PTC maneuver

The PTC maneuver requires the spacecraft to be rotated around the X, or longitudinal axis, in a slow roll of 0.35° per second. A complete roll of 360° should take about 17 minutes at that speed.

The X-axis of the CSM is an imaginary line which runs from the centre of the SPS engine bell, through the centre of the SM and out through the apex of the CM's cone. Incidentally, in the docked configuration, the LM's X-axis is colinear with the CSM's but is in the opposite direction, running from the descent engine to the docking hatch and, because of this, the Y- and Z-axes of each spacecraft are also opposed. The 'front doors' of each spacecraft are aligned 60° around the X-axis from each other. Readers should be careful to distinguish between the spacecraft axes and those of the IMU.

The last P52 aligned the X-axis of the IMU platform with the ecliptic and at right angles to the Earth-Moon line; its Z-axis was aligned southward, perpendicular to the ecliptic. If the spacecraft were aligned to match the platform, it would have its longitudinal axis aligned with the ecliptic with the Earth and Moon to either side. By pitching 90°, as called for in the Flight Plan, the longitudinal axis of the spacecraft is brought perpendicular to the ecliptic which guarantees that the Sun (which is always in the plane of the ecliptic) will strike the spacecraft side on as it rotates. (ap15fj)

The PTC mode is controlled either by the G&N (Guidance and Navigation system) or by the SCS, with the former being used in this case. The spacecraft was maneuvered to the PTC attitude at 11:46 using Verb 49, a routine which moves the spacecraft to a desired attitude. With the FDAI set to its maximum sensitivity and with a 0.5° deadband, the motion rates of the spacecraft are monitored until they settle down. This process is aided by using only two adjacent thruster quads on the SM so that each thruster firing comes from one, not two thrusters. The smaller impulses from the thrusters allow finer control of the spacecraft's attitude. After the spacecraft's attitude motions have been damped, with only two opposing roll thrusters enabled, a roll rate of 0.35° per second and an attitude deadband of 0.5° is entered into P20 (option 2) along with a time at which the maneuver is to commence. Program 20 in the CMC (Command Module Computer) sets up a desired rate of rotation; this program is also used to set up "orb rate" rotation (where the rate of rotation matches the orbital period, thereby keeping one side of the spacecraft facing the surface) when the spacecraft is in orbit around the Moon. (ap15fj)