The Pratt & Whitney F135 engine powering the Lockheed Martin F-35B short takeoff and vertical landing (STOVL) variant has exceeded the thrust requirement during hover pit tests leading up to airborne testing.

Lockheed’s first STOVL prototype – dubbed BF-1 – has demonstrated 41,100 pounds of thrust in vertical mode. The new benchmark represents a 1.3% improvement over the rebaselined thrust requirement, and a nearly 5.4% increase over the programme’s original requirement.

 bf1 hoverpit
 ©Lockheed Martin

That extra margin still provides an “excellent” buffer in case the F-35B’s customers need an additional boost of vertical thrust above the current requirement, which is 40,550 pounds.

In 2004, Pratt & Whitney demonstrated an engine that could meet the original requirement of 39,000 pounds of vertical thrust. That requirement became outdated after the F-35B experience a major redesign to offset about 5,000 pounds of extra weight accumulated during the design phase.

A shaft-driven lift fan manufactured by Rolls-Royce powers the F135 engine in vertical mode.

The hover pit tests are among the last steps before BF-1 can attempt to return to flight after a nearly year-long hiatus and transition from conventional flight to vertical landing mode.

Lockheed says it has also demonstrated several key test points before the hover pit tests, including inlet pressure recovery, pitching moment, rolling and yawing moment, effective vector angles of the exhaust and control-input response time.