grain, the burn time was reduced to 36 seconds with a very short tail-off.

The specific impulse losses from (say) geometry tail-offs are full of nuisances from flight dynamics and let's not forget that flow separation *contributes positively* to specific impulse. The primary issue with flow separation is when it's asymmetric and the nozzle is large and flimsy to cope with deformations from the pressure gradients.
Things like a lowering of static Pa with altitude and dynamic Pe from velocity both contribute to suppress flow separation with the former assisting specific impulse directly by pressure ratio terms in the isentropic Cf equation.
Yes, at ground level, you definitely want the highest Pc/Pa ratio (optimally expanded) to maximise thrust coefficient, but the importance of that is proportionally reduced with altitude.

TP

 

[rocket_troy]

This means that in addition to the preset geometries available in other software, it also allows you to load in a custom port geometry from a DXF file and simulate it just as easily.

"Other software" not being GrainsCAD which allows freehand geometry creation without the need to import

TP

 

[rharshberger]

Wonder what effect an aerospike nozzle would have in conjunction with moon burner? Has anyone tried it yet?

 

[pugachu]

Wonder what effect an aerospike nozzle would have in conjunction with moon burner? Has anyone tried it yet?

I watched a Formlabs webinar last week on 3d printed alumina.... i thought that would make a great aerospike nozzle. Problem is it's $1,300 for 750mL of resin

 

[rocket_troy]

Wonder what effect an aerospike nozzle would have in conjunction with moon burner? Has anyone tried it yet?

Not aware of any experiments, but the typical flight dynamics of (say) a 100Kft (say) single stage is actually quite suited to a typical de laval nozzle if coupled with a regressive burn profile. Okay, a moon burner might not provide the optimum profile for that, but it should generally be within the neighbourhood. So, given that aerospike nozzles are generally heavier, more complex and have much more thermal issues to contend with; the practical incentives for exploring that avenue aren't compelling for these kinds of projects.

TP

 

[robopup]

Hmm, not sure what core L/D would do, but I can see how tweaking the core shape might also help eliminate the tail-off. Maybe instead of a simple circular port, a teardrop with the point towards the center of the grain would eliminate some of the slivers (though it would also make it a faster burn).

Fortunately, openMotor can help answer questions like this, because rather than using closed-form equations for calculating regression, it calculates the distance transform of the core geometry and then finds level-set contours on it to determine the surface area of the grain at any regression depth. This means that in addition to the preset geometries available in other software, it also allows you to load in a custom port geometry from a DXF file and simulate it just as easily.

I like the teardrop idea, and didn't realize you were behind open motor. I've always been a burnsim guy but I'll give open motor the 'ole college try.

Wonder what effect an aerospike nozzle would have in conjunction with moon burner? Has anyone tried it yet?

Interesting. I poked around a bit looking into this, RCS even sells a 54, but sounds like our hobbyist level implementations wouldn't take the heat which is why you only (as far as I know) see them on fast motors.

 

[MClark]

A long tail-off is pretty much guaranteed with a moonburner or D grain, because of the way that the propellant burns away to a sliver. If you want to see why, you can download my openMotor software and use the "grain" tab to see the shape of the propellant grain as the burn progresses. The only way I can think of to get a clean, BATES-style shutoff is by casting something inert into the space where the slivers would occupy, so the motor runs out of propellant when it is still at a reasonable Kn. That would be a bandaid solution that reduces impulse and mass fraction, though if that sliver of propellant isn't helping you maybe you don't want it.

My old boss worked on design of the Mercury escape tower motor. It used was a star grain with inert inserts so there was no tail off. Could damage the parachute. IIRC he said they cast a grain without inserts and severed ends at desired burnout. Propellant snuffs and what remains is the shape for insert.

 

[Chad]

Camera window

I had a little extra time and wasn't quite happy with my first try at the payload tube with the camera hole so I took another shot at it. Original goal which worked on round two was to have the window and not use any screws to hold things together. This ended up being harder than I expected and I didn't find many detailed pictures showing how others did there windows so figured I'd throw these here.

Mount for the window is 3d printed and glues into a section of body + coupler tube:
View attachment 602570

Window is held by a lip in the above picture, and by the blue press fit piece below which will be siliconed in place:
View attachment 602571

I found this thread last night while looking for something else. First of all, what an amazing accomplishment! Very inspiring to see projects and results like this.

Do you have CAD or stl files for your camera window mount? I want to do something similar as I haven't flown with a camera on board yet.

 

[robopup]

I found this thread last night while looking for something else. First of all, what an amazing accomplishment! Very inspiring to see projects and results like this.

Do you have CAD or stl files for your camera window mount? I want to do something similar as I haven't flown with a camera on board yet.

Thanks! See attached for the CAD, let me know if you need anything else.

This project is still very much alive for another attempt - the 100k goal hasn't been met yet! Not sure whether it will be in 2024 yet, but it will remain my rocketry focus for the foreseeable future.