Upthread: Manual Steering for LM Ascent (May 12, 1969)
Downthread: Manual Ascent revisited (Jul 07, 1969)
See list attachedJune 19, 196969-PA-T-94APA/Chief, Apollo Data Priority CoordinationAscent with busted guidance and control systems
On June 11 we had a Mission Techniques meeting to discuss manual ascent from the lunar surface. The term manual ascent, though, is somewhat misleading since most of our discussion had to do with how the guidance systems should be operated if certain of its components failed prior to ascent. In summary, I think everyone generally agrees that:
a. Given a rate command attitude control system, the crew should be able to guide the spacecraft into orbit quite satisfactorily using the horizon viewed through the overhead window as his attitude reference. The resultant orbit will be far from nominal which could present rendezvous problems, but at least we feel fairly confident he can get into orbit. Manual steering in the “Direct” attitude control mode is considered pretty hopeless in the sense that it is probably impossible to control the space- craft at all – not in the sense that the insertion conditions are not acceptable.
b. Both the AGS/CES and the PGNCS have a substantial capability, even if the accelerometers are broken. However, special procedures are required to utilize this capability.
c. Gyro failures virtually wipe out the system with the possible exception of the rate gyros in the AGS/CES package.
The rest of this memo just adds a little detail to the above summary if you are interested.
Pure Manual Ascent using rate command and the horizon
Since our last meeting, Paul Kramer and Chuck Lewis have set up and run a series of simulations using CES rate command and the overhead window, which I understand were generally quite successful. They are in the process of documenting their results, so I suggest you contact them if you are interested. Briefly, they found that using the four step pitch profile MPAD/TRW has recommended works very well. They also found that it is possible to use the pitch angles in the current checklist that the crew uses to monitor a nominal guided ascent. These angles are tabulated for each 30 second time-hack. They found that letting the APS run to propellant depletion always resulted in an excessive overspeed – that is, yielding apogees up around 400 miles or so which suggests that it may be desirable to use the interconnect during manual ascent just as during nominal, thereby using APS propellant rather than RCS for attitude control. I expect we will all agree this is the right thing to do. Due to simulator limitations, they used the initial FDAI as an azimuth reference. It was the consensus of those at the meeting that if the inertial reference is not available, as could easily be the case, an acceptable alternate is for the crew to yaw the spacecraft during vertical rise to place the LPD line on the LM shadow. Given this initial launch azimuth as a reference, they should be able to choose prominent features downrange to head for in real time. In addition to the horizon angles,as viewed through the overhead window, corresponding angles as displayed on the FDAI are also available for the crew's use if an inertial reference is available. The reason we place greatest emphasis on the horizon is that it will always be there and a good FDAI may not be.
PGNCS with accelerometer failed still provides attitude hold rate command and FDAI
As well as anyone can determine, there is no reason why the PGNCS IMU cannot be aligned even with accelerometers broken. Of course, the gravity align is out, but it still should be possible to use the LM body attitude option and the AOT two star sightings option (alignment techniques 0 and 2). The accelerometers will cause program alarms but the alignment programs should still work. In either case, we would recommend aligning the IMU to the standard nominal REFSMMAT. No special procedures are required for this and the crew would be provided a perfectly nominal FDAI display.
Of course, no navigation or automatic guidance can be carried out without the accelerometer, but it still should be possible to get a rate command attitude, hold control capability provided we are able to manage the digital autopilot (DAP) in the LGC properly. Of specific concern is what special inputs, if any, are required to take care of vehicle mass as the ascent progresses. You recall, the LGC decrements mass as part of its DAP function but without PIPA's it won't. This also had some impact on which program the LGC should be operated in during ascent. It was our impression that the standard Ascent program (P12) is preferable. Alternates suggested were the Average G program (P47) or the Idling program (P00). MIT was assigned the action item of advising us precisely how we should handle the mass in the DAP and which program was best from their viewpoint. One thing, reasons for preferring P12 is that the PGNCS might offer a redundant Engine-On capability as well as a more favorable attitude deadband. If the PGNCS is used with a broken accelerometer, the crew should follow the standard four step pitch profile and fly to propellant depletion as noted above.
PGNCS–LGC failed leaves only an attitude reference – maybe
If the LGC has failed, it is impossible to realign the IMU. This presents two choices, if the alignment is known and favorable at the time of LGC failure, it may be desirable to leave it alone. If that is not the situa- tion, it is possible to cage the IMU thereby aligning it to the LM body axis, which may provide a useful reference if the LM has landed in a fairly level attitude with the z-axis close to in-plane. Obviously if the LGC has failed, the only capability the PGNCS can possibly offer is an inertial attitude reference since attitude control and navigation demand a functional LGC.
AGS y or z accelerometer failed – AGS can still go “Auto”
If either the y or z-axis accelerometer is broken, it is impossible to do a lunar surface gravity alignment. However, it is possible to align the AGS given two AOT star sightings and ground assistance to compute the LM body attitude. Given the star data, the MCC will compute and relay to the crew both the LM and CSM state vectors in the AGS coordinate system assuming a body axis alignment (DEDA entry 400 + 50,000). It will be based on the assumption the crew will select initial guidance (DEDA entry 400 + 10,000) at precisely two minutes before lift-off. By zeroing the bias and scale factor coefficients in the AGS computer for the failed accelerometer, it is possible to use automatic AGS steering into orbit with a guided cutoff. Of course, no out-of-plane steering will result since the spacecraft will always be oriented such that the broken acceler- ometer is oriented out-of-plane.
If it is the z-axis accelerometer which is broken, it would be necessary for the LM to fly into orbit on its side. It is instructed to do this by loading the so-called WB (Addresses 514, 515, 516) as relayed from ground to arm the WB (DEDA entry 623 + 10,000). It may be possible to load a pseudo bias to compensate for the 1½° APS engine cant angle. There is a real trade-off to be made here between using the manual guidance noted above with a resultant overspeed or to fly the automatic AGS guidance with the LM on its side. The crew would be unable to monitor its performance but, if it works as advertized it would produce good insertion conditions for the subsequent rendezvous.
If AGS x accelerometer is broken a good inertial reference is all that's left
If the AGS x accelerometer is broken, it is possible to perform a lunar gravity alignment using the standard procedures associated with broken PGNCS/good AGS. In this case, we are assured of a good initial attitude reference for use in flying the pitch profile, but the automatic guidance and navigation is completely lost by the AGS.
AGS/CES with a rate gyro broken
No one is able, at this time, to say whether or not the AGS can fly completely automatically with a rate gyro disabled. It is suspected that rate feedback is required to provide a stable system but we are not sure. Accordingly, some runs are planned on the GAEC facilities with the RGA disabled to see what happens. If it can't handle it, the crew will have to fly Direct in the channel with the broken rate gyro using the error as a reference. This will also be simulated.
One major open item coming from all this is how we should play the rendezvous game given any of the situations here. Specifically, should we bias the lift- off time either late or early to give more time to do the rendezvous or to put the command module behind the LM at insertion? Should some CSM maneuver be made prior to or immediately after launch? A number of people will think about this and we'll probably get together in the next couple of weeks to lay out some plans since this is just as important as knowing how to get in orbit in the first place.
In all of the above cases a number of action items were identified, primarily dealing with establishment of precise procedures for initialization of the systems. It is expected that the necessary information should be available within a few weeks so that we can document all this before the G flight.