The Apollo program spacecraft systems made creative use of discrete transistors and discrete logic gates.
I began my career in electronic circuit design in 1972; I remember what relatively few integrated circuits were available at that time. So, when I look back at the Apollo program that began in the early ’60s and put a man on the Moon in 1969, I am in awe of the creative use of that era’s technology put to use in that spacecraft system, using mostly discrete transistors and discrete logic gates to perform communication, guidance, and more on the journey to the Moon.
The first group of spacecraft in NASA’s Mercury series, which were launched in the late 1950s, had no computers on them at all. At that time, Jack Kilby, from Texas Instruments, had invented the first integrated circuit (IC) in September 1958; it used external wire connections. In 1959, Robert Noyce, from Fairchild Semiconductor, invented the “monolithic circuit,” which put all the components on silicon chip and connected them with copper lines printed on an oxide layer—the first microchip.
Figure 1 The first type ‘F’ flip-flop fit into a TO-18 can (Image courtesy of Fairchild Camera & Instrument Corp. and the Computer History Museum)
In 1961, the first practical commercial IC, the NOR Gate (made with three transistors and a load impedance into a TO-5 ‘can’ with 6 connection legs) was developed.
Figure 2 Most of the electronics on Apollo were composed of transistor circuitry (SC is the spacecraft, SCS is stabilization and control system, and CMC is the command module computer).
Figure 3 Logic circuitry mostly used micrologic1 (uLogic) ICs (NAA is North American Aviation)
The Apollo program was a major factor in the growth of Silicon Valley in California in the early 60s.
The Apollo Guidance Computer (AGC)
In order to achieve President Kennedy’s goal of putting a man on the Moon by the end of the 1960s, new technologies would be required. A prime need was for a small, lightweight guidance and navigation unit that could process complex trajectory equations and issue guidance commands to the Apollo spacecraft in ‘real-time’ during the flight. The AGC was created.
Figure 4 The AGC physical configuration (Image courtesy of NASA Archives “Apollo Guidance and Navigation System: Equipment and Familiarization Manual”)
In August 1961, NASA gave the opportunity of designing the AGC to engineers at the Massachusetts Institute of Technology (MIT). Those engineers began designing the first computer to go into space on the Apollo program. They needed to reduce the size and weight of that on-board computer and the emergence of the IC became the means to achieve that task by replacing many, many discrete transistors. The Apollo computer used few flip-flop registers due to size and weight considerations; however, seven key registers in the computer did use flip-flops.
It was 1962 when MIT became the first to use these new integrated circuits, introduced in 1961, in their AGC design because it would lead to that necessary lightweight compact unit. In 1963, MIT was testing and developing the AGC Block I units. They ordered about 60% of the world’s available ICs at that time!
MIT’s original AGC design called for only 4K words of fixed memory and 256 words of erasable (at the time, two computers for redundancy were still under consideration). In June 1963, the figures had grown to 10K of fixed and 1K of erasable. The next design jump was to 12K of fixed, and MIT was still insisting that the memory requirement for an autonomous lunar mission could be kept under 16K! Fixed memory jumped to 24K and then finally to 36K words, and erasable memory had a final configuration of 2K words.
The AGC system was built by Raytheon, and each system used about 4,000 “Type-G” (3-input NOR gate) circuits. They acquired 200,000 units at $20-30 each; the AGC was the largest user of ICs through 1965.
The Lunar Module (LM) weight and power requirements made an amplitron-tube design more attractive than the traveling-wave-tube design used in the Command Service Module (CSM). The amplitron-tube power amplifier weighed 16.8 pounds and required 72 watts; the CSM power amplifier weighed 32 pounds and required 90 or 167 watts, depending on mode selection.
The CW Amplitron was a continuous-cathode, crossed-field, backward-wave amplifier. This device had many characteristics in addition to high efficiency that made it perfect for space telemetry.
[Continue reading on EDN US: The CW S-band power amplifier]
Steve Taranovich is a senior technical editor at EDN with 45 years of experience in the electronics industry.