F/G cis-lunar midcourse correction mission techniques

See list attachedFebruary 6, 196969-PA-T-18APA/Chief, Apollo Data Priority CoordinationF/G cis-lunar midcourse correction mission techniques

This memo is to document the cis-lunar midcourse correction mission techniques we agreed to January 27 and 28 at the F and G Mission Techniques meetings. The translunar maneuvers are based on the follow- ing assumptions and guidelines:

a. We are not concerned about getting substantially further off the free return trajectory than on C' – primarily because we have the DPS backup.

b. We are especially anxious to conserve RCS propellant, which led to the procedures of allowing the midcourse corrections to grow to SPS size if possible.

c. In order to maintain best control over the situation we decided to use MCC₃ (at LOI – 22 hours) as the prime MCC, leaving MCC₄ essentially for fine trimming if necessary.

d. The minimum SPS burn is 0.5 seconds which is equivalent to approximately 3 fps.

Based on all that, we established the following:

a. MCC₁ (at TLI + 7 hours) and MCC₂ (at TLI + 24 hours) The need for these maneuvers will be based on how big MCC₃ would be if we did not make them. Specifically, MCC₁ and/or MCC₂ will not be executed as long as MCC₃ is less than about 25 fps without them. Furthermore, we will not make them unless we can use the SPS (that is, they must be bigger than 3 fps) and we will not trim residuals.

b. MCC₃ (at LOI – 22 hours) This is the prime maneuver to achieve the desired trajectory around the moon. It will be made if the predicted MCC₄ is greater than about 3 fps in order to avoid using SPS for MCC₄. Residuals will be trimmed to within 0.5 fps on this maneuver, which will most likely be made with the SPS.

c. MCC₄ (at LOI – 5 hours) By taking advantage of the significant flexibility provided with two-stage LOI maneuver in targeting the LOI maneuvers, we are often able to avoid making an MCC₄. That is, the LOI targeting can be done to achieve a 60 mile circular orbit in spite of substantial approach trajectory dispersions. This is done by rotation of the major axis of the initial 60 x 170 n.m. lunar orbit. However, we established that the apsidal rota- tion should be limited to less than 45 degrees. If it is necessary to use the SPS for MCC₄, the residual will be trimmed to within 1 fps.

Midcourse correction techniques on transearth leg phase of the flight were somewhat simpler. We are retaining the C' technique of utilizing transearth midcourse corrections only for corridor control. We have concluded that it is desirable to avoid making the last midcourse correction (i.e., MCC₇ at EI – 3 hours) if at all possible. Accordingly, we opened up the entry interface (EI) flight path angle limits a little more than on C'. Speci- fically, we will not execute MCC₇ if the flight path angle falls between 6.3 and 6.6 degrees (6.5 degrees is nominal). In order to minimize the probability of that midcourse correction, we set the threshold for MCC₆ (scheduled at EI – 15 hours) at .5 fps which is close to the MSFN target- ing accuracy at that time. The first transearth midcourse correction (MCC₅ at TEI + 15 hours) will not be executed unless it is greater than 1 fps.

The most significant change from C', of course, is brought about by the DPS backup which safely permits deviation from the free return trajectory. This makes the logic much simpler since we don't have to consider moving the maneuvers earlier to stay within RCS return-to-earth capability.