F-35B X-35B lift fan; the VTOL propulsion system is designed and manufactured by Rolls-Royce plc X-35B lift fan; the VTOL propulsion system is designed and manufactured by Rolls-Royce plc The F-35B is the short takeoff and vertical landing (STOVL) variant aircraft. The F-35B is similar in size to the Air Force F-35A, trading fuel volume for vertical flight systems. Like the AV-8 Harrier II, guns will be carried in a ventral pod. Vertical flight is by far the riskiest, and in the end, a decisive factor in design. The F-35's main power plant is derived from Pratt & Whitney's F119 or GE Rolls Royce fighter team's F136, with the STOVL variant of the latter incorporating a Rolls-Royce Lift Fan module. Instead of lift engines, or rotating nozzles on the engine fan and exhaust like the Pegasus-powered Harrier, the F-35B uses a vectoring cruise nozzle in the tail, i.e. the rear exhaust turns to deflect thrust down, and an innovative shaft-driven Lift Fan, patented by Lockheed Martin and developed by Rolls-Royce.[77] Somewhat like a turboprop built within the fuselage, engine shaft power is diverted forward via a clutch-and-bevel gearbox to a vertically mounted, contra-rotating lift fan located forward of the main engine in the center of the aircraft. Bypass air from the cruise engine turbofan exhausts through a pair of roll-post nozzles in the wings on either side of the fuselage, while the lift fan balances the vectoring cruise nozzle at the tail. This system is more similar to the Russian Yak-141 and German VJ 101D/E[78] than previous STOVL designs, such as the Harrier with thrust vectoring. In effect, the F-35B power plant acts as a flow multiplier, much as a turbofan achieves efficiencies by moving unburned air at a lower velocity, and getting the same effect as the Harrier's huge, but supersonically impractical, main fan. Like lift engines, this added machinery is dead weight during flight, but increased lifting power increases takeoff payload by even more. The cool fan exhaust also reduces the harmful effects of hot, high-velocity air which can harm runway pavement or an aircraft carrier deck. Though potentially risky and complicated, it was made to work to the satisfaction of DOD officials. This variant is intended to replace the later derivatives of the Harrier Jump Jet, which was the world's first operational short takeoff, vertical landing fighter, ground attack aircraft. The RAF and Royal Navy will use this variant to replace the Harrier GR7/GR9s. The F-35B variant was unveiled at Lockheed's Fort Worth plant on 18 December 2007.[79] The U.S. Marine Corps will use the F-35B to replace both its AV-8B Harrier II and F/A-18 Hornet fighters. The B variant is expected to be available beginning in 2012.