[mxwrench]

No need to open the doors with servos. Use a cam on the drum, and a follower on the doors. When the drum is rotated the followers will follow the cam lobe to the open position (timed moment) and light spring pressure can close them. Use a sail servo to rotate the drum. You could use the dial of a computer radio to index the drum to specified degrees, ie. open or closed position. Bombs can be released by gravity(doors open).

Good luck, great project!