 Simple rotatable aerofoils were installed on the ends of the ducts to exert control forces; the vertical aerofoil on the aft end was extended to form a pair of conventional aerodynamic rudders.The development of the SR.N1 had involved the development and registering of a number of patentable technologies, a portion of which being extensions of Cockerell's original concepts and patents.These patents included the craft's annular jet cushion, plenum chamber burning, compartmentation jets for stability, powered and unpowered recirculation techniques, and various configurations of flexible skirts.