Ballistic measurement systems: optically measuring the projectile velocity (1954 – 1985)

Optically measuring the projectile velocity (1954 – 1985)

This optical device measures flight times of projectiles coming from a cannon barrel. The first prototype with a bar length of 1 meter was tested in 1952. Two device sets were built for the Commission of Trials of the Royal Netherlands Army. The first ‘photo bar’ with a base distance of 2 meters (to achieve the desired accuracy) was taken into use in 1954. The measurements can be performed when shooting at elevations from 0° to 85°.
The equipment is composed of two parts, the photo bar and the main amplifier. That photo bar was used up to and including 1984. In 1961, the photo bar underwent a major overhaul. In 1962, a second photo bar was developed that was also used up to and including 1984.

The equipment is composed of two parts, the photo bar and the main amplifier. The photo bar is placed under the projectile track. The photo bar consists of two measuring heads that are connected to each other by a steel tube with a fixed distance of two meters. Each measuring head contains two photomultipliers, preamplifiers and a double optical system so that projectiles can be measured over two independent measuring systems.

Construction of the photo bar and the schematic device setup
Construction of the photo bar and the schematic device setup

When the projectile passes, the light from the sky that falls on the photomultiplier is partly blocked. The amplified detector signal is converted into pulses in the main amplifier, which corresponds in time with the passing of the projectile through the “watched sky” by the front and rear detectors. The different pulses are used as start and stop signals for four electronic time interval meters (two front and two rear measurements). The speed of the projectile can be determined from the known base length of two meters and the measured flight time.

The photo bar on tripod (right), a Doppler radar, and tailstock for measuring projectile velocities under elevation
The photo bar on a tripod, Doppler radar and tailstock for measuring projectile velocities under elevation

 

Adjustment possibilities of the optics
Adjustment possibilities of the optics (height, diaphragm, gap width)

 

Optical measurement device for projectile velocity (OSM 04-01) (1989)

In 1989, the Ammunition and Small Caliber Weapons (= Mun/KKW) section of the Army Material Testing Department 1 (MBA1) needed a computerised projectile velocity measurement system for taking measurements of small calibre projectiles (up to 20 mm) in shooting tunnels and in the open air. This replaced the old Cintel equipment that was not able to measure rapid-fire series and that was not equipped with registration equipment.

The system that was ordered from the Physics Laboratory TNO had the special requirement that it should be able to measure a 5.5 mm projectile at a height of 6.5 m above the detectors. A test model with optical detectors was developed. The results were positive. Subsequently, the construction of the definitive system “Optical Measuring Device Projectile Velocity” (=OSM 4-01) was carried out.
Projectile flight times are measured over a base of 2 m or a base length of choice. Two optical detectors, of which the fan-shaped detection field is placed perpendicular to the base, convert the passage of a projectile into impulses. The first detector gives the start impuls. The second detector stops the time measurement. The speed is calculated from the base length and the time to pass the detectors. The optical detectors can be placed at the ends of a 2 meter base tube (see photo), or detached at a distance of choice for the specific firearm. The optical detector uses an artificial light source in the shooting tunnel and the light from the sky background in the open air.
The processing unit can process a single shot, rapid fire, and burst shooting. An infrared mouth flame detector activates the processing unit to perform measurements. The maximum result of 600 shots can be saved. The average and standard deviation is calculated over a selected number of shots. The speeds of a series are recorded with the average and standard deviation on a teletype printer. The initial values ​​are recorded as well in a punch tape.

Optical measurement device for projectile speed (OSM 04-01)
Optical measurement device for projectile velocity (OSM 04-01)

 

2 m measurement base of the Optical measurement device for projectile speed (OSM 04-01)
2 m measurement base of the Optical measurement device for projectile velocity (OSM 04-01)

 

Detector at the 2 meter bar
Detector at the 2 meter bar

 

Processing unit Optical measurement device for projectile speed (OSM 04-01)
Processing unit Optical measurement device for projectile velocity (OSM 04-01)

 

Detector Optical measurement device for projectile speed (OSM 04-01)
Detector Optical measurement device for projectile velocity (OSM 04-01)

 

Papertape punch device
Papertape punch device