In-house Rocket Motor development for 3-D printed model rocket

[vishnu.vardhan.vithanala]

Hi all!

I've 3D printed a model rocket structure using ABS that's around 29.5 cm in height with a body radius of 5 cm. The overall structure weighs around 170 grams without onboard electronic & propulsion subsystems. Inside this, I plan to mount an IoT module for flight data collection.

But I'm worried on what kind of rocket motor I should use for this! I don't wish to purchase a standard motor and I'd rather make one myself. Since it's my first time doing this, I'd appreciate if anyone could help me with the rocket motor composition and the kind of grain configuration I'll have to use for this. Something simple yet powerful!

Also, the onboard electronic module I'm planning to use for this project is a simple Pi Pico as MCU with an MPU6050, BMP180, an SD card module for data storage. In addition to this, I'm now trying to use a SIM800L (I know it's not that good of a module but it just serves the purpose! ) to remotely start and terminate data capturing. As for the firmware, I'm using Micro python to integrate all of this. Any suggestions on this system are welcome!

Appreciate any kind of help!

Thank you!

 

[FredA]

Learn to walk before you run.
Buy and COTS motor and work out ALL the other stuff.
Then you can focus on motor development.

 

[cfb_rolley]

You can run simulations using OpenRocket to get an idea of how stable your rocket will be and how it will perform with various motors. Given that the weight is pretty high for a small rocket, I would be going with a fairly punchy D motor at least, but as long as it's stable, it'll fly. being ABS you shouldn't have to worry about the heat of the motor warping the rocket which is a big bonus.

I suspect you may have printed it a fair bit stronger than you need though - generally you can get away with 2 walls and around 8-10% infill. Printed body tubes are a lot heavier than cardboard and if the body is just a simple cylinder, you probably don't need to use a 3d printer to make the body tube. What I do is make as much of the body tube as possible out of cardboard and then only print the tail, the nose and any other small parts to keep the weight down. I usually also only use one wall but tick the "alternate additional wall" setting (I think that's the name of it...) This prints one outside wall every layer, and one extra inside wall every second layer. this ends up being lighter but almost as strong as 2 walls, as it the infill layers end up interlocking with the alternating inside walls

There are a few groups on facebook for 3D printed rocketry that can be super helpful too, definitely join them for some tips. 3D printed rocketry is really fun and so is developing your own electronics - combining the two... even more fun! if the flight computer doesn't work and the rocket crashes, you can just print another one!

 

[lr64]

I could see the structure weighing that much if the rocket was 100 centimeters long. Maybe even 75. Where did you find the room for all that plastic? ;-)

To save weight, a rolled balsa tube would probably be much lighter than a cardboard one, though it would need fiberglass or something running across the grain if there was going to be an ejection charge. Then again, it's clear cardboard is good enough.

 

[cfb_rolley]

To save weight, a rolled balsa tube would probably be much lighter than a cardboard one, though it would need fiberglass or something running across the grain if there was going to be an ejection charge. Then again, it's clear cardboard is good enough.

...rolled balsa you say? I've actually never heard of this, how is this done? I have a rocket on the build now that is CF over a cardboard postage tube but the tube is really just there for shape and something for the layer of carbon to adhere to.... rolled balsa instead of the cardboard tube in this setup would be way lighter and plenty strong enough I reckon...

 

[lr64]

I don't know of anyone who uses rolled balsa for rockets, but it's often used for model airplanes. Check out these P-30s with rolled tube fuselages. http://p30page.weebly.com/rolled-tube-fuselage-p30s.html
If I'm not mistaken, some of them have build articles with them.

It's not that hard to do, particularly if you boil the balsa before rolling. I have a quarter inch ID tube that's made from 1/32" balsa that I hope to finish up into a plane one of these days.

I'd guess that it's possible to make a double spiral wrap with balsa, which I think is how some cardboard tubes are done.

Because balsa is light, it can be thicker, which makes it stiffer and, in some directions, stronger. It can also work as a core material. A tube lightly glassed inside and out would be impressively stiff for its weight.

I don't know if there are any examples of rolled balsa tube rockets, but balsa can be an excellent structural material, and I have no doubt some very light, stiff rocket tubes can be made with it.

Your cardboard is probably helping the carbon to resist buckling, unless the carbon is pretty thick. If the cardboard really isn't necessary, you could put the carbon over a foam rod and then use a solvent to remove the foam. The foam could, of course, have various stiffeners inlaid into it before applying the carbon.

 

[lr64]

BTW, here's a very long video about making a rolled tube for a Wakefield model. I admit I ran out of patience, but, judging by his accomplishments, Mr. Finn seems to know what he's doing. He's also into boost gliders and such.


A more abbreviated note on how to make a smaller tube:
https://0201.nccdn.net/1_2/000/000/126/3be/Rolled-Balsa-Motor-Stick.pdf

The best tubes are made with c-grain (quarter sawn) balsa, but that's a bit harder to work with, at least if the thickness of the balsa is a significant fraction of the radius of the tube. OTOH, once formed, such a tube will be more resistant to buckling.

BTW, IMHO, like computers, CA is a modern miracle that lets us make mistakes faster than ever before.

