Build a Crayon Rocket

If you're like me, anything that looks like a tube and comes to a point can be made into a rocket.  Crayons fit that description perfectly, so when I found a giant crayon-shaped bank at the store, I knew its fate immediately.

This Instructable, created for submission to the Launch It! contest, describes how to turn a crayon bank into a high power rocket.  For regulations restricting the flight of high power rockets, see step 5.  There are many different ways to turn a crayon bank into a rocket.  I will be detailing the design I chose.  Step 2 will provide insight into how you can make your crayon rocket unique.

To make a crayon rocket, you will need the following materials and tools.  Flight hardware for various configurations is described in Step 5.

Building materials:
- Jumbo crayon bank (Toys 'R' Us, $7.99)
- 38 mm tubing (Wildman Rocketry, $6.25)
- 1/8" plywood (Hobbylinc.com, $11.19)
- Small strip of paper
- T nuts (optional)
- Screws (optional, same threading as T nuts)
- 2x 1/4-20 closed eye bolts (McMaster, $2.89/each)
- 20 ft climbing webbing (REI, $0.30-$0.36/ft)
- 2x 1010 rail buttons with well nuts (Doghouse Rocketry, $3.25/each)

Tools:
- Exacto knife
- Dremel rotary tool
- Laser cutter or hand saw
- Wood glue
- Epoxy
- Calipers

Start by disassembling the crayon bank.  The tip and end are friction fit into the main cardboard tube and should come out with a twist and a tug.  Measure the lengths and diameters of each component.  Most important are the inner diameters of each component of the crayon and the outer diameter of the 38 mm tubing.  These dimensions will govern the sizing of other parts made from the plywood.  The plastic pieces have a lip that should be cut off.  I used a Dremel to sand off the lip.  Sanding off this lip will allow you to more accurately measure the inner diameters of the plastic parts of the crayon.

Be sure to use the calipers to measure diameter.  It is important to accurately determine the diameter within 0.01 inches.  While it may not seem like a lot, you need a pretty good tolerance to get pieces to slide together.

The length is not as important, but you should be reasonably confident in the length of various parts for modeling purposes.  Similarly, it may be useful to measure and record the weights of individual components for modelling purposes.

It is generally best to design a rocket before construction, as this verifies stability and performance.  I use a piece of software called OpenRocket.  It's free and open source--great for college students like me who don't want to pay for more expensive software.  OpenRocket has most of the features contained in paid software like RockSim, so you don't lose much by going with the free option.  It is also fairly intuitive to work, though there are also some good tutorials on YouTube.

The most important use of software like this is in determining the stability.  Ideally, the stability of the rocket should be between 1 and 2 cal.  A stability of one caliber means the center of gravity is 1 body diameter ahead of the center of pressure, or about 4 inches for this rocket.  Stability less than 1 cal can have dangerous and disastrous results, typically ending in a hard crash landing.  Stability margins over 2 cal are fine, though the rocket can display some odd forms of motion, particularly on windy days.

There are two ways to increase the stability margin:  adding more fin area in the back of the rocket and adding weight to the nose.  Adding fins adds drag, and adding weight adds more downward force.  I tend to prefer to add fin area, as I don't have to adjust my parachute sizing to compensate for the added weight.  For this model, I use six fins, which is a lot for me given my usual 4-fin designs.

The fin and centering ring designs are attached in an AutoCAD drawing, which ultimately became the source file for the laser cutter used to cut out the parts.  Feel free to use that file for your own model, though beware that you may need to adjust some of the outer diameters on the circular parts to meet tolerancing for your rocket.

To make the centering rings, bulkheads, and fins, I used a laser cutter to cut the components out of a couple of sheets of 1/8" plywood.  This ensures the parts are exactly the right size and shape, and is a really fast way of manufacturing custom components.  Sadly, I made a few bad measurements that over-sized my components.  This was fixed by using the Dremel to sand components down to an ideal fit.  (Don't try sanding by hand.  I've done this before and become so discouraged by the slow rate at which plywood can be sanded  that I just started chiseling away with an Exacto knife.  Even that wasn't fun.)

I cut the 38 mm tubing down to 8 inches in length using an Exacto knife.  This saves me a bit of weight and space inside the rocket.

For this rocket, I decided to spray paint all exterior components before installation.  This makes finishing much easier since the main rocket body is already painted.

Assembly is by far the most tedious step of making your crayon rocket.  It takes patience waiting for glue to dry and occasionally a bit of force if your tolerancing is not just so.  To begin assembly, have all components fully disassembled in front of you.  If you have not already removed the the lip from the crayon tip and end, do so now using the Dremel sanding wheel.

