An ORDEAL to face a world

The attitude adopted by a spacecraft when it orbits a planet is completely dependent on its mission. Observatories like the Hubble Space Telescope usually hold a fixed attitude with respect to the stars to allow optical systems to gather light from distant objects. Their attitude control is said to be inertial and, as a result, they continuously change the face they present to the world below as they go around in orbit. But the vast majority of satellites in orbit around Earth are required to aim cameras, antennae or other paraphernalia at the ground, and this is usually achieved by having the entire spacecraft slowly rotate so as to ensure the appropriate equipment can be brought to bear on their subject. This method of flying around a planet or moon with one side constantly facing the surface is known as orbital rate because it requires an orbital rate rotation to be set up. If it takes 90 minutes to orbit Earth, as is typical for a low Earth orbit, then by pitching down at a rotational speed that also takes 90 minutes per revolution, one side of the spacecraft can be made to face the ground at all times. Apollo often used orbital rate motion in Earth orbit and in lunar orbit because so many operations required a ground-based frame of reference. As well as pointing cameras and instruments at the surface, the attitude required for many manoeuvres depended on where the ground was with respect to the spacecraft and the crews felt it would be easier to monitor and control the bums if they did so with reference to the ground.

As it happened, the guidance system for Apollo was designed by a team from MIT who thought entirely in inertial tenns. As trying to monitor an orbital rate rotation was a little problematic for a spacecraft that had been designed to show inertial attitudes, a means had to be found to make the 8-ball rotate at orbital rate in sympathy with the spacecraft’s rotation so that it, too, would display attitude with respect to the ground. The solution was one that reflected the hurried nature of the programme. There was
insufficient time within Kennedy’s end-of-decade deadline to redesign the guidance system to implement such a feature. Instead, engineers added a workaround; a little box. given the acronym ORDEAL, that the crews had to install after they attained orbit. The acronym stood for orbital rate display, Earth and lunar, and its operation was simple. On the assumption that an orbital rate attitude was simply a progressive pitch – down motion (which it w’ould be if the orientation of the S-IVB was as planned), the ORDEAL supplied a calibrated drive signal that caused the 8-ball to pitch at the same speed. With this box properly set up. the crews could read off their attitude with respect to the ground. For later flights, they learned how to use the ORDEAL to monitor their attitude during the 1 LI burn, which was also carried out in an orbital rate attitude. If the burn could be accurately monitored, the commander could take control of the stack in the event of a failure of the S-IVB guidance system, thereby further increasing the redundancy of the entire Apollo system.