Patriot District Pinewood Derby Nerd's Corner

So what's a retired Naval Flight Officer and pilot and nerd to do with his free time? Add a little solder, a huge battery and a ridiculously over-powered remote control airplane/jet turbine engine.. maybe a little creativity along with LEDs, a bunch of 3D printer filament (yea, there's not a single gram of wood in this beast—but the wheels and axles are 100% legal! Oh, and it might be a tad over 5oz), some prayers, a fire extinguisher (for the great disaster in late 2020 when version 1.0 of the JetCar turned into a molten, flame-sputtering, and spark-tossing pile of water-logged filament and LiPo battery after I mistakenly crossed the positive and negative battery terminals.. I don't recommend doing that! <Pffffft> "Rut-ro.")

But version 2.0 worked like a champ as you will see below. 

If interested, scroll down below the video and if you'd like to read more about the build.

For me to remotely feel like I am protecting everyone at the Derby, I first clear out anyone standing at the end of the track! Then I have the JetCar  timed so that I can turn it on, arm it on the start-peg, and then walk to the end of the track so I can "catch" the JetCar with a pillow when it crosses the finish line. I designed the JetCar so that the air is literally sucked in through the bottom of the car which ends up (essentially) vacuuming the bottom of the car to the track. I also have the turbine "exhaust" pointing about 15 degrees up so that the vector is pushing downwards on the car. These two things have kept the JetCar from zinging off the track and demolishing itself on the nearest wall and/or floor. <knocking on wood>. The JetCar 2.0 has been zipping down the track since 2021 at the Pack 1101 and Patriot District Derbies and bringing in gasps of surprise, wide-eyed amazement, time-shattering speeds, and a ton of screaming noise (those remote control ducted fan motors are really loud!).    

JetCar — The Build

The JetCar is designed off of a roughly similar design I found online (for the life of me, I can't find or recall the source to give him credit). Essentially, there are four components to the JetCar: body and cover, a LiPo battery, an engine speed controller, an Arduino Nano microcontroller, and the motor. 

Body and Cover
The body and cover are all entirely 3d printed. The body securely holds all of the electronic guts of the JetCar including the microcontroller, motor, battery and ESC, and the cover, well, just covers it all up. The 3D design was made in Fusion 360 and the 3d printer used was a Artillery Sidewinder printer with Overture PLA+ 3D blue filament.  The design of the body and cover was engineered to keep the car ON the track by placing the motor body tilted 15 degrees upwards at the business end (keep the thrust vector pushing a bit downwards), and pulling air in from the very bottom of the car which acts like a vacuum and sucks the car to the track. I was amazed at how much, um, sucking the motor does at full power! It really does stick to the track. The JetCar has been rock solid going down the entire track. 

 Engine Speed Controller (ESC)
Controlling the EDF between the battery and the motor is an 80 Amp engine speed controller (ESC). This bit of magic takes the output from the microcontroller and manages the amazing crap-ton of juice pumping from the battery to the EDF. The hair dryer you have in your bathroom pulls about 12-15 amps. This thing pulls 80 for that little iddy-biddy EDF.   

LiPo Battery
The battery is a 22.2V 550mAh LiPo. The battery is a 550 milliamp six-cell LiPo battery. It was the smallest six-cell I could find and is still waaayyy bigger than I needed. But the 70mm EDF <do your best Scotty from Star Trek impersonation> "Needed MORE POWER." So I had to use the six-cell 550mAh battery. Based on the program and time for the motor to run, I can get about 20 runs on a single charge. Since I run the demo between dens at our Pinewood Derby, that is plenty. 

Microcontroller
The "brain" of the JetCar is an Arduino Nano Microcontroller programmed using the Arduino IDE with it's instance of C++. I wrote the code so that the JetCar has four phases essentially: 1) ready-waiting, 2) ready-armed, 3) racing, and 4) finished-standby.   I programmed the Nano to first initialize the EDF/ESC through a normal-typical EDF sequence (each EDF is different). Once initialized, the Nano waits for the switch to be compressed when the JetCar is placed on the starting pin at the top of the track. Once the switch is triggered, a timer starts counting down from 20 seconds to give me enough time to walk down to the end of the track so I can catch the monster and keep it from flying off the end. At the end of the 20 seconds, the EDF spins up to about 10% thrust (just to get the EDF spinning to overcome the inertia going from no rotation to full power) and then waits for the switch to release (when the gate drops). Once that happens, the motor spools up to 100% power for 1.8 seconds and then shuts itself off. Magically, the 1.8 seconds is how long it takes it to travel the 40' of track. Once that happens, it begins another 20-second timer. At the end of that 20 seconds, the Nano returns to the ready-waiting mode.  

Motor
The "jet turbine" motor is a 70mm 12 blade 2300KV brushless ducted fan motor (DFM).  What else can I say about it aside from the above? It takes in a crap-ton of juice from the battery, turns it into a massive amount of moving air that, as Newton tells us, creates an equal and opposite reaction which, as a byproduct, provides a ridiculously loud and overpowered display of stupid-nerd-engineering. I don't have the heart to compete it against the other dads in the "Unlimited Class" competition at the Pack level... nobody ever stood a chance. Hehe.