In one aspect of the invention a drill string has a rotary valve disposed within its bore. The valve has a first disc attached to a driving mechanism and a second disc axially aligned with and contacting the first disc along a flat surface. Each disc has a port adapted to align with each other periodically when the discs rotate relative to one another. The aligned ports are adapted to direct fluid to at least one nozzle disposed in a working face of a drill bit of the drill string.