 

[vishnu.vardhan.vithanala]

You can run simulations using OpenRocket to get an idea of how stable your rocket will be and how it will perform with various motors. Given that the weight is pretty high for a small rocket, I would be going with a fairly punchy D motor at least, but as long as it's stable, it'll fly. being ABS you shouldn't have to worry about the heat of the motor warping the rocket which is a big bonus.

I suspect you may have printed it a fair bit stronger than you need though - generally you can get away with 2 walls and around 8-10% infill. Printed body tubes are a lot heavier than cardboard and if the body is just a simple cylinder, you probably don't need to use a 3d printer to make the body tube. What I do is make as much of the body tube as possible out of cardboard and then only print the tail, the nose and any other small parts to keep the weight down. I usually also only use one wall but tick the "alternate additional wall" setting (I think that's the name of it...) This prints one outside wall every layer, and one extra inside wall every second layer. this ends up being lighter but almost as strong as 2 walls, as it the infill layers end up interlocking with the alternating inside walls

There are a few groups on facebook for 3D printed rocketry that can be super helpful too, definitely join them for some tips. 3D printed rocketry is really fun and so is developing your own electronics - combining the two... even more fun! if the flight computer doesn't work and the rocket crashes, you can just print another one!

Hi cfb_rolley!

Thank you for the input. Actually, the infill is 40%...

I didn't really expect 200 grams to be that heavy for a model rocket! This might sound dumb, but I'd really like to build a rocket motor that is compatible with the 3D printed structure... I'll also try reducing some of the weight by removing some material at unnecessary places. Any suggestions as to how I can build a rocket motor (D class like you mentioned)? Any documentation/ reference on the composition?

I'll also have to consider the fact that the onboard electronic module might add more weight to the body!

 

[Handeman]

By "compatible with the 3D printed structure" I assume you mean a rocket motor similar to commercial motors and NOT one 3D printed by you.
The Admins here won't let us talk about formulas or motor construction details on this forum, except in the [Restricted] Research threads. I just don't think you can 3D print a motor case that won't rupture at ignition with any printer available for general use. $10K - $100K ones may be able to do it.

My suggestion would be to take it one stage at a time. Build and test your rockets on commercial D motors. Once you have the rocket part down, then you can move onto building motors.

Just remember, If you are launching in the USA, you must keep propellant weight down where it stays in the FAA Class 1 rocket category. If it gets bigger than that, you will need to get your own COA from the FAA, or comply with Tripoli Research motor use requirements to fly at one of their sanctioned launches.

I know, all the regulations tend to take the fun out of it. Good Luck.

 

[OzHybrid]

Which country are you in? Where?
provide an Open Rocket sim.
Otherwise we have to guess. Which wastes time.

 

[cfb_rolley]

I just don't think you can 3D print a motor case that won't rupture at ignition with any printer available for general use. $10K - $100K ones may be able to do it.

Can definitely be done on regular 3d printers, there’s a bloke that has posted stuff about the printed solid rocket motors he’s made on an experimental rocketry group on Facebook that I’m part of.

…That isn’t to say it’s a good or bad idea, because I don’t know much more about success rate or safety other than “it can work”. But, definitely possible on a regular printer.

 

[lr64]

I suspect a 3D printed rocket motor could be designed to be easily reinforced so as not to burst, but it might take some time and expertise to design. I suspect it would be a one use motor. OTOH, I could see 3D printing used to make tooling for a motor.

Might be better not to tackle everything at once, though.

 

[Handeman]

Can definitely be done on regular 3d printers, there’s a bloke that has posted stuff about the printed solid rocket motors he’s made on an experimental rocketry group on Facebook that I’m part of.

…That isn’t to say it’s a good or bad idea, because I don’t know much more about success rate or safety other than “it can work”. But, definitely possible on a regular printer.

Was that a sugar motor? They are much lower pressures for the most part. I only mix APCP so when I think of motor designs, I think along the lines of pressures at +500 psi.

 

[0011001100]

Can definitely be done on regular 3d printers, there’s a bloke that has posted stuff about the printed solid rocket motors he’s made on an experimental rocketry group on Facebook that I’m part of.

…That isn’t to say it’s a good or bad idea, because I don’t know much more about success rate or safety other than “it can work”. But, definitely possible on a regular printer.

You can print metal parts on a normal FDM printer. They just need to be sintered afterwards:
https://www.matterhackers.com/store/c/3d-printer-filament/metal-3d-printing-filament-from-basf

Was that a sugar motor? They are much lower pressures for the most part. I only mix APCP so when I think of motor designs, I think along the lines of pressures at +500 psi.

Not necessarily, I like running 1000+ psi to get more performance.

 

[cfb_rolley]

Was that a sugar motor? They are much lower pressures for the most part. I only mix APCP so when I think of motor designs, I think along the lines of pressures at +500 psi.

Yeah those ones that I was referring to were sugar motors. I’m uncertain whether a casing printed on a standard FDM printer could be used for motors with higher operating pressures, but a change in process might change that.

There is a video of a modified 3d printer that can be made to operate a bit like a cnc lathe, I’ll have to find it and post it but that would drastically increase the strength of a casing by removing the weak point of vertically stacked layers.

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