Where Are the Fastest Airplanes in the World Heading?

For years, passenger airplanes have moved away from achieving supersonic speeds. Among experimental or military aircraft, or some models being developed by NASA, surpassing speeds greater than five times the speed of sound has not been particularly achievable. Typically, military aircraft can reach a maximum of twice the speed of sound, while research aircraft can achieve speeds up to five times the speed of sound. Nevertheless, NASA is trying to slightly change these boundaries. But why is designing a supersonic aircraft so costly and technically challenging? Simply put, compressible fluids like air become aware of the sound waves from the flow path and adjust themselves accordingly. When speed exceeds the speed of sound, a collision occurs between the aircraft and the air before the air can synchronize with the flow. This collision creates shock waves, commonly known as breaking the sound barrier. Contrary to popular misconception, breaking the sound barrier does not only occur when the aircraft exceeds the speed of sound, as the aircraft generates pressure waves that cause an explosion-like sound while flying at supersonic speeds. Furthermore, when speed exceeds the speed of sound, many fluid physics laws operate in reverse. Due to such conditions, the design of wings and fuselage in supersonic aircraft differs from conventional aircraft. The formation of shock waves at supersonic speeds necessitates stronger fuselage reinforcement and better engineering in the aircraft structure, and fuel consumption significantly increases due to shock waves. Currently, due to high noise production, most countries do not allow supersonic passenger aircraft to operate. However, this is not true for military and research aircraft, yet aerodynamic issues and the design of new engines remain the biggest barriers to increasing the speed of supersonic aircraft. NASA currently has a program for the development and growth of supersonic aircraft. This speed increase is included in the X program. The X aircraft are a series of airplanes, rockets, and helicopters built to test and evaluate new technologies and aerodynamic concepts of the United States. Information about some of these projects is made available to the public, while others, like the X-16, are classified. The X aircraft project has significantly contributed to the advancement of aviation knowledge; for example, the X-1 was the first aircraft to break the sound barrier, and the X-2 was the first to reach three times the speed of sound, a speed still unattainable for most fighter jets worldwide. Among the X projects, the highest speed belongs to the X-15, which still holds the record for the fastest manned aircraft. The X-15 used a rocket engine as its propulsion system. In the early 1960s, the X-15 set records for the highest speed and altitude, reaching the edge of space. To this day, the X-15 holds the official record for the fastest manned aircraft, with a maximum speed of 7274 kilometers per hour. Among non-research aircraft, only the Blackbird has been operational at speeds three times the speed of sound for over 40 years, with a total of 32 units built. This aircraft's normal flight speed was 3.3 Mach at an altitude of 24,000 meters. Its high speed and altitude made it safe from any threats, whether from ground-based missile defense or enemy interceptors. In practice, during nearly four decades of operation, not a single unit was threatened by the enemy. The main issue in designing this aircraft was the heat generated at high speeds, making the use of standard materials impractical, so titanium and titanium alloys were used in its construction. After the Blackbird, no manned aircraft capable of exceeding three times the speed of sound was built. However, in 2001, NASA successfully tested the X-43 project. The initial version, X-43, experienced speeds exceeding seven times the speed of sound, around 8050 kilometers per hour, at an altitude of 30 kilometers. Three samples of the X-43 were built, with the initial sample crashing in the Pacific Ocean, but the other two flew successfully. Nevertheless, these aircraft still lacked the capability for long operational flights and remained in the laboratory phase. NASA is currently testing the unmanned X-51 aircraft, which can reach speeds above six times the speed of sound. The X-51 successfully conducted a supersonic test at six times the speed of sound (6400 kilometers per hour) in 2010. This aircraft also achieved the longest flight duration at speeds exceeding five times the speed of sound to date. Currently, the biggest challenge, alongside high temperatures and changing gas behaviors at multiple times the speed of sound, is designing an efficient engine under such conditions. For speeds above three times the speed of sound, ramjet engines are used. However, these engines, which do not use turbines, cannot operate at subsonic speeds, so engineers had to develop newer engines capable of functioning at subsonic speeds while providing the necessary thrust for travel at several times the speed of sound. Therefore, turbo-ramjet engines were used in the Blackbird. This engine consists of two components: the ramjet and the turbofan. In this type of engine, the pilot first starts the turbofan engine to initiate flight. Once the engine and aircraft reach a speed of Mach one or close to it, the turbofan engine automatically shuts off, and its valve closes. Then, the air enters the ramjet engine, and with the existing air pressure, the engine ignites, allowing the aircraft to accelerate from Mach one to Mach three or seven in a short time. In conventional turbojet engines, air is compressed and heated by a compressor and pushed into the combustion chamber, where it mixes with fuel and produces high-pressure gases after combustion. The exhaust of these gases generates thrust forward. In jet and turbofan engines, there is a limitation on the speed of incoming air into the compressor. As the aircraft approaches the speed of sound, the incoming air is accompanied by shock waves that can damage the compressor. In ramjet engines, there are no compressors or other moving parts like turbines. Simply put, air enters the combustion chamber, mixes with fuel, burns, and produces high-pressure gases. The compression of air is provided by the high forward speed of the flying object. Ramjet engines cannot operate below Mach one because they require higher speeds to compress air in front of the engine. Speeds above six Mach are also unattainable for them because they cause combustion gases to heat up to the point of decomposing combustion products. The solution lies in scramjet engines. In the scramjet engine used in the X-43 aircraft, the aerodynamic design of the scramjet engine's inlet is such that it compresses air only to the point of combustion in a matter of milliseconds while not significantly reducing the speed of incoming air, which is a very complex task. The engine body and other parts of the flying object must also be very robust to withstand aerodynamic and gravitational forces at high speeds. Scramjet engines can provide speeds up to 25 Mach. Their accessible flight ceiling is also much higher than other jet engines. While modern turbojet engines can reach a maximum altitude of 40 kilometers, this ceiling is 55 kilometers for ramjets and 75 kilometers or even higher for scramjets. The highest speed ever achieved with a scramjet engine was by the unmanned X-43A aircraft, which flew on November 16, 2004, by NASA using a B-52 as a mother aircraft, reaching a speed of 11858 kilometers per hour. With the help of new engines, NASA has recently announced a broad program for the development of supersonic aircraft. Although this research may not yield immediate results, in the long term, it could fundamentally change the nature and timing of air and space travel.