The helicopter rotor probably represents one of the most difficult of all power-transmission problems in the fact that high power is required at relatively low rotational speeds. High power at low speed defines high torque, and high torque in a mechanical transmission requires large, heavy gears. In addition, since every action has an equal and opposite reaction, the shaft torques must be reacted, and this requires a tail rotor system for the shaft helicopter.

A jet-propulsion system offers certain distinct advantages in comparison with a shaft-drive system for powering a helicopter rotor. It delivers a driving torque to the rotor in the form of a tangential force at the tip of the rotor rather than by twisting a shaft at the centre of the rotor.