This post is one in a series on the development of the F-35. Click here for a Flash graphic with system cutaways. Click on Read More to read the entire post on that system and check out next week's special coverage of the program in the Dec 12 edition of AW&ST.
It's relatively simple in concept, but in reality the powered-lift propulsion system in the short take-off and vertical landing F-35B Joint Strike Fighter has proved difficult, and expensive, to develop and produce.
The relatively simple concept is to take power from the engine, via a shaft, to drive a lift fan. This has the effect of increasing the amount of air accelerated by the propulsion system, and enables an engine that produces 43,000lb of forward thrust in full afterburner to generate more than 40,000lb of downward thrust without reheat in STOVL mode.
During development of the F-35B there have been challenges to overcome in design and operation of both the Pratt & Whitney F135 propulsion engine and Rolls-Royce lift system that we will look at in more detail in this and related blog posts.
Graphic: Pratt & Whitney
Making up the lift system are: the three-bearing swivel module (3BSM) between the engine and the nozzle that allows main engine thrust to be vectored downwards; roll posts that duct compressor air out to the wings for roll control and lift; driveshaft, clutch, gearbox and two-stage lift-fan; and the variable-area vane box nozzle (VAVBN) that vectors lift-fan thrust and controls its stall margin. Each element has posed its own development challenges.
In powered-lift STOVL mode, the engine nozzle provides about 18,000lb of vertical thrust, the lift fan just under 19,000lb and the roll posts 1,850lb each, for a total of 40,600lb (compared with the 23,800lb generated by the Harrier’s thrust-vectoring R-R Pegasus). In addition to thrust, the lift system provides flight control in STOVL mode: the 3BSM in pitch and yaw control, VAVBN in pitch and wing posts in roll.
Perhaps the biggest single development issue - and most costly in time and money - was the failure of low-pressure turbine (LPT) second-stage blades on two F135-600 STOVL engines during ground tests at Pratt in 2007-8. In STOVL mode, the two-stage LPT produces 29,000hp of shaft power to drive the lift fan.
High-cycle fatigue cracking was traced to vibration caused when the hollow unshrouded blades struck the wakes from stator vanes upstream. In the STOVL engine, the large pressure drop across the turbine when driving the lift fan intensifies these wakes. The fix was to switch to asymmetric vane spacing, to disrupt the excitation causing the vibration, and redesign the blade to eliminate a stress concentration at an internal cooling hole.
In 2011, Pratt and Rolls made changes to the engine and lift-fan digital controls to increase thrust by about 100lb to help meet the F-35B’s vertical-lift bring-back (VLBB) payload requirement. The weight of unused fuel and weapons that can be brought back to a vertical landing, VLBB is a key performance parameter for the F-35B and has always been close to the margin, even after the STOVL weight-attack redesign in 2004.
Return to F-35B graphic, or on to Lift Fan.