The Brains of the F-15 Eagle

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The vaunted American air superiority fighter, the F-15 Eagle, was a product of years of research and development. Its advanced and integrated systems were the central part of the Eagle’s ability to dominate the skies since its introduction three decades ago.

From the outset, the F-15 design team planned to incorporate the aircraft with the most sophisticated avionics and weapons packages available. In a departure from previous aircrafts designs, the F-15 would integrate those two systems through a complex Central Computer (CC). The initial batch of Eagles were fitted with IBM’s CP=1075-AYK central computer system. The AYK was a 48.5lb, high speed, and all purposes, analog platform that, at the time, had no equals in term of pure analytical and processing capability. The AYK possessed a “hard wired” memory chip of 16.K (34 bit) words, expandable to 24.6K, in order to store the weapons envelops for both air to air and air to ground ordinance deployment. The CC would also store all kinds of flight performance data. It had an amazing 340,000 per second instruction computation pattern that remained unrivaled for almost a decade. The CC used the impressive computation rate to convert data emanating from subsystems onboard the Eagle into coherent and readable information which was displayed to the pilots on three main displays. The Heads Up Display (HUD), the Vertical Situation Display (VSD) and the Horizontal Situation Indicator (HSI) which is the aircraft’s primary navigational instrument.

The CC concept was the brainchild of the innovating design team at McDonnell Douglas, McAir division. The original requirement sheet for the F-15 called for a “more deadly platform with just one occupant at the controls than the F-4 Phantom II had ever been with two”. In the late 1960s, it became apparent to Douglas’ engineers that computer technology had reached a point where it could be applied successfully to an integrated air combat platform such as the Eagle. The CC communication with the rest of the aircraft’s systems is done through an H-009 multiplex bus called MUX, which gave the means of data traveling and the scheduling of the transfer to two separate avionics interface units. The units operated as interfaces between all the F-15′s complex systems and the MUS.

Among other ingredients, the new CC housed the Weapons Engagement Zone (WEZ) data for the AIM7 Sparrow missile system which calculated the dynamic launch zone on a real time basis. The WEZ also contained all the ballistic and release information for a possible deployment of the F-15′s vast air-to-ground arsenal. Unfortunately, the integration of the ground component to the WEZ-CC platform meant that little memory was available for the aircraft’s main defensive weaponry: the Sidewinder missile. But since the missile was a rear aspect weapon (also at the time), this did not represent a major problem. Nevertheless, the Air Force wanted to increase the size of the AYK’s memory. Before the very first units of the F-15, the A/B versions, rolled out; they were upgraded with a new 24.6K bulk memory in order to expand the WEZ capacity to assimilate new data from “Lima”.

The “other” system which integration with the CC proved a pivotal step in the development of future fighter concepts was the Tactical Electronic Warfare Suite or TEWS. The TEWS, along with the Eagle’s powerful new radar array, was the eyes and ears of the F-15. Without either of them, the vaunted Eagle would not have been the air dominance weapon it became.

Story By: Raul Colon
e-mail:rcolonfrias@yahoo.com