[Apollo Program]

Originally from the personal collection of Dr. George E. Mueller (1918-2015).

FIXED MEMORY MODULE OF THE APOLLO GUIDANCE COMPUTER COMMEMORATING "THE TRIP" TO THE MOON

Joseph Shea was the chief engineer responsible for the Apollo spacecraft. He presented this fixed memory module to George Mueller upon Mueller’s retirement from NASA to honor the “memory” of their work together on the Apollo Program. 

In his address to Congress on May 25, 1961, President John F. Kennedy declared his famous national goal of landing a man on the Moon and returning him safely to Earth” before the end of the decade. With those words, President Kennedy set in motion what would become the most ambitious and challenging technological endeavor in human history.

Later known as “the man who put men on the Moon,” Dr. George Mueller was a key player in achieving the goal set by President Kennedy. Mueller was originally sworn in as Deputy Associate Administrator for Manned Space Flight on September 1, 1963, but his title quickly changed to Associate Administrator for Manned Space Flight after an internal reorganization and he remained in that role until he left NASA in December 1969. Joseph Francis Shea joined NASA in 1962, where he became the project manager at the Manned Spacecraft Center for the lunar module and capsule. Shea was chief engineer in the design and development effort for lunar spacecraft and was responsible for the test and evaluation program. It was Shea who championed the Apollo lunar orbital rendezvous technique that NASA adopted, which was essential to achieve the mission objectives within Kennedy’s timeline and with minimal mission risk. 

In a memorial tribute penned by Mueller in a National Academy of Engineering publication, Mueller shared that Shea made “key decisions regarding engineering and scientific applications and actively directed the critical system tradeoffs and integration among spacecraft development, flight crew operations, and overall Apollo flight operations.” 

THE APOLLO GUIDANCE COMPUTER

The Apollo program presented NASA with a seemingly impossible task given the computer technology available in the 1960s. One thing was certain: this mission outstretched human capacities, requiring calculations far too complex and time-consuming for the astronauts alone to handle while in flight. As such, on August 9, 1961—just six weeks after the commencement of the Apollo program—NASA awarded the MIT Instrumentation Laboratory with its first Apollo contract to design, develop, and construct an onboard computer, later named the Apollo Guidance Computer, that would assist with the navigation, guidance, and flight control functions of the spacecraft. This choice was in large part because MIT, under the direction of laboratory head Charles Stark Draper (who the lab is know named after) and lead hardware engineer Eldon C. Hall, had a strong record in developing inertial guidance systems for the US Navy’s Polaris Missile Computer in the 1950s. Raytheon, who played a key role in the construction of the Polaris computer, was brought on soon after to take over the manufacturing element of production. 

The AGC had two flight versions: Block I and Block II. Block I AGCs, MIT’s initial design, was largely influenced by the architecture of the Polaris computer and was used primarily for uncrewed and early crewed development flights (all relatively simple earth-orbital missions) between August 1966 and April 1968, including the uncrewed Apollo 4 and 6 flights and the ill-fated Apollo 1. The Block II design maintained the original architecture of the Block I design, however, it incorporated newer technologies that addressed issues arising from experience in early uncrewed flights with the Block I computer. In the end, the Block II AGCs had twice as much memory, more I/O capability, required less power, and was smaller and more reliable than the Block I computers. It was this design version that guided crewed missions and that landed men on the Moon in 1969. Of Block II design, the present AGC stands as one of the 57 Block II AGCs NASA produced throughout the Apollo program.

Throughout its nine-year lifetime from 1966 to 1975, the AGC flawlessly flew on over 15 crewed missions, including nine flights to the Moon, all six lunar landings, three Skylab missions, and the Apollo-Soyuz Test Project mission. While it was not the first computer ever to be flown in space, the AGC did achieve several firsts for the history of space exploration and computer technology. Unlike earlier space computers, the AGC was the first onboard computer where the lives of the crew depended on it functioning properly and without ground support; it was the first computer based on silicon integrated circuit (IC) chips, enabling it become the most advanced miniature computer at the time; and it was the first completely digital fly-by-wire system. An enduring legacy, the AGCs influence extended beyond the Moon landing, serving as the basis for the computer technology used on the Space Shuttle, a space vehicle entirely controlled by the onboard computer, and on the earliest computer-controlled aircrafts.

CORE MEMORY

As Apollo mission requirements continuously grew, so too did the need for additional memory storage within the AGC. While MIT’s original design for the AGC called for only 4K words of fixed, read-only memory (ROM) and a mere 256 words of erasable, random-access memory (RAM), the computer’s final configuration leapt to 36K words of ROM and 2K words of RAM, an increase 18 times over the original estimates for its fixed memory and 16 times over for the erasable. These quantities, still millions of times less than the memory storage of a single iPhone, allowed for just enough space for the mission-critical software.

To store the software, MIT engineers adopted the use of Core Rope Memory from the original Mars probe computer in 1958. With this method, the written software was woven, literally, onto handmade rope memory, contained inside the computer’s six fixed memory modules. Needles carrying half a mile of copper wire were run through or around 512 magnetized iron cores, with each wire threaded through a ring representing a binary “1” and each wire bypassing a ring representing a “0.” Dubbed the LOL method, for “little old ladies,” the rope memory was skillfully sewn by female workers recruited from local textile factories. While the core rope technology allowed more information to be stored in less space, it was a strenuous process. Because the software data stored in the wires could not be erased, altered, or corrupted once it left the factory, the assembly of the memory modules required absolute precision. All in all, it took about eight weeks for the workers to weave the memory for a single flight computer, at a cost of $15,000 per module.  

Mueller shepherded the agency through the completion of the Apollo program as well as the early development of Skylab and space shuttle projects before retiring in December of 1969. Shea left NASA in August 1967 and joined Raytheon in 1969 as Senior Vice President of Engineering, where he would be responsible for all engineering and quality assurance programs. 

REFERENCES

Mueller, George (2002). "Joseph F. Shea". Memorial Tributes: National Academy of Engineering. 10. National Academies Press: 210–214. ISBN 0-309-59422-7.