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 The third abort mode listed in the summary chart is a circumlunar abort available with the SPS, DPS, or marginally with the Service Module RCS continuously throughout this mission phase. Figure 22 shows the basic features of this abort mode to be a single unconstrained burn performed at some point along the translunar coast trajectory which returns the spacecraft to the earth after circumnavicating the moon. The altitude of pericynthion or perilune is allowed a certain amount of freedom in order to obtain landing area control upon return to earth. This mode of abort is not as yet thoroughly understood. Preliminary analysis indicates that this type of abort many times produces the absolute minimum ΔV required to return to a particular landing area, especially if the translunar coast profile is a non-free-return type. However, for just as many cases the minimum ΔV required to return to a particular recovery area has been found to be minimized by use of delay to pericynthion abort mode. A complete understanding of this effect is currently under investigation.

Figure 23 shows a special case of the circumlunar abort mode. This particular case consists of merely using mid-course corrections to correct back to the nominal free-return trajectory, assuming this type of return is the nominal profile. Use of the circumlunar abort mode in this fashion is essentially equivalent to the fuel critical unspecified area sub-mode described previously.

The last mode listed on the summary chart is a two-burn mode for the special case of when the translunar coast trajectory is on an impact course with the moon. As shown, this mode of abort is available continuously throughout the entire phase with either the SPS, DPS, or marginally with the Service Module RCS. Figure 24 shows the basic features of this abort mode. The first maneuver, usually a small one, is used to raise perilune or pericynthion altitude to an acceptable value. The second burn is then performed in the vicinity of perilune to return the spacecraft to earth in one of the seven sub-modes described previously. The most brobable need for this abort mode would be in the event of a badly executed second midcourse correction near the sphere of influence due to a G&N or an SPS failure.

Figure 25 shows a very significant characteristic f trans lunar coast abort trajertories, expecially as regards returns to a primary recovery site. As shown, the transfe angle from abort to reentry is relatively insensitive·t,ce time of abort or to the type of abort return. This angle varies from approximately 170° to 180° throughout the entire translunar coast phase. This fact, combined with the fact that the reentry ranging capability is also relatively fixed,