How composites, and an unlikely companion, make hypersonic flight a possibility.
How composites, and an unlikely companion, make hypersonic flight a possibility.
Hypersonic history and development.
Since the beginning of time, humans have been fascinated with flight. The first glider was designed and successfully flown over 200 years ago, in 1804. Self-propelled flight was then achieved nearly a century later by the Wright Brothers in 1903. Since then, technological advances have allowed us to create a world where travel and freight by air are commonplace and often taken for granted.
It makes sense, then, that the next step is to go hypersonic. To fly at speeds faster than light. The development of vehicles to reach hypersonic speeds can be traced back to 1949. Yuri Gagarin was the first person to travel at hypersonic speeds in his world-first orbital flight. In 1967, the legendary X-15 reached speeds of Mach 6.7. This translates to 4,520 MPH, 7,274 KM/H, or 2,021 metres per second. As technology has continued to advance, hypersonic jets are getting much faster and becoming increasingly fuel-efficient.
Challenges of hypersonic flight.
Hypersonic flight occurs at roughly 90 kilometres above earth. Flying faster than Mach 5 at this height causes stagnation of the air, with a velocity of zero, to surround the jet. This essentially creates a forcefield of air around the aircraft that the air it flies into must move around. This causes a shockwave cone that gets progressively larger, which, at hypersonic speeds, impairs the jet's flying capabilities. The insane speeds hypersonic aircraft reach also cause problems with temperature control. The surface of an aircraft travelling at Mach 5 and beyond can reach up to 3000 degrees Celsius. More than enough to destroy most materials used in standard flight. The first object to attempt hypersonic travel discovered this the hard way. It reached approximately Mach 6.7 but burned upon its re-entry to the atmosphere. The iconic X-15 used Inconel X to tackle the issue. It was used within a heat sink structure to successfully resist the impact of aerodynamic heating.
In recent times, researchers from the University of Manchester have created a material that can meet the demands of hypersonic travel. They knew that carbon-carbon composites are strong enough with sufficient melting points. But the oxidation occurring at hypersonic speeds causes too much damage when used on its own. They also knew that ultra-high-temperature ceramics (UHTC) are extremely hard but also too brittle to pierce through the atmosphere at hypersonic speeds. However, when put together, all these issues disappear. As always, it’s not quite as easy as putting the two materials together. Ceramics are typically made into powders and then pressed into a mould to form a coating. Unfortunately, this coating falls off easily. To combat this, the researchers used liquid metals to infiltrate the carbon-carbon composite. As carbon-carbon composites are extremely porous, the UHTC can permeate the fibres, creating a coating that can withstand the pressures of hypersonic flight.