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This Defense Intelligence Reference Document was prepared by the Defense Intelligence Agency's Defense Warning Office in fiscal year 2010 as part of the Advanced Aerospace Weapons System Applications program. It reviews how air flows around objects at subsonic, supersonic and hypersonic speeds, covering shock waves, wakes and ionization. It then compares ways to detect and track hypersonic vehicles, including radar, optical, infrared, LIDAR, infrasound and seismic methods. The report makes four recommendations, among them building a database of aircraft wake signatures and developing novel detectors.
From the source: Release of 2026-09-18 Incident: 11/20/10, Las Vegas, Nevada. Released with redactions. This document is a Defense Intelligence Reference Document (DIRD), a technical reference format used by the Defense Intelligence Agency (DIA) to capture baseline knowledge on a specific topic for later analytic use. DIRDs are best understood as reference and synthesis products rather than as original research. It is one of 38 DIRDs produced under the Advanced Aerospace Weapon System Applications Program (AAWSAP) between 2009 and 2011. Because AAWSAP’s scope permitted a broad range of supporting topics, not every DIRD in the series directly concerns aerospace systems or future threat assessment. The following summary reflects the DIRD’s scope and framing at the time of writing and should not be read as implying current validation of the concepts discussed. This DIRD surveys how hypersonic vehicles may be detected and tracked by exploiting the physical effects they create in flight, especially shock waves, ionized gas, hot surfaces, and turbulent atmospheric wakes. The report reviews a broad set of detection methods, including radar, infrared sensing, optical imaging, LIDAR, passive radio reflection, infrasound, and seismic techniques, and argues that the most effective systems will likely combine multiple sensor types, because each captures different features of a high-speed vehicle’s passage through the atmosphere. It presents radar and infrared sensing as the strongest existing tools for operational detection, while giving particular attention to wake-based methods such as LIDAR and passive radio techniques for improving tracking, identification, and discrimination of future hypersonic aircraft. The paper identifies hypersonic vehicles as an evolving surveillance problem in which future progress will depend on better multi-sensor integration, improved wake characterization, and novel signature-exploitation techniques.
UNCLASSIFIED/fFOR. QFFIEl.t.L: Uii QD:I! X Chapter 1: Theory Governing Objects in Flight Many of the detection technologies for hypersonic aircraft are based on the properties of the air flow around the object. The flu id mechanics affecting supersonic and hypersonic aircraft are well described in textbooks on compressible flow by J.D. Anderson;2•3 subsonic flow, including the affect of boundary layers, is covered in the textbook by F. M. White. 4 Hypersonic flow usually refers to the reg ime where objects are moving faster than Mach 5, but technically refers to the Mach range where pressure, temperature, and density ratios across shock waves reach constant values, and this does happen at about Mach 5. Fluid flow changes dramatically at Mach 1 defined at the point where the velocity of a projectile, V, equals the local speed of sound, a, in the atmosphere: M ='!_ (1) a Flow regimes are defined by the following: • M 1, supersonic flow • M > 5, hypersonic flow The speed of sound varies with temperature and with t he type of gases in the atmosphere. A simple equation for the speed of sound includes the ratio of specific heats, y, the gas constant, R, and the temperature, T: a=.JyRT (2) For air at room temperature, y = 1.4, R = 287 J/kg -K, and T = 20° C or 293 K. The subsequent speed of sound is 343 m/s (1,125 ft/s or 767 mph). It is easier for a projectile to exceed the speed of sound at higher elevations since the temperature of the atmosphere is lower, reducing the speed of sound and the subsequent velocity necessary to break the sound barrier. The viscosity of the fluid flowing around an object induces drag and retards its forward motion. Supersonic flow has some of the viscous characteristics of subsonic flow, but adds in the complication of shock waves. To understand the nature of how a projectile affects the flow of fluid around it, consider the case of a subsonic projectile. SUBSONIC FLOW AND DRAG Subsonic flows, such as the flow shown in Figure 1, are heavily influenced by collisions between molecules of air and the surface of the projectile. The equations for this interaction were first developed by Sir Isaac Newton in the seventeenth century, where he determined that fluids have a property called "viscosity" that is the cause of drag on projectiles. The key parameter that affects subson ic fluid flow is the ratio of viscous forces to inertial forces, where inertial forces are associated with the tendency for a moving fluid to keep moving in the same direction. The ratio of these two forces is defined as the Reynolds Number, Re. UNCLASSIFIED//FQA: QlililEl.t.L: W&li ONL'l 3
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