burn a lot of fuel and so flight times were short. Early turbojets were more fuel-efficient but did not have enough thrust and some experimental aircraft were fitted with both a turbojet for low-speed flight and a rocket engine for supersonic flight. The invention of the afterburner, in which extra fuel is burned in the jet exhaust, made these mixed powerplant types obsolete. The turbofan engine passes additional cold air around the engine core, further increasing its fuel efficiency, and supersonic aircraft today are powered by turbofans fitted with afterburners. Supersonic aircraft usually use low bypass turbofans as they have acceptable efficiency below the speed of sound as well as above; or if supercruise is needed turbojet engines may be desirable as they give less nacelle drag at supersonic speeds. The Pratt & Whitney J58 engines of the Lockheed SR-71 Blackbird operated in 2 ways, taking off and landing as turbojets with no bypass, but bypassing some of the compressor air to the afterburner at higher speeds. This allowed the Blackbird to fly at over Mach 3, faster than any other production aircraft. The heating effect of air friction at these speeds meant that a special fuel had to be developed which did not break down in the heat and clog the fuel pipes on its way to the burner. Another high-speed powerplant is the ramjet. This needs to be flying fairly fast before it will work at all. Supersonic flight Supersonic aerodynamics is simpler than subsonic aerodynamics because the airsheets at different points along the plane often cannot affect each other. Supersonic jets and rocket vehicles require several times greater thrust to push through the extra aerodynamic drag experienced within the transonic region (around Mach 0.85–1.2). At these speeds aerospace engineers can gently guide air around the fuselage of the aircraft without producing new shock waves, but any change in cross area farther down the vehicle leads to shock waves along the body. Designers use the Supersonic area rule and the Whitcomb area rule to minimize sudden changes in size. However, in practical applications, a supersonic aircraft must operate stably in both subsonic and supersonic profiles, hence aerodynamic design is more complex. One problem with sustained supersonic flight is the generation of heat in flight. At high speeds aerodynamic heating can occur, so an aircraft must be designed to operate and function under very high temperatures. Duralumin, a material