Defense Intelligence Reference Document Detection And High Resolution Tracking Of Vehicles At Hypersonic

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Introduction
An object is supersonic when its speed through the atmosphere is greater than the local
speed of sound. The Mach number is defined as the speed of the object divided by the local
speed of sound. For Mach numbers greater than 1 (supersonic flow), shock waves develop in
the flowfield and near the surface of the object due to the air's compressibility. Traditionally,
the lower Mach number limit for the so-called hypersonic speed regime is about Mach 5 (1.7
km/sec). "Low hypersonic" values range between Mach 5 to about Mach 10, while "high
hypersonic" values range between approximately Mach 10 to Mach 30 or above. Mach 30
(10 km/s), for example, is close to Space Shuttle reentry velocity. Few objects can travel at
hypersonic velocities. The most common object that we see moving at these speeds are
meteors entering the Earth's atmosphere. As meteors fall to the Earth's surface, their
velocities may reach 30 miles per second (48 km/s), 1 and their corresponding Mach number
as they enter the upper layers of the atmosphere will exceed 150. Meteors are preceded by
a bow shockwave as they compress the air immediately in their path. Temperatures and
pressures increase dramatically across the shockwave to a point where the gases in air ionize
and disassociate, leading to the emission of visible light and radio waves. These conditions
also lead to rapid heating of the meteor surface causing them to fracture and break up as
they enter the atmosphere. Optical and RADAR-based surveillance systems are now used to
scan outer space to detect asteroids and other objects with orbits that may lead them to
collide with Earth.
A second class of hypersonic objects includes reentry vehicles moving into the Earth's
atmosphere from orbit. In the case of a reentry vehicle returning from low-Earth orbit at
100 km altitude, the velocity of the vehicle will reach 8 km/s (about 5 miles per second).
The Mach number of this vehicle will exceed 26.5 in the upper atmosphere. As the vehicle
moves through the atmosphere, flow-induced drag forces will slow down the vehicle; if the
vehicle has sufficient thermal protection, it can survive reentry and be recovered.
A third class of hypersonic vehicles includes the reentry payload used in ICBMs
(intercontinental ballistic missiles). The payload in these missiles is launched using a rocket
that boosts them above the atmosphere to a suborbital velocity. The payload velocity may
exceed 7 km/s as they reenter the atmosphere, equating to M > 23. The coupled high
velocity and high kinetic energy of these objects creates an ionized wake and shock wave
that can be used to detect their position and to determine their velocity. This information,
along with their ballistic trajectory, is used to locate their target and the time to impact.
A fourth class of hypersonic vehicles includes manned and unmanned rockets and aircraft.
The North American X-15, for example, exceeded a speed of 7,274 km/hr in 1964, equating
to about M = 6.5. This aircraft also exceeded 100 km in altitude on two occasions, qualifying
the X-15 as a spacecraft. The SR-71 Blackbird, an air-breathing strategic reconnaissance
aircraft, has exceeded Mach 3.2 at 80,000-ft altitude, and its actual maximum speed may
have reached into the hypersonic flow regime. Other real and conceptual aircraft capable of
reaching hypersonic speeds include the alleged Aurora SR-91 (Mach 4 to 6), the Boeing X-
51, an unmanned Mach 6 scramjet, the HTV-2 from the DARPA Falcon project (designed for
M=20 flight to low-Earth orbit), the HyperSoar (M = 12), the Russian Leninetz Ayaks, and
the Skylon (designed for single-stage-to-orbit flights). Certain rockets are also designed to
travel at hypersonic speeds. The Patriot missile, for example, travels at Mach 5, a high
speed necessary for it to be able to intercept and destroy other missiles while they are in
flight.
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