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Ballistics Photography - Projectiles in Flight, The Synchroballistic System, The Flight Follower, Aeroballistic Ranges, Lighting for Ballistic Photography

film cameras image camera

PETER W. W. FULLER
Consultant in Instrumentation and Imaging Science

Ballistics is the study of the motion, behavior, and effects of projectiles of all kinds. It is one of the many areas of armament research. The application of photography to ballistics also extends to most other areas in the field. In many instances, photographic or photonic studies may be the only way to obtain some of the parameters required. Photographic methods are particularly important in ballistics research because of the multiple types of information that can be obtained from a picture. For example, a pressure gauge will usually only provide pressure readings, but a good interferometer photograph may provide information on the state of the projectile, the distribution and pressures in the flow field, the projectile velocity, and the angle of yaw and its general aerodynamic excellence at that velocity.

In ballistic research, events are of relatively short duration. The range of subject movement is significant, e.g., 2m/s is consistent with a parachute flare-burn observation and up to 7km/s for projectiles launched from two-stage light-gas guns. Exposure times will generally be very short to freeze motion and prevent blur; consequently, framing rates will be high. The ballistic environment can also be very hazardous to equipment and operators, due to blast, shock, high pressures and temperatures, flying debris, high noise levels, and light flash.

There are five main areas of interest to the ballistics researcher, internal, intermediate (muzzle exit), exterior flight, and impact or terminal for guns as well as dynamic processes for other areas such as explosives, propellants, ignition devices, mechanism action, rockets, wind and shock tunnel models, shaped charges, etc.

Many general, high-speed photographic techniques are employed in ballistics work, but other specialized high-speed techniques developed specially for ballistics applications are also used. Some of the more important ones are described below.

Projectiles in Flight

Probably the earliest ballistics photograph (of a cannon ball in flight), was taken using a simple home-made camera by T. Skaife in 1853 at Woolwich Arsenal, London. This was in an era when photography was in its early infancy and most pictures were made of static subjects using long exposures, and Skaife was repeatedly asked how he managed to stop the cannon ball in flight in his picture.

Until the end of the 19th century the only available way of imaging high-speed events was the use of electric sparks. In 1884, Ernst Mach began photographing bullets and their flow fields by using spark schlieren photography. By 1892, Charles Boys in England was doing similar work using shadowgraphs. At normal atmospheric pressures this method was as good as schlieren and much easier to set up. Modern research still uses both techniques, but with sparks, flash tubes, or lasers as the light source.

Producing a basic shadowgraph only requires a point light source and a sheet of film. The subject, e.g., a projectile in flight, is arranged to pass between them. At the correct instant, often triggered by the arriving projectile, the light is fired and the shadow of projectile and its wake is produced on the film. The area must be in darkness while the film is uncovered.

A more convenient method uses a retro-reflective screen and a camera by the side of the light source, which photographs the shadow projected onto the screen. The picture records the rate of change in air-density gradient caused by the passage of the projectile. There are other flow field techniques that are more sensitive for lower-than-normal atmospheric pressure
environments, such as schlieren and interferometry. Schlieren photos and shadowgraphs only portray a projectile and their flow fields as shadows. To obtain front-lit pictures of the projectile, other methods are required. Holography, a more recently developed method, is now used, which provides multiple three-dimensional images of the flow field and the objects in it.

When photographing projectiles, pictures are usually taken at right angles to the trajectory, requiring very exact synchronization using very short exposures. To allow for synchronization errors, the imaging system can be moved back from the trajectory to ensure the projectile is in the field of view, but with this method the image size will be smaller.

The Synchroballistic System

The required system uses a camera set to streak mode, i.e., without conventional shuttering. The basic principle is used in the sporting “photo-finish camera.” The camera is oriented parallel to the trajectory. The objective is to synchronize the speed of the film and the speed of the projectile image falling upon it while both move in the same direction, so that film and image are relatively stationary to one another. This is achieved by correct placement of the components, and the appropriate choice of characteristics for the lenses that are used. To prevent blur, the image passes through a fine vertical slit placed in front of the lens, this effectively reduces the exposure time because the image is formed by a series of narrow vertical strips. This offers reduced exposure time without increased lighting intensity. Slight synchronization errors will elongate or compress the image dimensions in the horizontal or length direction.

