Descent Abort Mission Techniques

See list attachedFebruary 20, 196969-PA-T-28APA/Chief, Apollo Data Priority CoordinationDescent Abort Mission Techniques

On February 13 we went over our Descent Abort Mission Techniques with the world. In general they were accepted as is. That isn't to say we didn't have some lengthy discussions resulting in some improvements and/or changes but we didn't make any substantial changes to the basic ground rules, philosophy, or overall procedures. I would like to list here some of the things we decided as well as some open items requiring work.

1. Although we didn't spend any appreciable time discussing this, it probably would be worthwhile to look into fixing the spacecraft computer program (LUMINARY) such that we could use the DPS and APS Descent Abort Programs (P70 and P71) before PDI (TIG). In other words, prior to PDI the crew and/or MCC-H may decide PDI is “no go.” Since the descent abort programs have the capability of targeting and guid- ing an ideal maneuver to set up the standard rendezvous sequence it may be quite an advantage if we are able to call upon those programs without actually having attempted PDI as the program is currently constrained.

2. It was agreed that if the steerable S-band antenna lock-on is lost during a descent abort, the crew will not attempt to reacquire with that antenna but rather will switch to the omnis as soon as it is con- venient for them to do so. Of course, this will only supply the ground with low-bit rate data but reacquisition with the steerable is considered to be almost impossible, particularly in an emergency situation like this. (Landing Analysis Branch was given the action item of determining if the initial descent abort attitude maneuver for any period in a nominal descent would cause the S-band steerable to loose lock.)

3. It was concluded that there is a significant advantage to having the AGS Mode Control switch nominally set to Attitude Hold during descent in order to permit the crew to complete a landing using the AGS if they have a PGNCS problem late in descent and consider it safer to land than to abort. Of course, this means that an extra switch setting must be made if it is necessary to abort on the AGS. Specifically the AGS abort sequence would be:

a. Set Guidance Control to AGS

b. Make a manual maneuver to approximately the abort attitude

c. Set Mode Control:AGS to Auto (This is the “extra”)

d. Push Abort or Abort Stage

4. We had a lengthy discussion about whether or not the DPS should be run to propellant depletion. The Propulsion people (who are never in attendance in any meeting dealing with how their systems are going to be used) have stated that running the DPS to propellant depletion should not be done unless crew safety is involved. There are obviously times in the descent aborts at which crew safety is decreased if we turn off the DPS any sooner than we have to. Accordingly, in order to avoid some sort of complicated logic to guide the crew in determining when they can or cannot run to propellant depletion, we all agreed that the DPS will ordinarily be run to propellant depletion if the guidance system does not shut it off first. The crew took proper note that there is some hazard incurred in doing that and plan to manually shutdown the DPS when the propellant gauge reads 1 or 2 percent remaining provided they are clearly in the region that shutting down the DPS is not going to increase the probability of hitting the moon AND it is clear an APS burn will be required to achieve orbit. Implicit, of course, is that they are not so busy in treating the cause of the abort that they fail to monitor and take this action.

5. In the event it is necessary to use the APS to achieve orbit, it was concluded that the crew will not attempt to provide ullage prior to pushing the Abort Stage Button. Although this is not accepted practice for an in-orbit maneuver, we could see no reason why it should not be perfectly safe to do this following a DPS burn of any magnitude with completely full APS propellant tanks.

6. By far our longest discussion dealt with how to handle the situation at insertion following an abort during the first 300 seconds of powered descent. Specifically, we are faced with the problem of how to jettison the DPS conveniently and safely and at the same time trim the ΔV residuals in order to get on the desired rendezvous trajectory. The results of this discussion were so meager that I will not report them here. Particularly since subsequent to the meeting several new proposals have been made that appear better than anything we considered. What I'm saying is that our discussion was fruitful to the extent that it got a lot of people thinking about this problem but we probably need to get together again to discuss all the resultant ideas and choose our course. I will set up a get together just for that purpose.