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 Voice communications, data transmission, and tracking cont inue during the second orbit. Another IMU alignment is performed 15 minutes before the planned injection time, the "go" decision is given by the ground, the crew secures the spacecraft, the S-IVB restart sequence begins, ullage rockets are fired, and the vehicle is ready for translunar injection (Figure 3).

TRANSLUNAR INJECTION

During the approximately 5½ minutes of S-IVB burn for translunar injection, the spacecraft velocity will increase by more than 10,000 ft/sec. Altitude will increase to about 160 nautical miles and about 50° to 60° of longitude will be transversed.

Tracking by ground based stations during translunar injection will be available on many missions, but due to the large envelope of translunar injection points, tracking, even with a limited number of ships, will not always be possible. In any case, however, the spacecraft will be acquired and tracked by a MSFN station within no more than 7 or 8 minutes after translunar injection. However, voice communications and data transmission will be maintained during the injection phase by means of relay aircraft.

Having completed translunar injection, the spacecraft is now on the next plateau, translunar coast (Figure 24).

TRANSLUNAR COAST

The translunar injection maneuver was configured such as to place the spacecraft on a circumlunar coast trajectory which circumnavi­gates the moon and returns to a safe entry condition back at earth with no major intervening maneuvers required. This is called a "free return" trajectory and will be discussed in more detail in a later paper by M. P. Frank.

Imme1iately after injection the hydrogen and oxygen tanks on the S-IVB will be vented to a low pressure to assure that uncontrolled venting will not occur during the critical attitude operations for the next two hour period.

Following a quick systems status check after the end of injection, the first operation will be for the crew to reorient the vehicle in a direction favorable for docking illumination while at the same time maintaining communications with earth during the next two hour period. One additional constraint is that the maneuver se­quences for this reorientation must avoid yawing the vehicle more than ±: 45° so as not to result in gimbal lock for the S-IVB inertial platform.