If the film motion is at right angles to the trajectory, and the slit placed parallel to the trajectory, the projectile forms a diagonal streak across the film. Projectile velocity can then be found, using the film velocity and the system optical magnification.

The synchroballistic camera produces high-quality, front-lit images making full use of the film frame size in the direction of film motion. The image can also provide projectile characteristics including velocity, attitude, and spin rate. The system also records all objects passing along the trajectory while the camera is recording. This is useful for detecting objects that may cause unexplained spurious triggering. Both film and electronic image-converter cameras can be used in the system.

Each system station only provides one small section of the trajectory. If a close-up cine record is required for a large part of the flight, the camera must be panned to follow the projectile. While possible in lower speed events such as motor racing, it is not possible in ballistic research where very high transitional rates occur. A special system must be used.

The Flight Follower

Originally developed at the Royal Armament Research and Development Establishment (RARDE) in England in 1962, this system keeps the camera stationary, while the image is conveyed to it by a very fast panning mirror mounted on a large galvanometer coil (Figure 14a). To track the projectile, the mirror must follow a parabolic curve of acceleration and deceleration, peaking when the projectile is normal to the system axis (Figure 14b). In the first systems, the coil was accelerated by a series of positive pulses, then decelerated by reversing the polarity. To allow for mirror inertia change, the system was triggered before shot exit, and the mirror view and projectile synchronized at the muzzle. The system worked well and has now been modernized using a servo-galvanometer controlled by a microprocessor. The cine record can be recorded on a film or video camera. In the newer system, the mirror is still triggered before shot exit, but now, feedback from velocity detectors near the muzzle can switch the computer to control the mirror acceleration curve to match the velocity, thus ensuring that the reflected image remains in the camera field of view. Mirror swing-angle can be in the region of 90 degrees giving trajectory coverage governed by the stand-off distance. For longer coverage, several stations can be set up and electronically triggered to take over as the previous camera reaches the end of its tracking swing.

Aeroballistic Ranges

If a full projectile analysis of projectile trajectory is required, a scale model will be fired in an aeroballistic range. Spaced at intervals along the range will be many orthogonal photographic stations, which will take two simultaneous spark or laser flash shadowgraphs of the projectile and fiducial markers. Marker beads will be strung at known positions on three catenary wires running the length of the range. The two orthogonal photographs will each contain a view of the projectile and two marker wires, one common to both shadowgraphs. The projectile will trigger its own photograph, and the trigger instants will be recorded. From the known range geometry and analysis of all the shadowgraphs, the projectile yaw, pitch, precession, mutation, velocity, and three-dimensional position at all stations can be obtained and the projectile’s full aerodynamic coefficients found.

For long range rockets or missiles fired on large open ranges, trajectory tracking can be achieved by triangulation from the results from widely spaced cine theodolite stations recording azimuth and angle. Tracking is often assisted by placing a bright flare on the missile tail.

Lighting for Ballistic Photography

For ballistics photography, daylight, tungsten halogen lamps, and xenon discharge lamps are used as continuous light sources. Flash bulbs are relatively cheap short-duration sources, but triggering must allow for burn-up time to reach full brightness. Overlapping ripple firing of a series of flash bulbs can provide a 0.5 second or more continuous light source, or many bulbs can be fired simultaneously to give intense light levels for a short time. For single pictures, spark sources, flash tubes (single or strobed), argon light bombs/candles, and pulsed lasers are used.

