An air vehicle includes a combustible fuse that is used for time-delayed release of a retention device. Combustion of a combustible propellant for propelling the air vehicle initiates combustion of the fuse. After a time delay following initiation of the combustion of the fuse, a retention device of air vehicle is released. In one embodiment, the air vehicle is a projectile which is fired from a propellant in a cartridge that is behind the projectile. The retention device may be used to release a cover for a sensor of the air vehicle, such as a cover for an optical sensor or seeker.