US Patent 7240744 Rotary and mud-powered percussive drill bit assembly and method

A method and apparatus for boring a hole in the earth including a drill bit assembly including a fluid-powered impact engine contained within a drillpipe sub between an ordinary drillpipe and a modified conventional rotary drill bit. The impact engine is powered by pressurized drilling fluid delivered via the drillpipe which acts on a piston to charge an energy accumulator, preferably in the form of a spring. Periodically, the pressurized drilling fluid in the impact engine is vented, allowing the energy accumulator to rapidly transfer its stored energy to an impact bit which is slidingly housed in a rotary bit to strike and fracture the formation.