Flash X-rays are used extensively for in-bore photography, in intermediate ballistics, to see the state of projectile arming mechanisms and internal integrity, and for impact experiments to show the penetration of projectiles and shaped charge jets into opaque targets. If the time history of such phenomena is required, cine X-ray will be employed, using a long duration X-ray pulse, an image intensifier, and an image converter camera which provides the required shuttering. Alternatively, multiple, short X-ray pulses are used with an image intensifier and conventional cine or video cameras. High-quality, high-accuracy triggering and synchronization is essential in ballistic research. Methods of projectile/event detection include make or break screens, electric or magnetic fields, interrupted light or microwave beams, emitted or reflected radiation from the object, detection of sound, local pressure changes, or shock waves.

Ballistic Cine/Video Photography

In the equipment used for high-speed cine/video photography, as framing rates increase and exposure time decreases, the available number of frames per event is reduced. As the very high framing rates are approached, direct film usage gives way to electronic imaging.

In both general and ballistic high-speed photography, the past decade has seen video systems make a strong challenge to conventional film cameras in the high-speed and very high-speed region. Their relative ease of use, versatility, and immediate results are very important in cutting costs and shortening the time for ballistic trials. Conventional cine cameras often have a correlation between exposure duration and framing rate, which limits the minimum exposure time available. They have been given wider application possibilities by coupling them with lasers. Pulsed metal vapor lasers can be triggered from these cameras at the point where the shutter is fully open, giving the possibility of a cine system with exposure times down to tens of nanoseconds.

Apart from some high framing rate but lower resolution video systems, beyond about 45,000fps, longer established systems such as drum cameras, rotating mirror/prism cameras (both framing and streak), and image converter and image intensifier cameras are still used.

Framing and Streak Cameras

Rotating mirror/prism cameras and image converter cameras are often used in either mode. In framing cameras the event is shuttered to provide recognizable pictures taken at discrete intervals. However, no information is obtained in the intervals between frames. If a continuous record is needed, e.g., in studies of propellant burn or explosive behavior, a streak camera is used. In this mode there are no lost intervals as there is no shuttering employed. The image is focused onto a slit and the slit can be parallel to the film motion or at right angles to it. If the system is at right angles to the motion, recognizable pictures are produced (see the section Synchroballistic System), if parallel-to-film motion pictures are required, the images will require interpretation as only the recorded moving boundaries between solids or incandescent gas and the surrounding air will be imaged.

Knowing the speed of the recording medium or sweep speed of the electronic system, highly time-resolved measurements can be made. Typical writing speeds for rotating mirror systems can be in the order of tens of millimeters per microsecond with a resolution of 2 to 3 nanoseconds. For electronic cameras writing time can be 1000 millimeters/microseconds with a resolution of 0.15 nanoseconds.

Combined systems that include the best attributes of film-camera mechanics and charge-coupled device (CCD) recording are now in use, e.g., in some image intensifier and rotating mirror/prism cameras where a limited number of sequential still exposures are combined to form a cine sequence. In these cameras, film has been replaced by multiple CCD detectors that offer great processing, versatility, and comparatively short turnaround times.

“Still” video cameras using CCD recording can give a limited number of high framing rates with short duration exposures onto one frame as the CCD detector is capable of fast response. The image is not downloaded as in video systems but all pictures are recorded and downloaded later. Operation modes are either two half-frame exposures, e.g., before and after penetration in an impact sequence, or several exposures where the subject remains in the field of view, but moves such that each image location does not obscure the previous image. This is similar to making a stroboscopic picture using film and multiple flashes.

No doubt, as high-resolution video capabilities and framing rates increase, video systems will be used increasingly in ballistics research, but replacement of ultra-high-speed film cameras still seems to be in the distant future. Meanwhile, video systems form a useful rapid access adjunct to film or electronic cameras to prove a technique that is practicable before using other high-resolution film or electronic cameras for the final results.

Precautions

Because of the inherent dangers in ballistic research, the cameras used must be robust and shock resistant. Often special protective housings or walls, temporary or permanent, are used to provide necessary protection. Cameras will often view the scene through very thick glass windows or through use of sacrificial relay mirrors, while the cameras remain protected from blast or debris.

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