Combat Aircraft Flight: You Cannot Sustain "Supersonic" Indefinitely

Anyone who drives a car understands that flooring the accelerator the entire way wastes fuel, wears out the vehicle, and creates safety hazards—and this principle applies even more so in the realm of combat aircraft flight. The maximum flight speeds of Mach 2 or Mach 2.5 listed in aircraft specifications are like the peak speed on a car's dashboard: they can be achieved only briefly and cannot be sustained for extended periods. In short, "supersonic capable" does not mean "capable of flying supersonic indefinitely."

For the vast majority of current operational combat aircraft, breaking the sound barrier requires engaging the afterburner—injecting additional fuel at the rear of the engine so that oxygen not fully combusted downstream of the turbine re-ignites, boosting thrust for a short period. Comparative tests conducted on the U.S. Air Force's F-16 show that over the same 30-minute flight, fuel consumption during afterburner-powered supersonic flight far exceeds that of subsonic cruise, and range is significantly reduced.

First, fuel efficiency and combat effectiveness are difficult to reconcile. Flying at full throttle does not get you far. Even heavy fighters with large fuel loads will exhaust their fuel quickly if the afterburner is engaged throughout a mission. External fuel tanks and weapons substantially increase aerodynamic drag, degrade high-speed flight performance, and diminish combat capability. Although the Soviet MiG-31 can sustain flight above Mach 2 continuously, its airframe empty weight and engine fuel consumption far exceed those of mainstream contemporary fighters; its operating and maintenance costs are high, and it cannot be widely fielded.

Second, there is the "thermal barrier" (热障) threshold that cannot be overcome. Once flight speed exceeds Mach 2, temperatures at the nose and wing leading edges surpass 120°C; as speed continues to increase, localized temperatures will reach above 200°C. The aviation aluminum alloys used in conventional fighters suffer a sharp drop in structural strength at around 150°C, and prolonged high-temperature flight can cause skin deformation, seal failure, and even canopy cracking—all potentially fatal risks. Although the SR-71 "Blackbird" can sustain cruise at Mach 3, the majority of its airframe is constructed from high-temperature-resistant titanium alloy, making its production and maintenance costs extremely high—something conventional fighters simply cannot replicate.

Third, there is the exponentially increasing wave drag. During subsonic flight, a combat aircraft primarily overcomes aerodynamic friction drag. Once it crosses the sound barrier, the air ahead is compressed into a dense shock wave, and wave drag rises exponentially with speed; the additional thrust required to accelerate supersonically is several times greater than at subsonic speeds, making prolonged high-speed flight extremely inefficient.

At present, countries are racing to develop adaptive variable-cycle engines, which can automatically adjust their operating mode according to flight conditions, balancing range and speed. These engines are expected to enable next-generation fighters to break through existing constraints and achieve sustained supersonic flight.