Page:LunarLandingMIssionSymposium1966 1978075303.pdf/359

 The first abort mode listed is a direct abort available with either the SPS or LM DPS propulsion systems. The basic features of this abort mode are shown sketched in figure 20. This mode consists of a single unconstrained attitude abort burn which returns the spacecraft directly to reentry without circumnavigating the moon. Shown sketched, are the two extreme types of returns possible in this mode--time critical and fuel critical. A fuel critical return usually passes through apogee following the abort maneuver; whereas, a time critical or a fast return usually does not pass through apogee following the abort maneuver. All of the seven sub-modes described previously for a translunar injection phase are available for this mode as well as for all of the other modes for this phase. If the SPS is available, this mode of abort produces the fastest possible returns to the earth for aborts performed prior to reaching approximately the lunar sphere of influence. This is the reason why this particular mode is considered prime for approximately the first three-fourths of the coast period from the earth to the moon.

The DPS direct abort mode, as shown in the summary chart, is available as a backup mode to the SPS direct mode for approximately the first two-thirds of the coast period from the earth to the moon. Use of this mode during the latter portion of the translunar coast phase would result in excessively long return times. Thus, the use of this mode as a backup during this latter period is not considered as a possibility. As will be shown in more detail later, the LM DPS direct abort mode, when used in the event of an SPS failure, will result in the fastest possible return time only if the abort maneuver is performed during approximately the first 25 hours of translunar coast.

The next abort mode listed in the summary chart consists of delaying the abort maneuver until the vicinity of pericynthion or perilune is reached, as shown sketched in figure 21. This mode of abort is available continuously throughout the entire translunar coast phase with either the SPS, DPS, or marginally with the Service Module RCS system. If the SPS is available, this mode of abort will generally result in the fastest possible return to the earth for abort decisions made near the sphere of influence or thereafter. For this reason, it is shown in the summary chart as the prime mode of abort for this portion of the translunar phase. In the event of an SPS failure, the LM DPS propulsion system used in this mode will result in the fastest possible return time for abort decisions made approximately 25 hours or later out along the translunar coast trajectory. In the event of an SPS failure and failure to obtain the LM DPS propulsion systems, the Service Module RCS would be marginally available for this mode of abort due to the fact that the nominal translunar coast is a free-return trajectory.