Upthread: No special visual reference will be provided for the final transearth midcourse correction maneuver (May 10, 1968)
Downthread: Transearth midcourse maneuvers are getting easier and cheaper all the time (Jun 18, 1968)
See list belowJUN 3 196868-PA-T-111APA/Chief, Apollo Data Priority CoordinationTransearth midcourse correction philosophy – a major operation break through!
1. In trying to establish mission techniques for the midcourse correction maneuvers on the way back from the moon, we reached a point beyond which we could not progress without first establishing some sort of maneuver philosophy, like:
a. Should the MCC's be carried out on a fixed or real time selected schedule?
b. Which propulsion system should be used?
c. Are these things dependent upon propellent available?
d. In fact, what should we be trying to accomplish with the MCC's?
Accordingly, on May 17, Ron Berry, Aaron Cohen, Harry Byington, Jon Harpold, Stan Mann, Harold Granger, and I got together to see if we could find some logical way to handle these maneuvers. Since then I've talked to others who agree with what we came out with. Personally I think it's just great and I hope you do too. I assume you'll let me know if it makes you unhappy.
2. In summary there are only two things to be accomplished through the use of the transearth midcourse corrections (MCC). The first and most important is to guide the spacecraft into the entry corridor. The second is to help to control the location of the landing point on the earth's surface. We quickly concluded that the latter is unnecessary after the first MCC. If we want the recovery force in the center of the reentry footprint, move the ships there rather than making spacecraft maneuvers to adjust location of the footprint. So, that leaves corridor control as the only MCC objective. We feel that the best way to do this is to make as many as eight small RCS burns whenever their need becomes apparent. It is our estimate that they would occur no more often than every 10 or 12 hours, would be less than 1 fps each and would be made using the SCS control system. Thus, the total transearth MCC cost should not exceed about 8 fps (aside from alignment and altitude requirements) and we would never use the SPS or the G&N on the way back except in some low probability contingency situation. Therefore, this procedure would really provide a minimum delta V return and would be consistent with a non-G&N situation which simplifies the decision logic and standardizes procedures. That is, we would use the same techniques regardless of the status of the propul- sion systems, the G&N, and/or the amount of propellants remaining. How could anyone ask for anything more than that!
3. The rest of this memo just gives the rationale and some interesting comments for the record. If you're busy, you should stop here.
4. Landing Point Control
The first midcourse correction has always been scheduled at about five or six hours after the Transearth Injection (TEI) maneuver which is near the sphere-of-influence of the moon. This maneuver is primarily to correct whatever dispersions have occurred during TEI but will unquestionably be primarily for landing point control. This is due to the fact that the MSFN is able to determine the spacecraft trajectory characteristics pretty well along the line-of-sight, but is relatively weak perpendicular to the line-of-sight at that point in the mission. It is the line-of-sight components that have the greatest influence on the transient time, which in turn controls where the entry footprint is located on the earth. Therefore, it is anticipated that this maneuver should do a pretty good job of setting up the entry footprint where we want it over the recovery force. It is our opinion that after this time the task of maintaining the desired relative location of the entry foot- print with respect to the recovery ships should be handled by moving the ships to wherever you want them in the footprint rather than maneuvering the spacecraft to move the footprint. A single exception to all this is the possible need for spacecraft maneuvers to insure that the non-G&N recovery area is free of bad weather. This will be discussed in more detail later.
5. Entry Corridor Control
As noted previously, we came to a very interesting and startling con- clusion with regard to how we should control the trajectory to hit the entry corridor. But, in order to understand how you arrive at this con- clusion it is important to first understand something of the character of corridor control midcourse maneuvers. Control of the flight path angle at the entry interface (i.e., corridor control) is achieved by almost exactly horizontal maneuvers with respect to the earth. Very small maneuvers in this direction have a very large effect. The following table illustrates this point:
Time of MCC Delta V EI – 2 hours 4 fps EI – 15 hours 1.2 fps
Time of MCC (cont'd) Delta V EI – 20 hours 1.0 fps EI – 25 hours .8 fps EI – 80 hours Teensy weensy
(The delta V listed is that required to change the flight path angle at the entry interface (EI) 0.36°. This is a typical value for “corridor width,” i.e., the maximum acceptable dispersion from the center.)
