In aerodynamics, hypersonic speeds are those that are highly supersonic. In the 1970s, new areas of mathematics and physics were developed to deal with flow fields at high Mach numbers (> 5), including asymptotic theory, direct simulation Monte Carlo methods, and large-eddy simulations.
The term “hypersonic” is often used to refer to atmospheric flight at Mach 5 and above. At these speeds, the heated air in front of a vehicle becomes ionized due to friction, making it electrically conductive. This conducts electricity away from the surface of the vehicle into the shocked airflow ahead of it, where it quickly dissipates. The result is a shock wave around the nose of the vehicle that limits its forward progress (drag), as well as a strong electromagnetic field. The effects become more pronounced as speed increases.
Hypersonic flight poses significant technical challenges because aerodynamic heating rates are so high that traditional materials such as aluminum alloys melt or vaporize before reaching their destination. For this reason, research into hypersonic propulsion has focused on developing new materials and innovative cooling schemes that can withstand these conditions. Additionally, guidance and control systems must be able to operate in an environment where there is no longer any atmospheric oxygen present for combustion purposes (i.e., ramjet engines).
Despite these challenges, there has been significant progress made in recent years towards achieving sustained hypersonic flight. In 2004, NASA’s X-43A unmanned experimental aircraft reached a top speed of Mach 9.6 (approximately 7200 km/h) using scramjet propulsion technology; this was followed by successful flights of the X-51A WaveRider in 2010 and 2011 which achieved durations of over 200 seconds at Mach 6+. Several countries are currently working on developing reusable spacecraft capable of operating in the hypersonic regime; examples include China’s Shenlong project and Russia’s Project Moskva