Evidently, airplanes as a mode of transportation are significantly faster than donkeys, camels, or automobiles. However, commercial airliners are still not fast enough. In a trip I took a few days ago from Incheon, South Korea to New York, USA, I was airborne for 14 hours in a nonstop flight via the Pacific Ocean route. Moreover, Singapore Airlines operated an 18-hour 45-minute nonstop flight from Newark, USA, to Singapore, a distance of 9,534 miles. These are very long hours to stay in a compartmentalized environment that offers only a little opportunity to move around during flight. We would like to reduce these travel times to under three hours.
In aviation, vehicle speed is sometimes stated in terms of the Mach number of flight, which is the ratio of the vehicle speed to the speed of sound. So a Mach number less than one indicate subsonic speed, while a Mach number of one indicates sonic speed, or the speed of sound. Supersonic speeds usually refer to Mach numbers between one and five, while higher Mach number speeds are called hypersonic. Commercial airliners are relatively slow since they operate at subsonic speeds, which are usually between Mach 0.75 and 0.85.
To appreciate the speed limitations of a jet engine, you need to take a look at its components. Outside air passes through a compressor, which compresses the air, raising its temperature and pressure. (These conditions enhance combustion.) It is then passed through a combustion chamber, where it mixes with a fuel, and burns. The combustion generates heat and significantly raises the temperature. Jet engines need to create a lot of propulsive force (or thrust). To do this, jet engines need very high energy flow at the nozzle exit. Combustion is used to provide a high-kinetic-energy (or velocity) fluid, which energizes the air and brings it to the speed of the vehicle at the exit of the engine. A part of the energy drives the turbine, which in turn, drives the compressor, while the remaining energy is used to propel the vehicle.
To make jet engines operate at supersonic or hypersonic speeds is not a walk in the park, and some innovation is needed. There are air vehicles that operate at supersonic or hypersonic speeds. An example can be found in rockets, such as the Space Shuttle. One important difference between a rocket and a jet engine is that whereas the former carries its own oxidizer, the latter uses the air outside the vehicle for oxidation. Both types of vehicle carry their own fuel on board. Air vehicles that use ambient air are known as air-breathing vehicles. The technology of ramjets and supersonic combustion ramjets (or scramjets) enables supersonic/hypersonic flight under air-breathing conditions. Unlike turbojets, ramjets and scramjets have no moving parts. Instead, air compression is achieved by specialized geometric designs of the air intake of the vehicle. In ramjets, the supersonic/hypersonic airflow is reduced to subsonic flow by the time the air reaches the combustor, so that combustion takes place under subsonic conditions. In scramjets, combustion takes place under supersonic conditions. One issue with ramjet and scramjet vehicles is that they cannot be started from rest, as another vehicle is needed to get them to supersonic/hypersonic conditions before they can take over the flight. On the other hand, jet engines can take off from rest, and so are quite attractive for this reason. Endowing jet engines with the additional capability of supersonic or hypersonic flight is the focus of some current research.
Although the atmosphere may be very cold – high negative degrees Centigrade, even when a vehicle flies at supersonic or even hypersonic speeds, it is a totally different matter when the air flows over a surface (internal or external). In this case, the flow generates an extremely high amount of heat (temperature) when the airflow is in the supersonic or hypersonic regime. The phenomenon of this heat generation is called viscous dissipation by aerospace engineers. To demonstrate this phenomenon, rub the palms of your hands against each other at a fast rate. Obviously the speed at which you rub your palms is nothing compared to the supersonic/hypersonic speeds of high-speed aircraft. Thus, the heat generated by the high-speed vehicle is significantly higher than that from rubbing your palms against each other. For air vehicles, the problem is that the generated heat will melt any material that the engine is made of, thereby necessitating pre-cooling of the air before it enters the compressor.
The pre-cooler technology is at the center of high-speed jet engine development, since without the pre-cooler, the jet engine is not capable of supersonic flight. A pre-cooler developed by the British company Reaction Engines, uses recirculated cryogenic (liquid) hydrogen fuel to cool incoming air down from 1,000° C to roughly 100° C in 1/20th of a second.
On 21 October 2019, Reaction Engines demonstrated a milestone in its turbojet engine technology that can handle the intense temperatures of hypersonic speed, thereby proving the viability of its pre-cooling system in conditions equivalent to a speed of Mach 5.