6. You will notice that dispersions (even 0.1 fps) at TEI and MCC1 will certainly make corridor control maneuvers necessary. But, you will also notice that even as late as 15 to 25 hours out from the earth an 0.8 fps error would only require a corrective MCC of 1.2 fps after a 10 hour propagation period, and further out it is much less. Therefore, intuition says that a sequence of maneuvers throughout the transearth coast should be capable of maintaining continuous corridor control at very little RCS cost – individually and collectively. Also, it is evident that misalignment during these maneuvers can only hurt to the extent the desired maneuver magnitude is reduced – a cosine effect. Very coarse orientation is good enough – even 30° error or more is acceptable. For example, suppose we want 1 fps and only get 3/4 fps. This should become apparent via MSFN tracking over the next 10 to 12 hours and can be corrected at a cost very little more than the 1/4 fps error just incurred. It doesn't hurt very much to do the wrong thing. Duration of the burn is not critical either for the same reason making it reasonable to control delta V by time (a clock) rather than with accelerometers. Therefore, there is no need to bring the G&N on line. SCS is good enough. Rather than scheduling two maneuvers (currently at two hours and 20 hours prior to entry) we tentatively propose that as many as eight RCS burns be planned, all of which should be less than one foot a second to be scheduled at intervals of about 12 hours apart throughout the transearth coast. Of course, any one would be omitted if its computed magnitude were so small that the “noise” in the targeting obscures it. The advantages to be gained by this technique are:
a. It continuously maintains a trajectory intercepting the center of the entry corridor which is advantageous from both a psychological and communication loss standpoint.
b. The procedure is the same as the one to be used in the case of G&N failure.
c. The G&N need not be brought on line which simplifies procedures and reduces consumable consumption.
d. The SPS engine need not be used simplifying the maneuver pro- cedures and eliminating concern over whether or not it will restart and perform properly.
e. The real time logic is very simple.
f. It is anticipated to be a minimum delta V technique or close to it, which means that the procedures and logic will not depend on the propellent situation.
In addition to the analysis currently underway to learn more about this technique, it is necessary to investigate the RCS cost associated with SCS alignment, maneuver control, etc. It is anticipated that with a little study, techniques may be developed which couple this with other activities such as spacecraft thermal control, which will minimize total delta V requirements.
7. Something else came out of this discussion that hadn't occurred to me before dealing with the problem of bad weather avoidance in the recovery area. All systems design and analysis have been based on providing adequate L/D reentry maneuverability to assure good weather at the landing point area without the need for maneuvering on the way back. As I understand it, that is where the thousand mile long foot- print came from. However, it is also necessary to make sure that good weather is available in the recovery area to which the spacecraft would go in the event of a G&N failure. You recall, the entry mode for this situation is to fly a constant range (1200 n.m.), constant “g” reentry. It is evident that the lifting reentry cannot help us here. Hence, it is not the prime area but rather it is this non-G&N area which must be protected for weather, if indeed either must be. Accordingly, it is proposed that one or two days prior to entry, based on the weather prediction for that area, it will be determined whether or not a mid- course correction maneuver should be made for that purpose. It certainly must be made if the weather is unacceptable and the G&N is busted; it probably should be made even if it is still working. If the maneuver were made 20 hours before entry, it should not exceed 170 fps. This is the amount required to move the entry footprint 300 miles, which is Recovery's estimate of weather disturbance radius including prediction uncertainties. Based on a trade-off of weather prediction uncertainties, which deminish with time, versus maneuver magnitude, which grows with time, it may be found cheaper to make the maneuver earlier than that. The number given is to give you a feel for the situation. If such an SPS burn were made, it would have to be followed by corridor correction maneuvers as described above, perhaps carried out at a greater frequency.
9. In summary, the first midcourse correction is likely to be an SPS burn compensating for whatever dispersions occurred in TEI. After that, all the rest of the midcourse corrections will be made solely for corridor control consisting of many very small RCS burns using the SCS. The “landing point footprint” would be accepted as is and the recovery force would be moved to compensate for its dispersions. The only exception would be to add a midcourse correction maneuver if necessary to provide good weather in the non-G&N recovery area if that is a requirement.