Recently, Riley and I completed the Cub Scouts’ Pack 32 Space Derby. Months ago, I told Riley’s den leader that I was enthusiastic to help with the derby, since it is one of my favorite scouting events. After a handful of emails, I found myself volunteering for a number of things related to the event, in addition to helping Riley with his rocket.
The derby itself is a race of propeller-driven rockets, flying down a length of taught fishing line. The rocket is made of two halves of balsa wood, hollowed out and glued together, then shaved and sanded to a rocket shape. A hanger is attached to the top of the rocket that allows it to slip into a cradle that hangs from the fishing line track. Rubber bands inside the rocket attach to the propeller in front and a small stationary dowel in the back. The propeller is wound, tightening the rubber bands, and when it is attached to the cradle and released, the rocket flies down the track!
Rocket construction is a technical thing, akin to the ubiquitous pinewood derby. Shaving down the rocket body so that it is as light as it can be without compromising it structurally is difficult. Make the body too light, and its torque will rotate the rocket around the line, losing that torque for momentum. Too heavy, and the rocket won’t be as fast as it can. There are other strategies involved, too, for making the fastest rocket. Balance, internal friction of the bands, aerodynamics… all play into the best rocket construction.
Part of the trick is building the rocket correctly at all. There are propeller parts that must be assembled in a certain counter-intuitive way, otherwise the rocket will not be nearly as fast as it should. The tension in the bands is so high that without some protection added to the metal propeller hook, the hook will tear right through the rubber bands. If you glue in the dowel instead of notching the rocket in the back, then you can’t remove the bands if they do break or become tangled. All of these are simple things that are included in the basic instructions for rocket assembly.
And this is where I found myself on Friday night before the derby, checking in rockets and inspecting them for these simple assembly issues. The number of scouts that assembled their rockets with the propeller on backwards was dismaying. (Yes, I realize that either way the propeller is on, it blows air in the same direction, but one side of the shaft is narrow and the other wide, creating a huge difference in friction, thus speed.) I spent a good amount of time with a pair of needle-nose pliers helping scouts and dads flip their props around.
Earlier in the evening, I was helping to set up the tracks for the rockets. The tracks consist of over-sized, custom-made saw horse-looking things. These stands sit on either end of the tracks, and the fishing line stretches between them. The line is tied at one end to an eye hook, and on the other end the line loops through the eye hook and is weighed down by some weights. This keeps tension in the lines so that they’re always taught.
Our pack’s track is constructed with some 2x4’s between that lay on the floor and keep the ends separated. The ends of the 2x4’s have numbered hinges that fit together so that there is a firm connection between the ends of the track to ensure that the lines - being as taught as they are - don’t pull the ends together. There are also some barbell weights on the back stand of each end so that the ends don’t tip forward. It seems rather overkill, but the track lines pull the ends together pretty hard.
One end of the track has a lever and a starting block that holds the rockets in place until they are launched. Dropping the lever lowers the block, and the wound rockets shoot out down the track. At the other end, electronic sensors sit on the track and determine the finishing order of the four rockets on the track. In the end, rockets are awarded points based on their placement in each heat, with the winner having the fewest number of points.
On race day, I tried to help wherever I could. I mostly felt like I was running around ineffectually most of the time, and probably should have settled in one place. But I really wanted to get an idea of how the electronics ran, since the guy running that component this year will not be around next year, since his son is moving on in scouts.
I ended up watching the race computer, trying to figure out how that worked, and winding rockets. We wound each rocket 80 times using a battery-powered winder, then placed it in the cradle on the starting block. We ate through batteries doing this, it’s amazing how much power it takes to “charge up” these rockets.