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How Fast is Hypersonic? Technically any speed above Mach 5 is considered to be hypersonic. Most flight testing and research has been done in the Mach 5-10 range; however, aerospace engineers are already talking about the capability of a vehicle reaching a speed of Mach 25. Hypersonic Propulsion Devices The most recent technology capable of propelling aircraft to hypersonic speeds is the scramjet. This is a version of the ramjet but the difference is that the airflow is not slowed down. All of the air going through the engine is travelling at supersonic speeds. This technology is mechanically the simplest of all the propulsion engines, but aerodynamically, it is the most complex. In order for the aircraft to reach these speeds, the total weight must be as light as possible. This is a large part of the reason why jet engine scramjets are used instead of rocket engines. If a rocket were used, the aircraft would have to carry its own fuel supply, making the weight heavier. A scramjet combusts the oxygen from the surrounding air. Hypersonic Flight Requirements The lightweight requirement even influences the type of material used for the plane. Research shows that graphite composites are essential because of both their ultra-lightweight and rigid characteristics. Not only does the weight present an obstacle, but at speeds around mach 10 the coolest part of the airplane can easily reach 2000° F. Therefore, it is important to also use advanced heat resistant material. However, the material is not enough, an active cooling system is necessary to prevent thermal damage to the airplane parts. Testing Problems The biggest problem that prevents rapid development of hypersonic technologies is the lack of testing capabilities. Most wind tunnels are not designed to provide hypersonic conditions. Heating problems within the tunnels occur, also air is not as predictable at such high speeds. 
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