In the previous newsletter, we talked about the Apollo program from Apollo 1 to Apollo 8. Before we move on further, it is important to know the computer technology used in Apollo Missions. This newsletter will be all about that.
Astronauts manually flew Project Gemini (It was NASA’s second human spaceflight program, conducted between projects Mercury and Apollo) with control sticks, but computers flew most of Project Apollo except briefly during lunar landings.
The Apollo Guidance Computer (AGC) is a digital computer produced for the Apollo program that was installed onboard each Apollo command module (CM) and Apollo Lunar Module (LM). The AGC provided computation and electronic interfaces for guidance, navigation, and control of the spacecraft.
AGC was designed to run a very small set of specifically designed programs needed to run a mission to the Moon, things like checking the guidance platform alignment and firing the engines. Everything on an Apollo mission was done through the computer and it took more than 10,000 keystrokes to get one mission to the Moon and back.
Astronauts communicated with the AGC using a numeric display and keyboard called the DSKY (for “display and keyboard”, pronounced as “DIS-kee”) as shown in Figure 1. The AGC and its DSKY user interface were developed in the early 1960s for the Apollo program by the MIT Instrumentation Laboratory and first flew in 1966. The AGC was the first silicon integrated circuit-based computer.
So how exactly did the astronauts use the DSKY? For this, let’s first understand the Apollo computer DSKY user interface unit in detail (refer to Figure 2).
COMP ACTY lit up when the computer was running a program.
UPLINK ACTY was lit when there was data being received from the ground.
TEMP lit up when the temperature of the platform was out of tolerance.
NO ATT lit up when the inertial subsystem could not provide an attitude reference.
GIMBAL LOCK lit up when the middle gimbal angle was greater than 70 degrees signifying that the spacecraft was close to hitting that deadly gimbal lock scenario.
STBY lit up when the computer system was on standby.
PROG lit up when the computer was waiting for additional information to be entered by the crew to complete a program.
KEY REL lit up when the computer needed control of the DSKY to complete a program.
RESTART was lit up when the computer was in the restart program.
OPR ERR error was lit when the computer detected an error on the keyboard.
TRACKER lit up with one of the obstacle coupling units that failed.
All data going through the Apollo guidance computer used verbs and nouns. The verb is defined as the action being taken and the noun is defined as the data set being acted on. It is a simplified version of giving commands, like: “Go home”, “Eat sandwich”, “Print picture” etc.
So the astronauts inputted all their data using a verb-noun interface, where two-digit numbers represented programs, verbs, and nouns. However, the DSKY consisted of a limited vocabulary of 99 nouns and 99 verbs which is shown in the Figures below.
The comparison is often made that a modern cell phone has more computing power than the Apollo guidance computer. Well, yes, that’s true, but your iPhone can’t get you to the Moon; it doesn’t exactly have that software… And really, the beauty of the Apollo guidance computer is in how tightly packed and specifically organized that software really was. Apollo computer never made a mistake, never made an error. But the computer did exactly what it was supposed to do. It generated alarms, but it is a different story to be discussed later.
Vintage Space: https://www.youtube.com/watch?v=b8S_T772H1c
In the next article, we will continue on the rest of the journey in Apollo Missions.