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.
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.