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 means that the inertial position of landing is approximately fixed for a given return plane. This means that the abort trajectory problem is essentially a timing or a rendezvous problem. In other words, the return must be timed such that, as the spacecraft reaches its relatively fixed inertial landing point, the desired landing area is just rotating underneath. If the fastest possible abort return just misses rendezvous with the desired landing area, then a delay in landing time of nearly 24 hours results in order to allow the desired landing area to make a complete revolution to again reach the desired inertial rendezvous point. Thus, more than one solution is possible, but the possible solutions occur in 24-hour increments as regards landing time.

Figure 26 shows that this 24-hour landing effect is also present when the time of abort is added as an additional degree of freedom to the problem. This effect occurs because the inertial direction of the abort point changes very little with abort time along the translunar coast trajectory. An abort from point 1, as shown in figure 26, to a particular landing site might require relatively low energy in order to achieve rendezvous with a desired landing site. An abort at a later time, as represented by point 2, would require a higher energy return trajectory if the spacecraft is to rendezvous with the same landing site at the same time as the abort trajectory 1. Similarly, an abort trajectory performed at still a later time, point 3, would require even more energy resulting in an even faster return trajectory in order to account for the time lost in delaying the abort maneuver. Obviously, a limit will be reached as regards delay time when either the A.V required for abort or the velocity at reentry violates its respective limit. When this limiting or critical delay time is reached, an abort would be forced to a landing 24 hours later, as represented by point h in figure 26.

An interesting feature of this 24-hour effect for aborts to a particular landing site is that returns will either all be in daylight or all in darkness. In other words, if an abort solution to a primary recovery site such as Hawaii lands in darkness, a solution cannot be found for any other abort delay time or any other AV required which will cause landing at Hawaii to be in daylight.

Figure 27 shows this 2h-hour effect in the form of actual abort trajectory performance data computed for a typical translunar coast trajectory. Shown plotted is the time

of landing measured from translunar injection as a function of the time of abort measured from translunar injection. Data is shown for three primary recovery sites--Indian Ocean, Hawaii, and Bermuda. The data shown for aborts