The launching of spacecraft to rendezvous and dock with the International Space Station (ISS) is considered routine today. Without this ability, new crews and vital supplies could not be delivered to the ISS during its ongoing mission in low Earth orbit. While this skill may be taken for granted today, a half a century ago a pair of orbiting spacecraft had yet to rendezvous and dock in space despite the fact that this activity was central to the lunar orbit rendezvous (LOR) mission architecture adopted for NASA’s Apollo missions to the Moon. Although there had been much theoretical work performed on orbital rendezvous and docking, actual experience was required to test these concepts and gain practical experience. With less than a half a decade left until the self-imposed “end of the decade” deadline for the first Apollo Moon landing, it fell to NASA’s Gemini 6 mission to be launched on October 25, 1965 to attempt the first rendezvous and docking of two spacecraft in orbit.

Gemini Program Objectives

The purpose of NASA’s Gemini program was to develop the technologies and techniques needed to fulfill President Kennedy’s goal of landing a man on the Moon by 1970. The major objectives of the program were:

– Demonstrate that humans and their equipment can survive up to two weeks in space

– Demonstrate rendezvous and docking techniques in orbit

– Demonstrate the technology and techniques needed to perform EVAs (Extra-Vehicular Activities)

Meeting all of these objectives was necessary if the Apollo lunar program were to be successful.

Gemini was a two-man spacecraft that was roughly conical in shape with a base diameter of 3.3 meters which stood 5.8 meters tall. Built by the McDonnell Aircraft Corporation (which merged with Douglas in 1967 to become McDonnell Douglas which subsequently merged with Boeing 30 years later), it consisted of two major sections. The first section was the reentry module which housed the crew, their equipment, food supplies and so on in orbit as well as the recovery systems needed to safely return them to Earth. The nose of this module also contained an L-band radar system for orbital rendezvous operations. Unlike today’s crewed spacecraft, the Gemini crew cabin was pressurized with pure oxygen at about one-third standard atmospheric pressure to save weight. The next section, the adapter section, connected the reentry module to the launch vehicle during ascent and housed equipment needed to support the crew while in orbit. It consisted of a retrograde section which held a set of four solid retrorockets used to start the descent to Earth from orbit and an equipment section which housed the in-orbit propulsion systems, life support, power systems and all other equipment not needed for the return to Earth.

With a typical launch mass of up to about 3,700 kilograms or more, Gemini needed the largest operational rocket available at the time to get into orbit: a modified Titan II ICBM built by Martin Marietta (which is now part of Lockheed Martin). A number of modifications were made to simplify the operation of this rocket, smooth out its ride and improve its reliability to support crewed missions. With the Gemini spacecraft mounted on top, the Titan II GLV (Gemini Launch Vehicle) was 33 meters tall and had a fully fueled launch mass of about 154 metric tons.

Rendezvous & Docking

The Gemini 6 mission was to be the fourth crewed mission of the Gemini program. The first three missions had steadily pushed the capabilities of the crews and spacecraft to meet the program’s main objectives. While the goals of the previous two missions were centered primarily on increasing mission lengths in a stepwise fashion, the primary objective for the Gemini 6 mission was to tackle the untried procedure of rendezvous and docking with another spacecraft in orbit.

As with the other objectives of the Gemini program with each successive mission building on the experience of earlier missions, the previous Gemini missions had already started testing the skills needed by Gemini 6. During the Gemini 4 mission, the crew attempted a station keeping exercise with the spent second stage of their Titan II launch vehicle immediately after reaching orbit. The problems they encountered stressed the need for better training and preparation for such a task especially in the sometimes anti-intuitive realm of orbital flight (see “The Forgotten Mission of Gemini 4”). During the Gemini 5 mission, the crew deployed a Radar Evaluation Pod (REP) to serve as a target for a simple rendezvous exercise as the next step. While Gemini’s radar system was briefly checked out, unfortunately problems encountered with the spacecraft’s power-producing fuel cells during the first hours in orbit delayed the actual rendezvous exercise and ultimately forced a change of plans. With the REP no longer available, the Gemini 5 crew performed a rendezvous with a virtual target using precise maneuvers calculated on the ground and by the Gemini’s onboard guidance computer (see “Eight Days or Bust: The Mission of Gemini 5”). Gemini 6 would now build on this earlier experience to perform the first rendezvous and docking – a vital step for the upcoming Apollo missions to the Moon.

