Upthread: Descent Aborts – Part II (Oct 25, 1968)
See list attachedNovember 25, 196868-PA-T-258APA/Chief, Apollo Data Priority CoordinationDescent Aborts – Part III
We have had a couple more Descent Aborts Mission Techniques meetings resulting in substantial progress which I would like to tell you about in this memo, if you haven't already heard.
A basic ground rule we have established is that these abort procedures go into effect at the time powered descent initiation (PDI) is attempted (i.e., starting at the time of PDI TIG). The point is, if the descent burn is not attempted at all another procedure is used (TBD). But once descent is started and an abort is required, the crew will always go to P70 or P71, the DPS or APS abort programs.
As noted previously we have eliminated the special abort zone during the first 50 seconds of powered descent which used to require special pro- cedures. A simple program change was made to LUMINARY to do this. In order to cause the system to work in an acceptable way, it is also neces- sary to increase the insertion apogee altitude in the PGNCS targeting. This is done by changing the value of an erasable memory constant in the LGC. (Insertion apogee altitude is now 100 n.m.; it was 60.) A prefer- able solution was considered for LUMINARY but must be delayed to LUMINARY II due to schedule impact. It is to have the PGNCS compute the optimum apogee insertion altitude in real time based on the phase angle between the LM and the CSM at the time of the abort. It is possible to do this such that the subsequent rendezvous sequence is almost identical to the nominal lunar landing mission rendezvous sequence – always providing a one rev rendezvous with a differential altitude of 15 n.m. This program change will likely be made in the AGS, too – perhaps even in time for the F mission since it is relatively simple. Assuming we are able to fix the PGNCS program for the lunar landing mission, it looks like we have a yery good, straight forward, simple and standardized abort/rendezvous procedure.
One caution must be observed since the DPS abort program (P70) commands full throttle immediately. Therefore, if the crew decides to abort on the DPS immediately after PDI they must at least await engine stability before hitting the Abort button. I should also point out that aborts during the first 40 seconds of powered descent will currently result in a spacecraft pitch maneuver which will cause the MCC-H to lose all telemetry until the crew can realign the hi-gain antenna or switch to the omnis. A program change request for LUMINARY II has been submitted to fix this.
Another area in which we have been working is the procedure following a descent abort using the DPS engine immediately after the engine cutoff. Like any other maneuver, the standard procedure is for the crew to call up the ΔV residuals on the DSKY and check the horizontal ΔV still required. Then:
a. If the horizontal ΔV to be gained is less than 5 fps, which should be the usual case for aborts prior to about 300 seconds into powered descent, the crew will trim it with RCS without staging the DPS. Out-of-plane and radial ΔV components will be left untrimmed and their effects will be eliminated by the subsequent rendezvous maneuvers.
b. If the ΔV in the horizontal direction at the end of DPS burn is more than 5 fps but less than 30 fps, we want to stage the DPS off prior to burning into orbit with RCS since RCS plume impingement pre- cludes dragging the DPS along. However, staging presents a problem since the PGNCS digital auto pilot (DAP) will not be aware it has happened. Since it would continue to assume the high inertia, unstaged spacecraft, it would command excessive RCS firing for attitude control. Like LM₁, it would really hose out the RCS fuel. The easiest way around this is to switch guidance control to “AGS” and attitude control to “AGS attitude hold” and then manually translate into orbit with RCS based on the PGNCS DSKY ΔV display. The procedure would be to manually stage immediately after initiation of the RCS trim burn. Again, there is no reason for trimming the out-of-plane and radial ΔV residuals.
c. If at DPS engine cutoff the ΔV residual in the horizontal direction exceeds 30 fps, the procedure is to simply hit “Abort Stage.” This will automatically separate the DPS and utilize the APS to complete the maneuver required to achieve the desired orbit. The ΔV required depends on the abort time and can range from as little as 30 fps all the way to a full Ascent duration burn. The 30 fps boundary was chosen because attempts to use P71/APS for smaller maneuvers can result in very large ΔV errors, in fact as much as 60 fps. Again, only the horizontal in-plane component of ΔV need be trimmed after the main engine cutoff.
Of course, in case “a” noted above it will be necessary to separate from the DPS sometime. There was considerable discussion as to whether a special post-insertion maneuver should be made for this or if it was preferable to await the first of the scheduled rendezvous burns – CSI. We finally concluded that the most straight forward procedure was to separate the DPS at CSI in order to avoid the need for more complicated special procedures for this special situation. Separation at CSI rather than immediately at insertion also provides the peripheral advantage of an extra hour use of DPS consumables. But that is not our reason for recommending this procedure. Of course, it will be necessary for the crew to carry out certain DPS safing procedures. Specifically, they must vent the tanks just as they do after a nominal lunar landing. One open item in regard to this is the determination of how propulsive this venting is. If it turns out to be unacceptable we may be forced to provide some special procedure to stage the DPS at insertion. FCD has the action item of determining the magnitude of venting ΔV.