Supersonic flight has always presented substantial technical challenges to engineers, as the aerodynamics of supersonic flight are dramatically different from those of subsonic flight (i.e., flight at speeds slower than that of sound). In particular,
aerodynamic drag rises sharply as the aircraft passes the transonic regime, requiring much greater engine power and more streamlined airframes.
Wings supersonic
reconnaissance aircraft To optimize drag, wingspan must be limited, which also reduces aerodynamic efficiency during subsonic flight, including takeoff and landing. Minimizing wave drag is a crucial aspect of wing design. Since a supersonic aircraft must also take off and land at a relatively slow speed, its aerodynamic design must be a compromise between the requirements for both ends of the speed range. One approach to resolving this compromise is the use of a
variable-geometry wing, commonly known as the "swing-wing," which spreads wide for low-speed flight and then sweeps sharply, usually backwards, for supersonic flight. However, swinging affects the
longitudinal trim of the aircraft, and the swinging mechanism adds weight and cost. Use of a
delta wing, such as those used on the
Aerospatiale-BAC Concorde, generates a
vortex which energises the flow on the upper surface of the wing at high speeds and attack angles, delaying flow separation, and giving the aircraft a very high
stall angle. It also solves the issue of fluid
compressibility at transonic and supersonic speeds. However, it is, of course, inefficient at lower speeds due to the requirement of a high angle of attack, and therefore requires the use of
flaps.
Heating Another problem is the heat generated due to air compression as well as friction as the air flows over the aircraft. Most subsonic designs use aluminium alloys such as
Duralumin, which are cheap and easy to work with but lose their strength quickly at high temperatures. This limits the maximum speed to approximately Mach 2.2. Most supersonic aircraft, including many military
fighter aircraft, are designed to spend most of their flight at subsonic speeds, and only to exceed the speed of sound for short periods such as when intercepting an enemy aircraft. A smaller number, such as the
Lockheed SR-71 Blackbird reconnaissance aircraft and the Concorde supersonic airliner, have been designed to cruise continuously at speeds above the speed of sound, and with these designs the problems of supersonic flight are more severe.
Engines Some early supersonic aircraft, including the first, relied on
rocket power to provide the necessary thrust, although rockets 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 Subsonic
aerodynamics is simpler than Supersonic
aerodynamics because the airsheets at different points along the plane often cannot affect each other in subsonic flight. 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
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 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. Additionally, 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 properly shape the overall aircraft to be long and thin, and close to a "perfect" shape, the
von Karman ogive or
Sears-Haack body. This has led to almost every supersonic cruising aircraft looking very similar to every other, with a very long and slender fuselage and large delta wings, cf.
SR-71,
Concorde, etc. Although not ideal for passenger aircraft, this shaping is quite adaptable for bomber use. In the 1960s and 1970s, many design studies for supersonic airliners were done and eventually two types entered service, the Soviet
Tupolev Tu-144 (1968) and Anglo-French
Concorde (1969). However political, environmental and economic obstacles and one fatal Concorde crash prevented them from being used to their full commercial potential.
Transonic flight showing flow patterns at and above
critical Mach number Airflow can speed up or slow down locally at different points over an aircraft. In the region around Mach 1, some areas may experience supersonic flow while others are subsonic. This regime is called transonic flight. As the aircraft speed changes, pressure waves will form or move around. This can affect the trim, stability and controllability of the aircraft, and the aircraft will experience higher drag than subsonic or fully supersonic speeds. The designer needs to ensure that these effects are taken into account at all speeds.
Hypersonic flight Flight at speeds above about Mach 5 is often referred to as hypersonic. In this region the problems of drag and heating are even more acute. It is difficult to make materials which can stand the forces and temperatures generated by air resistance at these speeds.
Sonic boom A sonic boom is the sound associated with the
shock waves created whenever an object traveling through the air travels faster than the
speed of sound. Sonic booms generate significant amounts of
sound energy, sounding similar to an
explosion or a
thunderclap to the human ear. The crack of a supersonic
bullet passing overhead or the crack of a
bullwhip are examples of a sonic boom in miniature. Sonic booms due to large supersonic aircraft can be particularly loud and startling, tend to awaken people, and may cause minor damage to some structures. Eventually, they led to prohibition of routine supersonic flight over land. Although they cannot be completely prevented, research suggests that with careful shaping of the vehicle the nuisance due to them may be reduced to the point that overland supersonic flight may become a practical option.
Supercruise Supercruise is sustained
supersonic flight of a supersonic aircraft with a useful cargo, passenger, or weapons load performed efficiently, which typically precludes the use of highly inefficient
afterburners or "reheat". Many well known supersonic
military aircraft not capable of supercruise can only maintain
Mach 1+ flight in short bursts, typically with afterburners. Aircraft such as the
SR-71 Blackbird are designed to cruise at supersonic speed with afterburners enabled. One of the best known examples of an aircraft capable of supercruise was
Concorde. Due to its long service as a commercial airliner, Concorde holds the record for the most time spent in supercruise; more than all other aircraft combined. ==Supersonic transport==