When discussions of 6th-generation fighter jets focus on stealth, speed, range, payload, and sensors, they often overlook a less visible but critical challenge: thermal management. Modern combat aircraft increasingly resemble airborne computing platforms, with powerful radars, electronic warfare systems, processors, and high-bandwidth communications that generate substantial heat. Future 6th-generation designs are adding even greater electrical demands, including possibly directed-energy systems (aka lasers).

While all these regularly discussed aspects (range, RCS, etc.) are important for engineers, one of the most difficult aspects and limits to building next-generation fighter jets (and upgrading 5th-generation fighter jets) is thermal management. When assessing the capabilities of new stealth-like fighter jets like Turkey’s Kaan, China’s J-36, and China’s J-50, a revealing question is “how are they cooled?” Unfortunately, thermal performance is one of the most important yet least visible aspects in assessing advanced combat aircraft capability. Getting thermal management right is critical to building a true 6th-generation fighter jet.

Data Centers & Smartphones

Credit: BAE Systems

Perhaps the most relatable analogy for understanding the engineering challenge of 6th-generation fighter jets is overheating smartphones. Modern fighter jets can be thought of as flying smartphones, overheating from being on charge, making a call, running high-power apps, using the camera, and being exposed to direct sunlight all at the same time. Additionally, fighter jets have to hide from thermal cameras while also generating heat friction at supersonic speeds.

A more salient analogy is to think of 6th-generation fighters as modern data centers. Around 30–40% of a data center’s energy use is spent on cooling the system. The system’s servers, GPUs, and other hardware generate massive amounts of heat. The rise of AI and high-performance computing is compounding the issue. Data centers are mostly cooled using air-based, liquid-based, or hybrid approaches.

A medium-sized data center can use 300,000 to 500,000 gallons a day (about the same as 1,000 households). Hyperscale facilities can reach five million gallons a day (around the same as 50,000 people). Northern Virginia data centers used around two billion gallons in 2023. But fighter jets are not large, spacious data centers able to use millions of gallons of water. That said, interestingly, one of the ways fighter jets are designed to disperse heat is through their onboard fuel.

The Overheating Fighter Jet Problem

Credit: BAE Systems

Simple Flying has previously reported how the F-35 Lightning II was able to solve a number of heat-related problems, over the older Harrier jump jet, from downward thrust heat to electronic heat. Thermal management systems enabled the F-35 to push the boundaries of what was possible and incorporate advanced AESA radars, sensor fusion, and more. Even so, the ongoing Block 4 upgrade is straining the aircraft’s advanced thermal management to the limit.

But this is set to be even more difficult for incoming 6th-generation fighter jets like the F-47 and GCAP/Tempest. These aircraft will carry larger and more capable AESA radars, far higher electronic warfare output power, advanced communications and networking, AI processing, an infrared search-and-track system, potentially lasers, and more.

Not only will these aircraft generate unprecedented amounts of heat, but they also need to incorporate all-aspect stealth, which requires a very low infrared (heat) signature. To overcome these challenges, engineers have to get creative with dispersing heat. There is no single answer, and engineers are looking for a ‘holistic’ solution, with adaptive cycle engines and the aircraft’s fuel as two important pillars. Engineers are conceptualizing the whole aircraft as a vehicle to disperse and dispose of heat.

Adaptive Cycle Engines

Credit: US Air Force

Adaptive cycle engines are one of the key solutions engineers are looking to solve the F-47’s heat problem. Adaptive cycle engines add a “third stream” that provides additional airflow for cooling and heat rejection. Both GE Aerospace (XA100) and Pratt & Whitney (XA101/XA103) are developing adaptive-cycle engines. They are key not only to managing heat but also to extending the range of fighter jets by allowing the engine to operate in the most efficient mode for that phase of flight.

GE Aerospace claims not only are they 30% more fuel efficient, they “provide an extra source of cooled air to improve propulsion and fuel efficiency. Most importantly, it enables a step change in power and thermal management capability that will be required for next-generation mission systems.” Rolls-Royce is likely developing adaptive cycle engines for the GCAP/Tempest fighter jet, but it has been much more coy about it than GE Aerospace or P&W.