Before applying any adhesive, it is a good idea to dry fit all of the circular components.  This will save you a lot of frustration later when a part tolerance is off, as it is much easier to sand off excess material when all the parts are exposed.

First, we're going to cut the fin slots.  Mark off six equally spaced points along the outer diameter of the cardboard crayon tube at the bottom of the tube.  I used a piece of paper wound about the outer diameter to ensure that the marks were equally spaced.  I then marked six more ticks about 1/8" to the right side of each tick.  (You may also choose the left, but be consistent.)  This represents the thickness of the fin.  It doesn't hurt to slightly oversize the thickness of the fin slot, particularly if this is your first time building a rocket.  Using a door frame, extend a line up the body using a pen or pencil.  This line only needs to be a few inches long--long enough to trace with a knife.  While you are at the door, draw a longer line up the entire length of the tube in between two of the fin slot locations.  This will help us align the rail guides for launching the rocket.  Determine the location of the upper end of the fin tab and close off the lines each 1/8" apart using a pen or pencil.  This will help you cut the right amount of body tubing out.  Using an Exacto knife or Dremel, cut the fin slots out.  Because my fin slots interfered with the shoulder of the plastic end cap, I inserted the plastic end cap into the cardboard tube so the fin slots could be cut uniformly into the plastic.

Mark six equally spaced lines on the 38 mm tube, using a door frame as a straight edge.  By placing the tube in the door frame, you get not only a straight edge, but also a plumb edge.

Insert the aft centering ring onto the 38 mm tube.  This will likely be an inch or so above the bottom of the tube.  The top surface should be flush with the bottom surface of your fin tabs, providing an extra gluing surface.  Glue the centering ring into place using wood glue.  You may opt to add an epoxy fillet later.

To glue the fins onto the tube, use superglue to tack the fins to the tube.  The fins should all be glued with the left edge of the fin tab aligned on the right side of each line.  Again, this pattern should remain uniform so that the fins can easily slide into the cardboard outer body.  After the fins are tacked onto the 38 mm tube, generously apply epoxy to fillet the fins to the tube and the aft centering ring.  Slide the middle centering ring down so it is flush with the top surface of the fin tabs and glue into place.  Apply epoxy at the joints of the fin tabs and middle centering ring.

Prepare the forward centering ring by drilling a 1/4" hole and installing the eye bolt.  It may not hurt to coat the threads in epoxy or thread lock to prevent the loosening of the eye bolt in flight.  An eye bolt that becomes detached will be difficult to reattach and will likely lead to the failure of the rocket.  Tie one end of the webbing to this eye bolt (securely!), and epoxy the centering ring to the top of the fin assembly.

Apply a layer of epoxy to the outer edges of the middle and forward centering rings and slide the fin assembly into the outer body tube.  This may take a bit of effort, especially if the fins are not glued on perpendicular to the 38 mm tube.  Apply wood glue fillets to the joint between each fin and the outer body tube surface.  You may also want to apply wood glue fillets to the joints between the centering rings and the interior surface of the outer body tube.

Cut a 38 mm hole in the bottom of the plastic end cap.  This will allow you to insert and secure your motor into the rocket.  Insert the end cap into the bottom of the body tube and glue in place with wood glue or epoxy.

Epoxy a slip of paper on the interior of the nosecone to cover up the bank slit.  This should improve aerodynamics of the rocket.  Glue the two upper bulkheads together to create a 1/4" thick bulkhead.  Drill a 1/4" hole and install the other eye bolt using the same procedure as before.  You may want this bulkhead to be removable.  If so, insert 4 tee nuts into the nosecone centering ring and epoxy the centering ring into the nosecone.  This will allow you to screw the bulkhead into the nosecone.  If you have no desire to access the space inside the nose cone, you can simply epoxy the bulkhead into the nose cone.

Tie the other end of the webbing to the nosecone eye bolt.  Do not glue the nosecone in.  It is designed to detach from the main body to allow the safe recovery of all components.

The most fun part of any rocket project is flight.  Rocket flight, however, can be dangerous.  This rocket was designed for what is known as high power applications.  In other words, the quantity of propellant, vehicle mass, and altitude performance combine to create a regulated hobby.  By joining a club through the National Association of Rocketry or Tripoli Rocketry Association, you can fly at events where the regulation is more or last covered by the sponsor clubs.  You will probably need a High power Level 1 certification to fly your crayon.  The procedures can be found on the websites of NAR and TRA.

As far as flight hardware goes, you will need the propellant (I used an AeroTech H242 motor.), a parachute (I used a 42-inch diameter parachute), and fireproof cellulose insulation to prevent your parachute from burning up.  I also included an altimeter on-board my rocket, but it is not necessary.  If it is your first time flying, seek the help of a more experienced flier to review the preparation of your rocket.  Good luck and happy flying!