Rendezvous in Space: The Gemini 6 Launch Abort

With the loss of the Agena target vehicle shortly after it was launched on October 25, 1965 and no replacement immediately available, NASA scrapped the original Gemini 6 rendezvous and docking mission and quickly devised an alternate mission (for a complete description of the original Gemini 6 mission and its aftermath, see “The Unflown Mission of Gemini 6”). The crew of Gemini 6, Wally Schirra and Tom Stafford, would now fly a new “Gemini 6A” mission that would rendezvous with the Gemini 7 long-duration mission already scheduled for an early December launch. The actual docking would have to wait for the Gemini 8 mission scheduled for the first quarter of 1966.

Portrait of the prime crew for the Gemini 6A mission: Thomas P. Stafford (left) as pilot and Walter M. Schirra, Jr. (right) as command pilot. (NASA)

Training for the Worst

After the decision was made on October 28, 1965 for the new Gemini 6A mission to rendezvous with Gemini 7, the Gemini 6 launch vehicle and spacecraft were cleared from the pad at Launch Complex 19 to make way for the immediate preparation for the launch of 14-day long Gemini 7 mission on December 4 (for a full description of the preparations and first few days of the Gemini 7 mission, see “Rendezvous in Space: The Launch of Gemini 7”). This would be followed by the launch of Gemini 6 from the same pad just nine days later.

Schirra and Stafford shown in a Gemini trainer practicing launch procedures and abort options. (NASA)

As ground crews rushed to tackle the logistical challenge of launching a pair of crewed spacecraft from the same pad just days apart, the crews of the Gemini 6 and 7 missions as well as their backups continued their training. Included among these numerous training sessions was time on simulators to practice various aspects of the mission especially the fast-paced and potentially dangerous launch phase of the mission. The crews would be required to train to solve a range of problems that could crop up during powered ascent including aborting the mission if required.

Diagram of the Gemini ejection seat. Click on image to enlarge. (NASA)

Unlike the Mercury before it or Apollo to follow, Gemini did not use a launch escape tower to pull the capsule to safety in case of a problem during the earliest phases of launch. Instead military-style ejection seats were used – an arrangement also employed by the Soviet Union’s Vostok spacecraft. After much study and given the relatively low volatility of the propellants used by the Titan II launch vehicle (which tended to burn rather than explode), it was decided that ejection seats offered the best (and lowest mass) abort option for the Gemini crew for a “Mode I” abort. Mode I covered aborts on the launch pad out to about 50 seconds after liftoff when the Gemini, still under power of the Titan II first stage, was below an altitude of about 4,600 meters.

A schematic of the various Gemini launch abort options. (NASA)

During a typical Mode I abort, the doors of the Gemini capsule would swing open and lock in place as the ejection seats, built by Weber Aircraft, catapulted the pilots clear of the spacecraft. The pilots would then be pulled from their seats and descend on their own parachutes. During ejection the maximum stress would be 7 Gs with a maximum of 4 Gs during descent. The system was capable of lifting the pilot clear of the Gemini-Titan II even on the launch pad with the hatch just 46 meters off of the ground.

The Mode I abort option using ejection seats was only available from the launch pad to an altitude of 4,600 meters. (NASA)

After 50 seconds of powered flight, it was no longer safe for the astronauts to use their ejection seats. For the next 50 seconds of flight, the “delayed Mode II” abort option was available while still under the power of the first stage. In this abort mode, the launch vehicle’s engines would be shutdown and the crew would wait five seconds as the ascending spacecraft slowed down to minimize aerodynamic loads. Gemini’s four solid retrorockets would then fire simultaneously to push the reentry module clear of the launch vehicle. The Gemini capsule would then perform a normal parachute landing. Between 100 and 310 seconds after launch when the ascending spacecraft’s altitude had exceeded 23,800 meters, the Mode II abort option without the five-second delay could be employed.

A schematic showing the details for the Mode II and III abort options. Click on image to enlarge. (NASA)

After about five minutes and ten seconds of powered ascent when the velocity of the ascending Gemini and its second stage had exceeded 6.3 kilometers per second, the crew could use the Mode III abort option. In Mode III, the second stage engine would be shutdown when a problem was detected and the Gemini would use its thrusters to push itself clear of the rocket. The Gemini then turns 180 degrees and fires its solid retrorockets one at a time. The Gemini reentry module would then follow a normal descent and landing profile, albeit with a steeper trajectory than a normal reentry from orbit. This more punishing descent trajectory was tested during the unmanned Gemini 2 suborbital test flight in January 1965 (see “The Mission of Gemini 2”). In order to support the various Mode II and III abort options, recovery crews were stationed from 66 kilometers off the coast of Cape Kennedy and across the Atlantic to the coast of Africa.