The docking target chosen for the Gemini program was a modified Agena D upper stage known as the Gemini Agena Target Vehicle (GATV) launched into orbit using the SLV-3 (Standard Launch Vehicle-3) version of the Atlas booster. Built by the Lockheed Missile and Space Company (which is now part of the aerospace giant, Lockheed Martin), development of the Agena had started back in 1956 for use as an upper stage with the Thor and Atlas missiles initially to orbit defense-related payloads such as the Corona spy satellites and the MIDAS early warning satellites (see “The First Discoverer Missions: America’s Original (Secret) Satellite Program“). In addition to serving as an upper stage for these missions, the Agena remained attached to the these payloads as well as others while in orbit to provide support functions such as power and attitude control (see “The First Nuclear Reactor in Orbit”). Later the Atlas-Agena was chosen to launch a variety of NASA missions including the Ranger lunar missions (see “The Prototype That Conquered the Solar System”) and the first Mariner spacecraft to Venus and Mars (see “The Launch of Mariner 3”).

Although it had experienced a number of problems since its first launch in 1959, the Agena had developed into a versatile and increasingly reliable tool to support a wide range of missions. The Agena D, like the earlier models, was 1.5 meters in diameter and used a Bell 8000-series rocket engine which employed UDMH (unsymmetrical dimethyl hydrazine) and IRFNA (inhibited red fuming nitric acid) as propellants. These hypergolic propellants ignite on contact thus simplifying engine design. The standard, 6.3-meter long Agena D included options for four modules containing guidance, beacon, power, and telemetry systems, a standard payload console, and a rear rack above the engine for plug-in installation of optional equipment such as solar panels and piggyback payloads including deployable subsatellites (see “Vintage Micro: The Original Picosatellite”). The standard Agena D of this time employed a 71-kilonewton Bell 8096 engine for its primary propulsion system which was capable of up to three starts for a total burn time of 265 seconds. This highly adaptable stage was ideal for serving as a docking target.

Modifications made to the standard Agena D to support its new role in the Gemini program included replacement of the standard Bell 8096 engine in the primary propulsion system with the 8247 model which was capable of up to 16 starts in space. Such a capability was required not only to insert the stage into orbit and perform subsequent trajectory modifications needed for rendezvous, but also to support the option of sending the combined Gemini-Agena into higher altitude orbits after docking. The ullage propulsion system normally carried in the rear to keep the propellants settled at the bottom of their tanks during the weightless coast in orbit prior to reigniting the main engine was replaced with a pair of new secondary propulsion modules. Each of these modules was equipped with 71 and 890-netwon engines burning UDMH and a nitrogen oxide mixture. These engines were capable of multiple restarts to perform small orbit adjustments in addition to ullage functions.

The forward end of the modified Agena D was fitted with an auxiliary rack holding special rendezvous and telemetry equipment. Also added were strobe lights and an L-band radar transponder to aid in rendezvous operations as well as command equipment to allow the GATV to be controlled from the ground or by the Gemini crew. A cone shaped target docking adapter (TDA), which was under a shroud during launch, was added to the forward end of the stage to allow the nose of the Gemini reentry module to dock with the Agena and lock the two spacecraft together. Once in orbit, the GATV was 9.7 meters long with a mass of about 3,175 kilograms.

