As 1966 began, NASA was well on its way towards its goal of reaching the Moon before the end of the decade. During 1965, a total of five crewed Gemini missions had steadily increased NASA’s practical experience in space with the first EVA, the first orbital rendezvous and a series of long-duration missions culminating with a two-week stay in orbit. On February 26, 1966 NASA also launched an Apollo-Saturn IB designated AS-201 for the first unmanned test flight of this vital hardware in preparation for the first crewed Apollo mission planned to fly before year’s end (see “The First Flight of the Apollo-Saturn IB”). Despite the impressive progress that had been made, there was still much work to be done including five more Gemini missions which would continue the vital task of developing and refining the skills necessary to reach the Moon. The first new mission of 1966 would be Gemini 8.

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 system called OAMS (Orbital Attitude and Maneuvering System), 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 and Docking

The Gemini 8 mission was the sixth crewed flight in the Gemini program. The earlier flights in the series had begun to address in a careful step-wise fashion all of the objectives of the program save for one: docking in orbit. 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. 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. 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 was specifically engineered to be a standardized production model which, 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 for Gemini.

Modifications made to the production model 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 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. When the propulsion systems were not operating, attitude was controlled using a set of cold gas thrusters.

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 mechanically lock the two spacecraft together. Once in orbit, the GATV was 9.7 meters long with a mass of about 3,175 kilograms.

The original objective of the two-day Gemini 6 mission was to rendezvous and dock with the first flight-ready GATV, Agena GATV 5002 (see “The Unflown Mission of Gemini 6”). Unfortunately, the Agena was lost during launch on October 25, 1965 when its main engine failed upon ignition. Without a replacement GATV available, a new “Gemini 6A” mission was quickly devised where the spacecraft would rendezvous with Gemini 7 during its already planned two-week mission. Gemini 6 successfully performed its rendezvous with Gemini 7 on December 15 but no docking was attempted because of the lack time to build and qualify an appropriate interface on the crewed target vehicle (see “Rendezvous in Space: Gemini 6 and 7”). Performing the first docking in orbit now fell to the Gemini 8 mission.

The Gemini 8 Mission Plan

The crew of the Gemini 8 mission consisted of Neil A. Armstrong as the command pilot and USAF Major David R. Scott as the pilot. A naval aviator who flew 78 combat missions in Korea, the 35 year old Armstrong earned a BS in aeronautical engineering after leaving the service then became a civilian test pilot working for NASA before joining the astronaut corps in the second group announced in September 1962. During his career as a test pilot at the High Speed Flight Station at Edwards AFB, Armstrong had flown a number of different aircraft including the famous X-15 rocket plane. Armstrong flew the X-15 a total a six times between December 1960 and July 1962 reaching speeds as great as Mach 5.5 and altitudes as high as 63 kilometers – just 17 kilometers shy of qualifying for USAF astronaut wings. Scott, who was 33 years old, was a West Point graduate and received a masters in aeronautical engineering from Princeton University in addition to graduating from the USAF Test Pilot School and Aerospace Research Pilot School. Scott joined NASA with the third group of astronauts selected in October 1963. Neither Armstrong nor Scott had previously flown into space. The backup crew for the mission was Charles “Pete” Conrad, Jr., who had served as the pilot on the Gemini 5 mission (see “Eight Days or Bust: The Mission of Gemini 5”) and Richard F. Gordon, Jr. who was a rookie from NASA’s third group of astronauts like Scott.

The Gemini 8 mission was scheduled to start 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 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. As in the earlier Gemini 6 mission, the “coelliptical” method was chosen instead for the Gemini 8 mission where the active spacecraft would first be placed into a circular orbit below and some distance behind the 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 early attempts of the orbital rendezvous procedure.

Gemini 8, with Armstrong and Scott on board, was scheduled to lift off at 11:40:53 AM EST just as their Agena target was completing its first orbit (the precise liftoff time would vary depending on the exact parameters of the Agena’s actual orbit). If Gemini 8 missed this 6¼-minute launch window, there were various launch windows, each of about 47 minutes duration, available over the next four days. Once in orbit, the Gemini 8 crew would perform a series of maneuvers over the following four orbits to close in on the GATV similar to the sequence successfully performed during the Gemini 6 mission. After completing the rendezvous about 5½ hours after launch, Gemini 8 would fly in formation with the Agena for about 45 minutes and finally dock in sunlit conditions. The astronauts would then perform a series of checks and engineering tests including three additional dockings by Armstrong. The two spacecraft would remain docked as the astronauts began their 7½ hour rest period at the end of the first day.

The EVA

On the second day of the mission, the next primary objective would be addressed: Scott would perform an EVA. In the original plans for the Gemini program, the first EVA was suppose to be a simple “stand up” EVA where the astronaut would expose himself to space but not totally exit his ship. Only in a subsequent mission would the astronaut completely exit the spacecraft. In later missions, increasingly complex tasks would be performed during ever longer EVAs. In this way, each mission would build on the lessons learned earlier.

