An actuator includes first and second linear motion devices, each able to extend the actuator and arranged such that jamming of one of the first and second linear motion devices does not preclude operation of the other. The first linear motion device includes a first linear screw, a first nut, and a first prime mover arranged to impart relative linear motion between the screw and nut. Similarly, the second linear motion device includes a second linear screw, a second nut, and a second prime mover arranged to impart relative linear motion between the second screw and nut.