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 of sun elevation angles where a flight crew's ability to select an acceptable touchdown point is markedly improved over angles outside that range. Again, present understanding of the nature of the lunar surface leads to the conclusion that it is necessary to provide a high capability in this area in order to enhance mission success and crew safety. The first lunar landing mission is now being planned for sun elevations between 7° and 20° to take maximum advantage of the crew's visibility in the landing operation (Figure 5).

Since there is only a range of 13° in the planned sun elevation angle, and since sun elevation angle changes at the rate of 26° for a single 48 hour recycle, 52° for two recycles, and 78° for three recycles, then this obviously leads one to look at multiple landing sites.

Some slight flexibility in launch to a single site could be realized if accumulators we're used as built-in holds. That is, plan to launch early after a successful countdown, and wait at some point in the mission for the planned landing site to catch up to the correct lighting condition. For example, additional earth orbits and lunar orbits could be used as built-in holds at the rate of l/2° per hour (3/4° per earth orbit and 1° per lunar orbit). Similarly, trans-lunar transit time could be used as an accumulator at this same rate. Since there is a limitation of only three earth orbits (from S-IVB consumables considerations), and since the free return trajectory requirement restricts trans lunar transit time to a narrow range, then about the only significant flexi­bility item is in the number of lunar orbits prior to LM descent. However, to make full use of this, one would sometimes launch so early that more than 70 hours in lunar orbit would be required to rectify the lighting condit ions at the landing site. Con­sumables and systems limitations would then become a problem.

Therefore, the multiple landing philosophy be comes an inherent feature in lunar mission planning.

Apollo mission planning personnel have, of course, been involved in Lunar Orbiter and Surveyor site selection. The sites shown on Figure 6 are from the Orbiter A and Orbiter B missions as planned to be flown later this year. These particular sites have been identified at this point in time as most probable to contain acceptable touchdown points in a large dispersion ellipse and radar approach terrain.