Humans have always had an indisputable desire to go bigger, better, and faster. Once limited to the ground, human ancestors looked to the sky and imagined flight. Over many years, the idea was developed until humans, at last, left the ground over 100 years ago.
It was not long before scientific minds pushed the limits farther, and reached for the stars, sending rockets into space. To this day, engineers and scientists still push the limitations of flight by building bigger, and faster vehicles- most recently with a new experimental jet that will bring humans one step closer to hypersonic flights.
Previous experiments led scientists to believe that the faster an object traveled, the more air resistance it encounters, pushing back on the object. In the case of a motorized vehicle, this would mean that the faster a vehicle travels, the less fuel-efficient it becomes. However, once the first prop planes were developed, engineers wanted to go faster.
In the early 1900s, a new engine was developed that worked by burning fuel in air to release hot exhaust gas - the jet engine. Where a car engine uses the explosions of exhaust to push its pistons, a jet engine forces the gas past the blades of a windmill-like spinning wheel (a turbine), making it rotate.
The concept of jet engines for flight
The first turbojet engine of its kind was developed by Frank Whittle, although there is some controversy over this, as it may have been first developed by Dr. Hans von Ohain first, or they may have both developed turbojet engines independently of each other in the 1930s.
The turbojet functioned by mixing compressed air and fuel and igniting the mixture to create hot gas (see illustration below). It was soon discovered that a turbojet has an optimal speed where it is the most fuel efficient, any faster or slower and efficiency is severely impacted.
However, despite fuel inefficiencies, engineers and scientists alike continued to push the limits of aviation and speed thresholds. It was this innovative mindset that allowed aircraft to be propelled even faster, and unintentionally, in an even more fuel efficient manner. Mechanical components tended to fail at supersonic speeds inside a jet engine - obviously not a great situation. A new engine would be required to achieve speeds above the traditional turbine engine.
Ramjet & Scramjet propulsion
It was in the early 1900s that scientists began drawing up ideas concerning ramjet propulsion. Ramjets work by taking in relatively low speed air and expelling the air at a higher speed. The difference in speed results in a forward thrust, and thrust is produced by passing hot gasses through a nozzle.
The burning fuel creates higher pressures inside the engine, causing higher exhaust speeds. But the thrust of the engine depends entirely upon how much air flows through it. To maintain thrust, the combustion must remain at a pressure level that is higher than the pressure at the exit nozzle.
In the case of a ramjet, the pressure is produced by ramming external air into the combustion by using the speed of the craft, much like the way you would cause a garbage bag to inflate by moving it to force air inside.
The ramjet engine is remarkably simple; it has no real moving components and is incredibly fuel efficient since it does not have any turbines, compressors, stators or any other part that obstructs the flow of the engine as well as it does not have to carry an oxygen supply like a rocket, greatly reducing weight and the hazards associated with having one of the most flammable cargoes on the planet.
The first ramjet engines were developed in the late 1930s in the Soviet Union, and the world's first ramjet-powered airplane flight took place in December 1940. In 1949, the Leduc 010 proved its extraordinary capabilities with the first flight powered solely by a ramjet engine. The jet had to reach Mach 0.5 before the air was sufficiently compressed to enable enough air for the engine to start combustion.
The jet was first strapped onto a rocket to accelerate it up to speed but was later set atop a Languedoc 31 to get it to the appropriate altitude where it would detach and fall until it reached the critical speed where the engine could start. The ramjet proved a milestone in avionics for flights up to Mach 5, but a major flaw was discovered in speeds any faster.
The combustion inside a ramjet must occur at subsonic speeds. For a vehicle exceeding the speed of sound, the air is slowed down to subsonic speeds due to the intense shock waves generated in the aircraft inlet. Much above Mach 5, the shock waves cause performance losses so great that the engine will not produce thrust, forcing the jet to slow down.
Better designs for the future of hypersonic flight
In the 1960s, an improved ramjet was devised in which the burner could function supersonically. The modified ramjet, or scramjet, minimizes the losses due to the shock wave flow loss allowing the engine to produce enough thrust for hypersonic travel or speeds up to Mach 15.
Since the scramjet uses external air to power the combustor, it is much more efficient than a rocket, which must carry all of its oxygen on board. Scramjets are optimized for hypersonic flight and will not function at speeds below Mach 3, requiring a rocket to accelerate it up to critical engine stall speeds.
The first successful test flight of a scramjet took place on July 30, 2002, by the University of Queensland's HyShot team and their international partners.
In 2004, NASA demonstrated it X-43A scramjet-powered aircraft, achieving speeds almost at Mach 7. Recent advancements in scramjet technology have seen the HIFiRE 5b jet travel at Mach 7.5. Although ramjets and scramjets still require immense acceleration to achieve ignition, scientists are currently devising engines that utilize a turbine engine with retractable blades that can turn into a scramjet at appropriate speeds.
Engineers continue to push the limits of flight, from taking the first tentative steps in the 1900s, to traveling over 7 times the speed of sound today. Aeronautical engineering is improving every year, making jets more fuel efficient, and more importantly, faster. The development of scramjets could once again revolutionize travel with superbly efficient engines capable of traveling around the world in hours. Although most modern scramjets remain un-piloted, the technology remains promising for future aviation travel.
What is hypersonic flight?
While we've talked about the history of hypersonic flight, we haven't really described the process of it in great detail or what hypersonic flight means. Hypersonic flight is specifically a flight that occurs above Mach 5 or 3836.35 miles per hour. This flight has to occur below about 55 miles (90 km) above the earth, well within the atmosphere for it to be considered hypersonic flight.
At these speeds, the air begins to disassociate, and there becomes a massive heat load on the aircraft. As aircraft fly at hypersonic speeds, there becomes a portion of stagnant air around the aircraft that has a velocity of zero.
The air that the craft flies into has to move around this stagnant air, which forms the shock wave cone you often see with the supersonic and hypersonic craft. This cone gets larger as the craft goes faster and at a point, at hypersonic speeds, this begins to severely inhibit the lift, or the flying capabilities, of the craft.
It's this problem of stagnant air that ties back to why ramjet engines can't achieve supersonic speeds, rather scramjet engines are needed to do so. Ramjet engines slow the air down to subsonic, which can cause the craft to stall when traveling at hypersonic speeds.