Tachymetric anti-aircraft fire control system

From Wikipedia, the free encyclopedia

A tachymetric anti-aircraft fire control system generates target position, speed, direction, and rate of target range change, by computing these parameters directly from measured data.[1] The target's range, height and observed bearing data are fed into a computer which uses the measured change in range, height and bearing from successive (or later, continuous, radar) observations of the target to compute the true range, direction, speed and rate of climb or descent of the target. The computer then calculates the required elevation and bearing of the AA guns to hit the target based upon its predicted movement.

The computers were at first entirely mechanical analog computers utilizing geometric model mechanisms, disk–ball–roller integrators, and (later, for antiaircraft, 3–D) ballistic and prediction cams to physically perform the calculations of what had been done earlier with protractors, straightedge and compass construction of lines of symmetry, and slide rules, using moving graph charts and markers to provide an estimate of speed and position.

Complex 3D math calculations such as star tracking for navigation are also possible with a spherical / tangential gearset performing the functions of an astrolabe inclinometer.

Variation of target position over time was accomplished with one (USN Mk. 1/A) or more constant-speed motors to run the integrators in the mechanical simulation.

The term "tachymetric" should more properly be spelled as "tachometric"[2] which comes from the Greek "takhos" = speed, and "metric" = measure, hence tachometric, to measure speed.

An alternative, non-tachometric, gonometric [3][4] method of AA prediction is for specially trained observers to estimate the course and speed of the target manually and feed these estimates, along with the measured bearing and range data, into the AA fire control computer which then generates change of bearing rate and change of range data, and passes them back to the observer, typically by a "follow the pointer", indicator of predicted target elevation and bearing or by remote power control of the observer's optical instruments.

[5] The observer then corrects the estimate, creating a feed back loop, by comparing the observed target motion against the computer generated motion of his optical sights. When the sights stay on the target, the estimated speed, range, and change of rate data can be considered correct.[6]

An example of tachometric AA fire control would be the USN Mk 37 system, which used the Mk. 1/A computer muntioned later. The early RN High Angle Control System (HACS) I through IV and the early Fuze Keeping Clock (FKC) were examples of non-tachometric systems.[7]

By 1940 the RN was adding a Gyro Rate Unit (GRU)[8] which fed bearing and elevation data to a Gyro Rate Unit Box computer (GRUB), which also received ranging data to calculate target speed and direction directly, and this tachometric data was then fed directly to the HACS fire control computer, converting the HACS into a tachometric system.[9]

The Ford Instrument/US Navy Mk. 1 (Note: NOT "Mk. I"! Article title is wrong.) and Mk. 1A computers each included a complete target simulator (The routine type B, (dynamic) tests validated simulation).

Aided tracking fed to the gun director moved the line of sight (and could update the optical rangefinder) as the computer converged to a solution. Essentially, the ongoing target simulation was progressively brought to match the target motion vector.

The US Navy Mk. 37 GFCS also used tracking feedback from the computer. At first, the gun director operators did all the tracking, squeezing a trigger–like handwheel–crank switch only when on target. Those switches closed feedback clutches in the computer to refine the simulation. This was sometimes called "aided tracking".

Radar, however, closed clutches when the target tracking began.

Computed mechanical fuze times were sent to the gun mounts for automatic setting.

Back then, aerial and surface attacks required avoiding maneuvers or changing (air)speed, which validated predictions that assumed no changes in target speed and direction.

Assuming no changes, the converged simulation could be valid for half a minute, in (rare?) cases of gun director disability.

However, this simulation feedback was too slow, and the Mk. 56 fire control system, the early close–in weapons system, was made to cope with WW II kamikazes. It operated on very different principles; its analog computer (for most functions, notably ballistics) was a linkage type. A 3–D geometrical model developed gun order offsets. It had no ballistic cams nor ordinary vertical gyro.

The successor to the Mk. 1A was the Computer Mk. 47, which used a basically different scheme. It was hybrid mechanical (and electrical, using compensated resolvers for trig. functions). While repair (or even readjustment) (apparently quite rarely needed!) of the Mk 1/A could take days or even weeks, the Mk. 47 provided access in seconds.

Notes[]

  1. ^ Weapon Control in the Royal Navy 1935-45, Pout, p126-127, from The Application of Radar and other Electronic Systems in the Royal Navy in WW2 (Kingsley-editor)
  2. ^ Weapon Control in the Royal Navy 1935-45, Pout, p127, from The Application of Radar and other Electronic Systems in the Royal Navy in WW2 (Kingsley-editor)
  3. ^ from Greek gōnon "angle" + metron "measure"
  4. ^ BRITISH MECHANICAL GUNNERY COMPUTERS OF WORLD WAR II, Bromley, p17
  5. ^ The RN Pocket Gunnery Book, p153-154, paragraphs 432-435
  6. ^ The RN Pocket Gunnery Book, p153-154, paragraphs 432-435
  7. ^ Weapon Control in the Royal Navy 1935-45, Pout
  8. ^ Weapon Control in the Royal Navy 1935-45, Pout
  9. ^ Weapon Control in the Royal Navy 1935-45, Pout, p104

External links[]

Retrieved from ""