posted 12-05-2005 03:31 AM               

The point where individual crews took over (P66) varied though.

posted 12-07-2005 02:42 AM               

quote:

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Originally posted by astroborg:
I think it is: Descent Propulsion System.

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Yep, that's right. When they had the Lunar Contact light illuminate, they would shut off the DPS exactly the same way whether in P65 (Auto) or P66 (Semi-auto).

posted 12-22-2005 03:45 AM            

Basically, they said that the landing phase was designed to be automatic but would provide for crew visual assessment of the landing site and, if required, to provide capability for pilot take-over from the automatic control — which is what happened on the actual Apollo missions.

Under the automatic guidance, a vertical descent was initiated when the TGO (time to go to landing) was less than 12 seconds or at an altitude of 200 feet and would terminate on landing. A rate of 5 fps would be used throughout an automatically controlled vertical descent. The crew would need to disable the descent engine when the footpad probes touched the surface. (See notes 1 and 2 below).

The vertical descent guidance used a routine that nulled the lateral (horizontal) velocities while accepting the commanded vertical rate from the ROD (Rate Of Descent) switch. The vertical rate was also controllable by manual throttle control.

Note 1: An earlier NASA MSC document said that the LM vertical descent would have started at 100 feet altitude and rate of descent would have been 3 fps.

posted 12-23-2005 09:46 AM            

Reading that book will give you a fairly good starting point and understanding as to why each LM had "unique" software for its mission.

Another book worth having, especially if you're not a guidance engineer but have an insatiable appetite for understanding of how do we "fly" to the Moon, is "Lunar Landing and Return: A Simplified Physics and Mathematics Investigation" by Donald C. Lundy. Read through the chapters of this book and carryout the exercises and your half way to becoming a G&N programmer.

posted 12-24-2005 10:30 AM            

• The gyros in the IMU spun at 24,000 rpm.

• The P64 program began at about 1.36 miles altitude above lunar surface.

• The commander could start the Terminal Landing Phase at any time during the running of the P64 program.

• LGC coding was done in LGC basic and interpretive language - there was no LGC compiler.

• The commander could redesignate the landing site upto 10 seconds before reaching the P64 'target site' point, when P66 was initiated.

• A LM negative pitch rotation gave a new site beyond the 'present' site.

• A LM positive pitch rotation gave a new site short of 'present' site.

• A LM positive roll gave a new site to the right of the 'present' site.

• A LM negative roll gave a new site to the left of the 'present' site.

• During the P64 program, when the LGC estimated the altitude to be below 30,000 feet, X-axis override was NOT provided to the crew.

• ROD was not enabled in the P64 program.

• The P64 program could not be restarted once program P66 initiated - so any site redesignation must have been undertaken before P66 initiated.

• The P66 terminal descent phase began at 100 feet altitude and 36 feet ground range from the landing site.

• Whenever the commander raised or lowered the 3-position ROD (rate of descent) switch with his left hand the altitude was raised/lowered by 1 foot/second.

• The P66 programs vertical channel was accessed once per second.

• During the P66 terminal descent phase there were no 'window commands' allowed - yaw was done manually.

• During the P66 program 0.33 seconds elapsed between the accelerometers reading and then issuing the throttle increment command.

• Further delays were added when the thrust increment command was issued. Because guidance needed data responses as short as 1.5 seconds, the P66 throttle routine (THROT) had to take into account these delays and calculate (in advance) a correction for thrust movement for the NEXT cycle.

• During P66 program only the landing radar (LR) could update the LM state vector.

posted 12-27-2005 04:32 AM               

If they wanted to update the LM position, it was thought they'd have to go through a tedious routine of putting in new positions to the GNC. This involved several entries into the computer.