Journal of the Society of Motion Picture Engineers (1930-1949)

The photographically obtainable data which are required to determine the cause of failure are the missile velocity, acceleration, direction of motion and attitude. From the difference between the attitude of the missile and its direction of motion can be determined the direction in which aerodynamic forces are applied to the fins. From the missile velocity and acceleration can be determined the magnitude of the forces. The direction of the force may be needed to an accuracy of ±0.1° or better. The acceleration may be required to an accuracy of ±1 G. To obtain free-flight data with the above accuracy at rates of 1000 frame/ sec would require impossible accuracies in position determinations. Random errors of ±0.00001 ft in position would result in an error of ±1 G in accelerations obtained by differencing of position data from successive frames. Using specially designed cameras, it is often possible to hold random errors in missile position determinations to approximately ±0.04 ft. When position determinations are held to this accuracy, frame rates in the order of 20 per sec result in acceleration accuracies of ±1 G. If long focal length lenses are used with these special cameras or if the cameras can be placed close to the flight line, random missile position errors in the order of ±0.001 ft can be obtained. Frame rates of 90 per sec can be used with this position accuracy to obtain ±1 G acceleration accuracy. This accuracy, however, can be achieved only with such cameras as the Bowen Ribbon Frame camera and then only if the film can be read to an accuracy of ±2 microns. Camera Parameters Some of the cameras in use at NOTS are standard high-speed machines but others such as the Bowen ribbon frame cameras1 and Askania Cinetheodolite2 were originally designed for metric purposes. Table I compares their operating characteristics. The table does not include all cameras in common use at NOTS but merely gives single examples of each type. Likewise, only the most frequently used lens-and-shutter combinations are given. These cameras differ widely not only in their ' operating characteristics but in the accuracy limits for data obtained from them. In determining the accuracy limitations of a camera and consequently its usefulness as a metric tool, the following factors are considered : 1. resolving power, 2. accuracy with which the lens and film orientation can be determined, and 3. accuracy with which the time of exposure of the missile can be determined. Resolving power is considered here as the resolving power of the lens and film combined when photographing missiles under usual lighting. The resolving power is then defined as the reciprocal of twice the diameter of the finest line that can be resolved on the film under these conditions. Expressed mathematically R = \/2w (1) where R is resolving power in lines per millimeter and w is the width of the line in millimeters. This definition is essentially in agreement with standard methods of determining resolving power with the exception that this is the resolving power under ordinary medium contrast, not with high contrast. Since high contrast in the negatives is rarely possible, the use of a resolving power based on that condition is invalid. Resolving power is linked to accuracy in missile position in the following way. It has been experimentally determined that a line of medium contrast to the general background can be bisected to an accuracy of about 1/20 its width. Thus (using standard film-measuring devices) an operator can bisect a reference line and the missile line on the Griffin and Green: Accuracy Limitations 487