This memorandum was also circulated under the erroneous title “Aborts to the lunar surface from powered descent” in the tindallgrams01.pdf source.
See list belowMAY 14 196868-PA-T-101APA/Chief, Apollo Data Priority CoordinationAborts from powered descent on the lunar landing mission
1. We spent the entire May 8 Ascent Data Priority meeting discussing mission techniques associated with aborts from powered descent on a lunar landing mission. This discussion led to some pretty simple procedures which are outlined in this memo. They are based on some assumptions which I've also listed below. If you feel that they are in error, please let us know.
2. The basic assumptions we made are:
a. From a DPS engine performance and dependability standpoint, it is preferable to operate the DPS at full thrust throughout the abort ascent trajectory rather than at some lower level. (Is this okay after operating for awhile at reduced thrust? Also, we must make sure there are no bad guidance system transient problems at staging.)
b. The low level sensor light comes on when there is 1200 pounds of propellent remaining, which is equivalent to about 120 seconds burn time at 25% thrust, and 30 seconds burn time at maximum thrust.
c. It is operationally acceptable to run the DPS to fuel depletion. That is, there is no reason for the crew to prematurely shut down the DPS engine if there is an advantage to be gained by running it to fuel depletion. (I'll bet I hear something about this!)
d. Use of the “Abort Stage” automatic sequence is as safe or safer than manually proceeding through it one step at a time. (Someone's not going to like this either.)
e. The crew can make a go/no go decision one minute after the DPS low level sensor light comes on, at which time they should be prepared to either commit to landing or to abort immediately. (At least we are recommending this if it is at all possible. Of course, they may abort after that, but it's getting hairy.)
f. There is a very great advantage to be gained by keeping the variety of abort modes to a minimum – that is, always do the same thing as often as possible. The point is, there may be some special cases in which some benefit could be gained by doing things a little differently. But, we always felt the advantage of standardized procedures outweighted them in those cases we recognized and discussed.
3 . The abort procedure is really very simple, at least if the above assumptions holdup. So simple, in fact, that I'm sure you'll wonder how we spent the day! Basically, whenever an abort situation arises at any time during descent, the crew will hit the “Abort” button which will automatically put the PGNCS (or AGS) into the DPS abort program (P70) and the DPS should be run to fuel depletion or to a guided cutoff at orbital conditions, whichever occurs first. If fuel depletion occurs, the crew should then “Abort Stage,” which will automatically cause separation of the DPS and will put the PGNCS (or AGS) in the APS abort program (P71), leading to a guided insertion into orbit. We propose never initiating an abort with “Abort Stage” as long as the DPS is still operating okay.
4. There is one special case requiring attention which occurs with an abort approximately five minutes into power descent. It is at about that time when the DPS is able to return the spacecraft all the way to nominal orbit. If the DPS does make it all the way to orbit, all is well and good. If, however, fuel depletion results in DPS shut down just shy of that, something must be done of course. The procedure we propose if the velocity required to get into orbit is less than 10 fps, is for the crew to remain in P70, not to stage the DPS, and to use four jet RCS to achieve orbit. This requires approximately a 15 second burn. (This value was selected in deference to the problems brought about by a spacecraft whose thrusters shoot at itself.) If the velocity required to achieve orbit is in excess of 10 fps, which would require an APS burn of one second duration or greater, the procedure is as before – “Abort Stage” and use the APS.
5. One item requiring some research is to make sure that the spacecraft computer program (P71) will provide proper guidance to the APS for a “small” maneuver following DPS shut down. Another is to confirm that 10 fps is within the APS minimum impulse mode capability.
6. Consideration was given to establishing a special procedure in this region where the RCS would be used to insert the staged spacecraft. However, there was no advantage apparent to avoiding use of the APS unless there is some sort of freezing problem for short burns. In addition to keeping the procedure simple and standard, this technique should reduce the demand on RCS propellent and thruster lifetime. As a matter of interest, the magnitude of the remaining APS and/or RCS maneuvers in the coelliptic rendezvous sequence for an abort at that time are approximately as. follows: CSI 35 fps, CDH 100 fps, and TPI 30 fps.
7. The only other situation I'd like to discuss deals with aborts late in the descent phase after the DPS low level sensor light has come on. There is a real advantage to be gained if the crew spends no more than about 60 seconds in that state before aborting since after that time the DPS will have less than 15 seconds of burn time remaining at full thrust. This duration would assure getting through “vertical rise” and pitchover before DPS fuel depletion. After that, it's cutting things pretty close. However, even then, it stills seems best to always attempt “Abort” on the DPS in order to get as much out of that engine as possible – if it's only a cough. The full thrust DPS acceleration is over twice that of the APS and if it's ever needed it's there: The only disadvantage occurs with a more-or-less simultaneous “Abort” and DPS fuel depletion causing a delay in “Abort Stage” with no engine on. If the crew has been watching the fuel gauge, etc., he should never let this situation arise and special procedures should not be required to handle it.
8. Finally, I'd like to outline the alternate techniques we established if fuel depletion DPS is not acceptable. As before, we always recommend “Abort” rather than “Abort Stage.” The modified procedures are based on providing the equivalent of at least five seconds of DPS burn time at maximum thrust as a pad against fuel depletion. This is equivalent to shutting down the engine with about 120 fps DPS remaining. There are two classes of abort which must be considered:
a. The first is if the abort situation is detected before the low level sensor light has come on. In this case after “Aborting” into P70, it is necessary to monitor the inertial velocity in the DSKY (or the DEDA) at the time the light comes on. If the inertial velocity is less than 5,000 fps, the astronaut should “Abort Stage” 25 seconds after the light comes on and proceed into orbit on the APS. If the inertial velocity is greater than 5,000 fps, it is possible to proceed into orbit on the DPS without fuel depletion occurring. (Note: it is only necessary to monitor the “thousands” digit to make this decision.)
b. If the abort situation arises after the low level sensor light has came on, the crew should “Abort Stage” immediately after the pitch- over maneuver following vertical rise. This would occur about 10 seconds after the “Abort,” if the abort is from hover.
9. In summary, if the DPS is still working, always use the DPS to initiate the abort and after getting as much as possibly from the DPS, “Abort Stage” if necessary to achieve orbit. This provides the following advantages:
a. Avoids shutting down and changing engines at a time critical point and insures a positive altitude rate before staging.
b. Obtains the maximum delta V available from the DPS.
c. Produces the greatest possible acceleration at the abort time to get the heck out of there.
d. Makes the procedure standard for all cases – and simple!