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 and that the engine thrust vector is not acting through the c.g. Since the engine gimbals are used for trim control only and are relatively slow, the engine thrust is reduced to insure that the moment control of the RC8 is not exceeded during the initial travel. The high thrust at the end of the burn allows a check to be made on the engine through its full throttling range.

The fuel expended during the insertion maneuver is about 330 lbs., and the corresponding delta V is slightly less than 100 ft/sec.

The spacecraft is now on the next plateau, descent coast (figure 39). At this point, the LM is on a slowly descending trajectory which will continue for about the next hour until it reaches its pericynthion altitude of 50,000 ft. During this period, the LM will track the CSM with its Rendezvous Radar and determine its descent trajectory onboard. Similarly, the CM can track the LM flashing light with its sextant and perform an independent deter— mination of the LM orbit. Finally, when the LM comes within line of sight of earth, then the ground station will track the vehicle and provide the LM with the final source of navigation data. As in all operations discussed up to this point, the ground based navigation is the primary source of data.

If the decision is made not to initiate powered descent, the LM is in a safe orbit from which it could subsequently rendezvous with the CSM, or if necessary, the CSM could perform a rescue. Having decided to continue descent, however, another IMU alignment will be made and preparations for powered descent begin.

Up to this point in the coasting descent, the LM has been leading the CSM. At pericynthion, this lead angle is about 10 degrees. From this point on, however, once the LM begins powered descent, the CSM will catch up and finally go ahead of the LM during the latter part of the landing maneuver.

At the 50,000 ft. pericynthion altitude, a propellant settling maneuver is performed with a S-second RCS firing, followed by descent engine ignition. The thrust profile is the same as

before with a 3-second burn at 30% thrust, followed by a 28-second period at 10% thrust, and then increased to 92½% thrust (9700 lbs.). The powered descent phase will be discussed in detail in a later presentation by Mr. Cheatham, so only the gross profile will be described here.

The powered descent is divided into three distinct portions, called the braking phase, the final approach phase, and the landing phase.