FINAL FOUR HOURS

The crew’ spent the next few’ hours of their approach by making extensive checks of their electrical power, environmental and propulsion systems as well as their caution and warning system. If a final mid-course correction was required, this was the time to make it, and if so then additional onboard navigation sightings w’ould be taken in case communications w’ere lost in the final hours.

Before they reached the waiters of the Pacific Ocean, a coordinated sequence of events had to occur in the correct order. A number of those events included explosive devices. These would separate the command and service modules before re-entry and operate the various subsystems of the ELS during the final drop through the denser layers of the atmosphere. This included the jettisoning of the forward heatshield or apex cover to uncover the parachutes, and their subsequent deployment. These complex events were under the control of the sequential events control system (SECS), whose status could only be verified via telemetry. To give these circuits an early check, the crew7 temporarily armed them and asked the flight controllers to inspect them.

At about the time the PAD was being passed to the crew’, flight controllers were granted direct uplink access to the onboard computer’s memory to change the

REFSMMAT to that required for re-entry. With an hour and a half remaining, the guidance platform was given its final realignment to this REFSMMAT.

It was usual throughout a flight that, when the orientation of the guidance platform had to be changed, it would be realigned twice. The first would be according to whatever orientation had been in use up to that point, which, prior to re-entry, was likely to be the PTC REFSMMAT. This was simply to determine how far the platform had drifted since its last realignment. It gave engineers an additional ‘data point’ in their record of the mechanical characteristics of the IMU in case this data had a bearing on subsequent flights. The second realignment swung the platform around to its new orientation as defined by the entry REFSMMAT. *

As a backup, the same alignment was passed to the spacecraft’s secondary attitude reference, which consisted of the body-mounted gyroscopes and their associated electronics system, the giro display couplers (GDCs). This just required a press of the ‘GDC Align’ button, and although this backup system was more prone to drift, if all went well the spacecraft would be in the water before it became an issue.

Because he had received most training for the re-entry procedures, the command module pilot took the left couch. None of the Apollo crew s w ore their pressure suits for re-entry after the Apollo 7 commander rebelled, citing his head cold as a reason not to have to wear the bulky garment. The CMP manoeuvred the spacecraft to an attitude that put the heatshield forward with the crew hcads-down and looking back. It wasn’t their final entry attitude; a further pitch-up w;ould be required for that a manoeuvre they would execute a few minutes before reaching entry interface. But in this attitude, they could make checks of their attitude control and trajectory, and it w as a starting point for the procedures to jettison the service module.

The first of these checks required that the sextant be aimed at an angle given in the PAD, with the expectation that, if their attitude was as it should be. a specified star would be visible in the instrument’s narrow field of view-. With that check made, the spacecraft’s optics had performed their final task, and were sw ung to a shaft angle of 90 degrees and powered down. Earlier flights also included an additional check of attitude using the COAS – an optical sight, similar to a gun sight, mounted in one of the rendezvous windows. Again, with the sight mounted at specified angles, a named star was expected to be visible through the instrument.