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By John Tylko
As NASA was planning the Apollo project in the early 1960s, one of the greatest technical challenges was the problem of navigating a spacecraft from Earth to the moon. To solve this challenge, NASA selected the MIT Instrumentation Lab to design and develop the onboard guidance, navigation and control systems for both the Apollo command and lunar modules.
The first major contract of the Apollo program was awarded to MIT on August 10, 1961. This milestone occurred just 10 weeks after President John F. Kennedy announced the national goal of landing a man on the moon before the end of the decade.
The late Institute Professor Charles Stark Draper recalled a pivotal meeting in August 1961 with NASA Administrator James Webb and Deputy Administrator Robert C. Seamans Jr. (SM 1942, ScD in Aeronautics and Astronautics, 1951) and later MIT Dean of Engineering.
"After some preliminary explanations of the mission plan being considered for Apollo, (we were asked] if guidance for the mission would be feasible during the 1960s decade," Professor Draper said. "We said 'Yes.' When we were asked if the Instrumentation Laboratory would take responsibility for the navigation and guidance system, we again said 'Yes.' They asked when the equipment would be ready. We said, 'Before you need it.' Finally, they asked, 'How do we know you're telling the truth?' I said, 'I'll go along and run it.'"
Draper followed up with a formal letter to his former student, Robert Seamans, dated November 21, 1961. “I would like to formally volunteer for service as a crew member on the Apollo mission to moon … if I am willing to hang my life on our equipment, the whole project will surely have the strongest possible motivation.” At the time, Draper was head of the MIT Aeronautics and Astronautics Department, a post he held from 1951 to 1966.
Although NASA did not take Doc Draper up on his offer to fly to the moon with the astronauts, the MIT Instrumentation Lab delivered on its promise of building a reliable guidance and navigation system using computer technology available in the early 1960s.
The Apollo Guidance Computer weighed 70 pounds, consumed 55 watts of power and occupied only 0.97 cubic feet inside the spacecraft. These first digital flight computers were limited to only 36,000 words of fixed memory and 2,000 words of RAM, and operated at a 12-microsecond clock speed.
The AGC was one of the first computers to use integrated circuits. During 1963, the MIT Instrumentation Lab consumed 60 percent of the integrated circuit production in the United States. By 1964, Fairchild Industries had shipped more than 100,000 ICs for use in the Apollo program. Approximately 2000 man-years of engineering were consumed in the development of the Apollo computer hardware.
Software for the Apollo Guidance Computer was developed using a mix of assembly language and an interpreted mathematical language. Defined program milestones were adopted for preliminary design reviews, critical design reviews, first article configuration inspection and customer acceptance readiness reviews. Processes for software validation and verification were developed, making extensive use of hardware and software simulators. Before the first lunar landing, more than 1400 man-years of software engineering effort had been expended, with a peak manpower level of 350 engineers reached in 1968.
During the first manned lunar landing, Apollo 11 astronauts Neil Armstrong and Buzz Aldrin (ScD ’63 Aeronautics and Astronautics) reported a series of program alarms indicating that the AGC was overloaded. An erroneous switch position caused the computer to process inputs from the lunar module’s rendezvous radar, overwhelming its computational throughput. Despite its overloaded condition, the AGC continued to function properly. With input from flight controllers in the Mission Control Center in Houston, Neil Armstrong made the determination to proceed with the landing.
Perhaps the most remarkable example of the capabilities of the Apollo Guidance Computer and the ingenuity of the engineers at the Instrumentation Lab occurred during the Apollo 14 mission, when a faulty abort switch in the lunar module threatened a successful landing. Within two hours, engineers wrote and tested new software commands which allowed the computer to ignore the erroneous abort signal and continue the lunar landing sequence. These commands were verbally transmitted to the astronauts and manually entered into the lunar module's computer, allowing astronauts Alan B. Shepard and Edgar D. Mitchell (ScD ’64, Aeronautics and Astronautics) to execute a flawless landing on the moon.
The Apollo Guidance Computer performed flawlessly on 15 manned flights, including nine flights to the moon and six successful lunar landings. It was used for the three manned Skylab missions and navigated the final Apollo spacecraft to a docking with a Russian Soyuz spacecraft in 1975.
