Friday, April 20, 2012

Program 66

Or: Chauffeur or Pilot?

I just got finished reading Digital Apollo, where one of the major themes is how much automation is the right amount. The author refer to the two poles as "Chauffeur", someone who just wants to get a job done, and "Airman" (I will say "pilot" from here on out) who flies for the joy of flying and personally controls a complicated machine. This debate can be easily seen in other machines, as well. For instance, do you like a manual or automatic transmission?

Are you worthy of the term "driver" if your idea of driving is getting on I-80, turning on cruise control, and doing only what is necessary to stay on the road and not hit anyone? Would you like to turn those tasks over to your car as well?

On the other hand, do you love choosing the exact shift points to get the most out of your car? Do you want to be able to do things in a manual that you just can't do in an auto, like a push start or engine braking? Why don't you use a manual choke as well as a manual transmission? Is there a manual timing adjust in your car? Fuel injector control? Theoretically, a manual transmission can be more efficient than an auto, but is it really more efficient in your hands?

I for one am squarely in the automation camp. I drive because I want to get somewhere. I program for the fun of it, the way that some people drive for the fun of it. After being around and fascinated by the aerospace business over my whole life, I finally discovered that I am not a pilot. I like the idea of space travel, and I wish I could do it, but I'm not one who likes to control my machines that much.

Digital Apollo is mostly about the struggle between the computer and software engineers, who just wanted to get the job done, and the astronauts who wanted to actually fly in space, rather than just ride. In the late 50's and early 60's, it was becoming increasingly difficult to justify human controllers. For instance, no US rocket has ever been manually flown from the ground into orbit. Ascent guidance was developed for ballistic missiles which obviously couldn't carry a pilot, and this guidance was adapted to launch vehicles targeting orbit instead of targeting flaming death on our enemies.

Even if a human was to control a rocket, they would not be able to navigate or guide it. Navigation is done by the inertial measurement unit (IMU) and some software to integrate its measurements. Guidance is done by some complicated formulas which take the current and target state vector as inputs and produce a vector to align the thrust to. All that a human would be able to do is point the rocket in the direction that the automatic system already calculated.

All this brings us to the most complicated maneuver ever attempted in space, deorbit and landing on an airless world at a particular target in a safe spot. Surveyor performed landings, but had an error ellipse on the order of tens of miles, and it landed directly from an impact orbit. Apollo was a complicated coordination of humans and machines, each doing what they were best at. The program was broken up into several major modes:

