Upthread: Lunar Mission Reentry Mission Techniques meeting (Feb 23, 1968)
Downthread: Lunar Reentry Mission Techniques meeting – March 21 (Apr 04, 1968)
See list belowMAR 13 196868-PA-T-60APA/Chief, Apollo Data Priority CoordinationLunar Reentry Mission Techniques meeting – March 7
1. On March 7 we had a Data Priority Mission Techniques meeting on lunar reentry. This was the first on this mission phase with contractor partici- pation. Our objective was to understand the current status of the business and to begin pinning down the operational procedures to be used onboard the spacecraft and on the ground. We were particularly interested in data flow, decision points and logic, and the actual detailed techniques to be used during this phase of the mission. Although we intended for it to start just prior to the final (third) midcourse correction on the way back from the moon, it turned out the discussion unavoidably included activities earlier in the flight, starting with the Transearth Injection (TEI) maneuver itself. Generally speaking, I would say this mission phase is better understood and more completely developed than any other in the lunar mission. A reasonable set of mission techniques is more or less in hand right now. Of course, there is no question that significant changes will be made based on further analysis and actual flight experience.
Paragraphs 2 through 5 deal with ‾‾‾‾‾‾the midcourse correction maneuvers‾‾‾‾‾‾
2. Jerry Yencharis (MPAD) briefly discussed the second midcourse correction maneuver (MCC2). It is a maneuver to be made entirely in-plane designed to achieve specific entry interface conditions consistent with a safe reentry and controlled landing point. Analysis summarized in Figure 1 has shown that this maneuver can be made efficiently anytime in the period between 15 and 25 hours before entry, and so it should probably be scheduled to fit the crew work/rest cycle. However, some consideration is being given to rescheduling it in real time based on its magnitude. Obviously, both the nominal time and real time decision logic must be worked out before that (i.e., now). One question to be resolved involves basic “small maneuver” philosophy. Specific- ally, should maneuvers of a magnitude less than the targeting uncertainty be made? We have generally said that they would be so that dispersions would be equally distributed plus and minus. This, however, is not the currently proposed technique for these midcourse corrections, and deserves further examination. It is clear that if this maneuver is made we'll use the External Delta V guidance mode with the SPS engine (if it is in excess of 8 fps). And it will be targeted from the ground. Platform orientation can be determined either onboard or on the ground; this appears to be pretty much a crew preference, and we'll be interested in their decision.
3. A third midcourse correction (MCC3) is scheduled in the timeline 2 hours prior to entry. The real time decision as to whether or not this maneuver need be made is carried out as follows: The desired flight path angle at the entry interface is compared to the predicted value assuming no MCC3. Only if the difference in these two exceeds .36° will the maneuver be executed. This limit has been selected to insure a safe reentry but is large enough to make the need for this maneuver extremely small. For example, a 200 sample Monte Carlo study was made, and in no case was the MCC3 required. In fact, the largest flight path angle difference was only about .25° (see Figure 1). It has been established that this maneuver will be entirely inplane, targeted from the ground to achieve the desired flight path angle and will utilize the External Delta V guidance mode. Of course, the inertial platform must be aligned prior to this maneuver. Its orientation will not be constrained to provide any particular pitch attitude display on the FDAI 8-ball during the burn. Of course, the ORDEAL could be used to give all zeros on the 8-ball. The actual REFSMMAT to be used during the MCC3 and reentry will be computed and relayed to the crew from the ground to provide 0, 0, 0 on the ball at 400,000 feet altitude when the spacecraft is in a heads down, in-plane, horizontal, wings level attitude, heat shield forward.
4. It has also been established that preparations for all maneuvers are begun 2 hours and 40 minutes before time of ignition to allow sufficient time to activate the systems from a standby state, to get all of the initialization data input into the system and to make all of the various checks to develop confidence that the burn will be made properly. It was also decided that the same timeline for bringing up the system, aligning the platform, etc., would be utilized regardless of whether the MCC3 maneuver is made or not. FCSD people involved in crew timeline development took the action item of making sure this is an acceptable approach.
