Ballistic range

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A long, instrumented enclosure wherein tests of gun-launched projectiles are conducted. Ballistic ranges were originally used for study of projectile flight characteristics such as the rate of velocity loss and the dispersion of trajectories, that is, the imperfect following of the bore sight line of the gun. Ballistic ranges are now used for the measurement of aerodynamic characteristics of projectiles and of scale models of vehicles intended for flight through the atmosphere, including missiles, aircraft, and space-flight capsules.

Ballistic ranges may vary in length from less than 10 ft (3 m), for limited investigations of small arms projectiles, to greater than 1000 ft (300 m), for detailed studies of projectiles from large-bore guns. Instrumentation may be as simple as a series of sheets of paper hung normal to the flight path (yaw cards) in which the projectile, by punching its outline, gives evidence of its vertical and lateral position and its angular orientation at the instant of penetration. The more refined instruments required in modern practice are usually photographic. A spark of extremely short duration (from 1 to 0.1 microsecond) is discharged when the projectile reaches a position between the spark and a piece of film (Fig. 1). The resulting shadow photograph (shadowgraph) records in projection two components of the linear position of the projectile and one component of angular orientation. Two orthogonal pictures, as in the simple system shown, completely define the position in space of the model. A number of shadowgraph stations placed in sequence along the flight path will define, as a function of distance flown, the history of position and attitude of the projectile in flight. Corollary timing apparatus is used to measure the precise times, frequently accurate within microseconds, at which the pictures are recorded.

Simple shadowgraph system.

The shadowgraph pictures further aid aerodynamic studies by making visible certain details of the airflow about the projectile (Fig. 2). Refraction, or bending, of light rays by the variable-air-density field about the model makes visible the shock waves, turbulence, boundary layers, and boundaries of expansion fans. The flow visualization is most valuable for the study of aerodynamics.

Shadowgraph of a cone in supersonic flight. (NASA, Ames Research Center)

In modern practice the velocity performance of ballistic ranges has sometimes been enhanced by placing the instrumented range within the test section of a supersonic wind tunnel, which provides a high-velocity airflow opposite in direction to the model's flight direction. The earliest facility of this type used a conventional supersonic wind tunnel with a very long test section, driven by a high-pressure reservoir of air at room temperature. Later devices have been driven by high-temperature air generated in a shock tube. See also Wind tunnel.