3D Printed Micro Rocket

The goal of this project is to design a model rocket that:

• Flies over 40 m high
• Is easy and quick to 3d print
• Works on the smallest class of rocket motors (Micro=1/8A)

• 3D printer
• Micro Maxx-1 model rocket engine( asp-rocketry.com/ecommerce/Micro-Maxx-Model-Rocket-Engines-6-Pack-.cfm?item_id=416&parent=88)
• Paper Straw
• 2mm Garden Wire

This model rocket will work on the Micro Maxx 1 or n.2 engine. This engine has an diameter of 6mm and a length of 16 or 26 mm depending on the type.

So before jumping to fusion 360 to CAD model the rocket, I tested some designs in the simulation software openRocket.

The most important thing here, is to design it in such a fashion that it is passively stabilized. This is achieved by positioning the center of lift (from now on abbreviated as CoL) well below the center of mass (CoM). Why? We want the model rocket to keep pointing up during the duration of the flight, especially when the motor is still burning, but any misalignment of the motor in respect to the CoM will induce a torque on the model rockets body causing it to rotate. To counteract the rotation a larger area of the model rockets body under the CoM is needed then above it (CoL below CoM). Incoming air will hit the rotated rocket body causing drag, a resistive force will act at every point along the body. The CoM will act as a fulcrum and because there is a larger area under the CoM, there will be a stronger force under the CoM then above it thus causing a rotation. This rotation will move the rocket upright again, if instead the CoL was above CoM the rotation will only exaggerate the initial rotation instead of counteracting it.The distance of the CoL to the CoM is also important, a longer distance between the points will result in a longer moment arm and thus making the rocket more stable during flight. In openRocket a stability rating is given in the topright corner as can be seen in the screenshots I added above.

The rocket motor will sit at the bottom of the rocket shifting the CoM towards the bottom, we need CoL under CoM so a set of 4 fins is necessary to add the extra area to make it passively stabilized.

I came up with a design that would be 11 cm in length, weigh under 4 grams and, in theory, fly to a height of 97 m

I ran simulations for both type 1 and type 2 motors on the same rocket, and although the type 2 packs double the power it does not deliver a higher altitude. The type 2 motor has got a much higher initial acceleration but this also causes the rocket to experience a lot more areodynamic drag effectively cancelling out the extra power this motor has. In our case, the type 1 motor will work best.

In fusion 360, I started off with a 7.6mm (6 inside + 2*0.8 wall thickness) cylinder and a length of 80 mm for the main body. The nose cone is made using the spline tool and then using the revolve tool on the sketch and the y-axis. The nose cone has a length of 20 mm and a radius of 3.8mm.

The circular pattern tool can be used to make 4 identical fins from its sketch and then finished up using the fillet tools.

Finally, the shell tool is used to make the rocket hollow with a wall thickness of 0.8mm. A bulkhead can be added at 16 mm from the bottom up for the micro motor to push on.

The STL files for the rocket and the support can be found in the attachments.

The 3d printing is pretty straightforward, load the two stl files into your slicer. Set the infill to 100% and 0.2 mm layer height. Also make sure to select print as support for the second support object.

For the rocket to be passively stabilized it needs some initial velocity. This can be achieved by using a launch rod, the launch rod guides the rocket straight up for the first 20 cm. After this the rocket will have sufficient velocity to stabilize its own flight. For the launch rod a 2mm garden wire can be used, and a small section of paper straw can be superglued onto the rocket for the rod to slide over.

The model rocket is finished!

Press fit the motor engine in the bottom of the rocket, hook it up to your electrical igniter and let it fly!