Haven't posted in while on this forum but: How much more complex would AGC need to be than the guidance systems on, say, a Surveyor or Lunakhod lander?
Complexity is part of the solution space, but the key factor is reliability. One system is human-rated while the others are not. Since the Surveyors were never outside the realm of ground control, ground control factored into the overall plan. The automated portion used a simple slant-range computation and closed-loop control of a throttled engine. Closed loop means the throttle setting is an ongoing function of the velocity and distance along any slanty trajectory toward the surface. If a crater or a boulder happens to be there—oh, well.
For LM guidance, pinpoint landing was the goal—and scrupulously avoiding killing the crew. That means a descent from a precisely measured orbit and lots of fail-safe thinking. Computers are really good at flying orbits and nominal braking maneuvers, while human pilots are not. Conversely, human pilots are really good at avoiding obstacles they can see with the Mark I eyeball once they arrive on station. The goal was to put a human on the Moon, not just land something on it. So make that human a very skilled pilot and give him a ship that can fly itself to a point where the pilot's skills become necessary. This is why the AGC needs the different modes and is therefore more complex than the single-program automation of the Surveyor.
My dude seems to be only an Apollo hoax believer, who hangs his hoax belief on both his alleged computing skills and his personal incredulity.
So, on nothing. It's one thing to say, "I don't believe they did it." That can translate to, "I have no idea what needed to be done or how the people who say they did it, did it." For the argument from authority to have any teeth, there need to be some receipts for that authority. From such an expert I would expect a more detailed explanation of what parts of the problem were intractable and what specific aspects of the computer system were measurably incapable of it. If his argument is just, "I'm very smart, and I can't see how it could have been done," then we have a whole bunch of demonstrably very smart people who can explain in great detail how it was done, and actually recreate the experience with surviving hardware and existing specs. Then the most parsimonious explanation for handwaving disbelief is ignorance.
He scoffed at the idea of engine burn parameters being read up to the astronauts...
That factually happened. Denying that it did is irrational. Reducing a simple maneuver to a discrete set of numbers is how astrodynamics works, whether someone wants to believe that or not.
...and at the lack of computing power of the AGCs.
Compared to what? The common mistake is to compare the computer to other computers. The proper method is to compare the computer to the required task. I can say that my smart watch has more computing power than the engine control module in my car. But does that prove my car won't work?
I use dirt-simple MCUs (microcontrollers) in many of my designs and products. These are inexpensive, to be sure, but also consume far less electrical current than the much faster processors I could use if I wanted. Right-sizing components is part of control system design. The testament to how carefully the computing load on the AGC was planned is the series of 120x program alarms that indicated that even a marginal increase in computing requirements (i.e., the rendezvous radar) would overload the system.
Again, most of the "computer people" you meet online have zero understanding of control theory and little if any understanding of real-time embedded systems. I've explained the control laws involved and how little computing power they required. The AGC was an embedded system running a discrete and deterministic software load with predictable, simulatable, and measurable performance requirements. It's not a web server or a smart phone running heaven-knows-what job mix. This is why I tend to hire crusty old game programmers for my embedded systems software. These are the guys who know how to shoehorn functionality into a deterministic hardware environment.
Also, the AGC was not a general-purpose computer. It was a computer created for the specific task of spacecraft guidance and control. As such it had things like counter interrupts and shift memories that you don't find on general processors. There was a lot of bolt-on hardware that offloaded the tasks that we would these days implement in software on a general MCU. Unless your guy has really done a deep dive into the AGC architecture, I wouldn't consider him an expert.
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