This conservative, step-wise approach was altered when Soviet cosmonaut Alexei Leonov performed the first EVA during the Voskhod 2 mission on March 18, 1965 (see “The Mission of Voskhod 2”). With no problems reported, Gemini officials scrapped the planned standup EVA for their upcoming Gemini 4 mission. Instead, Edward White completely exited his spacecraft for the first American EVA on June 3 (see “The Forgotten Mission of Gemini 4”). Having leapfrogged ahead in their plans, the EVA originally planned for Gemini 5 was cancelled since the gear needed to meet more advanced EVA objectives was not yet available. With the primary objective of Gemini 6 limited to the untried rendezvous and docking procedure while Gemini 7 focused on a two-week long-duration mission, Gemini 8 would be the first opportunity in nine months for NASA to continue working up the EVA learning curve.

Scott’s EVA was scheduled to start 20½ hours after launch and last two hours and ten minutes. Scott would wear a modified G4C spacesuit similar to the variant worn by Ed White during the Gemini 4 EVA. Scott would initially get his life support through a 7.6-meter long umbilical line attached to the spacecraft. Scott would also wear a 19-kilogram, chest mounted Extravehicular Life Support System (ELSS) similar in function to the one worn by White which would allow Scott to control his life support and provide 30 minutes of emergency oxygen in case an issue arose with his umbilical-supplied oxygen from Gemini. Initially Scott would perform some simple tasks like retrieving samples from the spacecraft exterior and test a power tool. Before the beginning of the first night pass, Scott would work his way to the back of Gemini’s unpressurized adapter section to a special station installed on the underside where he would don the Extravehicular Support Pack (ESP).

The 42-kilogram ESP backpack was designed to provide the astronaut with life support functions for extended periods of time. In addition to a 79-minute oxygen supply, the ESP also carried 8 kilograms of Freon 14 to serve as propellant for a Hand Held Maneuvering Unit (HHMU). The HHMU was first used during the Gemini 4 EVA but its limited supply of pressurized gas was quickly exhausted. The HHMU propellant supply carried by the ESP was about 15 times larger and would allow Scott to maneuver for much longer periods of time. Also stowed with the ESP was a longer 23-meter umbilical line that would allow Scott to move much farther from the spacecraft.

As Scott donned the ESP and prepared for the next phase of his EVA, Armstrong would undock from the Agena GATV and move 18 meters out of plane to fly in formation. Scott would then use the HHMU to maneuver away from Gemini 8 and in formation with the Agena in a series of steps. After Scott had maneuvered to the end of his 23-meter tether, Armstrong would then move the Gemini towards Scott to recover him. The EVA would be completed during the 15th revolution with Gemini flying about 76 meters from the Agena. With the EVA completed, Armstrong would then redock with the GATV to begin another round of engineering tests.

Afterwards, Gemini 8 would undock from the GATV and at about 27 hours, 28 minutes after launch perform a 6.1 meter per second translation burn to place Gemini 8 into a new 292 by 303 kilometer orbit. After moving 21 kilometers from the Agena, Gemini 8 would then start a series of maneuvers to rendezvous with the target once again. Gemini 8 would redock one last time at a mission elapsed time of 29 hours and 5 minutes. After spending 45 minutes docked, Gemini 8 would undock and take up station 460 meters from the Agena to observe it and its rocket plume as the primary propulsion system was ignited. Over the next three days, ground controllers would command the GATV to perform a series of maneuvers to test the spacecraft’s propulsion system. With the propellants of its primary propulsion system depleted, the Agena would be left in a 407-kilometer circular storage orbit which was expected to decay to 370 kilometers over the next four months. At that point, the Agena could be used as a passive target for a future Gemini mission.

With the primary objectives involving the Agena and EVA completed, Armstrong and Scott would spend their third day in orbit performing other experiments and preparing for their return home. Retro fire was scheduled for 70 hours, 12 minutes and 30 seconds mission elapsed time for a splashdown in the primary west Atlantic recovery area where they would be recovered by the aircraft carrier, the USS Boxer. The total mission length was planned to be 70 hours and 48 minutes.

The Mission

The first piece of flight hardware for the Gemini 8 mission to be shipped to Cape Kennedy (as today’s Cape Canaveral was then known) was the Atlas TLV-5302 which would launch the Agena GATV. It arrived on August 11, 1965 and, after undergoing tests and the installation of flight-specific hardware, was erected on the pad at Launch Complex 14 (LC-14) on January 5, 1966. The Agena GTV-5003 target vehicle was shipped to the Cape on January 21 and was mated to the Atlas on March 1 after it had undergone a series of tests. The Gemini’s GLV-8 launch vehicle, Titan II serial number 62-12563, reached Cape Kennedy on January 6 and was erected on its pad at LC-19 a week later. Gemini spacecraft number 8 arrived on January 8 and was temporarily mated to its launch vehicle on February 25. Final mating took place on March 6 followed by more testing. With the final launch simulation completed on March 10, all was set for the actual launch scheduled for March 15.