The typical landing sequence for the Gemini reentry module. Click on image to enlarge. (NASA)

Getting Gemini 6 Off the Ground

With a veritable army of welders, technicians and other workers waiting in the wings, the clock for the launch of Gemini 6 started ticking on December 4, 1965 at 2:30:03.97 PM EST when Gemini 7 was successfully launched from LC-19. Immediately after given the “all clear”, workers descended on the launch pad to assess the damage and make any repairs needed so that the Gemini 6A mission, as it was officially called, could be launched nine days later.

The launch of Gemini 7 at 2:30:04 PM EST on December 4, 1965 cleared the way at LC-19 for the launch of Gemini 6A scheduled for nine days later. (NASA)

It was found that the launch of Gemini 7 had left the pad in very good condition. On December 5, Titan II GLV-6 serial number 62-12561 was installed on the pad at LC-19 followed later that same day by Gemini spacecraft number 6. Testing of the newly erected rocket and spacecraft began immediately. Preparations for the next launch proceeded better than anyone had dared hoped and it soon became apparent that Gemini 6 could be ready for launch a day earlier than originally planned. While a computer problem briefly dampened those hopes, the issue was repaired and the final simulated flight test was completed ensuring the early launch at 9:54 AM EST on Sunday, December 12 just 26 seconds before Gemini 7 passed overhead.

Stafford and Schirra along with other officials at LC-19 shortly after the launch of Gemini 7 reviewing preparations for their upcoming launch. (NASA)

Schirra and Stafford were up before dawn to prepare for their two-day flight to rendezvous with Borman and Lovell on Gemini 7. The flight’s backup crew, Gus Grissom and John Young, had already checked out and prepared spacecraft number 6 for launch by the time Schirra and Stafford were sitting down for their traditional launch day breakfast of steak and eggs with fellow astronaut, Gordon Cooper. Afterward, it was off to LC-16 where the crew donned their standard G4C spacesuits. Then it was on to LC-19 when Schirra and Stafford were strapped into their seats about 90 minutes before launch.

Stafford and Schirra with their G4C spacesuits in the transfer van on their way to LC-19 on December 12, 1965. (NASA)

As the largely uneventful countdown reached “zero” at 9:54:06 AM EST, the now familiar shriek of the Titan’s turbopump gas generator was heard followed by first stage engine ignition. With the spacecraft clock now running, automated systems detected a problem and immediately aborted the launch. After running for just 1.2 seconds, the first stage engines had shutdown with the announcement from the blockhouse,“we have a shutdown, Gemini 6”. Under mission rules, the crew should have ejected. But the cool headed command pilot, Wally Schirra with his hands on the D-ring between his legs that would activate the ejection seats, did not feel the Titan II leave the pad despite the start of the clock which would normally indicate otherwise. The veteran astronaut gambled that he and Stafford were safe… and the gamble paid off. Had the ejection seats been activated, the Gemini 6 mission would have been over before it started.

Gemini 6A on the pad at LC-19 on December 12, 1965. (NASA)

After confirming that the Titan II was safe, the tower was erected to extract the crew from Gemini 6 and figure out what had happened. With the ejection seats safely deactivated, Schirra and Stafford finally exited the spacecraft at 11:33 AM. Work began to turn around the hardware as quickly as possible for another launch attempt. Normally this process would take four days. By this schedule, Gemini 6 could still fly an abbreviated one-day rendezvous mission to be launched on December 16 followed by recovery the next day. The Gemini 7 crew would return to Earth from their two-week mission on December 18 as originally planned.

A disappointed Schirra and Stafford shown leaving the pad at LC-19 after their launch abort. (NASA)

It was quickly discovered that a loose fitting electrical plug had been shaken free too early at engine ignition and caused the premature start of Gemini’s onboard clock. By the next morning, an investigation had shown that a cover had been inadvertently left over the inlet port of the first stage gas generator during its manufacture. Either the plug or the errant cover could have triggered the automated abort. The gas generator would be cleaned out and prepared for another launch attempt. Within a few hours, NASA and Martin engineers were certain not only of the causes of the launch abort, but that they could be ready for another attempt after only a three-day turnaround. But this would be the last chance at the rendezvous mission before Gemini 7 had to return.

The story of the Gemini 6 and 7 missions is concluded in “Rendezvous in Space: Gemini 6 and 7“.

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General References

David Baker, The History of Manned Space Flight, Crown Publishers, 1981

Barton C. Hacker and James M. Grimwood, On the Shoulders of Titans: A History of Project Gemini, SP-4203, NASA History Division, 1977

David J. Shayler, Gemini: Steps to the Moon, Springer-Praxis, 2001

“Gemini 7/6”, NASA Press Release 65-362, November 29, 1965