In the first decade of the 21st century, rising jet fuel prices increased emphasis on engine/airframe efficiency to reduce emissions, which renewed interest in the propfan concept for jetliners beyond the Boeing 787 and Airbus A350XWB. For instance, Airbus patented aircraft designs with twin rear-mounted contra-rotating propfans.[57] Rolls-Royce had the rear (pusher) configured RB.509-11 and front (tractor) configured RB.509-14 geared propfan designs, which produced 15,000–25,000 lbf thrust (6,800–11,300 kgf; 67–111 kN) using the gas generator from its XG-40 engine[58] with 13,000 hp (9,700 kW) of shaft power.[59] It became lukewarm on propfan technology in the 1980s[60]although it developed an open rotor design that was thought to be a finalist for the Irkut MS-21 narrowbody aircraft.[61] The Rolls-Royce RB3011 engine would have a diameter of about 170 in (430 cm; 14 ft; 4.3 m) and require a 16,000 shaft hp (12,000 kW) gearbox.[62]