The Gemini 6 Mission Plan

The primary crew for the Gemini 6 mission consisted of US Navy Captain Walter M. Schirra, Jr. as the command pilot and USAF Major Thomas P. Stafford as the pilot. Schirra, a US Naval Academy graduate who served as a jet fighter pilot in the Korean War, was one of the original seven Mercury astronauts chosen by NASA in 1959. He had flown on the six-orbit Sigma 7 mission in October 1962 logging over nine hours in space. Stafford, who was also a US Naval Academy graduate, was a pilot who served as the chief of the Performance Branch of the USAF Aerospace Research Pilot School at Edwards Air Force Base before he was selected as an astronaut in September 1962. This would be his first space mission. The backup crew were veteran astronauts USAF Lt. Colonel Virgil “Gus” Grissom and US Navy Commander John W. Young who had previously flown together during the Gemini 3 mission launched on March 23, 1965 (see “The Mission of Gemini 3”).

Since the untried rendezvous and docking procedure with the GATV was the primary objective of the Gemini 6 flight, it was decided that this mission would last just for two days so that training could focus on this complex task. The Gemini 7 mission, scheduled for December 1965, would be the next long-duration mission to continue the ramp up of mission length started by the Gemini 4 and 5 flights. If for some reason the objectives of the Gemini 6 mission were not fully met, the Gemini 8 scheduled for the first quarter of 1966 would try again.

According to the original plan, the Gemini 6 mission was scheduled to start on October 25, 1965 with the launch of the Atlas-Agena GATV at 10:00 AM EST from Launch Complex 14 (LC-14) at Cape Kennedy, Florida. The GATV would place itself into a 298-kilometer circular orbit with an inclination of 28.87°. A number of different rendezvous modes with the GATV had been identified during theoretical studies. The most aggressive and fastest of these, which would be preferred for the Apollo missions when the Lunar Module departed the Moon’s surface, was the “first apogee” method where the active spacecraft would be launched directly into an elliptical orbit that would allow a rendezvous with the passive target spacecraft during the active spacecraft’s first apogee. This method required a fast-paced sequence of precisely timed and executed maneuvers in order to be successful. For the Gemini 6 mission, the “coelliptical” method was chosen instead where the active spacecraft would first be placed into a circular orbit below and some distance behind the passive target spacecraft. The active spacecraft would then catch up to its target over the course of several orbits then maneuver to match the target’s orbit in order to perform the actual docking. While this approach took longer, it was much more flexible and allowed more time to plan and execute maneuvers – a desirable characteristic for the first attempt of the untried orbital rendezvous procedure.

The launch of Gemini 6 would take place at 11:18 AM EST from LC-19 as the GATV was completing its first orbit. After almost six minutes of powered ascent, Gemini 6 would be placed into an initial 161 by 270-kilometer orbit about 1,930 kilometers behind the GATV. At the end of its first orbit, Gemini 6 would perform a trim maneuver to raise its perigee by almost one kilometer and correct for any trajectory deviations from launch while it was now 1,185 kilometers behind the GATV. The first major rendezvous maneuver would take place at the second apogee about 2 hours and 19 minutes after launch. This 16.3 meter per second “Phase Adjustment” maneuver would raise the spacecraft’s perigee to 215 kilometers and slow the closing rate with the GATV. One orbit later 3 hours and 48 minutes into the flight, Gemini 6 would execute a its “Coelliptical Maneuver” of up to 16 meters per second to circularize its orbit at an altitude of 270 kilometers – 28 kilometers below that of the GATV. At this point, Gemini 6 would be 259 kilometers behind its target with the closing rate slowed even further.

About five hours after launch, Gemini 6 would execute the “Terminal Phase Initiation” with the GATV 63 kilometers away. This was the beginning of a series of final maneuvers that would move Gemini 6 into the same orbit as its target and allow docking in daylight conditions about 5 hours and 45 minutes into the flight near the end of the fourth orbit. The requirements of this sequence of maneuvers and the need to perform the initial docking in daylight limited Gemini 6 to a 2½ hour launch window on the day of the GATV launch with a series of “panes” for each of several alternative sequences of mission events. In case the Gemini launch was delayed beyond the first day, launch windows with a length of 4¼ hours were available for the next five days.

