The history and research of flight started with observations of birds soaring the sky, therefore early designs tend to mimic bird’s wings. Ornithopters, machines with flapping wings generating both lift and propulsion were one of the earliest designs for heavier-than-air flight. Scientists such as Leonardo da Vinci, Galileo Galilei, and Isacc Newton provided ways to calculate aerodynamic forces (History of flight). George Cayley was crucial in bridging the theory of flight and engineering research (Sir George Cayley). He was a pioneer who first experimented fixed-wing aircraft and explained that a successful flying machine would have separate systems for control, lift, and propulsion (History of flight). After the first meeting of Aeronautical Society of Great Britian, various advances such as wing tunnels, airfoil designs, and performance calculation methods allowed researchers such as the Wright brothers to develop gliders and later airplanes (History of flight).
In the beginning of the 19th century, the crucial problem of sustained powered heavier-than-air flight is the lack of suitable power plants. Spring powered systems did not have enough power for flight while electricity solutions have poor power-to-weight ratio. The most optimistic choices were the steam and internal-combustion engine. Steam engine prototypes proved to be disastrous and by the end of the 19th century internal-combustion emerged as the most promising. At the same time, car engines were being developed and internal-combustion engines were being constantly improved to be lighter and more powerful. Research and breakthroughs in control systems lead to new monoplane designs (History of flight).
At the end of World War I, rail tracks and bridges were destroyed and political conditions disrupted train schedules. This allows the airline industry to advance as more money were invested. Two basic engines were installed on aircrafts: inline engine where cylinders aligned one behind the other or two banks in V-type install; radial engines where cylinders arranged in a circle around crankshaft. An inline engine requires a radiator and circulation of liquid coolant while the radial engine has numerous small fins on cylinder to radiate heat to passing airstream (History of flight). At the end of 1930s, superchargers were developed to enhance high-altitude engine performance. Superchargers compress the air and increase the pressure into the combustion chamber, thus letting it burn more fuel and do more work (Smithsonian). This improved piston engine performances and later with jet engines.
The idea of propulsion by gas dates back to days of Hero of Alexandria’s steam powered aeolipile, a radial steam turbine that spins when the central water tube is heated with torque produced by exiting steam jets. In 1940 the idea of a turboprop engine, similar to a turboshaft engine, was proposed. The turboprop engine does not produce significant thrust with exhaust gases. Instead most of the energy generated from the turbine goes into driving the propeller (Turboprop). To increase propeller efficiency, a variable pitch propeller can be used to adjust pitch relative to airspeed. A turboprop engine can fly at the same speed as small jet airliners using less fuel. But a propeller aircraft cannot fly as high as a turbojet which jets do for enhanced speed and fuel efficiency (NASA).
With military needs for faster and higher aircrafts, a different type of engine was proposed, the turbojet. A turbojet engine consists of a compressor, combustion section, a turbine, and exhaust nozzle. Because of fuel inefficiency and noise at low Mach numbers, turbofans were favored over the purely turbojets. The turbofan engine is similar to a turbojet but with a large fan at the front to provide thrust. Different kinds of bypass turbofan engines were developed to fly in a defined spectrum of Mach number. The bypass ratio means the mass flow rate of the stream to the mass flow rate entering the turbine. A medium to ultra-high bypass turbofan engine would be more suited for transonic flights while a low bypass turbofan engine can fly more efficiently at supersonic. To counter act insufficient thrust of low bypass turbofan and turbojet engines in low Mach number regimes, afterburners were installed. Afterburners are extensions on the back of the engine that adds more fuel to the exhaust stream and ignites the mixture to boost the thrust. A pilot would not leave the afterburner on because it consumes up to three times as much fuel and not efficient (Air and Space).
For higher Mach numbers up to Mach 5, ramp pressure ratio becomes so high air does not need to go through a turbo compressor anymore. The inlet is designed such that the airflow is less than Mach 1. This is done to increase efficiency and prevent shockwaves in the engine. The main thrust producer is the after burner. It is lightweight and has a simple power plant. For even higher Mach number flights, a scramjet is used for better efficiency. Scramjet stands for supersonic combusting ramjet, meaning the combustion takes place in supersonic airflow (NASA ramjet). Main issues of this engine are that it cannot produce thrust at zero airspeed, and it needs to be assisted to take off, and the heat produced due to high Mach number. To solve the issue of assist take off, a turbo ramjet engine was developed to bring the aircraft from subsonic to supersonic. A turbo ramjet has a turbo engine in the inlet of the ramjet engine. This configure allows the turbo engine feeds the ramjet section with high enough pressurized air. After crossing the transonic region, the turbo engine intake can be closed to avoid damages. (Cutler)