Apollo Guidance Computer History ProjectSecond conferenceSeptember 14, 2001
Computer SimulationsFRED MARTIN: I want to get back to something that Jim said and what he did in
that group. There was another fellow who perhaps should be here as well who also
contributed to that group, Bill Widnall. We needed those simulations so much because it
was the only thing that grounded us to what the truth was. You could never tell if what
you were doing was going to work until you banged it against a simulator. We did a lot of testing in this project. And a lot of testing with these simulators. I think that if all we ever did was fly the nominal mission, with nothing wrong, it would have been pretty boring after a while if you had this just one set of parameters and you always fly this one set of parameters. I think that part of the fun that we had was thinking up and creating these crazy off-nominal cases that probably could never, never happen. And then bang that against the software that we designed and design certain margins into the software that would make sure that whichever way the gimbal would go, that you'd be able to get out of it. For one nominal case, we ran a hundred off-nominal cases. JIM MILLER: The saving grace of the Apollo computer was that it was slow. Why I
say that is that the simulations that we ran and the assemblies that we had to do were
very long. And if the AGC had been ten times as fast, it would have taken ten times longer
to run the simulations. We would never have been able to run enough of them. They would
have just taken more time or machines than we could possibly have had. Sometimes
limited resources are a blessing in disguise. JIM MILLER: Houston wanted to have an independent computer simulation from the one we had, as a cross check. So they assigned somebody to write a simulator. And they wrote it in Fortran. I had personally modified our MAC executive so that some things we needed to do in the simulator could be done by run-time software in this part of the simulator's run-time environment. One involved stealing some functions that I knew nobody would ever use, like the hyperbolic cosecant function in the language. So if you put hyperbolic cosecant of minus 1 in there, or something, it would type a message on the console typewriter or something. I got a look at the listing of their 'independent' simulator written in Fortran. And right there was hyperbolic cosecant of minus 1 to write on the console of a machine that didn't even have a console typewriter. This was certainly not what you'd call an independent simulation. (Laughter) They were copying code directly into Fortran without the slightest idea of what it was doing. I thought, this is not much of a cross check. Oh well. We did get there. We were running lots of simulations on the IBM system, and a lot of other lab stuff as well, when we realized we had to have more simulation time than that on one machine could provide. So I went to Chris Kraft and said, "We've got to have a second machine." It was absolutely astonishing. Within six weeks, that machine was running in our place. The prior one had taken probably ten months. He just was able to make things happen for us and for everybody else in the project that really gave it what it needed to succeed. And I think without the influence of Chris Kraft, both inspirationally, because he really was a guy we all respected and liked, and his clout, the project could have not made it. There were just lots of things that happened to come together. site last updated 12-08-2002 by Alexander Brown |
|