  • P63 was called Braking, and lasted from before powered descent initiation (PDI) to the "high gate", where the landing site came into view. P63 was basically full automatic, where the only job of the astronauts was monitoring. Its job was to get from orbital speed, about 1.8km/s, at about 16km altitude, to almost stopped, only 200m/s or so at 3km altitude, in the most fuel-efficient way possible, and without hitting the ground. Calculus of Variations provides a unique trajectory, which the guidance system attempted to follow. It carried an approximation of this trajectory, perhaps similar to the powered explicit guidance routine. Early in the descent, the surface of the moon was too far out of range for the landing radar, so the guidance source was the inertial platform. After descending lower, the radar became active and the Kalman filter navigation system used the radar and inertial observations to produce its estimate. The guidance equations took the Kalman filter output as its estimate of where the vehicle is, and calculated the correct descent engine pointing to fly the best approximation of the optimum trajectory from the current location to the high gate.
  • P64 was the visual approach. At this point, the lander tipped over so that the commander could see the landing site through the windows. Before this point, the vehicle was pointed almost level, with the standing astronauts' feet towards the direction of travel and faces straight up into space. P64 was targeted to a low gate point, and flew the best trajectory it could, with the constraint that the commander could see it through the window. It also calculated where it would land if allowed to land automatically, calculated where in the window that would be, and displayed the Landing Point Designator (LPD) angle, which the lunar module pilot read aloud. The commander would use the angle scale engraved on the windows to see where the vehicle thought it was going to land, and could change this point with the rotational controller. If the commander did so, the program would recalculate the location of the low gate, so that guidance would fly to the new point instead of the old. This is an example of cooperation between manual and automatic processes. The commander used his senses, particularly vision, to evaluate the landing site. The machine can't see at all, it is basically a blind machine with a radar as a white cane. The human chooses and evaluates a landing site, and the machine figures out the best way to get there.
  • P65 was the automatic landing program. After passing low gate, the program nulled the horizontal speed and followed an altitude versus vertical speed profile to land. At this point, there was no targeting involved. The program was committed to the last LPD in P64, and just used the radar to measure altitude and run the vertical speed profile, and to measure horizontal speed to null it. No mission was ever landed with P65. During P64, the commander always switched to...
  • P66, altitude rate control. In this mode, the commander chose a descent rate, and changed it by clicking a switch up or down. He also chose the attitude by using the rotational controller. The machine calculated whatever thrust was necessary to hit the commanded descent rate, controlled the descent engine throttle, and calculated but did not control horizontal velocity. It displayed that on the cross pointers on the commander's instrument panel, and left it up to the commander to control horizontal rate. This is almost always referred to as the commander taking manual control, but as we can see, again there is a strong automatic assist, the machine doing the calculating, the human doing the guidance. Horizontal speed and position was controlled similar to a helicopter, by pitching the correct direction to use some of your thrust to add or subtract to your horizontal speed. It was up to the commander to make sure that by the time the vehicle was on the ground, its vertical speed was slow enough and its horizontal speed was close to zero. All Apollo missions were landed with P66. There was also...
  • P67, Full manual. The machine used the radar to calculate altitude, vertical and horizontal speed, and so forth, but only to display them. The commander had full control of the throttle and attitude. This mode was never used.
  • Actual attitude control, even in the semi-manual and full manual modes, was provided by the digital autopilot, in what we would now call fly-by-wire. This program figured out which RCS jets to fire, based on inputs. The autopilot itself could be driven by the guidance system, which calculated the best way to get to a chosen attitude (KALCMANU routine) and drove the jets that way. It could go on semi-automatic Rate Command, where when the commander moved the rotational controller, the machine would figure out the axis the commander wanted, then use the jets to get the vehicle rotating around that axis at a given rate. As the commander held the rotational controller, the vehicle would continue to rotate, with the jets quiet, since the vehicle is now up to speed. When the commander released the controller back to center, the digital autopilot would stop the vehicle rotation. There was also an almost full manual mode, where the jets were on as long as the controller was moved. This is most similar to the native Orbiter mode.
Similarly, the Space Shuttle guidance system could do deorbit braking, entry, descent, and landing all the way to touchdown, all on automatic. The only thing it couldn't do automatically was lower the landing gear. This was for safety so that the gear was not lowered in space, since they could not be raised once lowered.

The shuttle was flown in autopilot through 134 of 135 entries, but 0 of 135 landings. The commander and pilot always flew the final approach, from near or in the HAC to touchdown.

The automatic entries are a little bit surprising, as in Orbiter, I like to fly space shuttle entries on almost full manual, and I don't even like to fly. Skidding across the atmosphere at mach 25, 40deg nose up and 80deg banked, may be some of the most fun and exciting flight possible. I use the Glideslope MFD to see the reentry corridor, but stay on it myself. I'm not just flying to pre-calculated needles either, like I would in an ascent. I am actually doing what pilots do - flying by the seat of my pants.

There was a proposal to do a fully automatic unmanned landing of the lunar module, but among other things the astronauts hated it. I think the same thing happened with the shuttle. If the full auto system is proven, then why do we need pilots?

Automation is your friend, not your master. Besides, if you are the one building the automation, then it is the friend you built. I have heard it said that you can program a computer to dance and Irish jig, but only if you know how to dance an Irish jig. I would add further that while you have to know how to do it, you don't have to be able to do it yourself.

If you are buying your own car or plane, for the fun of driving or flying it, be my guest and have it however manual or automatic you want. If you are flying for me, whether I am a passenger on the aircraft with you or just a taxpayer paying for your ride, you are obligated to do whatever will maximize the chance of mission success. If that's full manual, so be it. If full auto, so be it. Some combination? Choose the right combination. Don't be like some astronauts who thought that they were destined to fly a lunar module and wanted as little automation as possible, to prove how macho they were as pilots. You are there in support of me, the science-consuming public. We did not spend billions of dollars so that you could fly. We spent it so that we would get the mission results.

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