After the initial docking of the Gemini and GATV, a series of tests and exercises would be performed. This included having Gemini 6 undock, separate and then redock with the GATV up to three more times allowing both Schirra and Stafford the opportunity to attempt the procedure in both day and night conditions. This would provide the maximum experience from this first ever docking mission.

At 10 hours 35 minutes into the mission, the crew would begin their seven-hour sleep period with the two spacecraft still docked with each other. At a mission elapsed time of 18 hours and 20 minutes, Gemini 6 would undock for the last time from the GATV and perform a two meter per second separation burn to maneuver into a 289 by 298-kilometer orbit that would allow Gemini to pull ahead of the GATV at a rate of 33 kilometers per revolution. The astronauts would then conduct a series of inflight experiments like those performed on early Gemini flights until the start of their second sleep period 33 hours into the mission. Retrofire was scheduled for a mission elapsed time of 46 hours and 10 minutes during the 29th revolution with the splashdown taking place in the West Atlantic Zone 530 kilometers southeast of Bermuda after a flight of 46 hours and 45 minutes with the recovery handled by the US Navy aircraft carrier, USS Wasp. There was also an option to end the mission after one day with retrofire at 23 hours 41 minutes after launch to allow splashdown in the same primary recovery zone.

The day after the end of the Gemini 6 mission, the GATV would begin its “solo” mission. With a number of favorable passes over North America, the Agena’s systems would be put through a series of tests including firings of its propulsion systems to simulate various rendezvous maneuvers expected in future missions. At the conclusion of this solo mission, the GATV would be placed into a circular 445-kilometer storage orbit. Over the course of the following four months, the orbit would be expected to decay to an altitude of about 271 kilometers at which point the now inactive GATV could be used as an inert target for future Gemini rendezvous exercises.

The Attempt and a Change of Plans

Schirra and Stafford had already been training together since the spring of 1964 as the backup crew for the Gemini 3 mission with Grissom and Young as the prime crew. After the successful completion of that mission in March 1965, they shifted to training for the Gemini 6 mission which, although brief in duration, would execute the most complex sequence of tasks ever attempted during a crewed spaceflight.

As the crew for Gemini 6 trained for their flight, the hardware for their mission was assembled and tested. The first piece of flight hardware to arrive at Cape Kennedy was the Atlas booster for the GATV on December 4, 1964. Designated Target Launch Vehicle (TLV) 5301, the Atlas remained in storage until it was erected at LC-14 in October 1965. The GATV, Agena number 5002, was delivered to the Cape on July 25 and started a series of prelaunch inspections and tests on August 23.

In the mean time, Titan II GLV-6, serial number 62-12561, was shipped from Martin’s Baltimore, Maryland plant to Cape Kennedy on August 2, 1965 and was erected on the pad at LC-19 for the first time at the end of the month for its own series of prelaunch preparations. After completing a series of post-assembly tests at the McDonnell’s plant in St. Louis, Gemini spacecraft number 6 followed on August 4. Among the many tests conducted were compatibility checks with the Agena 5002 target involving the crew on August 25. After further work, spacecraft number 6 was mated to its Titan launch vehicle on September 18 with the Agena 5002 mated to its Atlas booster on October 10. After a planned series of launch readiness tests, the crew and the two spacecraft were ready for the first launch attempt scheduled for Monday, October 25.

Around 7:00 AM EST on launch day, Schirra and Stafford were woken up and began their preparations for launch starting with a brief medical examine and breakfast. An hour and a half later, they were taken to the ready room facility at LC-16 (a former USAF Titan launch complex transferred to NASA to support the Gemini program) where they donned the G4C spacesuits that they would wear for their two-day mission. By 9:45 AM, the two astronauts had boarded their spacecraft at LC-19 to begin their prelaunch checks.

