Rear Thruster v2.0 – Building

Before reading this you should read about the CAD design for the rear thruster. As a brief into, the rear thruster body consist of 4 different parts: the nozzle, the fins to connect the nozzle, the cone, and the motor mount.

Cone

As described in this post the new rear cone design is larger than our previous cone. This caused a few issues in its manufacturing. We first tried to cut the cone out of a large 5"x6"x6" piece of polyethylene on a CNC machine that has a rotary axis. Unfortunately the piece weighed a bit to much and was very unwieldy so we had to try something else. Before we move on here is a picture from when we bored out the polyethylene bloc with a hole for the motor to sit in...it makes a flower!

Polyethylene "flower"

We decided to use the 3D printer instead (older version of MakerBot Replicator); surprisingly enough, it worked extremely well. You can see an image of the print with threaded inserts below, as well as a image of the cone inside the body to show how nicely it fits.

Cone w/ Inserts onto of Aluminum Body

Cone in Aluminum Body

Quick tip

We are printing this piece with the large flat side facing down. This is because it leaves only a little bit of overhang, which is not a problem for a printer with decent scaffolding. If your printer does not have that ability then, according to one of our advisers, adding a chamfer to the overhang should allow you to safely print the piece.

Motor Mount

The motor mount for the rear thruster was cut out of Delrin, the same material we used to make our small thruster mounts. This is a major improvement from last year as it gives us much more freedom while mounting the motor. Last year we used the small cross mount that comes with the Turnigy SK3 motor which we didn't like very much because it barely sticks out beyond the edge of the motor – meaning we had to have a very small compartment for the motor. This year we are able to have a larger compartment for the motor, which we wanted for cooling and flow purposes, as well as a much more secure connection to the rear cone.

The mount itself is a pretty simple piece. It has four holes near the middle for attaching the motor and four holes  around the edge for attaching the mount to the cone. The spokes are as large as we felt comfortable making them, while allowing for water flow but keeping enough strength to hold the motor in place.

Motor Mount

Motor on Mount

For information on the motor, shaft, and propellor that will be mounted to this check out our post on preparing the motors.

Nozzle

We kept the nozzle that was made last year in this year's prototype. We decided to do this because it is a hard part to manufacture, last years version came out nicely, and is still in good condition. It was printed in two parts and then melted/epoxied together and, finally, painted red.

Nozzle

Fins

We also used the Delrin plastic to make the fins that attached the cone and the nozzle. There are three fins, two of which are the same while the third is slightly different – the different sizes are needed because of the asymmetry of three fins in a four sided cone. 

Fins

Assembly

There were a few different tasks required to assemble the rear thruster. First we had to press the threaded inserts into the cone. We used threaded inserts to avoid cross threading and stripping our threads on the relatively weak PLA.

To press the threaded inserts into the printed PLA we heated them with a heat gun and easily pressed them into place. 

Two Threaded Inserts and Two Empty Holes

Using a Bolt to Install Threaded Inserts

We then epoxied the fins into the slots in the cone and nozzle to ensure a strong and lasting connection between the two. We also used the same epoxy mixed with acetone to create an epoxy paint. We then used this to paint the cone, getting ride of the shiny PLA look and giving it a smoother look.

Painting Cone w/ Epoxy

Painted Cone

Next we used epoxy to mount the nozzle onto the rear cone. After that we mounted the motor mount/motor to the cone using 12mm M4 bolts. And the thruster is complete (finally)! Below are a few profile shots and pictures of the finished thruster.

Rear Thruster Profile