Bertha Shark

by Greg Elder

Parts

• 1 - Estes Big Bertha Kit
• 1 - BT-60 body tube, 2 3/4" long
• 1 – JT-60C tube coupler
• 2 – CR2060 centering rings
• 1 – BT-20 motor tube
• 3/32" balsa fin stock

This rocket is a bash between a Big Bertha and Estes Industries Rocket Plan No.22, Tiger Shark. Hence, the name Bertha Shark.

Construction, Upper Stage

Build the Big Bertha kits according to the instructions, with the following exceptions:

1. Do not use the engine hook in the motor mount construction.
2. Glue the rear centering ring for the motor mount 1 1/2" from the rear of the motor tube.
3. Mark the body tube for three fins, instead of four.
4. Trace the fin pattern for the upper stage fins onto the balsa stock, cut, and sand. Make three fins and glue onto the body tube aligned with the marks you made for the fins on the body tube.
5. Cut two 1/2" ling launch lugs. Glue one launch lug along side one fin, against the body tube. Glue the other launch lug 8 1/2" from the rear of the body tube, aligned with the rear launch lug.

Construction, Booster Stage

1. Glue the centering rings onto the BT-20 motor tube, 1/8" from either end.
2. Glue the motor mount into the short BT-60 body tube, such that the rear motor mount centering ring is 1/8" inside the body tube.
3. Mark the short body tube for three fins.
4. Make two wing booster fins and one rudder booster fin.
5. Glue the fins along the marks you made on the short body tube.
6. Glue the tube coupler into the top of the short body tube, such that the back of the coupler touches the top centering ring.

Finishing

1. Apply sanding sealer to the balsa fins and sand the fins smooth. Repeat this process until the fins are sealed and smoothed as desired.
2. Paint the rocket colors of your choosing.
3. The booster stage should fit loosely into the upper stage. If the fit is too tight, sand the coupler.

Flight

I have flown this rocket three times. Once with a B6-0/A8-5 combination. The other two times with a B6-0/B6-6. All flights were great, with the exception of the last flight. During the last flight, the booster stage did not separate at ignition of the upper stage motor. The Bertha Shark still flew straight and recovered fine. However, since the upper stage motor was exhausting through the booster stage motor mount, the inside of the booster stage motor mount was "toasted."