68-PA-T-54A.md

layout	date	from	serial	subject	tags
tindallgram	Mar 7 1968	PA/Chief, Apollo Data Priority Coordination	68-PA-T-54A	Sixth "C" Mission Rendezvous Mission Techniques Meeting	SPS PNGCS rendezvous

1. This March 1 meeting conflicted with the President's speech but a few of us dedicated jokers pressed on as follows.

2. It had been stated that all "C" mission SPS burns would be performed in a heads down attitude (that is, 180° roll). This presents a problem on one or two of the SPS burns in the rendezvous sequence--NCC1 and maybe NCC2--since to constrain ourselves in that way would make it impossible to do the final sextant/star burn attitude check. These burns are expected to be within 15° radial which makes heads up/heads down rather meaningless anyway, except for the FDAI 8-ball presentation. Phil Shaffer checked with Tom Stafford and got agreement that the attitude check was of more value than the standard 180° roll indication. Accordingly, it is our plan to make NCC1 and NCC2 (if it is downward) in a heads up attitude and include the sextant/star check in the sequence.

3. As reported in the last meeting's minutes it is our proposal that if a platform failure is detected just prior to NCC1, it will be necessary to delay the rendezvous exercise a day. This ruling does not necessarily apply to the PNGCS attitude tests prior to NCC2 and NSR since after NCC1 has been performed we are committed to the rendezvous exercise. Accordingly, if we can assume the GDC is aligned we probably should press on with the rendezvous using the SCS, at least through NCC2 and NSR.

4. Apparently, consideration is being given by someone to extending the launch window. In particular, it is apparently being proposed to launch earlier in the day. It appears to us that to launch prior to local noon would preclude making a platform alignment between NCC2 and NSR. This alignment is thought to be essential for terminal phase. Accordingly, we would like to request that very serious attention be given to this matter prior to choosing a launch time earlier than currently planned.

5. An item came up concerning real time selection of the elevation angle to be utilized in determining TPI time. As you recall, it is intended to utilize the elevation angle option in the TPI targeting processors such that if everything works properly TPI will occur when the line-of-sight to the target vehicle coincides with the maneuver thrust vector (spacecraft x-axis). According to Ed Lineberry, if dispersions in the trajectory are not taken into account in designating this elevation angle, the thrust direction at TPI will be substantially off the line-of-sight. The elevation angle adjustment should be less than ½°. Apparently, the RTCC/MCC is capable of determining the optimum value by means of manual iteration. Since the effect of this on crew TPI backup charts may be unacceptable, FCSD was given the action item of checking into that.

6. Another action item assigned the FCSD was to establish which was more important---lighting conditions during the braking maneuver or thrust vector coincident with the line-of-sight at TPI. If the lighting conditions are the more critical, it may be necessary to include a decision point in our operations to assure proper lighting at braking by not allowing the TPI time to slip more than some specified amount---probably about 10 minutes. If the TPI time based on the elevation angle option slips too much the crew would have to utilize the TPI "time option" for targeting. Obviously, the decision would have to made onboard the spacecraft after sextant data had been incorporated into the PNGCS.

7. There have been a number of comments regarding the TPI backup charts and their usefulness on the "C" and "D" missions. At the next meeting, currently scheduled at 1:00 p.m. on March 8, we will review this subject and try to establish the role of the PNGCS, MSFN and backup charts for the TPI maneuver. The primary questions to be answered are: shall there even be TPI backup charts, and if there are, should they or the MSFN computation for TPI be used in the event of PNGCS failure. It is evident that in either case the subsequent midcourse correction will have to be based on charts, since the MSFN has no capability for computing that maneuver.