How To Build A Rocket Derby Rocket.


The Cub Scout Rocket Derby is as fun as the Pine Wood Derby but much easier for a boy to make. This little guide will go through the steps of building a rocket using the rocket derby kit available from the BSA. The goal of this guide is to point out the steps that are not obvious in the page of instructions that come with the kit. Making the fastest rocket in the world is not the goal of this guide. I have not discovered how to do that yet. But, I do know this, the condition of the rubber bands is the key to a rockets' speed. I have raced the same rockets against each other hundreds of times and had each rocket out perform the others. The one item of note was always the condition of the rubber bands. After a few runs the rubber bands wear in to peak performance, then after a time they wear out, eventually breaking.


Lets' build a rocket!


This is the BSA Rocket Derby kit. They are available from scoutstuff.org and local scout shops.




Inside the kit you will find the following parts:





The bag of parts has the following items in it:





It is best to leave these parts unopened in the bag until you are ready to assemble the propeller. Only remove parts from the bag as they are needed. This way nothing will be lost. I have had boys come to pack meeting to race their rocket without the dowel pin, brass bushing, and red plastic tube. Each part has a special purpose and should not be left out.


The first step is to dry fit the two balsa wood halves of the rocket. This is to make sure that the holes at the front and back of the rocket line up properly. I have built rocket kits where the outer edges of the balsa block did not fit flush to each other when their front and back holes were lined up for proper circles. This is OK because we will be sanding the rockets round later any way. Especially make sure that the small hole on the top of the rocket is lined up properly such that the nose button fits.




The second step is to glue the two balsa wood halves of the rocket together. I do this a day before we are going to shape the rocket. First I run a bead of white glue along the inside edge of one of the balsa halves as shown below.




Then I evenly coat the glue on the surface with a finger.




Now fit the two halves together and line up the holes on the top and the bottom so that they are properly round. I push the nose button into the top hole while I do this so that it lines up the top hole while I line up the bottom hole.






Then clamp the two halves LIGHTLY together. Don't crush the balsa wood in the clamp. I have used books in the past when I didn't have these clamps.




After the halves are clamped together I carefully remove the nose button from the small top hole so that it does not become glued in place. If any glue got onto the nose button, wash it off now. The nose button is the mount for the propeller assembly that must be removable for servicing the rubber band motor.




Next we will shape our rocket.


I use two grades of sand paper to shape the rocket. 60 grit for the shaping because it takes the surface down fast with out being so fast that one mistakenly sands things too thin. 220 grit is used for the finish work to smooth the surface out. Do all of the shaping by hand. Don't use a a power sander or you will soon have no rocket left. I do attach the 60 grit sand paper to a wood block to make it easier to hold onto the sand paper. I let the 220 grit sand paper wrap around the rocket surface so that the curve of the surface turns out smooth.


First reinsert the nose button into the top of the rocket and trace around it. This gives you a guide of how small the sanding of the nose can go.




Do not sand smaller than this circle.




Now mark the location for the dowel pin and shape of the bottom of the rocket. Notice that I have placed a straight line across the center of the rocket to show where the dowel pin will go. I then added a circle to show the size of the rocket at the bottom in the same way as the circle for the nose. My mistake at this point was that I planned to put my dowel pin lined up with one of the fin placements. I have made this mistake before. It is better to place the dowel pin offset from the fins so that the fins do not become damaged by repeated removal and placement of the dowel pin when winding the rubber bands. Don't place the dowel mark along the glue joint for the two halves of the rocket. The dowel pin place along the glue joint could lead to the dowel under rubber band tension splitting the rocket along the glue joint.




Now, let your cub scout go to work shaping the rocket with sand paper.




Here is the finished shaped rocket. Notice that is is kind of fuzzy from the 60 grit sanding. This fuzzy texture is removed by sanding with 220 grit sand paper.




Notice that I sanded down to the guide circles for the top and bottom of the rocket.






Then I smoothed out the surface with the 220 grit sand paper.




Now it is time to place the hanger and fins. First I mark the placement for the hanger. Like the dowel pin, I don't want to place the hanger on the glue joint between the two halves of the rocket. I have chosen to place the hanger centered on the rocket half that seemed to be denser wood of my two rocket halves. The instructions with the rocket kit then say to place the hanger three inches from the top and three inches from the bottom of the rocket with one inch for the hanger itself. The rocket is a total of seven inches long. I have used a square measure three and four inches up from the bottom of the rocket and make some marks. A tape measure works just as well here.




Then I use a trick I learned from building Estes rockets. Place the rocket in corner of a door frame and line up the two marks with the edge of the door frame. Then draw a line between the two marks for three and four inches. This one inch long line is where I will cut out a notch to push the hanger into. The trick with the door frame is it gives a guide for drawing a line straight down the axis of the rocket.




This is where the hanger mark is.




Now I used a small screw driver to push into the wood along the line I made. I have made sure not to make my cut at an angle or too wide for the plastic hanger that will go into it. The blade of the small screw driver is thinner than the hanger so that I can keep the cut only as wide as the hanger. I also checked that I was making my cut straight into the rocket by looking down the rocket as I pushed the screw driver blade into the wood. I also used an hobby knife to chip out small wood pieces after the notch got too deep for the screw driver blade. The finished hanger notch goes all the way through to the inside of the rocket.




