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Gemini-11: To unbelievable heights

Anatoly Zak, Spaceflight Correspondent
Sep 20, 2015, 14:49 UTC

Sen—The success of Gemini-10 prompted NASA to raise the bar even higher for the next mission on practically all aspects of the program, including docking operations, spacewalks and orbital maneuvering.

Racing to transition to the Apollo project by Jan. 31, 1967, NASA made the first attempt to launch the final bar one Gemini mission on Sep. 9, 1966. However it had to be scrubbed due to a minor leak in the first stage of the Titan-2 rocket assigned to carry Gemini-11 into orbit. The next day, the flight control system on the Atlas rocket, which would boost a fresh Agena space tug for the rendezvous mission, caused another two-day delay just as Charles Conrad and Richard Gordon were ready to board their capsule.

On Sept. 12, 1966, there was a last-minute air leak problem in the Gemini's cabin but, fortunately, it was resolved in just 16 minutes by simply opening and closing the entrance hatch over Conrad's head.

Finally, the Atlas-Agena thundered into clear skies at 08:05:02 local time from Pad 14, just two kilometers away from the ready-to-go Gemini-11. After the Agena had made its first orbit and re-appeared over Florida, Gemini-11 lifted off at 09:42:27. This launch needed a two-second accuracy to attempt a fast-track rendezvous and docking during the first orbit, simulating an urgent departure of the lunar module from the Moon and docking with its mothership in the lunar orbit.

Six minutes after liftoff, Gemini-11 ended up 430 kilometers behind its target but just 3.43 degrees away in the so-called phasing angle—a crucial parameter for a quick rendezvous. Rehearsing the lunar escape scenario, the crew was expected to rely exclusively on its onboard instruments, with no help from mission control. On the ground, flight managers were betting on how close to the target the crew could make it in the assigned time.

Against all odds, just slightly more than half an hour after leaving the Cape and a couple of radar-guided maneuvers, Conrad reported to Houston that he had spotted his target 93 kilometers away, as the two spacecraft were flying over Madagascar.

At 11:16:42, or just 94 minutes after liftoff, Gemini-11 successfully docked at the Agena as the duo was passing over the Southern United States. Illustrating an ever-increasing experience in space maneuvering, the Gemini-11 crew spent less propellant on its energy-hungry maneuvers than did Gemini-8 during its more relaxed approach to Agena. Ecstatic NASA officials let the crew practice undocking and re-docking four more times—two each by Conrad and Gordon in daylight and in the darkness, to ensure the upcoming Apollo program would have experienced cadre of rendezvous pilots.

Before the end of the day, the crew also made a 3-second test firing of Agena's engine but directed it sideways, so it could only affect an inclination of the orbital plane toward the Equator rather than flight altitude of the joint spacecraft.

The next day, after several hours of preparations, Richard Gordon began his action-packed spacewalk to Agena. As in previous missions, it took a lot of efforts and sweat even for most basic tasks and every wrong move was unforgiving. Gordon's first attempt to get to Agena carried him too far and required Conrad to reach out to the spacewalker's tether to pull him back. Finally, Conrad managed to crawl to the docked space tug and, like a true space cowboy, sat on the back of the rocket to rest. Close to exhaustion, he managed to attach an end of a 30-meter tether stored on the nose of the rocket to a special post on the nose of Gemini.

Gordon's 1.5-hour planned spacewalk also called for a test of a space screwdriver and of a hand-held maneuvering device stored in Gemini's aft trunk, however extreme fatigue forced him to abandon both tasks and close the hatch after only 33 minutes.

On September 14, the third day of the mission, the Agena fired its engine, pushing an apogee (highest point) of the stack's orbit to an unprecedented 1,369 kilometers above the Earth's surface. Like their predecessors on Gemini-10, the two astronauts took hundreds of spectacular photos from their unique vantage point. At the strategic level, the maneuver demonstrated a potential capability to send a beefed-up version of Gemini spacecraft all the way around the Moon—the idea which had some supporters at NASA including Conrad.

After two orbits in the dangerous area of our planet's radiation belts, the spacecraft maneuvered back to a low Earth orbit. Gordon then conducted another spacewalk, but this time just standing on his seat and taking photographs out of an open cabin hatch.


An Agena rocket orbits the Earth over Gulf of California while tethered to Gemini-11 on Sept. 14, 1966. Image credit: NASA

Last but by far not least, the Gemini-11 undocked from Agena, while still remaining connected to the rocket with a 30-meter tether. The unique configuration enabled two very important experiments for the future of space exploration.

First, the crew tried the so-called gravitational stabilization. In theory, an object at the bottom end of the tether (in this case Agena) would pull the top object due to a difference in orbital speed at different altitudes. As a result, two linked spacecraft would remain in stable position with the tether always pointing toward the center of the Earth. The phenomenon could be used to keep spacecraft in stable position without much propellant.

In practice, however, the Gemini-11 pilots found it difficult to fully stretch their cable without causing Agena to swivel around before the orbital dynamics could have an effect.

The crew then switched to an attempt to spin the two linked spacecraft around a common center to create an artificial gravity in space, which had long been a hallmark of science fiction novels and advanced space station concepts.

After some tricky maneuvering and struggle with the spiraling and looping tether, the pilots were finally able to put the tandem into a slow rotation reaching just 55 degrees per minute. Though unnoticeable to the crew, the invisible force caused floating objects in the cockpit slowly settle to the far end of the capsule. After a three-hour exercise the crew cut the connection and began preparing for landing.

On the morning of September 15, after another improvised rendezvous practice with Agena, Gemini-11 finally completed its event-packed mission with a successful water landing in the Atlantic Ocean at 13:59 UTC. The capsule splashed down 4.9 kilometers from the calculated point after two days, 23 hours and 17 minutes in flight.