Inertial or horizontal?

As the design of the G&N system was being finalised in the early 1960s, a eonllict arose between those who designed the equipment and those who would fly with it. The designers at the Instrumentation Laboratory at MIT led the field of applying mathematics to the problems of guidance, whether in a submarine, an aircraft, a nuclear-tipped missile or the exploration of space. They saw the problem from a wide perspective in which all guidance could be reduced to equations that modelled the solar system as it sat surrounded by the stars. Their fundamental point of view’ was an inertial one. which was expressed during Apollo’s gestation by an intention that the spacecraft’s attitude should be displayed as a set of numbers with respect to inertial space. The crews, on the other hand, w’ere pilots, and pilots see flight largely in terms of movement with respect to the horizon of wTiatever planet (usually Earth) they are flying over. Their point of view’ dealt with a local frame of reference that stayed aligned with the ground beneath their spacecraft, even as they flew around a curved planet.

These two viewpoints on spacecraft control influenced the Apollo guidance and navigation system as it evolved at a tremendous pace during its development; the inertial point of view’ dictating its fundamental structure, but w’ith the astronauts’ preferences heavily influencing the final mode of operation because they had experience on their side. They had cut their teeth on the Gemini flights of 1965 and 1966 during which NASA learned how to fly in space. They pointed out that most manoeuvres needed to be carried out with respect to the ground below, especially the all-important rendezvous manoeuvres on which the Moon-bound flights relied. As the design of the Apollo G&N system had been largely settled by the Lime this operational experience w-as gained, both hardware and software modifications had to be made to meet the expectations of the crews. These included the ‘8-ball‘ display and the ORDEAL add-on to turn it into an artificial horizon that the pilots preferred.