traditionally used in aircraft manufacturing, starts to lose strength and deform at relatively low temperatures, and is unsuitable for continuous use at speeds above Mach 2.2 to 2.4. Materials such as titanium and stainless steel allow operations at much higher temperatures. For example, the Lockheed SR-71 Blackbird jet could fly continuously at Mach 3.1 which could lead to temperatures on some parts of the aircraft reaching above 315 °C (600 °F). Another area of concern for sustained high-speed flight is engine operation. Jet engines create thrust by increasing the temperature of the air they ingest, and as the aircraft speeds up, the compression process in the intake causes a temperature rise before it reaches the engines. The maximum allowable temperature of the exhaust is determined by the materials in the turbine at the rear of the engine, so as the aircraft speeds up, the difference in intake and exhaust temperature that the engine can create, by burning fuel, decreases, as does the thrust. The higher thrust needed for supersonic speeds had to be regained by burning extra fuel in the exhaust. Intake design was also a major issue. As much of the available energy in the incoming air has to be recovered, known as intake recovery, using shock waves in the supersonic compression process in the intake. At supersonic speeds the intake has to make sure that the air slows down without excessive pressure loss. It has to use the correct type of shock waves, oblique/plane, for the aircraft design speed to compress and slow the air to subsonic speed before it reaches the engine. The shock waves are positioned using a ramp or cone which may need to be adjustable depending on trade-offs between complexity and the required aircraft performance. An aircraft able to operate for extended periods at supersonic speeds has a potential range advantage over a similar design operating subsonically. Most of the drag an aircraft sees while speeding up to supersonic speeds occurs just below the speed of sound, due to an aerodynamic effect known as wave drag. An aircraft that can accelerate past this speed sees a significant drag decrease, and can fly supersonically with improved fuel economy. However, due to the way lift is generated supersonically, the lift-to-drag ratio of the aircraft as a whole drops, leading to lower range, offsetting or overturning this advantage. The key to having low supersonic drag is to