F-35’s Cooling System Cracked the Problem That Grounded America’s Harriers for Decades

The Harrier’s single-column hot-blast scorched flight decks, recirculated exhaust back into the engine, and made hover treacherous. The F-35B’s distributed-thrust cooling system fixed all of it — mostly.

The U.S. Marine Corps on June 3 retired the AV-8B Harrier at Marine Corps Air Station Cherry Point, N.C., closing four decades of short takeoff and vertical landing operations defined — and repeatedly constrained — by heat.

The last operational AV-8B unit, Marine Attack Squadron (VMA) 223, the “Bulldogs,” had returned from a deployment aboard the amphibious assault ship USS Iwo Jima with the 22nd Marine Expeditionary Unit before the retirement ceremony. According to Aerospace Global News, citing the 2026 Marine Aviation Plan, the event “will provide an opportunity for active-duty personnel, retirees, contractors, and civilian communities to celebrate the historic contributions of the aircraft and the Marines.”

The retirement came ahead of original scheduling, driven by the operational advantages of the F-35B — a jet whose cooling system architecture was built specifically to address the thermal failures that grounded, damaged, and restricted the Harrier throughout its service life.

A Single Blast That Burned Decks and Engines

The AV-8B’s thermal problems originated in its propulsion design. The Rolls-Royce F402-RR-408 Pegasus engine generated approximately 23,500 lbs of thrust, all directed downward through four rotating nozzles during hover and vertical landing. That concentrated column of superheated gas scorched flight decks, buckled deck plating, and drew its own exhaust back into the engine intake — a condition called hot gas recirculation (HGR) that degraded thrust and accelerated engine wear at the most critical phase of every vertical landing.

Heat also constrained when and where the aircraft could operate. High ambient temperatures reduced hover duration, restricted maximum payload, and drove up maintenance costs. The Harrier’s dependence on a single air column for vertical lift made hover inherently unstable, demanding constant pilot compensation that engineers and aviators consistently described as difficult and dangerous.

F-35B: Distributed Thrust, Cooler Deck

The F-35B’s solution is the Rolls-Royce LiftSystem — formally the Integrated Lift Fan Propulsion System, which won the Collier Trophy in 2001. The system distributes lift across three spatially separated exhaust columns through four integrated components.

The 48-inch-diameter LiftFan, positioned directly behind the cockpit, receives 29,000 shaft horsepower from the F135-PW-600 engine via driveshaft and clutch, producing approximately 20,000 lbs of cool, unburned air directed downward beneath the forward fuselage. The three-bearing swivel module (3BSM) vectors the primary engine exhaust downward at the rear at approximately 18,000 lbs. Two wing-mounted roll posts bleed engine fan air for lateral stability, contributing a combined 3,900 lbs of thrust.

Together, the system generates approximately 40,000 lbs of combined vertical thrust — roughly 70% more than the Harrier — while spreading heat across separate exhaust zones rather than concentrating it in one column. During the Joint Strike Fighter competition, Lockheed Martin’s X-35 demonstrator outclassed Boeing’s competing X-32 in STOVL evaluations. X-35 test pilot “Rowdy” described the design as relying on “17 Miracles” made possible by the massive 48-inch Rolls-Royce LiftFan.

The thermal improvements are significant but not total. Peak rear-nozzle exhaust temperatures on the F-35B can match or exceed the Harrier’s in some tests. Amphibious assault ships still required structural modifications to operate the jet: the USS America (LHA 6) was the first West Coast ship to receive thermal spray coating on key landing areas of its flight deck.

The Electronics Problem the Harrier Never Had

The F-35 introduced an entirely different category of thermal challenge: the heat generated by its own electronics. The jet carries an active electronically scanned array (AESA) radar, advanced electronic warfare suites, a distributed aperture system (DAS), and high-performance mission computers — heat loads that the Harrier’s comparatively basic avionics never imposed.

At the center of that solution is Honeywell’s Power and Thermal Management System (PTMS). “Our Power and Thermal Management System (PTMS) integrates a conventional auxiliary power unit, environmental control system and emergency power into a single system,” Honeywell states. “On the F-35, the PTMS integrated power package delivers electrical power for the aircraft main engine start, auxiliary, and emergency power needs, while simultaneously providing thermal management of the aircraft heat loads.”

That capacity — currently approximately 32 kilowatts — is already insufficient for post-Block 4 upgrade requirements. The F-35 Joint Program Office has identified a need for 62 to 80 kilowatts to support expanded planned capabilities. Honeywell is proposing a phased path: a software upgrade achieving 40 kilowatts by 2029, followed by a hardware modification using 95% of existing PTMS components to reach the 62-to-80-kilowatt range. Collins Aerospace is competing with its Enhanced Power and Cooling System (EPACS), which Collins says demonstrated 80 kilowatts of capacity in laboratory testing.

F-22 Lessons Carved Into the Design

The F-35’s PTMS also incorporated direct lessons from the F-22 Raptor program. The Raptor fleet was partially grounded in May 2011 after pilots reported dizziness, confusion, blackouts, and oxygen deprivation. Investigators documented 12 or more such incidents between 2008 and 2011, attributing them primarily to failures in the Raptor’s Environmental Control System (ECS) and On-Board Oxygen Generation System (OBOGS), with a faulty valve in pilots’ upper-pressure garments also identified. The Air Force restored normal operations by April 2013.

Because the F-35 was designed after the F-22, Lockheed Martin built those lessons directly into the PTMS — integrating avionics cooling and cockpit environmental conditioning into a single architecture and designing in upgrade capacity from the outset.

The Transition, and What Comes Next

The Royal Navy adopted the F-35B in 2020, the Italian Navy in 2024, and Japan’s Maritime Self-Defense Force in 2026. The Italian Navy plans to complete its AV-8B transition by 2028. Spain’s navy — the last confirmed Harrier operator — has ruled out purchasing the F-35B and plans to sustain its aircraft at least through 2032. Going forward, the Marine Corps’ fixed-wing fleet will be built around the F-35B, with legacy F/A-18 Hornets continuing service until they are gradually phased out toward the end of the decade. Rolls-Royce remains the sole manufacturer of the F-35B’s critical LiftSystem components, a position it has held since the program’s inception.

The thermal demands will only intensify. Programs including the F-47, the Navy’s F/A-XX, and the international GCAP/Tempest will combine more powerful radars, AI-assisted computing, directed-energy weapons, and greater electrical generation — while preserving infrared stealth. Many aerospace engineers now consider thermal management as strategically important as thrust, stealth, and aerodynamics in defining what sixth-generation combat aircraft can achieve.