Intro: High Power Cardboard Rocket
This instructable shares one of the more extreme uses for cardboard. Namely, the construction and repeated flight of a high power sport rocket used to obtain my Level 1 and Level 2 National Association of Rocketry certifications.
The primary build material for this rocket is a 4 inch diameter cardboard Crayon Bank manufactured by the Ralphco Company and sold in various places including Toys R Us.
Bill of Materials:
Two 4" Crayon Banks (one for body, one for tube fins)
Plywood for fashioning centering rings and nosecone bulkhead
38mm Motor Mount Tube
U-Bolt to Secure Recovery Harness to Motor Mount
Carriage Bolt to Secure Bulkhead inside of Nosecone
Eyebolt to attach Recover Harness to Nosecone Bulkhead
Tubular Nylon strapping for Recovery Harness
T-Nuts for Securing Motor Retention
Large Fender Washers to Provide Motor Retention
Bolts to Attach Fender Washers to Rear Centering Ring
Miscellaneous bolts, nuts, and screws to attach Tube Fins and Rail Buttons
Parachute (36" or larger, depending upon flight weight)
Two Part Epoxy
Step 1: The Cardboard OnBoard
As stated above, the primary construction material for this rocket is a pair of cardboard Crayon Banks manufactured by Ralphco. The cardboard tube is relatively thick, dense cardboard, with a lot of longitudinal strength.These banks have a tapered plastic top, and a blunt plastic base. The plastic pieces are press fit into the 4 inch diameter cardboard tube.
One bank tube is left uncut. The other bank tube is sliced to create 6 tubes that are each 4 inches long. These short tubes will be used to form the tube fins that stabilize the rocket in flight. I used a miter box and fine toothed saw to cut the sections from the thick walled cardboard tube.
The other piece of cardboard onboard is the 38mm diameter motor mount tube. This is a high density cardboard mail ordered from a hobby rocket supply site.
Step 2: Finding Your Center
The motor mount tube is mounted in the bottom of the rocket with a series of plywood centering rings. The plywood needs to be thick enough to drive wood screws into their edges through the rocket tube wall. Each ring is cut to a specific outside diameter, and has a hole cut in the center into which the motor tube will be glued.
The centering rings are slightly different diameters depending upon where they are to be placed in the bottom of the rocket. The rings within the widest portion of the plastic base are the largest. The ring inside the portion of the plastic base that fits inside the cardboard tube is smaller. The ring at the top of the motor mount is a middle size that fits snuggly inside the cardboard tube.
The relatively large diameter precluded me from using a hand drill and hole saw to cut the outside diameter. A saber saw was used to rough cut the outside diameter. A benchtop belt sander was used to tidy up the cut to achieve a snug fit for each ring.
Step 3: Assembling the Motor Mount
The center of the bottom of the plastic end cap was cut away using a razor knife.
The plastic end cap is squeezed from the sides to force the bottom and middle centering rings inside the end cap. Press fit the rings into their appropriate places. The largest rings at the bottom and just below the cap shoulder. The smaller ring at the top of the section of the endcap above the shoulder that fits inside the cardboard tube.
Two T-nuts were installed on either side of the motor tube hole in the bottom centering ring. These are used to bolt the motor retention hardware to the bottom centering ring before flights.
Two part epoxy is used to securely glue the motor mount tube inside the holes of the centering rings. An approximately 3/4 inch tall cardboard ring cut from the same tube the tube fins were cut from is used to recess the rear centering ring slightly up from the bottom of the endcap. This recessed mount protects the motor nozzle when the rocket is standing on it's base. Cut a piece out of the ring to form a "C", and epoxy the "C" into the bottom of the endcap. Make sure the rings are firmly pressed against the bottom and top curled edges of the end cap.
Once the epoxy sets the motor mount tube and centering rings are physically locked between the top and bottom edges of the end cap, with one centering ring also trapped just below the end cap shoulder.
The top centering ring is similarly epoxied to the near the top of the motor mount tube. The U-Bolt for attaching the recovery harness is installed in the top centering ring.
Step 4: Attaching Tube Fins
The six tube fins form a hexagon around the base of the rocket.
Pairs of fins are bolted together to align them. Each pair is then attached to the rocket by using wood screws that go through the fin tube, through the rocket tube, and into the plywood centering rings. Bolts are then added to connect each pair of tubes to the neighboring pairs.
One tube fin is attached using longer wood screws that attach the rail buttons to the rocket body inside the tube fin. The rail buttons are what guide the rocket up the launch pad rail until the rocket is flying fast enough to be stable without the rail.
Some thin cyanoacrylate glue (superglue) can be used to toughen up the exposed edges of the tube fin cardboard. It seals the cardboard and toughens it. The same can be done for the top edge of the main body tube.
Step 5: Attaching the Recovery Harness
Tubular nylon strapping is used to connect the nose cone to the motor mount. The parachute is connected to the harness at a point between the nosecone and body.
To attach the harness to the nosecone, use a carriage bolt through the tip of the cone. The carriage bolt goes through the center of a plywood bulkhead inside the cone. The bulkhead also has an eye bolt to which the recovery harness is attached. Quick link hardware is used to connect the harness to the U-Bolt on the upper motor mount centering ring and the nosecone bulkhead eyebolt.
Step 6: Prepping for Flight
This rocket is a fairly heavy build. It requires at least an "H" sized quantity of thrust to fly safely. A motor of that size requires you to have a Level 1 Certification from either the National Association of Rocketry or the Tripoli Rocket Association. I used this build as the certification flight rocket to obtain my L1 NAR certification.
Prep the motor load and install it in the motor mount tube. Firmly secure the motor casing using the motor retention hardware bolted into the T-nuts on the bottom centering ring.
Attach the parachute to the recovery harness, optionally securing it in a protective wrap to prevent any damage from the motor's ejection charge.
Make sure the nosecone fits firmly enough within the rocket body that it will not come out due to inertial separation, and not so firmly that the motor's ejection charge cannot push the nosecone out of the rocket tube.
Visit your friendly neighborhood NAR or TRA Range Safety Officer to have them approve your rocket for flight.
Set up the RSO approved rocket on the pad rail, connect the igniter wires, and return to the flight line to wait for the Launch Control Officer to launch your high powered cardboard.
Repeat as often as possible.
Step 7: Follow Up Projects
As follow ups to the basic tube finned crayon rocket, I have added an avionics bay to support use of electronics for dual deploy. This enabled recovery from higher altitudes and enabled me to fly the rocket on a "J" powered motor and thus get my Level 2 NAR certification. The video at the start of this writeup is of that stretched rocket on an "I" motor.
I also found a source for some crayon banks by Ralphco that are 5 inches in diameter. I upsized the previous design to this larger diameter cardboard crayon bank and have had several high power flights with that, as well.