5. Although major emphasis at this meeting was devoted to nominal reentry procedures with all systems working properly, we did depart long enough to discuss briefly current plans for handling communications failure occurring at about the time of the second midcourse correction or later. Specifically, it was stated that if the ground has transmitted to the spacecraft its MCC2 state vector and targeting command load prior to communications failure, there should be no attempt made onboard the spacecraft to perform onboard navigation using the sextant. The point is that onboard navigation can foul up the state vector and some of us intuitively feel it better to stick with the last set sent from the ground for entry if it is that current. Various people did not agree with this rule, of course, and so an action item was promptly levied upon them to determine a superior alternate approach in detail. In the meantime, we will continue on as described above.
Paragraphs 6 through 12 deal with ‾‾‾‾‾‾ entry preparation ‾‾‾‾‾‾
6. At present the reentry guidance philosophy includes two planned landing areas (PLA) which are illustrated in Figure 2. (All figures attached are courtesy of MPAD's Lunar Mission Analysis Branch.) PLA 1 is a thousand mile band including the primary landing point and giving the capability of bad weather avoidance. In the event of PNGCS failure a shorter range landing point, PLA 2, is designated consistent with a no skip, constant g reentry which is the planned backup reentry mode. Efforts are being made to determine if the PLA 1 range can be made to include PLA 2 with current PNGCS hardware and software implementation. If so, it is probable PLA 2 would be selected as the primary recovery area in order to make PNGCS, EMS and backup techniques all compatible.
7. With regard to the constant g reentry, the MPAD reentry people have the action item of preparing and delivering updated constant g reentry load factor profiles to FCOD for their evaluation and, hopefully, buy off. We anticipate no problem on this. Typically, they are a 4 g reentry with a 4 minute duration or a 3 g reentry with a 5 minute duration, sometimes preceded by a high acceleration, short duration spike (See Figure 3).
8. It was established that as long as communications exist with the ground, MSFN data will be used for EMS initialization. This activity will be scheduled at some convenient time, probably an hour or so before entry, since it is not time critical. Although the PNGCS computer is programmed to provide this data, there is no need to pay any attention to it unless communications prevent receipt of the ground update.
9. Command module/service module separation will be carried out using manual attitude control and will occur approximately 15 minutes before EI. It was stated that the Descent program (P-61) will be called up approximately 2 minutes prior to that event. This will enable the PNGCS to accept accelero- meter inputs making it aware of any small spacecraft translations due to separation itself and/or due to subsequent attitude control. (Recall command module attitude control is not done with balanced couples.) Since accelero- meter bias could accumulate over a period of time as a significant contributor to missing the landing point, we spent some time discussing the question of whether or not allowing the guidance system to accept accelerometer input for 20 or 30 minutes prior to entry interface is acceptable. According to recent analysis (summarized in Figure 4), down range miss distance due to a 3 sigma accelerometer bias (calibrated inflight) would be about 10 miles, and cross range would be about half that much, even if Average G is enabled by the Descent program (P-61) 30 minutes prior to entry. Some consideration is apparently being given to adding an accelerometer threshold limit into the computer program to avoid this small error. Since this worst case error is really quite acceptable, I would oppose any such program change which I assume would only be made after approval of a formal program change request.
10. Claude Graves' people presented some data to chow the magnitude of landing point miss due to platform misalignment, the major contributor (see Figure 5). He showed that with 3 sigma gyros the miss distance was nearly linear at the rate of about .6 of a mile down range and 3 miles cross range for each hour spent between the last platform alignment and the entry interface. Since a 3 or 4 hour period of drift would only result in about 12 miles miss at the worst, we felt it unnecessary to make any further platform alignments after the third midcourse correction.
11. Some thought was given to making a spacecraft attitude check using the sextant prior to reentry; however, it was concluded that this really accomplishes very little. Confidence has been developed in the PNGCS prior to the MCC3 maneuver and so we would only be uncovering failure subsequent to that. Furthermore, there are a whole series of PNGCS performance evaluation tests associated with the reentry itself made before committing to the PNGCS and there is nothing that could be done to fix the system if it has failed in that short time. All of which says, the test is useless. Accordingly, although FCSD has not completed the detailed timeline yet, as of now there is no known reason for the crew to leave their couches after MCC3.
