See list belowMAR 12 196868-PA-T-57APA/Chief, Apollo Data Priority CoordinationSixth Midcourse Phase Mission Techniques meeting
1. The Midcourse Phase Data Priority meeting of March 6 was devoted to the earth orbital phase evaluation of guidance systems to make the translunar injection (TLI) Go/No Go decision. It was reported at the outset that a new set of ground rules have been established with regard to this subject. They are:
(a) A TLI maneuver will not be attempted if there is any indication that the S-IVB IU guidance system is not working properly.
(b) A properly operating CSM PNGCS is not mandatory for TLI. That is, it is acceptable to make a TLI maneuver with a failed CSM PNGCS if the subsequent alternate mission is considered more valuable than remaining in earth orbit.
Rationale for these ground rules is given in Memorandum No. 68-PA-T-56A (H. W. Tindall), dated March 7, 1968. The manner in which we may detect S-IVB IU failure is also presented in that memo and is partially reproduced in this one to make the rest of it more understandable.
2. There are two sources of S-IVB IU failure indication. The first is by the S-IVB's own failure detection system which indicates failures via telemetry. The second is by comparison with the CSM PNGCS and MSFN tracking. These comparisons, it must be emphasized, are extremely gross. That is, the S-iVB IU is designed to be at least an order of magnitude more precise than the CSM PNGCS and the MSFN. Thus, these monitoring systems— telemetry, CSM and MSFN—do not provide data to prove that the IU is performing normally but rather are only able to show us when it has degraded very badly—for example, 30 to 100 sigma! Whereas, MSFC's definition of a definitely and absolutely broken IU is anything beyond 3 sigma. Therefore, the actual limits we would select for TLI Go/No Go based on the S-IVB IU performance evaluation can only be the smallest, dependably, detectable failure. That is, we would use the smallest failure which we can confidently attribute to the S-IVB rather than the comparison system itself. Deviations in excess of that amount are certainly true S-IVB IU failures and would result in a No Go for TLI.
3. At this meeting we added another ground rule regarding guidance systems checks prior to TLI. The question was whether or not the sextant and scanning telescope are mandatory for TLI Go/No Go. If they were it would be necessary for the crew to check them out. However, analysis performed by MPAD has shown that LM PNGCS alignment with the AOT and undocked COAS alignment of the PNGCS and SCS can be accomplished with sufficient accuracy to ensure safe return to earth after TLI. Accordingly, as of this time we are proceeding under the assumption that checkout of the sextant and telescope need not be performed prior to TLI. If you do not agree with this decision, you should say so immediately.
4. By accepting these ground rules neither a platform alignment nor sextant check need be included in the timeline. Therefore, it should be possible to establish a monitoring technique which would permit performing TLI on the first opportunity even for an Atlantic injection (i.e., about 100 minutes after lift off). The technique would be to compare the CSM PNGCS and the S-IVB IU during the launch phase and perhaps in earth parking orbit. If this comparison is favorable, it can be assumed that both the S-IVB IU and the CSM PNGCS are performing well and we would execute TLI. If the comparison were not within those limits, one of the systems must have failed by our definition, but we may have insufficient knowledge to determine which one without performing a CSM PNGCS platform alignment in earth orbit. This would probably require going another revolution and, thus, TLI could not occur until the second opportunity. If the failure turns out to be in the IU, we would not perform TLI.
5. The Guidance and Performance Branch outlined their proposal for processing and displaying launch phase telemetry in the control center to evaluate S-IVB and spacecraft guidance systems performance as the prime TLI Go/No Go data source. They recommend plotting differences in the three components of velocity as determined from the PNCGS and S-IVB IU state vectors. These would be plotted in real time on strip chart recorders in the Mission Control Center. It is felt they would be extremely effective in not only detecting system failures but also for isolating exactly what type of failure has occurred. Limits would be established on three differences based on accuracy of the CSM PNCGS in accordance with the philosophy noted in paragraph 2. There is one big source of “error” in these plots resulting from our inability to align the CSM PNGCS accurately in azimuth on the launch pad. We had a similar problem, you recall, on Gemini and our solution here is about the same. It is proposed that a simple computation be performed similar to the platform alignment update carried out in the Gemini program at 100 seconds after lift off. This computation was based on the assumption that any difference in the horizontal velocity vectors detected when comparing the spacecraft to the S-IVB is due to gyro compassing misalignment of the spacecraft IMU on the launch pad. It is a simple way of determining what this misalignment is in order to improve the comparison by mathematically accounting for it in the plots during the remainder of the launch phase. In addition, it makes the magnitude of the misalignment available for use in later guidance systems performance evaluation tests inflight.
6. Flight Control Division and MPAD are jointly engaged in preparing RTCC program requirements based on this technique including display format and equation formulation. These program requirements should be in a form suitable for transmittal to FCD within 2 weeks, and negotiating will then begin to determine whether or not they can be included in the earlier manned Saturn 5 launches. They would be desirable on the “D” mission, possible mandatory for the “E”, and certainly mandatory for the “F” and “G”.
7. We next discussed the question of whether or not the launch phase systems evaluation for first opportunity TLI Go/No Go described above is sufficient all by itself. That is, platform drift checks based on telemetry gimbal angles from the SIV-B IU and the spacecraft PNGCS have been proposed as a supplement to the launch phase comparison, but there are obvious problems associated with that procedure which may make it rather useless—although I'm sure they will be monitored grossly. For example, the data is not time synchronized nor even homogeneous. As a result, it would be necessary for the spacecraft to maintain minute attitude rates over the measurement period which is almost impossible to achieve. Furthermore, structural bending and thermal warping also create very large errors—comparable to the differences we're looking for. MPAD was given the action item of determining if such a test contributes significantly to our confidence for TLI since if it doesn't we can simplify things a great deal by eliminating the whole procedure.
8. Finally, we investigate how we would determine which system had failed if the launch evaluation shown disagreement. It appears necessary to perform a platform alignment, or at least a determination of its orientation. This would probably force delay of TLI until the second opportunity. We are currently investigating the following approach:
(a) Evaluate the CSM platform torquing angles. The x and y axis should be near zero and the z axis should equal the pad misalignment as detected in the launch phase. If not within limits it has failed.
(b) Using MSFN tracking for orbit determination, compare the actual trajectory against the S-IVB IU insertion state vector trajectory. This requires 1½ revolutions on tracking. Disagreement beyond limits indicates S-IVB IU failure.
9. Obviously, we have a lot to do. But if the ground rules are accepted it is mostly a matter of implementing a technique we understand. Believe it or not, I think we've got this TLI thing pretty nearly licked. I hope so!
- Mar 07, 1968 – Guidance system oriented ground rules for TLI Go/No Go (12.6σ)
- Apr 10, 1969 – Descent monitoring at MCC (3.6σ)
- Apr 08, 1968 – Some lunar mission earth orbit phase ground rules (3.6σ)