Astronaut David R. Scott (SM and EAA in Aeronautics and Astronautics, 1962), who used the Apollo Guidance Computer to navigate on two Apollo missions said, "With its computational capability, it was a joy to operate -- a tremendous machine. You could do a lot with it. It was so reliable, we never needed the backup systems. We never had a failure, and I think that is a remarkable achievement."
When a team of engineers from NASA’s Flight Research Center visited NASA Headquarters in 1970, they met with the newly appointed associate administrator for aeronautics, Neil Armstrong. They described their interest in demonstrating a fly-by-wire aircraft control system using an analog computer. Armstrong challenged them to consider using a digital computer instead. “I just went to the Moon and back on one,” said Armstrong, who suggested they contact the MIT Instrumentation Lab.
In 1972, a modified Navy F-8C Crusader made its first successful flight at NASA’s Flight Research Center using a digital fly-by-wire system based on a modified Apollo Guidance Computer and software developed by the Instrumentation Lab.
As the Apollo program came to an end in the early 1970s, NASA asked the MIT Instrumentation Lab to begin developing the space shuttle avionics system. Following its divestment from MIT in 1973, the renamed Charles Stark Draper Laboratory continued the development and testing of the space shuttle's flight control system for both on-orbit and powered flight operations. Draper Laboratory continues to play an active role in each space shuttle mission, verifying that the payload configuration won't cause adverse dynamic interactions with the flight control software.
Today, most members of engineering team at the MIT Instrumentation Laboratory that developed the hardware and software for the Apollo Guidance Computer consider working on Apollo to be the highlight of their engineering careers.
Software engineer Margaret Hamilton captured the spirit of working on Apollo at the MIT Instrumentation Lab. “How fortunate I was to work with and share this experience with the many talented and dedicated people who made this possible. There was no second chance. We knew that. We took our work seriously, many of us beginning this journey while still in our 20s. Coming up with solutions and new ideas was an adventure. Dedication and commitment were a given. Mutual respect was across the board. Because software was a mystery, a black box, upper management gave us total freedom and trust. We had to find a way and we did. Looking back, we were the luckiest people in the world; there was no choice but to be pioneers.”
Alex Kosmala started working on Apollo software at the Instrumentation Lab in 1963. “No one in those early days had any notion of the magnitude and complexity of the programming task that Apollo would eventually demand. The word "software" had not even been coined at this time. It was only later that it became evident just how major a component the software would assume in the scheme of things. For a long time after I joined the Apollo program I had no faith that its objectives could ever be accomplished. Although we put in prodigious efforts, each in our assigned area of effort, the scope and magnitude of what yet remained to be achieved never seemed to diminish. And the speed with which President Kennedy's "by the end of this decade" was fast approaching was truly frightening.”
“In an incredible and audacious task, the landing of men on the moon, the guidance equipment for the mission was created out of primitive principles, prolific imagination, and a lot of hard work,” wrote David Hoag (SB ’46, SM ’50, Aeronautics and Astronautics), Apollo Technical Director at the MIT Instrumentation Laboratory during the 1960s.
In the foreword of the 1972 report “MIT’s Role in Project Apollo,” Charles Stark Draper summarized his view of the MIT Instrumentation Laboratory’s accomplishments. “Man’s rush into spaceflight during the 1960s demanded fertile imagination, bold pragmatism, and creative extensions of existing technologies in a myriad of fields. The achievements in guidance and control for space navigation, however, are second to none for their critical importance in the success of this nation’s manned lunar landing program, for while powerful space vehicles and rockets provide the environment and thrust necessary for space flight, they are intrinsically incapable of controlling or guiding themselves on a mission as complicated and sophisticated as Apollo. The great achievement of this Laboratory was to supply the design for the primary hardware and software necessary to solve the Apollo guidance, navigation and control problem. It is to the credit of the entire team that this hardware and software have performed so dependably throughout the Apollo program.”
"The Apollo Guidance Computers were early examples of what we would today call 'embedded' computers -- which now appear in everything from iPhones to automobiles,” said MIT Professor David Mindell, author of “Digital Apollo” which explores how human pilots and automated systems worked together to achieve the successful lunar landing. “The MIT machines showed the world that computers, previously known as refrigerator-sized cabinets, could be made small and reliable enough for even the most demanding, and life-critical applications."
John Tylko is a graduate student in MIT’s Science, Technology, and Society Department and is vice president of business development for Aurora Flight Sciences.