The U.S. Department of Defense and Pratt & Whitney finalized a $508-million contract modification that authorizes the jet-engine builder to proceed with construction of the sixth series of F135 propulsion systems — the engines that power the F-35 Lightning II Joint Strike Fighter.

The so-called “low rate initial production” (LRIP) contract covers 38 total engines, and the new costs include program management, engineering support, production non-recurring effort, sustainment, and spare parts.

This contract modification pushes the total funding for the sixth series of engines to $1.1 billion.

The F-35 Lightning II is a single-engine jet with stealth capability used by the U.S. Air Force, U.S. Navy, U.S. Marine Corps, and Great Britain’s Royal Air Force, for ground attack, reconnaissance, and air defense missions. Lockheed Martin is the primary contractor for the jet-building program. The Defense Dept. stages its construction orders, to manage costs and to accommodate design changes and other supply and demand issue.

Pratt & Whitney builds the engines at its Middletown, Conn., complex, the site of several production programs for military and commercial contracts. To date, it has delivered 115 F-35 engines.

Deliveries of LRIP 6 engines will begin in the fourth quarter of this year.

The 38 engines in this contract include 36 engines in different variants for installation, and two of the conventional takeoff and landing (CTOL) version, for use as spares.

The U.S. Air Force will receive 18 CTOL engines in the contract. The U.S. Navy will receive seven carrier variant (CV) engines, and the U.S. Marine Corps will receive six short takeoff and vertical landing (STOVL) engines. The LRIP 6 contract also includes the three CTOL propulsion systems for the Italian defense forces, and two more for the Australian forces.