https://www.skytamer.com/English_Ele...tning_F.6.html
“Supersonic speeds also threatened inlet stability; the inlet's central shock cone served as a compression surface, diverting air into the annular inlet. As the “Lightning” accelerated through Mach 1, the shock cone generated an oblique shock positioned forward of the intake lip; known as a subcritical inlet condition, this is stable but also produces inefficient spillage drag. Around the Design Mach speed, the oblique shock is positioned just in front of the inlet lip and efficiently compressed the air without any spillage. As speed increases beyond Design Mach, the oblique shock becomes supercritical, where supersonic airflow enters the inlet duct. The “Lightning's” inlet was designed to handle only subsonic air, a supercritical state not only drastically reduced engine thrust output but could lead to surges or a compressor stall, which could result in engine flameout and/or damage.
Thermal and structural limits were also present; as air is heated up when compressed by the passage of an aircraft. This heating increases considerably when at supersonic speeds. The airframe absorbs heat from the surrounding air, the inlet shock cone at the front of the aircraft becoming the hottest part. The shock cone was composed of fiberglass, necessary because the shock cone also served as a radar radome; a metal shock cone would interfere with the AI 23's radar emissions. The shock cone would be eventually weakened due to the fatigue caused by the thermal cycles involved in regularly performing high-speed flights. At 36,000 ft and Mach 1.7, the heating conditions on the shock cone would be similar to those at Sea Level and 650 KIAS, but if the speed was increased to Mach 2.0 at 36,000 ft, the shock cone would be exposed to temperatures more than 70% higher than those at Mach 1.7. The shock cone was strengthened on the later “Lightning” F.2A, F.3, F.6, and F.53 models, thus allowing routine operations at up to Mach 2.0.”.....