Bottom hole assembly with closed loop control

A bottom hole assembly comprising a drill bit, a mud motor to rotate the drill bit, a thruster (501) to apply an axial force on the drill bit (540, see fig 5C), sensors (550 etc.) to measure drilling parameters and a processor (562 and fig 8) to determine the magnitude of the force to be applied to the drill bit by the thruster. The processor uses dynamic models (812, fig 8) to calculate the force to be applied to the drill bit from measurements gathered by sensors (814-822 fig. 8) such as rotational speed of the drill bit, weight on bit, pressure differences, torque, rate of penetration, vibration or formation characteristics. The force application device is typically either hydraulically, mechanically or electro-mechanically operated. The invention is used to compensate for changes in pressure drop through the drilling motor automatically avoiding a situation where wear on the bit or motor can be excessive with communication with the surface only required to change the dynamic models or other limiting values.