How the F-35's cooling system solves problems that plagued its predecessors for years

Harrier heat problem

The Harrier family of aircraft is notorious for high exhaust temperatures and concentrations that can cause severe damage to the flight deck, runway and the aircraft itself. This limits where and how long the aircraft can hover and makes it more difficult to use in hot climates and when carrying heavier loads. This also increases the maintenance burden.

The Harrier engine directs extremely hot engine exhaust gases downward through its rotating nozzles, producing large amounts of infrared signals, intense thermal stress, drawing hot gases into the engine, and more. The Harrier basically creates a huge downward jet of hot air from a Rolls-Royce Pegasus via four rotating nozzles. The F-35B produces more balanced thrust, distributed across a central lift fan, rotating exhaust ducts and wing roll columns.

It's worth noting that the F-35B hasn't completely solved the thermal problem, as thermal management remains an issue for the aircraft. But it did improve them greatly and made it possible to build larger, heavier, and more electronically equipped STOVL aircraft. The Harrier also had to balance on a single column of air, making landing difficult and dangerous. The F-35B's columns are much more stable.

F-35B replaces Harrier fighter

The British Aerospace Sea Harrier (the British variant of the McDonnell Douglas AV-8B Harrier II) became an iconic aircraft during the 1982 Falklands War, performing well against Argentine forces. It was retired from the Royal Navy in 2010 (now replaced by the F-35B) and from the Indian Navy in 2016. The upgraded AV-8B II is still in service with the Italian Navy, Spanish Navy and U.S. Marine Corps.

Still, now in its twilight years, the Italian Navy is phasing them out and replacing them with F-35Bs by 2028, while the Marine Corps is expected to retire its last Harriers in 2026. The Marine Corps is finalizing plans to modify its amphibious assault ships to carry F-35Bs. Going forward, the Marine Corps' fighter fleet will consist of legacy F/A-18 Hornets and F-35Bs, with the Hornets being phased out. The Spanish continue to operate the AV-8B and plan to maintain it into the 2030s, with no clear replacement after ruling out the purchase of the F-35B. The F-35B is a generational leap ahead of the Harrier in many ways.

The fighter is not only a stealth fighter and a super-sensory computer with a “God's Eye” view of the battlespace, but it also has a greater payload capacity and a greatly expanded combat range. In terms of STOVL demonstration, Lockheed's X-35 demonstrator completely surpassed the competitor Boeing X-32 demonstrator. While the X-32's system has been described as an update of the old Harrier, the X-35 is revolutionary, It relies on what test pilot Rody called the “17 Miracles.” This is achieved through the giant Rolls-Royce LiftFan, which is 48 inches in diameter.

F-35 improved thermal management

this The F-35B's lift production is primarily distributed by a rear rotating exhaust nozzle and a large Rolls-Royce lift fan Behind the cockpit. Using this method, the downward exhaust temperature is lower, the heat is dispersed, the hot gas recirculation is reduced, and the thermal strain on the ship deck is reduced. This is not only beneficial to the ship, but also necessary for the F-35B itself.

It should be emphasized that the F-35B's exhaust system is still very hot, with peak temperatures sometimes even hotter than those of the Harrier in some tests. It still causes deck heating issues, and amphibious assault ships still require modifications, thermal coatings and other measures to operate the F-35B.

Rolls-Royce is currently the only contractor manufacturing certain key components for STOVL operations. Its LiftSystem is a descendant of the Pegasus engine and consists of a LiftFan, propeller shaft, 3-bearing rotating module (3BSM) and rolling columns. The F-35B's F135-PW-600 engines and Rolls-Royce LiftFan produce a combined vertical thrust of approximately 40,000 pounds. The F402-RR-408 engine of the later AV-8B Harrier II variant produced approximately 23,500 pounds of thrust.

Thermal management is a challenge for 4th and 6th generation

The F-35B has more electronics that can be damaged by heat than older Harriers. This is true for all F-35 variants. Modern fighter jets such as the F-35 are equipped with powerful electronics (e.g. sensors, radars, electronic warfare systems, processors, computers). These generate a lot of heat that the aircraft needs to deal with.

Ongoing F-35 Block 4 upgrades provide a more capable electronics suite, but also exacerbate thermal issues. The heart of the F-35 cooling system is Honeywell Integrated Power and Thermal Management System (PTMS). Power generation and thermal management are two key engineering challenges faced Current Sixth Generation Fighter Plans. These are expected to significantly increase demand for electricity, which will generate heat, which in turn will need to be managed.

Contractor Honeywell said: “Our Power and Thermal Management Systems (PTMS) integrate traditional auxiliary power units, environmental control systems and emergency power into one system. On the F-35, the PTMS integrated power package provides power for the aircraft's main engine starting, auxiliary and emergency power needs while providing thermal management of the aircraft's thermal loads“.

F-22 Lessons Learned

Thermal management is also an issue affecting the F-22. In 2011, the Raptor fleet was partially grounded due to an hypoxia-like incident in which pilots experienced dizziness, confusion, fainting and hypoxia. These incidents were primarily caused by problems with the Environmental Control System (ECS) and Onboard Oxygen Generating System (OBOGS). At the same time, the Raptor's avionics, radar, sensors and computers generate a lot of heat. We spent some time solving this problem by modifying the pilot's pressure suit, a backup oxygen system, and changing the Raptor's filters and airflow.

Because the F-35 was designed after the F-22, it was able to learn lessons from the Raptor. This has contributed to the development of PTMS that better cools avionics and manages climate control in the cockpit. The F-35 is also designed to account for future upgrades that will generate more heat that will need to be managed.

The F-22 may not be a predecessor to the F-35 because the F-35 replaced it (they play complementary roles), but it is a predecessor because it was developed about a decade ago. Thermal management is far from the only lesson the F-35 can learn from the Raptor. Another major improvement to the F-22 assist is Radar absorbent material (RAM) or “stealth paint” on the F-35. They are more durable and easier to maintain and upgrade.

Thermal management and sixth generation

While many headlines focus on the stealth, speed, combat cloud, MUM-T, and sometimes tailless features of the sixth-generation fighter, one of the biggest engineering challenges and bottlenecks is thermal management. Aircraft such as the upcoming Tempest/GCAP and F-47 will need to withstand greater thermal loads than today's fifth-generation fighters while maintaining stealth in the infrared spectrum.

Many in the aerospace industry consider thermal management to be as important as thrust, stealth and aerodynamics. In engineering, many of the core challenges are problems that many people may consider mundane and problems they would never think of. US Navy F/A-XX and Troubled Franco-German FCAS sixth-generation fighter (That is, having a French aircraft carrier variant) also needs to solve the exhaust problem of its carrier-based operations. That said, they are not expected to be STOVL aircraft.

The F-35 has already pushed the limits of onboard cooling, and the sixth-generation jet will push that even further. These aircraft will incorporate more powerful radars, larger sensor suites, advanced electronic warfare systems, artificial intelligence-assisted computing, lasers, more powerful engines, more power generation and more. If World War II fighter jets were compared to flying tractors with machine guns, sixth-generation fighters will fly overheated smartphones that need to hide the heat signature of thermal imagers. They would be flight computer command centers that happened to carry missiles.