Downthread: Ascent with busted guidance and control systems (Jun 19, 1969)
See list attachedMay 12, 196969-PA-T-77APA/Chief, Apollo Data Priority CoordinationManual Steering for LM Ascent
Over the years various groups have attacked the problem of if and how the crew can manually steer the LM back into orbit from the lunar sur- face. These studies were started before GAEC was even selected to build the LM and some analysis is still going on to define the optimum pitch attitude profile, which should be used in this mode. On May 8, I invited representatives of the MSC groups I knew had been involved in this business to a discussion – the purpose of which was to pin down just what the status is today. We were also interested in deter- mining if something useful could be done between now and the G mission. In summary, I think we all agreed that:
a. We should certainly not count on a manual operational backup mode for lunar ascent in the same sense that manual modes backup some other critical mission phases such as rendezvous targeting, burn control, etc. However, it's better than nothing and we ought to be prepared to do something.
b. Without a rate command attitude control system, it is extremely doubtful they could achieve orbit even if they had trained thoroughly in the technique. (Currently there is no training planned for the G crew.)
c. There are some things we should and will do before the G mission to prepare for this contingency, since it is an unfortunate fact that there are apparently quite a variety of two-failure combinations that can put us into this serious situation.
One of the first impressions you get when you start looking into manual ascent is that the procedures which should be used are strongly dependent upon the character of the system failures. That is, there are many different combinations of failures, each of which should be handled in a different way. As a matter of fact, the multiple-procedure-sets idea, combined with the low-probability-of-occurring idea has probably been the major reason we haven't got this whole thing all worked out in detail now. However, Jack Craven has finally convinced me the situation is not that remote and a worse situation can hardly be imagined. Further- more, our discussion leads me to believe that these multitude of procedures don't really present an insurmountable problem that can only be resolved in real time. I get the feeling that the “variation in procedures” which come about from many of the component failures is primarily a reconfigura- tion of spacecraft switch settings and the crew procedures probably aren't too different than for the nominal ascent itself. Of course, in that case the MCC must be prepared to advise the crew exactly how the spacecraft should be configured to best support ascent in one of these degraded modes. It was interesting to find that the method which must be used for the next level or class of failures essentially boils down to the following few options:
a. Prior to lift-off, some sort of initial azimuth reference must be chosen such as a prominent landmark or probably the LM's shadow on the lunar surface. Immediately after lift-off, the crew would yaw the space- craft to place the LPD line on the shadow prior to initiating pitchover, after which a landmark to aim for could be selected by the crew in real time.
b. After manual “Engine Start”, the crew would hold the vertical rise pitch/roll attitude for 15 seconds. They would then pitch the spacecraft in accordance with pre-selected four step pitch profile. These angles are essentially known today both:
(1) In inertial coordinates for use if a spacecraft inertial reference system is available and
(2) In a relative coordinate system – that is, the overhead window marks which should be held on the lunar horizon.
c. Propellant depletion should probably be used as the “Engine Off” technique and it is recommended that the interconnect not be used for attitude control since APS propellant is marginal to start with and should be utilized exclusively for getting into orbit. The “Engine Off” command could possibly be issued manually using the DEDA output of ΔVX provided the AEA and x-axis accelerometer are functional but probably shouldn't be.
This procedure, which essentially targets the spacecraft to the nominal insertion altitude and flight path angle most likely will result in a large dispersion in velocity, which of course would foul up the subsequent rendezvous. At least it provides the greatest chance of achieving orbit at all and probably minimizes the dispersions to give us a reasonable whack at rendezvous.
It is evident the two things that the crew needs to do on this job are an attitude reference and an attitude control mode. I was very interested to find that if we constrain ourselves to talking about pure manual as opposed to the various levels of degraded automatic ascent modes, we really came out with a very short list of candidates for these two things. Specifically for attitude reference, we have the following:
a. If the CES is broken, but the AEA, ASA, FDAI, and needles are available, they provide an excellent attitude reference. In fact, in this case, the crew should fly the needles as opposed to the four step pitch profile noted previously since they are driven by the actual ascent guidance error signal. (Unfortunately, it probably means having to fly in Direct Attitude Control – heaven forbid!)
b. If only the LGC is broken, we can use the IMU and GASTA driving the FDAI to provide a good inertial attitude reference if we can align it somehow (caging, probably) and can figure out how it is aligned.
c. The overhead window has been especially configured for use with the horizon during ascent, which fortunately is sunlit throughout the nominal ascent. (A sunlit horizon is not always available for descent aborts or lift-off immediately after touchdown.) Spacecraft pitch is controlled using the horizon and window marks; spacecraft yaw utilizes the horizon tilt and roll (that is, azimuth) must use some landmark as noted previously.
Those are all the choices we could think of for an attitude reference if automatic control has been lost. Furthermore, we found there are only three manual attitude control modes, which I will list in order of preference:
a. If a PGNCS accelerometer is broken, it is possible to use the LGC, IMU gyros, and hand controller to obtain a DAP rate command mode.
b. If the ASA and/or AEA is broken, it is possible to use the ATCA, rate gyros, and hand controller to obtain a rate command mode.
c. The rotational hand controller (ACA) can be used in either of two Direct Attitude Control modes, both of which are probably unacceptable. They are four jet – 12° (hardover) and two jets – 2½°.
Following is a list of things we are going to do:
a. MPAD/TRW will recommend the final angles – inertial and horizon – to be used for carrying out the four step pitch profile.
b. FCSD will check with the crew to determine if they want to add these numbers into their checklist along with the nominal attitude profile check points they have already, or if they want to leave this for a real time voice relay from the MCC.
c. Clark Hackler and Jack Craven are going to develop a complete matrix defining the preferred spacecraft configuration and capability remaining for degradation or failure of each component. This should be done by the first week in June. Incidentally, something along this line has apparently been worked out by GAEC already.
d. I am going to see if it possible for some experienced pilot, pref- erably Pete Conrad, to run a few simulations of some of these manual abort modes, particularly to evaluate using the overhead window attitude reference with the three rate command and direct attitude control modes noted above.
In mid June, we will set up a Mission Techniques meeting on this subject with world-wide participation – particularly MIT, TRW, and GAEC – to see where we stand at that time. Considering the catastrophic nature of the situation under discussion here, it seems some effort is certainly justi- fiable to get prepared. I would recommend that it be an effort equivalent to manual TLI steering. In other words, a blank check. Everyone at MSC and particularly the prime crew can spend full time on it, if they want to. And, I currently plan to have a Mission Techniques document prepared specifically for it, too – prior to G.