Downthread: “C” Mission Retrofire and Reentry Mission Techniques meeting (Jul 23, 1968)
See list belowJAN 22 196868-PA-T-15APA/Chief, Apollo Data Priority Coordination“Final” review of Spacecraft (Mission “C”) Retrofire and Reentry Mission Techniques
1. On January 10 we had a “final” review of Spacecraft 101 (Mission “C”) Retrofire and Reentry Mission Techniques as documented in the logical flow charts prepared by TRW based on a series of meetings we have had here at MSC. These flow charts, which show the operations starting several hours prior to the retrofire maneuver itself, had been distributed to all interested organizations, but this was the first time MIT, North American and others had an opportunity to thoroughly understand the rationale and to provide their comments. The charts are now being updated based on our new agreements and will be distributed within the next several weeks.
2. Although we were addressing ourselves specifically to the Spacecraft 101 flight, we are all in hopes that the techniques developed for that mission can be used on all subsequent earth orbital missions. Crews for Missions “C”, “D”, and “E” were in attendance and it looks like we may have accomplished that objective.
3. Probably the most controversial item of all dealt with the manner in which the Entry Monitoring System (EMS) is to be used. It was stated that if it is recognized that the primary guidance system (PNGCS) is not working for some reason in sufficient time prior to retrofire to respond, we would bade our retrofire maneuver on a ballistic reentry and would not attempt to use closed loop reentry guidance for ranging control. If it is recognized that the PNGCS has failed in the last moments prior to retrofire, following retrofire or during reentry itself we are obviously committed to a lifting reentry if we hope to reach the desired landing point. It is our current plan to utilize the essentially open loop bank/reverse bank backup technique similar to that employed for Gemini. The point to be made is that in both cases the EMS is only being operated as a systems test and is not utilized in the actual control of the reentry. Some parties contested these decisions, in particular North American, and it is recognized that a number of things are going on at present which may make it desirable to reverse the second one regarding use of the EMS during a backup mode lifting reentry. Specifically, analyses are being carried out in a number of places to improve our knowledge of the EMS capability and to verify its readiness for flight. In addition, simulated reentries both with and without the flight crews themselves are schedules and will have an influence on the way we ultimately go. And finally, G&C Division has been requested to state positively their position with regard to flight readiness of the EMS. But as of now, we aren't using it! It should be emphasized, however, that our current plans for reentry at lunar reentry velocities definitely involve use of the EMS is a vital role and confidence must be developed in that system partially from these earth orbital development flights.
4. It had been states that if the decision to retrofire is within 2 hours and 40 minutes of the time it is intended to do so we would not utilize the PNGCS. It is evident that there could be times wherein that system could be used in the unlikely event it was already in use and well aligned at the time of an immediate reentry decision. Accordingly, additional logic will be included in the flow to indicate the various conditions under which we would use the PNGCS, without a full 2 hours and 40 minutes for its preparation.
5. There are two ways of performing a ballistic reentry. One is to continuously roll a spacecraft as we did on Mercury. The other is to dissipate the lifting capability by banking the spacecraft 90° to one side and then 90° to the other causing the spacecraft to move in an out-of-plane direction. It was established that the primary mode for flying the ballistic reentry would be the latter based on a definite crew preference for that technique. The Control Center is prepared to handle either way including computation of the best bank angle to use taking into account cg offset in order to align the “lift” vector horizon- tally. Furthermore, if the recovery forces are known to be to the north or south of the ground track, the time at which the spacecraft reverses its roll attitude will be determined to minimize that miss distance.
6. It was determined that under no circumstances would we modify the retrofire maneuver targeting or state vectors in the spacecraft computer when the time to that maneuver is less than 30 minutes.
7. We decided to modify the final check of the PNGCS from that shown in the proposed flow. Specifically, it had been suggested that the spacecraft be pitched down from its inertial retrofire attitude 20° at 5 minutes before retrofire ignition time to verify that the horizon coincided with the window marking. If it did not, it was proposed to declare the PNGCS NO GO and use the SCS instead. In order to avoid the obvious undesirable change in spacecraft pitch attitude at this critical time, this was changed to utilize the COAS for the horizon check rather than the line on the spacecraft window. It was noted that with the spacecraft in retrofire attitude the horizon should pass through the middle of the COAS at approximately 7¼ minutes before retrofire providing just as accurate a check, earlier and without changing spacecraft attitude. It was established that the horizon check with the COAS should be to within 3° at the precise time the horizon should be centered in the COAS in order to commit the use of the PNGCS.
8. It was reported that during the retrofire maneuver 1σ dispersions in the guidance and propulsion system result in 2° pitch and yaw excursions during the maneuver and Delta V residuals of 6 fps. Rather large.
9. In preparing this flow we had convinced ourselves that the orbit rate ball device (ORDEAL) should not be used. The crews feel differently and so we are in the process of including initialization functions etc., in accordance with its use.
10. At present it is planned that the reentry will be flown with the crew actually controlling the spacecraft attitude manually in accordance with the PNGCS recommended bank angle just as they did on most of the Gemini flights. Those responsible for the reentry Digital Autopilot (DAP) maintain that it will not be possible to evaluate the performance of the DAP if the spacecraft is flown in this way. Since this mode has been agreed to by the Program Manager, those organizations who are concerned (e.g., G&C, MIT, MPAD, etc.) were advised to prepare their case and submit it in writing to the Program Manager.
11. There was a question as to what should be done about starting the EMS if it does not start automatically as it is supposed to. There are some who say to manually start it at a computed time, and there are others who say not to do that. Dave Heath (MPAD) was given the action item of determining what the mission requirements imply and North American was requested to present their position on this matter.
12. Items recorded above are obviously just highlights of the meeting – that's primarily significant things which were changed in the flow as it was presented and those things on which controversy still exists. Some of the decisions may be changed, of course, but as of now I think everything is covered pretty well and everyone can understand exactly how we are going to do this mission phase unless changes are approved. Once the comments noted have been incorporated, it is my intention to present this to the MSC management for their information, criticism and final approval.