Then I fit the hanger into the notch to check for a tight fit. But I waited to glue it in place until everything else was done.




Next I turned my attention to the dowel pin notch on the bottom of the rocket. The dowel pin needs a notch as deep as the pin is thick. I used a metal file to make this notch along the line I made earlier.




Notice how the dowel pin fits fully into the notch. A good notch keeps the dowel pin in place when is is under tension from the wound up rubber band.




Then I started making my fins and made the notches for them.


First I marked the shape I wanted for my fins on one of the three fin sheets and took a good look at it to see if I liked it..




Then I cut out the fin and copied it for the other two.




I also rounded the sharp corners of the fin with fine sandpaper because I wanted a smooth look and feel. I'll also be using this rocket a lot and don't want the sharp edges.




Then I marked the arrangement of the fins that I wanted on the bottom of the rocket. This is when I noticed that I had put the dowel pin lined up with one of the fins. This is going to cause me trouble later as I did this on another rocket and have always had problems breaking the fin that is lined up with the dowel pin on that rocket. The dowel pin when under tension from the rubber bands needs to be placed in its notch and the fin lined up with the notch often gets caught when doing this. It would be better to set the dowel pin line after setting the fin arrangement and make the dowel pin notch just before gluing the fins on. I set the bottom fin along the left edge of the dowel notch to minimize my mistake.




I then used the door frame method of drawing lines along the axis of the rocket again to draw the lines for the fins. I measured up from the fin mark on the bottom of the rocket to where I wanted the fins to set. Then I made marks for this length on the rocket for each fin. I lined up the fin mark on the bottom of the rocket with the door frame and drew a line from my length mark to the bottom using the edge of the door frame. Then I used a fin to score into the wood along these three fin lines to about 1/16 of an inch deep.




With the last cutting of the wood done it was time to start gluing the hanger and fins on. First I glued the hanger in place. I used super glue and put glue on the tab of the hanger and quickly pushed it into the hanger notch. Remember that I did this quickly because of the super glue, it dries quick and the hanger can end up only half way in.

NOTE THE DIRECTION OF THE HANGER. The top of the hanger looks like a bullet. The bullet end is to the front, the primer end is to the back. When the hanger is inserted into the rocket carrier the flange at the primer end pushes against the carrier and the rocket flies down the wire. If the hanger is installed backward the rocket can fly out of its carrier and smash into walls, floors, people, etc.




To glue the fins in place I first press fitted them fully into place without glue. Remember the super glue dries fast. I have found it to be too fast for the fins to get in place properly. After a fin is in proper place I then run a line of glue, bead by bead, along each side of the fin at the wood joint.




Now all the fins are in place.




Now it is time to paint the rocket. The only two places that paint is not wanted is inside the top hole where the nose button goes and the bullet portion of the hanger. We don't want a lot of paint in the nose button hole because it will make it too tight for the nose button to be removed. We don't want paint on the hanger bullet because the bullet will then be too thick for the rocket carrier and it will make the connection too tight as well. I mask off the bullet portion of the hanger as shown.




Then I roll up a piece of paper length wise and insert it into the bottom hole of the rocket. This makes a great handle while painting.




I chose to spray paint my rocket. Outside. Here is the finished paint job.




While the paint is drying you can assemble the propeller. Here is how it is done.

Take the red plastic tube and push it over the straight end of the propeller wire.




Then grab the bottom of the straight end of the propeller wire and continue pushing the red tube up over the hook in the propeller wire.




Stop when the red tube reaches the end of the hook. The red tube protects the rubber bands when attached to the hook and helps hold the rubber bands on better. It should look like this.




Next put the brass bushing into the nose button like so. Without the brass bushing the propeller will not spin as cleanly and fast. The bushing reduces friction.




Then run the straight end of the propeller wire through the back of the nose button and brass bushing and slide the propeller on top.




Now a quick check of the propeller direction is due here. There are two ends to the propeller. There is a rounded end and an end with a notch to catch the propeller wire when it is bent. Make sure the rounded end goes toward the brass bushing and the notched end is on top. Here is a close up picture of the propeller axis with the notched end up. The propeller wire would insert through the bottom of the propeller as shown here.




Double check that the propeller is in the right direction because the propeller wire can easily break if you try to bend it back after bending the acute L in the end that we are about to do. Grab about 1/8 of an inch of the straight end of the wire and bend it over into an acute angle.




It should look like this when done.




Now the rocket is done. Time to put the rubber bands onto the propeller hook, put the propeller onto the rocket, fish the rubber bands through the rocket, and wind it up.


Hook two rubber bands onto the propeller hook.




Push the rubber bands down the nose button hole and fish them out. I have a long hook made from coat hanger for this.




Then I put in the dowel pin and wind up the rubber bands. This can be done from the front with a finger, or from the dowel pin with a geared winder. I have a winder and wind from the back the put the dowel pin into its notch with the rubber bands under tension.




Then the rocket is hooked onto its carrier in its starting gate and launched!




I hope to see many great rockets at pack meeting.


Have fun building your rockets.




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