RR speaks of the need for Tempest to generate tremendous amounts of electrical power, but it is less clear how this will be managed. It tends to use terms like “advanced power and propulsion,” “integrated power management,” and “thermal management,” but hasn’t used the term “adaptive cycle engine.” It is worth noting that Tempest has enormous heat management, electrical power, and range requirements that will need to be resolved, perhaps similar to the F-47. China is known to have a program to develop adaptive cycle engines.

Using Fuel As A Heat Sink

Credit: Boeing

Another key way next-generation fighter jets intend to manage heat is by using fuel as a heat sink. Historically, jet fuel has been used as the primary liquid coolant for onboard electronics before it is pumped into the engine and combusted. But a challenge is that the sheer amount of heat from the subsystems heats the fuel to the point that it reaches its maximum safe thermal limit long before it actually reaches the engine. The thermal limit is the point at which fuel begins to cook, varnish, or pose a fire hazard.

Fuel can absorb heat from the AESA radar, electronic warfare equipment, mission computers, and other systems. A simplified heat flow chart goes something like radar→heat exchanger→fuel→engine→combustion. There are several factors that make jet fuel an attractive way to dispose of heat. One factor is that aircraft carry several tonnes of it, providing a huge amount of heat-absorption capacity. Another benefit is that it saves the aircraft from carrying extra weight of a dedicated cooling fluid.

Finally, as the fuel is continuously burned, it (with the heat) can be expelled from the aircraft. Early in a fighter jet’s mission, it has a large heat sink available from full fuel tanks. Late in the mission, it gets worse as the tanks empty and the heat sink declines. This is made worse when the aircraft flies at higher speeds. Flying at higher speeds increases aerodynamic heat, engine heat, and skin temperatures.

Modifying Jet Fuel To Absorb More Heat

Credit: Airbus Defense

Engineers are looking to modify the jet fuel to allow it to absorb more heat. There are two main approaches: one develops fuels that tolerate higher temperatures before breaking down, and the other develops fuels that actively absorb heat through chemical reactions. The second approach is through developing endothermic fuels and is considered more revolutionary.

Without modifications, standard jet fuel (e.g., JP-8) starts to degrade at around 163°C / 325°F. By adding specialized chemical additives that improve the thermal oxidative stability, the tolerance can increase to around 201°C / 425°F. Endothermic fuels are specially formulated to undergo chemical reactions that absorb heat instead of simply heating up.

A useful analogy is ice. Ice can absorb a large amount of heat, yet its temperature barely changes as it melts. The heat or energy is consumed during the phase change. The technical process of how endothermic fuels work is far too complicated to explain in this article. The ACS Omega paper entitled “Heat Management in Supersonic/Hypersonic Vehicles Using Endothermic Fuel: Perspective and Challenges” provides a technical description for readers wanting to explore it in greater depth.

Other Ways To Disperse Heat

Credit: Boeing

There are various other methods engineers are developing to cool next-generation fighter jets, in addition to adaptive engines and advanced fuel heat-sink technologies. One important approach is the use of advanced materials such as Ceramic Matrix Composites (CMCs), which can withstand significantly higher temperatures than conventional metallic components while requiring less cooling air. This not only improves engine efficiency but also reduces the thermal-management burden on the aircraft.

There are efforts to use air-based cooling, including optimized airflow management throughout the aircraft and the use of weapons bays, inlets, and other internal volumes as part of the overall thermal-management architecture. Advanced heat exchangers, more efficient heat-transfer surfaces, and integrated thermal-management systems are being developed to move heat away from critical components more effectively. Liquid cooling loops—often using fluids such as polyalphaolefin circulating through cold plates attached to avionics, radar arrays, electronic warfare systems, processors, and power electronics—are expected to play an increasingly important role.

Other technologies under investigation include heat transfer enhancements, holistic systems, thermoacoustic cooling, phase-change materials that temporarily absorb thermal spikes, and more. The airframe itself is being seen as a thermal reservoir. Exact methods remain classified and under development. Rolls-Royce describes its approach only in broad terms: “Our optimised Thermal Management System will utilise the gas-turbine as a heat ‘sink’ to recycle thermal energy around the platform. This removes the need for overboard venting and improves overall system efficiency.”