Introduction: 3D Printed Funnels for Rocket Motor Propellant Casting
Research rocket motors are made by casting various formulas of Ammonium Perchlorate Composite Propellant (APCP) into tubes - usually cardboard tubes through which a mandrel runs to form a core. One of the more useful things I've been able to print with a 3D printer is a set of funnels for casting rocket motors in tubes meant for use in standard 75mm or 54mm rocket motors.
In this article I will share the files needed to print your own set of funnels. I recommend using PETG. It holds up well, has a smooth surface, and withstands the cleaning fluid (charcoal starter fluid) that we typically use to stop further curative action and to clean mixing and casting equipment.
Step 1: Design Files
Attached to this step are the .123d and .stl files used to create the funnels, as well as links to the 123D Design files on the site associated with 123D Design.
You may open these files in your favorite 3D Design program to alter them, or open them in your favorite Slicer to print them.
The 54mm funnel is designed with a spout that has a 44 mm outside diameter. This fits perfectly in 54 mm casting tubes. The 75mm funnel spout has a 63.5 mm outside diameter to fit standard 75mm casting tubes.
Step 2: Printing the Funnels
They were printed on a glass bed topped with blue painters tape at 40 mm/second.
The 54mm funnel completed in 15 hours and 21 minutes. The 75mm funnel took 16 hours 54 minutes. Both were set with about 22% fill. More fill will extend the print time but will produce a slightly stronger nozzle, which is not necessary unless you expect to throw it around the shop. :)
Step 3: Casting Motors and Clean-up...
Rocket motor casting is well beyond the scope of this article, but if you are interested you should join the Tripoli Rocketry Association prefect nearest you, obtain your level 1 and level 2 certifications, and then work with the Research Rocketry groups in your prefect or in a nearby prefect. From them you will be able to learn how to safely make and test rocket motors. There are substantial costs in terms of equipment and knowledge that need to be acquired to practice this hobby safely. Don't take shortcuts. Learn from those with experience. Doing this will keep the hobby legal and safe and not destroy a hard fought reputation and relationship with authorities having jurisdiction. Follow your laws.
The video and photos attached to this step show a motor being cast using Tru-Core mandrels on a home-built vibration table. Mandrels have being sprayed with Stoner E236 Urethane Mold Release. The formula being poured is a high aluminum content formula that has been previously characterized. No, I will not post or give away the formula, but for those with the proper certification level the formula and characterization data is available on the Rocketry Forum.
Clean up is done first with charcoal starter fluid. This prevents the HTPB (composite rubber) from further curing. After this dish soap and water will finish the job.
Step 4: Results...
The photos attached to this step show the cast grains after they were cut and as they were assembled into a motor. The nozzle was made using the instructions in my article entitled Build a High Power Rocket Nozzle.
The remaining photos show the rocket at take off and in flight along with one during descent under the parachute.
If you are already a Tripoli Rocketry member that works with Research Motors, I hope these 3D printed funnels will be beneficial to you. Otherwise I hope this article will stimulate your interest in high power rocketry and may prove useful at some point in the future. Just always be safe, follow the rules and laws that apply to the sport, and be wise.