Upthread: Ascent with busted guidance and control systems (Jun 19, 1969)
See list attachedJuly 7, 196969-PA-T-104APA/Chief, Apollo Data Priority CoordinationManual Ascent revisited
On July 2 we had another meeting regarding Manual Ascent. As I have pointed out previously, the consensus is that the crew should have an excellent chance of achieving a safe orbit by manually steering the LM from the lunar surface if they have a rate command attitude control system by using the horizon view in the overhead window as an attitude reference. The two primary facets we discussed this time were:
a. What sort of ground support could be provided to the crew during powered flight and
b. What sort of rendezvous sequence would be pursued following the LM insertion.
This memo is to summarize the results of this session. Briefly though – the ground assistance can be substantial and the rendezvous can be a fairly standard CSM rescue requiring one or two extra revs.
As you recall, the flight controllers on the ground have a substantial capability for monitoring the LM's trajectory during powered ascent, even with the guidance systems broken, providing the RTCC powered flight processor (the “Lear”) is working. This program provides a complete up-to-date state vector to drive the analog and digital displays in the control center. As a result it is possible for the Flight Dynamics Officer (FDO) to monitor the ascent trajectory continuously and to dis- cern deviation from the nominal. For example, by monitoring the altitude vs. downrange distance plot and the velocity vs. flight-path-angle plot, he will be able to advise the crew if the radial velocity (altitude rate) becomes unacceptably dispersed. Specifically, starting about three and a half or four minutes into ascent, after the trends are well established, he should be able to advise the crew to bias the remainder of their pitch profile up or down probably using 2° increments. Given this assistance, it is anticipated that the crew should insert with a nearly nominal flight-path-angle.
It is also possible for the FDO to assist the crew in maintaining a near nominal out-of-plane velocity. That is, once the crew has keyed their initial launch azimuth on their shadow and then aimed for a prominent landmark (such as the south rim of Crater Schmit for landing site 2), the FDO will call out 2° north/south (or left/right) attitude changes when- ever his digital display of out-of-plane velocity exceeds 50 fps. This vectoring of the crew can start very soon after lift-off if necessary.
A major problem we feel we have now resolved has to do with when the crew should shutdown the APS. Analysis has shown that a continuous pitch angle bias of 2° can result in an unsafe perigee unless the APS is run to propellant depletion. Therefore without ground vectoring, as noted above, we feel it is advisable to permit the APS to operate until pro- pellant depletion; a 2° bias does not appear to be out of reason for manual steering using that weird lunar horizon as a reference. However, given ground assistance in attitude control a propellant depletion cutoff will certainly result in an excessively high apogee, which makes the rendezvous situation more difficult and costly. Accordingly, we propose that as long as the ground monitoring of the trajectory indicates that it is reasonably close to nominal, the FDO will voice command engine “Off” when his display of safe velocity (Vs) equals zero. (Briefly, Vs is the ΔV required to assure a 35,000 feet perigee at the current altitude and flight-path-angle.) A call at this time, assuming a 15 second delay, will produce an overspeed of about 300 fps yielding about 200 miles of excess apogee which should be adequately safe. The important thing is that it protects against apogees in excess of 250 n. mi. (which have been regularly occurring in simulations). Although these high orbits can be handled, there seems to be no reason to accept them. In this same vein, analysis has shown that we have been unduly conservative in proposing use of the RCS propellant for attitude control during ascent. We now feel confident that it is safe to stick with the nominal procedure of using APS propellant for attitude control during manual ascent and saving the RCS for whatever comes next.
Just about any failure combination which makes it necessary to perform a manual ascent will also demand a CSM rescue sequence. The sequence which seems to suit the situation best is as follows:
a. CSM performs a phasing burn (NC1) on the LM's major axis “maneuver line” approximately one rev after LM insertion.
b. CSM will perform CSI ½ to l½ revs after NC1 depending on how high the LM apogee turns out to be.
c. CSM performs CDH ½ rev after CSI.
d. CSM performs TPI at nominal elevation angle which should occur about midpoint of darkness.
e. Braking can be done by the LM and/or CSM at the crew's discretion, based on the real-time situation.
f. Plane changes should be handled in the standard way – that is, combined with the other CSM maneuvers and with the extra plane change burn between CSI and CDH performed by the CSM if it is necessary. (It is to be noted that any large out-of-plane situation must almost certainly be due to a velocity error at insertion and not an out-of-plane position error.) This would cause the node of the orbital planes to fall near the major CSM burns such that most of the plane change required would be efficiently combined with them. Given control center assistance in ascent steering though, a large out-of-plane situation seems unlikely. To insure that even a very low insertion orbit can be handled, it was decided to bias the LM lift-off late, approximately three and one-half minutes. Specifically, the FDO will compute a LM lift-off time consistent with a l0 mile circular insertion orbit and a nominal rendezvous sequence. However, since it is most desirable to utilize the sequence noted above rather than having to make rendezvous maneuvers soon after insertion if a low orbit is achieved, we feel the best course of action is for the LM crew to be advised to make whatever ground computed maneuver is required at insertion to achieve an orbit equivalent to at least l0 x 30 n mi. orbit. That is, if they truly burn out very low, they should boost their orbit with RCS to permit use of the CSM rendezvous sequences noted above. Incidentally, they will also be advised to make an apogee maneuver to pull up perigee to about 16 n. mi. as a safety measure in any case.
If for some reason the LM does not achieve a safe orbit with or without the control center assistance noted above, we still have a straw to fall back upon. The flight controllers have the capability immediately after insertion of computing a maneuver to insure at least a 35,000 feet perigee based on the Lear Processor. This maneuver will be scheduled at three minutes after APS shutdown or at apogee, whichever is required. It is to be noted that ample RCS should be available to execute this maneuver.
Although we have nowhere nearly the same confidence of success, procedures have been established for the crew to execute manual Descent Aborts. The problem here, of course, is that a single pitch attitude time history can- not be established for aborts occurring at any time in powered descent. However, the necessary work has been done by MPAD and TRW to provide the flight controllers with an acceptable pitch profile as a function of abort time in powered descent using the horizon attitude reference which would provide a safe orbit if the crew were to follow it. Accordingly, if communications are retained or regained after a descent abort, the crew can be informed of a pitch profile to follow to achieve orbit.
One other item we discussed was the relative merits of flying a completely manual ascent vs. a completely automatic ascent using the AGS with a broken z-axis accelerometer. You recall in this event it would be necessary to fly the LM into orbit on its side in order to place the broken accelerom- eter in the out-of-plane direction and bring the good y-axis accelerometer into plane to provide the automatic AGS capability. If the AGS works, everything should be just fine, but the crew will be unable to monitor its performance which leads to consideration of a completely manual ascent with its horrible overspeed problem. However, given ground monitoring we feel confident that a malfunctioning AGS can be detected and it is our strong recommendation that it be used. If the control center detects an unacceptable failure, the crew would be advised to yaw in-plane and pro- ceed into orbit using the standard manual ascent technique.