Enlargement drilling system

The invention concerns a drilling apparatus for enlarging a borehole in the ground. The drilling apparatus includes a drilling head which has a drilling bit, a percussion piston which is driven by a drive medium and impacts against the drilling bit, and a hollow drill string to which the drilling head is fixed and through the cavity of which the drive medium is passed to the percussion piston. The invention concerns a drilling apparatus for enlarging a bore in the ground, comprising a drilling head () having a drilling bit (); a percussion piston () which is driven by a drive medium () and which impacts against the drilling bit (), and a hollow drill string () to which the drilling head () is fixed and through the cavity of which the drive medium () is passed to the percussion piston (). The drill string () is surrounded by a hollow outer string () enclosing an annular space which surrounds the drill string () and through which the drive medium () is passed out of the borehole. The invention further concerns a method of enlarging an existing borehole, using a drilling head of that kind.