====The other abort mode possible for this phase is a DPS coplanar deorbit burn. This mode, as shown in figure 6, is available throughout the entire phase, but obviously requires transposition and docking in earth parking orbit.
Figure 8 shows the basic difference between this mode and the previously described one. Note that the deorbit maneuver is such that the spacecraft passes through apogee in order to provide enough time for the crew to transfer from the LM to the CSM prior to reentry. Also, the horizon cannot be easily monitored during the abort burn due to the spacecraft docked configuration. Thus, this attitude restrictionis deleted for this mode. As for the S-IVB mode, the use of this mode is very improbable if the Service Module RCS deorbit proves feasible due to the same type of recontact problems upon reentry as would be experienced with the S-IVB deorbit mode.
An alternate DPS abort mode not shown on the summary chart is currently being investigated. This new DPS mode would consist of using the DPS to lower perigee to very near the atmosphere so that the resulting spacecraft trajectory would then be within Service Module RCS capability to deorbit. In other words, the procedure would require maneuvers by both the DPS and the Service Module RCS. This procedure would eliminate recontact problems during reentry, since the LM would be jettisoned between the DPS burn and the Service Module RCS burn.
Summarizing for this phase, one can say that more than ade-quate abort capability exists from strictly a performance standpoint with essentially three independent propulsion systems capable of providing the abort required, continuously through the mission phase. However, the use of the S-IVB and DPS, as described in this section, would be very undesirable due to the recontact problems during reentry. If use of the Service Module RCS system to deorbit proves feasible, then the abort modes using the S-IVB or the DPS can essentially be eliminated from consideration.
Since the SPS mode is very similar to that planned for Projects Mercury, Gemini, and the early Apollo orbital flights, the detailed procedures and computer programs are already available and checked out for the ground computers. The SPS deorbit modes could be executed using either the onboard G&N system, the SCS system, or a strictly manual-type abort using visual attitude reference. The DPS abort mode would normally be executed using the onboard G&N system, although it could also be executed using the backup AGS system.
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