See list attachedNovember 21, 196868-PA-T-256APA/Chief, Apollo Data Priority CoordinationC’ entry initialization with no communication
Bob Weber, Jon Harpold, and some of their friends got together to work out the procedure for the crew to determine the landing point for targeting the GNCS entry program (P61) in the event of a no communica- tion/return to earth. The procedure is essentially as we laid it out during one of our big meetings a couple of weeks ago with a few minor embellishments. The procedure is as follows:
1. The crew will use the Return-to-Earth program (P37) to obtain the predicted landing point longitude and inertial velocity at the entry interface.
2. Using a chart, they will determine a longitude bias based on the inertial velocity. For lunar return velocities this bias will be approximately three degrees. Since P37 always plans for an entry range which is too long – that is, too far to the east – this bias must be applied such as to result in a more westerly landing point. The resultant longitude is the one we're looking for.
3. Using the Ground Track Determination program (P21) the crew will iterate to determine the latitude of the point on the ground track which is compatible with the biased longitude. This is necessary in order to avoid having the GNCS attempt to reach a target point substantially off the ground track and perhaps even outside the available footprint.
4. The values of the latitude and longitude thus obtained are com- pared to the last Block data values received from the ground prior to communication loss. If the values of latitude and longitude in the Block data are both within one-half degree of those determined by the process outlined above, the Block data values will be used. If either differs by more than one-half degree, the P37/21 values will be used to target P61.
5. The Range-to-Go display obtained in P61 after input of this target point shall be checked to make sure the GNCS will be attempting to fly a reasonable trajectory. Specifically if the Range-to-Go display used for initialization of the EMS is less than 1350 n.m., the G&N shall be used. If the displayed value is larger than 1350 n.m., the crew should assume something is wrong with the G&N and should manually fly a constant 4 g entry.
Rationale is selecting the above procedure is primarily directed toward providing a safe entry by making all G&C systems available for the crew's use. That is, we want to use the G&N provided it can be initialized properly. Furthermore, we want it to be operated nominally such that the EMS monitoring techniques are standard. To accomplish this it was most important to select a landing point assuring a nominal entry range regard- less of where the landing point turns out to be. Of course, if the crew navigation and maneuvers subsequent to communication loss are done properly, the landing point should be predictable and acceptable. Accordingly, the pro- cedures were set up, not to land in any particular place, but rather to provide approximately a 1350 n.m. entry range. By attempting to make the chart as simple as possible consistant with that, we ended up with a linear longitude bias as a function of inertial velocity. Specifically, the chart is a straight line between zero bias at 34,000 fps to three degrees bias at 36,300 fps. It is recognized that we are not taking into account the affects of heading angle and latitude at entry, both of which can affect the entry range somewhat. However, it did not seem to us that the extra complexity in compensating for them was justified. This chart will be relayed to FCSD by Harpold via Mike Collins.