posted 05-28-2010 01:44 AM               

From here: Apollo GNC Software

Basically P66 can operate in four different ways:

• The computer controls both vertical speed as set by the crew and nulls the horizontal speed,
  
• The computer control only vertical speed as set by the crew, while the crew selects the attitude with the hand controls, to null the horizontal speed,
  
• The crew controls the throttle of the engine to control the descend rate, while the computer nulls the horizontal speed,
  
• The crew controls both the engine throttling and the attitude, and thereby lands the LM themselves. This is only practical for the two crewmembers at the same time.

posted 09-14-2018 06:02 AM               

Here's the excerpt:

Since there is no apparent reason P65 would ever be preferred to the new Auto P66, the PGNCS logic is being fixed so that if the P64 target conditions are met prior to the crew taking over in P66, the automatic program switching from P64 will be to P66 Auto rather than P65. Thus, with this change and the one previously implemented so that the PGNCS ignores the throttle mode switch position, we have essentially eliminated both P65 and P67, and have remaining two modes of operation in P66.

posted 09-14-2018 07:00 AM               

quote:

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Originally posted by Marc05A:
Any idea why and/or when Jim Lovell made this statement about automatic landing, since an automatic landing was not possible anymore?

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Also, because the crater wall slope surface return acts like the surface is falling away at an angle, the computer could mistake this for a required attitude change. The result of these two misdiagnosed events, could cause the spacecraft to drop rapidly in a rapid sideways/forward or backwards impact with the surface.

The software change introduced a limit to the pitch or roll rate of change input, placed a limit of throttle change, and ensured that any control inputs at lower altitudes received dampened control response.

The computer retained the ability to fly the spacecraft to the designated touchdown point at the determined rates automatically, and control throttle control.

All control inputs went via the computer, as the system was fly-by-wire, so the computer was always in the loop.

As indicated in the Tindalgram, the plan was to let the computer control the decent, at about 100 feet above ground, the crew confirm there is no boulder field below, and control the throttle with a switch that could increase or decrease the rate of decent by "toggling" up or down. If the Rate of decent were low enough at this height, a hands off landing was possible, with engine cutoff triggered by the contact light probe.

posted 09-14-2018 12:48 PM               

Correct me if I'm wrong, but I thought that if the crew did nothing (i.e. not engage P66) the computer would automatically switch from P64 to P65, and the LM would fly to the designated touchdown point, and slowly descend to the surface. All the crew would have to do in that scenario is to wait for the contact light and shut down the engine.

posted 09-14-2018 09:40 PM               

High gate was the point where a transition from braking phase to final approach and landing phase was initiated, Program 64. At this time the computer runs the approach phase program, which is basically the same as program 63, with the addition of Landing Point Indicator "LPI" signals, and a new set of mathematical limits. The computer also monitors Vectors, ROD and Altitude until the constraints of Low Key are met, at which time the computer loads Program 65.

Program 65 nulls the cross-range velocities (the moving forward or sideways vectors) to zero, and controls the rate of descent to landing. If all went to plan, it could be hands off from here. No control of landing position was possible by the crew during P63, P64 or P65. However, if the landing site in front of the astronauts was deemed unfit (a boulder field), Program 66 was selected, throttle adjustments meant the rate of decent could be slowed, and P67 was used to allow a manual input of controls to steer the spacecraft forward, aft or sideways. Due to the field of view from the LM, going forward was the best option.

Following a review of previous flights, the issue of potential serious issues overflying certain craters was identified. This issue would be worse for landings in rough terrain of highlands, and could result in an auto flight into terrain.

It should be noted that if crew made throttle of control inputs during these programs, nothing happened, because the computer was driving.

I believe Alan Bean gave an interview where he talks about how violent he found the control response that Pete Conrad was getting during the Apollo 12 landing, with the sensation that the spacecraft was going to tip over.

With the lessons taken from 11 and 12, plus the need to make software changes, Program 65 was skipped at low key, going straight into program 66. P66 was called Rate of Descent, because this was the prime concern at this stage of the landing. The crew could now make throttle adjustments using a switch, with the computer maintaining attitude control. One concern during this phase was the landing radar receiving strange returns from the dust being scattered by rocket efflux, so if the computer made bad decisions at this time, or the crew wanted to land somewhere else, they simply selected P67, which allowed nulled manual control inputs. If they did nothing during P66, and the rate of decent was within limits, the LM would land all by itself.

It could be compared to riding an elevator down as fast as possible, with you manually gauging how fast the thing is falling, and deciding when and how much to apply the brakes. If you get it wrong, you hit before you slow, or, you stop before the ground, but you have no frame of reference about how far from the ground you are, because you do not know how big the rocks and craters really are. (Is it a toy car or a real car below your elevator?) This is why the LM shadow was an extremely useful aid. It gave a reference of surface scale.

There are many great resources on this subject, and it takes some time to understand. Each mission had different software configurations that could be bolted into the computer before launch. (Yes bolted, physically climbing into the LM and bolting in new memory modules.)

So, the auto land option was absorbed into P66 by default, because no change of throttle by the crew resulted in the computer controlling landing, but the crew could put in a request for a throttle position change to the computer is desired.