12. We had a lengthy discussion with regard to initialization and use of the EMS roll stability indicator (RSI), also known as the roll attitude indicator and lift vector indicator. Apparently, this device is merely a repeater from the FDAI roll bug driven by the GDC. It was originally included in the EMS when there was only one FDAI in the spacecraft. However, now that there are two FDAI's its purpose and value are rather nebulous. Actually, the discussion took a surprising turn. We started out trying to figure out how to initialize the damn thing and after much emotional, confused talk we seemed to arrive at the conclusion that it really has very little value. Mike Collins intends to obtain a crew position on this, and Clyde Paulk was requested to pulse G&C on the same subject. The thing that bugged several of us is that we shouldn't have something displaying wrong information in the cockpit, and so we should either cover it up with masking tape or else we should line it up properly, no matter how useless it is. The problem is that the way the PNGCS controls attitude is not consistent with the RSI alignment procedure. Therefore, it requires the crew to control spacecraft attitude manually until .05 g. Actually, I am not so sure if that ought not to be the procedure anyway, in order to utilize the horizon as an independent check that the spacecraft is in proper pitch trim attitude to insure aerodynamic capture of the spacecraft in the proper attitude. Left unresolved was whether we should submit a program change request to make the Colossus lunar return reentry program compatible with that procedure.
Paragraphs 13 through 19 deal with ‾‾‾‾‾‾ entry proper ‾‾‾‾‾‾
13. The remainder of this meeting dealt with reentry procedure based on Figures 6 and 7 which are attached to this memorandum. Generally speaking, these procedures for monitoring a nominal reentry and carrying out a backup reentry seem to be well thought out and complete. Obviously, there are still a number of relatively minor refinements or changes which have to be made. Some of these are the items reported in the following paragraphs.
14. Probably the most important decision to be made during reentry occurs when the reentry program changes from P-64 to P-65 which occurs just about at the time of peak g's. At this time, a display of predicted exit velocity and drag level (VL and DL) appears on the DSKY. The crew must determine if these values are within limits determined by the ground and relayed to the crew as part of the standard entry preparation procedure. If they are within bounds, the crew commits to the PNGCS. If they are outside, the PNGCS has failed and the crew takes over and flies constant g reentry to PLA2. An important point to be made here is that the primary PNGCS Go/No Go check is based on a comparison with the ground and that this is considered absolute! Of course, the crew does monitor the EMS for scroll line violation which also could result in abandoning the PNGCS, but that is not a comparison of one system against the other for performance evaluation. The criteria on which this test is based is expected to be tied to the accuracy with which the ground is able to predict these parameters as opposed to being selected to establish such things as 3 sigma PNGCS performance, assurance of landing within some specified distance of the recovery force, or assuring reentry itself—although it better do at least that! Graves' people are in the process of determining values for these limits and then we will know what sort of reentry may be assured. They expect this work to be completed at least six months prior to the “E” mission.
15. It was noted in this discussion that a second set of DSKY display parameters are available in P-65 by a crew input of “proceed” to the computer. It is evident that the crew is not likely to perform that operation while experiencing 5 g's, so Graves was given the action item of determining whether these display parameters (inertial velocity and altitude rate) are of any real use to the crew. If they are, it will be necessary to submit a Colossus program change request to make their appearance automatic probably after display of VL and DL for a fixed length of time.
16. Another PCR Graves intends to submit for Colossus No. 2 would make PNGCS control of attitude be lift vector up until .2 g's during “second entry” following a skip. This is felt to be mandatory since a pitch trim attitude check on the horizon is critically needed at this time. At present the computer program will drive the spacecraft attitude to whatever bank angle is consistent with the reentry guidance objectives even though prior to .2 g's the aerodynamic forces contribute very little to landing point control.
17. Graves' people were requested to examine the EMS scroll lines to make sure no EMS line violation during the second entry would cause the crew to take over from a perfectly operating PNGCS. That is, we want to make certain that sufficient margin is provided to prevent this from happening.
18. Both MPAD and G&C were requested to develop some sort of tests to be included in the reentry procedure to determine if the EMS is performing properly. NR will probably do some work on this, too. The point is, it was apparent from our discussion that all performance evaluation was centered on examination of the PNGCS with switchover to the EMS in the event of its failure. What seemed to be missing was performance evaluation tests of some sort to make sure the EMS, was working well enough to be used.
19. Based on this day's discussion TRW will prepare a mission techniques flow diagram to start the review cycle on this mission phase. After a couple of internal MSC meetings, I expect we will again call in MIT and NR and see if we can't put this business on ice.