Upthread: Sixth “C” Mission Rendezvous Mission Techniques meeting (Mar 07, 1968)
Downthread: Eighth and Ninth “C” Mission Rendezvous Mission Techniques meetings (Mar 27, 1968)
See list belowMAR 13 196868-PA-T-59APA/Chief, Apollo Data Priority CoordinationSeventh “C” Mission Rendezvous Mission Techniques meeting
1. Except for the first item listed below, the entire “C” Mission Rendezvous Mission Techniques meeting of March 8 was devoted to the terminal phase. Based on the very great importance FCOD is putting on proper lighting during the braking phase, a proposal is being considered for including another little tune up maneuver between NSR and TPI. I mention this here to make sure you don't overlook it since it is a rather significant item.
2. Considerable attention is being given by those responsible to lengthening the “C” mission launch window. Apparently, the constraints for the beginning and end of this window are almost solely associated with lighting, and MPAD is in the process of compiling all of these constraints on a single plot. It will probably be used as a basis of determining nominal lift off time and launch window duration as a function of launch date. One constraint, which should be included, is of particular interest to our “C” Mission Rendezvous Mission Techniques Panel. Namely, a period of darkness is mandatory between the NCC2 and the NSR maneuver in order to provide an opportunity to fine align the spacecraft platform in preparation for the sextant rendezvous navigation and actual execution of the terminal phase. Specifically, darkness must be available during the period from NCC2 plus 5 minutes until NCC2 plus 20 minutes. Of course, if in real time conditions prevent making this platform alignment the rendezvous would not necessarily be abandoned. The point is this constraint should be considered mandatory for launch but not mandatory for rendezvous. This constraint has been relayed to the MPAD mission engineer.
3. Now, on to the terminal phase. Our first discussion dealt with the use of the TPI crew charts. It is FCSD's desire to prepare and utilize TPI charts on the “C” mission rendezvous in very much the same was as they were used on Gemini. That is, they will develop procedures for the crew to obtain an onboard solution for TPI based on both the charts and closed loop PNGCS results. It is this integrated solution which would be compared to the MSFN for determining whether the onboard or ground solution should be utilized for TPI. The exact procedure for all of this will be the subject of further meetings. An important point to be made, however, is that onboard TPI charts do play a part in the “C” mission. This was questioned since apparently on the “D” rendezvous, there is some indication no TPI charts will be used in the command module since the one-man crew is too busy with Sundisk to work them.
4. As you know, it is intended that the TPI solutions always be based on a particular elevation angle of the target vehicle with respect to the local horizontal. The value to be used now and forever more on the “C” mission shall be 27.45°. The onboard charts have been developed in accordance with this and it is FCSD's desire that all future reference trajectories, mission planning, etc., use this same value. All other organizations have agreed to go along with this, however, Ed Lineberry's people (OMAB) are emphasizing the fact that using this value of elevation angle will nominally result in a situation where the TPI thrust vector will not be along the line of sight to the target. It is expected to deviate by about 6° from that alignment and everyone should clearly understand that at this time. Furthermore, some consideration has been given to actually modifying the value of elevation angle to be used in real time as noted in last week's minutes. It has been reported that on adjustment in the elevation angle in the order of ½°, to be determined at the beginning of the rendezvous exercise, would ensure that the thrust vector for TPI would have been along the line of sight. However, FCSD maintains this would foul up the charts and that they were more anxious to avoid that than to maintain that particular thrust alignment during the mission. Accordingly, no further consideration will be given to changing the elevation angle from 27.45°, either in advance of the mission or during it with the single, possible exception discussed next.
5. In response to last week's action item, FCSD reported that lighting conditions during braking have a higher priority than sticking to the 27.45° TPI elevation angle. The important point to be made here is that trajectory dispersions could cause the TPI time, based on the 27.45° elevation angle, to slip to such an extent that lighting at braking would be unsatisfactory. There are apparently two alternatives which can be considered to avoid ? from occurring. One is a real time adjustment in the TPI elevation angle if it becomes apparent that the TPI maneuver has slipped beyond acceptable limits and the other is a new proposal by Ed Lineberry that a small adjustment maneuver be made between NSR and TPI to return conditions to nominal at both TPI and braking. The rest of this memorandum is devoted to these two alternatives.
(a) Alternate One. Immediately after the NSR maneuver the crew takes about 10 sextant observations of the S-IVB to update its state vector, and then calls up the TPI targeting program (P-34) to obtain the TPI time. It is anticipated that this onboard solution for TPI time based on when the CSM will arrive at the position giving the desired elevation angle should be quite accurate. That is, all previous computations were based on ground determined state vectors which have a relatively large error in determination of TPI time. The onboard determination using sextant observations should be an order of magnitude more accurate. Accordingly, at this time in the operation we would have our first accurate indication of TPI time. The crew would compare this TPI time with the ground relayed ???????? of darkness ??????? determine if it has slipped excessively such that lighting ????? ??????????? maneuver ???????? unacceptable. In the event the slip is excessive the crew would terminate and recall P-34 utilizing the “time option” to determine the elevation angle consistent with an acceptable TPI time. Since it is necessary to utilize the “elevation angle option” in P-34 to permit a comparison of the onboard and ground solutions, the crew would have to terminate and recall P-34 using the elevation angle just determined as input. They would also have to relay the value of elevation angle to the ground for their computations. Obviously, the thrusting would not be along the line of sight for would TPI occur at 27.45°, but proper lighting conditions at braking would be assured.
(b) Alternate Two starts out the same way as One. That is, the crew updates the S-IVB state vector using the rendezvous navigation system and determines the TPI time and its slippage. Then, using this value of slippage they should be able to make a simple computation to determine a maneuver to adjust the TPI back to nominal time while retaining the nominal elevation angle. This maneuver would be horizontal and inplane to be made 30 seconds after NSR, probably using the Average G program (P-4?). The Orbital Mission Analysis Branch (formerly Rendezvous Analysis Branch) was given the action item of determining the computational technique or chart giving maneuver magnitude vs. desired change in TPI time. Since it is expected to be in the order of 1/3 fps for each minute TPI time change desired, and since we are talking about TPI time adjustments less than 15 minutes, the maneuver should be less than 5 fps. FCSD was asked to work out a detailed timeline which includes this maneuver to see if it presents any problems. One thing we are particularly interested is in whether it should be done as a standard procedure regardless of whether TPI time slippage was acceptable or not, or if it should only be done if some limit has been exceeded. Assuming the procedure is not too complex, my personal preference would be to make it a standard procedure. Phil Shaffer expects to spend some time next week with the “C” crew at the Florida AMB and will discuss this with them at that time.
6. Consideration has been given to having the crew call up the TPI targeting program (P-34) immediately after NSR in order to determine TPI time in order to set the spacecraft clocks. Currently the consensus is that this is not a useful operation and it should be dropped from the timeline.
7. As you can see, we are really getting into the fine detail on the “C” rendezvous and I predict that if we spend the next two or three sessions going through the mission techniques flow charts, we will be ready to call in the rest of the world and we could then ice this whole thing down within the next couple of months. Right now I expect the next meeting will be devoted to the review of the flow charts.