See list attachedApril 11, 196969-PA-T-59APA/Chief, Apollo Data Priority CoordinationF Rendezvous Mission Techniques Clean-up
On April 5 we had what I expect is the last of the F Mission Rendezvous Techniques meetings. We resolved a number of open items which had not been covered before, or which popped up during simulations. This memo is to list them for the record. Some are trivial, some are really quite significant.
1. Since the first planned DPS maneuver is DOI, it was agreed that the gimbal angles included in the LGC erasable memory load should be right for that maneuver. These values should also be included in the crew check list (Stan Mann please respond).
2. The LM attitude and attitude rate limits for the DPS burns are 5° and 5°/sec unstaged and for the APS are 10° and l0°/sec.
3. We agreed upon the following course of action regarding imperfect DOI maneuvers.
a. First of all, only the x-axis residual shall be trimmed. The y and z-axes residuals shall be left untrimmed since they do not bother anything and trimming wastes RCS propellant and can result in excessive plume impingement.
b. Underburns – Underburns less than 5 fps will be trimmed up to 7 seconds plus x RCS burn duration, which is a plume impingement constraint. (Note: that's only 3 fps of trimming and thus can leave a small residual which will force retargeting the later burns) Under- burns in excess of 5 fps will not be trimmed and will result in a “PDI abort.” (A PDI abort, you recall, involves making a maneuver at about PDI time yielding CSI one-half rev later. In other words, it eliminates the rev in the nominal mission between phasing and insertion. The PDI abort will be made with DPS if it is considered an operating system, otherwise with the APS.)
c. Overburns – Overburns less than 12 fps will be trimmed with minus x RCS. Again, this limit is based on a RCS plume impingement constraint. It should never occur since this is about a 4 second over- burn which could have been manually stopped before reaching this value. Overburns greater than 12 fps result in lunar impact and therefore call for a direct return of the LM to the command module by the immediate, brute force technique discussed in previous memos.
d. I guess it goes without saying that any PGNCS failure during DOI also dictates a direct return abort.
4. The following agreements were reached regarding the phasing maneuver:
a. It was emphasized that at least 40 fps should be achieved by the LM somehow if at all possible.
b. Underburns – Trim underburns less than 5 fps with plus x RCS up until the 7 second plume impingement limit. If the underburn is greater than 5 fps, but less than 25 fps, stage and complete the maneuver with RCS. If the underburn is in excess of 25 fps, stage and complete the burn with APS using the AGS.
c. Overburns – Trim overburns less than 12 fps with minus x RCS. For overburns in excess of that, trim out 12 fps and standby for an update of the Insertion targeting.
a. Underburns – If the total velocity gained is less than 45 fps, take it out using minus x RCS. This limit is based on the 30 second minus x RCS plume impingement constraint. In this event, the CSM does the insertion burn three minutes later. If the underburn is less than 80 fps, use the plus x RCS to complete the maneuver. (This limit is based on the 55 second RCS plume impingement constraint.) For the approximate 100 fps band of cutoff velocities in between these two limits, the LM should do nothing immediately and the command module will have to rescue.
b. Overburns must be removed somehow to avoid lunar impact.
6. It has been said repeatedly before, and I say again here today, that there is no such thing as a 200 n.mi. range limit on the VHF ranging by the CSM. That is merely a fictitious design value which has no bearing on how the operation should be conducted. VHF ranging should be used to its full 327 n.mi. recycle limit provided the data is good. The ΔR ΔV limits, which the CMP should use to decide if it's good or not are currently set at 0.5 n.mi. and 3 fps. (These values may be changed this week following a rendezvous navigation meeting of the experts.) It was agreed that the CMP could do P20 rendezvous navigation, updating the LM state vector in the CMC, between DOI and phasing, if this does not conflict with other more urgent activity.
7. The TPI window has been established as being from minus 8 minutes to plus infinity. The nominal TPI location is at the time the target vehicle is 23 minutes before sunrise. The significance of the window is that if after CSI it is discovered that the TPI associated with the elevation angle option has slipped earlier than 8 minutes, the crew will recycle the TPI program (P34) using the time option with nominal TPI minus 8 minutes on the input time.
8. The CSM always uses the LM computed CDH time for input to P33 as long as the LM PGNCS is assumed to be working okay.
9. It was agreed that all CSM mirror image targeting (that is, for CSI, CDH, and TPI) shall use the same TIG as the LM. That is, mirror image targeting will not be delayed one minute or three minutes as had previously been considered. This technique considerably sim- plifies procedures and results in (minor) difficulty only if the LM failure, which forces the CSM to become active, becomes apparent when the LM attempts to make the maneuver. Such a last instant failure on an RCS burn is considered very unlikely and does not result in too bad a situation if the command module then executes the maneuver late.
One of the simplifications obtained by eliminating TIG delays is the elimination of all biases that need to be applied to the CSM solu- tions for use in the LM with one exception. It is necessary to subtract 1 fps from the CSM CSI (P32) solution when the LM uses it for comparison with their own solutions or for execution.
10. Comparison limits were established for evaluating the acceptability of the various rendezvous maneuver solutions. In each ease, it is most desirable to use the LGC if possible. Accordingly, it will be used if it compares favorably with either the CSM or the LM chart solution. If it fails, the LM chart is compared with the CSM solution and is used if acceptable. If both the LGC and chart solutions fail their test, it is recommended that the LM execute the maneuver computed by the CSM since a rendezvous radar failure is the most likely cause of trouble. The comparison limits are 2 fps, 5 fps, and 6 fps for x, y, and z-axes, respectively, in both local vertical and in line-of-sight coordinates.
This comparison technique shall be used for the CDH and TPI burns for sure. It may also be possible to use it for CSI, provided analyses between now and the flight show that the CSM will have an acceptable performance. Since it is not certain that the CSM will shape up, we have adopted the following weird technique which should be used for CSI unless the CSM is eventually certified to be okay. It is based on use of three possible solutions – the PGNCS, the LM chart, and the pre-separation canned burns. It is also based on a desire to insure too large a CSI burn, if anything, in order to avoid having TPI slip early, which is considered a serious dispersion, as noted in paragraph 7 above. The rule is that the LM crew should execute the latest of these solutions, provided it is no more than 2 fps bigger than the next-to-largest solution! If the rendezvous radar has failed, it wipes out both the PGNCS and chart solution, the LM crew uses the same compari- son scheme, only in this event it is a comparison of only two sources – the pre-separation canned burn against the CSM CSI solution after it has been biased 1 fps as noted in paragraph 9.
11. There were at least two situations in which it seems desirable for the CSM and LM to share the braking task and it was agreed that they would do so if either occurs. If the LM fails to stage the DPS or if the LM is not able to visually acquire the CSM during braking, lateral line-of-sight control by the LM is not practical and the CSM shall do it. The LM will continue to be responsible for performing the actual braking maneuver provided the rendezvous radar is working.
And that's how we spent Saturday.