Brief:
Big Bertha modified for rear ejection.

Modifications:
This is a Big Bertha modified so that the motor tube extends nearly the length of the body, and acts as a piston upon ejection. Recovery is nose first, making the rocket appear to be coming in upside down. I call it Ahtreb, which is Bertha backwards.

Essentially the motor mount assembly is not glued into the body tube, but it is allowed to slide freely (although not loosely) so that the motor tube slides backwards from the ejection charge and frees the parachute for recovery. The nose cone is glued to the body tube and acts as a bulkhead to keep the ejection charge from pushing forward. There is a small piece of BT-60 coupler that retains the motor mount from fully exiting the aft end of the body tube.

Construction:
Requires one Big Bertha kit of equivalent parts, plus:

• 12" of BT-20 coupler tube stock
• 4" of BT-60 coupler, cut into 3 pieces--1.5", 1.5", & 1"
• 2 20/50 centering rings
• 1 mylar chute kit
• Small piece of thin, flexible plastic or vinyl approx. 1/2" by 1"

The difficult part about this mod was making sure prior to any gluing that all steps were in the proper order. Getting anything out of order could have prevented completing the build.

Getting the thrust ring/piston stop glued strongly in the proper place without getting excess glue smeared in the tube that might prevent full travel of the piston took great care. Planning for all situations, including being prepared to reach in a very tight space and clean up excess glue, was essential.

Flight:
Preparing for flight means getting the chute and shrouds tucked inside the body before the aft end without getting them stuck between body and piston. Wrapping the chute with a sheet of paper, just as the protector used during building, makes this easy to get started. Then, after pulling out the paper, gently pushing the remaining chute and shrouds into the tube while working the piston forward, completes the process. No wadding is required; the piston protects the chute and shrouds from ejection gasses. At ejection, the piston moves from fully forward against the thrust ring to rearward, where air flow opens the chute. If the piston did not move fully back against the thrust ring, the deployment will force this.

First flight was on a B6-4. Boost was straight and just like an unmodified Bertha. Unfortunately, this was on a previous build using a plain BT-20 for the piston rather than BT-20 coupler stock. The was too thin and came apart in a spiral. Deployment was not correct but descent was still nose first and no damage other than the piston core occurred.

On the second build, the torn BT-20 was removed and BT20 coupler glued in its place.

Second flight was on a B6-4. The flight was good and this time deployment and descent were as planned.

Third flight was on a C6-3. Boost was the same as previous flights except higher, but deployment was very early, causing deployment at high speed and tearing the chute. Descent was still nose first and no other damage occurred.

Recovery:
The chute has been replaced (by cutting at the existing shroud connection points and around the center, and gluing a replacement into place in 2 pieces) and the rocket is ready for flight again. Medium to long delay engines are recommended so that deployment can occur at or after apogee, not before apogee.

Summary:
This is a complicated and difficult build but is a rewarding challenge for the experienced modeler. This ejection method could be repeated in builds of most mid- to large-diameter LPR birds.