Gemini's onboard digital computer
May 4, 2015
by Mark Ollig

Many of us can nostalgically recall the feelings of excitement and adventure during the early years of NASA’s space program.

President Kennedy made a bold challenge in 1961, for the United States to commit itself, before the decade of the ‘60s was over, to achieving the goal of “landing a man on the moon and returning him safely to the earth.”

The US was up to this challenge.

July 20, 1969, Apollo 11’s Lunar Module (LM) called Eagle, carried astronauts Edwin “Buzz” Aldrin and Neil Armstrong, to a safe landing on the surface of the moon.

Instead of “a man” or one person landing on the moon as Kennedy envisioned, the US came through with two people landing there at the same time.

Not bad, not too bad at all.

A third astronaut, Michael Collins, orbited the moon in the Apollo 11 Command Module (CM) called Columbia.

Aldrin and Armstrong would blast off from the moon to rendezvous and dock with the CM after completing their extravehicular activity (EVA) on the lunar surface.

All three astronauts did “return safely to the earth.”

In order to achieve this historical accomplishment, much testing in space was needed.

In 1962, the computer folks at IBM, in their Space Guidance Center in Owego, NY, began building an on-board guidance computer (OBC), to be installed in NASA’s Gemini spacecraft.

This computer would assist Gemini astronauts in completing a number of important accomplishments during the early days of NASA’s manned space program.

The Gemini 3 astronauts first used the OBC March 23, 1965.

A similar, but more advanced onboard computer, would later be built for the future Apollo spacecraft moon missions.

In preparation for the moon trip, a series of Gemini spacecraft missions successfully tested numerous procedures, and completed various objectives.

Successful firsts during the Gemini space program, using its onboard digital guidance computer, included:

• Maneuvering an orbiting spacecraft.

• Rendezvousing procedures with another spacecraft for docking.

• Navigating in space.

• Maintaining a controlled earth-reentry using an onboard computer.

For its time, Gemini’s onboard guidance computer was considered an advanced technological piece of electronics.

It was designed with a pushbutton data input interface, making it easily operable by the spacecraft’s crew.

The OBC could perform highly-complex calculations (over 7,000 per second) used for space maneuvering, course guidance, navigation, and other operations.

Physically, the Gemini OBC weighed almost 59 pounds, and its dimensions measured 18.9 inches high by 14.5 inches wide by 12.75 inches deep.

The OBC was made up of peripheral hardware devices, and was located in an equipment bay wall, to the left of the Gemini spacecraft commander’s seat.

The computer’s guidance control-panel contained a seven-segment display readout, a modular display keyboard with push buttons labeled: “Zero” and “1 through “9,” along with three other push buttons labeled: “READ OUT,” “CLEAR,” and “ENTER,” and a variety of indication lamps, and rotary switches.

Its high-level flight operational software was designed to be used within assigned program modules; whereby each module performed a specific logic-processing function.

The Gemini OBC could process data sent from computing telemetry systems on the ground, or via keyboarding of data by an astronaut inside the spacecraft.

During liftoff from the Earth, a Gemini spacecraft would be attached atop a Titan II rocket called the Gemini Launch Vehicle (GLV).

The Titan was equipped with its own guidance computer.

This computer adjusted the rocket’s velocity, and made any course corrections needed for getting the Gemini spacecraft into the correct earth-orbiting path.

Information from the Titan computers was relayed to Gemini’s onboard computer.

In the event of a Titan computer failure, the onboard computer could take over and calculate any needed altitude course changes.

One special mission saw the Gemini 6 spacecraft using its onboard computer for not only supporting its orbit, but for calculating rendezvous maneuvering trajectories used to align itself with another spacecraft in Earth orbit; Gemini 7.

Dec. 15, 1965, 160 miles above the Earth, Gemini 6 and Gemini 7 conducted the first successful maneuvering and rendezvous between two earth-orbiting spacecraft.

Gemini 7 was used as the rendezvous target for Gemini 6’s onboard computer.

Both spacecraft came within 1 foot of each other, and could have physically hard-docked, if they had been so designed.

This ship-to-ship rendezvous maneuver needed to work in order to prove a future Apollo CM and LM spacecraft could successfully rendezvous with each other, while orbiting the moon.

You can watch the orbital rendezvous of Gemini 6 and 7 on the Discovery YouTube channel: http://tinyurl.com/npeyw96.

President Kennedy’s speech before a joint session of congress May 25, 1961, included his strong support of our country’s space program.

The portion of his speech urging the US to go to the moon can be viewed at the John F. Kennedy Presidential Library and Museum website: http://tinyurl.com/nxbaa4k.

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