In the mean time, the countdown for Atlas TLV-5301 carrying the Agena 5002 GATV was proceeding in parallel at LC-14. At 10:00:04 AM EST, TLV-5301 lifted off the pad and proceeded to follow its planned trajectory downrange over the Atlantic and towards orbit. After a nominal performance of the Atlas whose sustainer engine shutdown 4 minutes 41 seconds after liftoff, the Agena separated and began a preprogrammed coast. At the 6 minute 20 second mark when the Agena’s primary propulsion system was to ignite for the burn of over three minutes that would place it in orbit, all telemetry was lost. Fourteen minutes after launch, still with no telemetry being received from downrange tracking stations, a hold was called for the Gemini 6 countdown as the situation was assessed. A later investigation concluded three weeks later that the Agena’s main engine had experienced a “hard fire back start” at ignition (essentially equivalent to the backfiring of an automobile engine) which resulted in a cascade of events ending with a fuel tank rupture and the destruction of the ascending stage. With no further word from the Agena 5002, the Gemini 6 launch was officially scrubbed at 10:54 AM and the astronauts exited their spacecraft shortly afterwards.

With the loss of the first GATV, NASA officials had found themselves in a bit of a quandary. In an attempt to reign in the spiraling costs of the Gemini program, they had not planned for the loss of a GATV and did not have backup hardware readily available. With the completion of Agena 5003 meant for the Gemini 8 mission still months away, the other option was to refurbish the already-built Agena 5001 which had been used for ground testing at the Cape over the previous five months. But even this option would require months of work that threatened the Gemini program’s increasingly aggressive schedule to support Apollo.

As an official Agena Review Board was formed on October 27 to examine the failure of Agena 5002 and recommend corrective action, another option was already being seriously examined by Gemini program participants. Earlier in 1965, engineers at Martin had considered the option of launching a pair of Gemini spacecraft a few days apart from LC-19 with the second spacecraft rendezvousing with the first. Docking might even be possible using an inflatable collar mounted in the rear of the Gemini’s adapter section, although this was not a particularly popular option given the safety issues as well as the lack of time to develop the needed hardware and new procedures.

Within days, the idea that Gemini 6 could at least rendezvous with the already planned Gemini 7 long-duration mission quickly gained support at all levels. While it would be a challenge to quickly refurbish the pad at LC-19 after the first launch and control two crewed spacecraft in orbit simultaneously, the proposal was found to be feasible especially given the planned two-week length of the Gemini 7 mission. And while it was decided that no docking would be attempted, at a minimum the complex rendezvous procedures could be tested leaving the first actual docking to the Gemini 8 mission scheduled for early 1966.

On October 28, 1965 the new plan was publicly unveiled: Gemini 7 would be launched first on its original long-duration mission on December 4 with the second mission, now officially designated “Gemini 6A”, launched from the same pad nine days later. Since the Titan II GLV-6 did not quite have the performance needed to launch the long-duration spacecraft which was almost 120 kilograms heavier than the one for the rendezvous mission, spacecraft number 6 was destacked on October 28 and its Titan II removed from the pad. The way was now clear to start preparations for the Gemini 7 launch and the beginning of the most ambitious crewed mission ever attempted by the United States.

The next part of this story, “Rendezvous in Space: The Launch of Gemini 7“, covers events leading up to the launch and first few days of the Gemini 7 mission which served as the rendezvous target for Gemini 6A.

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Related Reading

“Rendezvous in Space: The Launch of Gemini 7”, Drew Ex Machina, December 4, 2015 [Post]

“Eight Days or Bust: The Mission of Gemini 5”, Drew Ex Machina, August 21. 2015 [Post]

“The Forgotten Mission of Gemini 4”, Drew Ex Machina, June 3, 2015 [Post]

“The Mission of Gemini 3”, Drew Ex Machina, March 23, 2015 [Post]

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 6 Press Kit, NASA Press Release 65-327, October 20, 1965