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 EARTH PARKING ORBIT

Following this confirmation of a good orbit, the spacecraft is on the second mission plateau which was discussed earlier (Figure 21). The capability exists to spend up to three complete revolutions in the earth parking orbit, but normally the plan will be to execute the next commit point, translunar injection. during the second revolution. The limitation of three revolutions is associated with considerations of S-IV consumables limitations. A later paper in the Symposium by M. P. Frank will discuss the geometry of trans­ lunar injection opportunities will occur every day over both the Atlantic area and the Pacific area, but one of these will be pre­ferred from a performance standpoint. In addition, it is not planned to have support aircraft covering both areas simultaneously. There­ fore, having planned the mission for a specific period during a specific month, and have launched at a specific time of day, then the mission is committed to either a Pacific or Atlantic injection. For reference purposes, the Pacific injection will be considered as the appropriate windown, and for the reference mission, it will occur over the Western Pacific Ocean, near the equator. The only parameter left to choose is which of the three opportunities during the three revolutions translunar injection will occur.

The answer to this question is dependent upon how extensive a set of operations is planned during earth parking orbit. These operations will be discussed below. Figure 22 shows the ground track and station coverage for the typical reference missions; it will be useful to refer to this figure during the discussion of earth orbit operations.

Immediately after S-IVB cutoff at earth orbit insertion, the launch vehicle propellant tanks are vented of hydrogen and oxygen gases to relieve the pressure buildup. Venting is performed at this time in order to prevent unpredictable vents from interferring with sensitive attitude operations later on. The venting sequence is preceded by ullage rocket firing to assure propellant settling so that only vapors are vented. After settling the propellant, the oxygen tanks are vented for about 15 seconds. Hydrogen venting will be done continuously throughout earth orbit, along the spacecraft's thrust axis; however, the propulsive force of the hydrogen venting is extremely low and, therefore, would not interfere with other operations during earth orbit.

Following earth orbit insertion the crew remains in their couches until the Manned Spaceflight Network (MSFN) has verified that the spacecraft is in a safe orbit. This confirmation is provided after about three minutes of tracking by the insertion ship in the Atlantic. At this time a sate vector update is provided by the ground which the crew inserts into the onboard computer.

Following a brief onboard checkout of spacecraft systems, the navigator will leave the center couch and go the lower equipment bay of the Command Module and prepare for the first operation--