Ch5-1

Computers in Spaceflight: The NASA Experience
- Chapter Five -
From Sequencers to Computers: Exploring the Moon and the Inner Planets


[140] One organization more than any other has dominated the exploration of deep space: the Jet Propulsion Laboratory (JPL) of the California Institute of Technology. JPL was responsible for the Ranger and Surveyor series of lunar exploration spacecraft, the Mariner and Viking Orbiter explorers of Mercury, Venus, and Mars, and the Voyager and Galileo probes of the outer planets. As a result, the evolution of on-board computers for deep space operations took place at JPL.

JPL's chief contribution to computing on unmanned spacecraft was in leading progress from hard-wired sequencers to programmable sequencers to digital computers. The Pioneer spacecraft developed mostly at NASA's Ames Research Center and the Lunar Orbiters used to map the moon in the 1960s did not carry on-board computers. Like their earth-orbiting cousins and the first JPL probes, they used sequencing devices to activate and command experiments. Later the Mariner spacecraft acquired more autonomy and flexibility by using machines that stored command sequences in changeable software. Finally, sophisticated spacecraft flew with special-purpose digital computers.

Unique in its relationship to NASA, JPL is not solely a government installation in the same way as, for example, the Johnson or Marshall Space flight Centers. JPL's personnel receive their pay-checks from Cal Tech, yet almost every piece of equipment on the site has a NASA property tag, since, for over a quarter of a century, Cal Tech has administered contracts that have paid for all research and development of the many spacecraft originated at JPL.

Another way in which JPL is unique is its products. Whereas thousands of earth-orbiting satellites have been launched, less than a dozen each of Rangers, Surveyors, and Mariners were constructed, and just two Vikings and Voyagers and one Galileo were sent into space. Not only were few spacecraft built, but the interplanetary launches were separated by years and had to be on strict deadlines due to the realities of celestial mechanics. This created a completely different development environment than that at other NASA centers. The emphasis on basic research at JPL has perhaps been stronger than at any other NASA installation. This orientation and its application in spacecraft forms a special part of the story of JPL.

JPL's computer development activities were shaped by its organizational structures. When a project is started at the Laboratory, an office is established to house the project manager, key systems managers, and staff. Offices have come and gone with the projects themselves. The Ranger office, for example, has been closed for nearly 20 years, whereas the Voyager office is likely to be open for as long as that. Most personnel are housed in divisions and sections relating to specific discipline or system functions, as, in 1984, the "Technical Divisions" contained sections on "Guidance and Control" and "Spacecraft Data Systems." When a project office needs a component or service, it "subcontracts" it to the appropriate technical [141] sections. For instance, Spacecraft Data Systems supplies on-board computers, whereas the Navigation Systems Section does the trajectory calculations needed for a specific mission. In this way, specialists can be kept busy on a series of projects over a period of years without depending on a specific project for their jobs. Competition between sections to develop related components can also exist, as on the Voyager project, when the attitude control staff wanted to make their own computer for their system while the data systems people claimed sole domain over computer development. Within this setting, JPL has produced high quality on-board computers that have demonstrated outstanding reliability. ^*

* JPL's roots and its role in NASA receive excellent treatment in Clayton Koppes' The Jet Propulsion Lab and the American Space Program, Yale University Press, 1982.