Problems with Gemini’s environmental control system and difficulties fueling the Atlas pushed the first launch attempt out a day to March 16, 1966. At 7:00 AM EST, Armstrong and Scott were woken up for breakfast and to begin preparations for their launch. At 8:17 AM, the astronauts made their way to a facility located at the deactivated LC-16 where they donned the G4C spacesuits they would wear throughout their three-day mission. By 9:46 AM, the Gemini 8 crew had been strapped into their spacecraft to begin their prelaunch checks.

As the astronauts and ground crews continued the countdown for Gemini 8 at LC-19 and dealt with the usual mix of minor issues including a short in an OAMS thruster heater, the simultaneous countdown at LC-14 for the launch of the target vehicle proceeded smoothly. At 10:00 AM EST, Atlas TLV-5302 lifted off on schedule to begin the first part of the Gemini 8 mission. After the Atlas finished its job, the main engine of the Agena GTV-5003 ignited and successfully placed itself into the planned 298-kilometer circular orbit.

As the GATV was about to complete its first orbit, it was time for Gemini 8 to get off the pad. Right on schedule at 11:41:02 AM EST, Gemini 8 with Armstrong and Scott on board lifted off. After six minutes of powered flight, the second stage of the Titan II rocket shut down placing the 3,788-kilogram Gemini 8 into an initial 160 by 272 kilometer orbit some 1,963 kilometers behind the Agena. With this good start, it was time for Gemini to start catching up with its target.

One hour and 34 minutes after launch as Gemini 8 neared the end of its first orbit, Armstrong fired the OAMS thrusters for five seconds to slow the spacecraft slightly and lower its apogee. During the Gemini 6 mission, the crew was so focused on their tasks that they failed to take a meal break leaving them hungry and tired by the time they rendezvoused with Gemini 7. Learning from this experience, Armstrong and Scott started preparing some lunch as they headed towards their second apogee.

At two hours, 18 minutes and 25 seconds mission elapsed time as the crew was waiting for their meals to rehydrate, Armstrong fired the OAMS thrusters to increase the spacecraft’s velocity by 15 meters per second. Just over 27 minutes after this maneuver while over the Pacific, Armstrong turned the nose of the spacecraft 90° towards the south of its flight path and fired the thrusters once again for a delta-v of 8 meters per second. An additional trim maneuver of 0.6 meters per second a short time later lined up Gemini’s orbital plane with its target. This was followed by a rendezvous radar test with a lock established with Agena’s radar transponder at a range of 332 kilometers.

As Gemini 8 continued to close in, the astronauts visually spotted their Agena target at a range of 140 kilometers as it gleamed in the sunlight. Armstrong continued the series of preplanned maneuvers to line up Gemini’s orbit with the GATV with Scott reading out the range and range rate figures. Finally at 5 hours, 43 minutes and 9 seconds, Armstrong fired the thrusters one last time to bring Gemini 8 to a halt some 46 meters from GATV 5003. The Gemini program had completed its second successful rendezvous. For the next 36 minutes, Armstrong maneuvered the Gemini around the Agena as he and Scott inspected their target. Finally, Armstrong eased the nose of his spacecraft into Agena’s TDA at a speed of just 8 centimeters per second and at 6:15 PM EST announced “Flight, we are docked!”. The first docking of two spacecraft in orbit had been completed.

The first order of business was for Scott to issue a sequence of 16 commands to the GATV so that the astronauts could use it to control the combination’s attitude. As the combined spacecraft began its pass over the Indian Ocean, the Agena’s cold gas thrusters were used to yaw by 90°. After this turn was completed 55 seconds later, commands were issued to the Agena to start its tape recorder. At this point 27 minutes after docking while out of communication range with ground controllers, Scott noticed that the spacecraft had rolled out of its intended attitude by 30°. Armstrong used the thrusters to correct the roll but it soon began to move out of its proper attitude again. Thinking that the problem was with the GATV, Scott commanded the Agena’s attitude control system off after Armstrong had once again steadied the craft.

For the next four minutes the joined spacecraft were in a stable attitude. With the problem apparently passed, Armstrong started to maneuver them into the proper horizontal orientation. At this point, the spacecraft began to roll at an ever increasing rate. As the astronauts cycled through various switches in an attempt to isolate the problem, Armstrong noticed that the OAMS propellant level had dropped to 30% of its original 308-kilogram load strongly suggesting that the problem was with the Gemini. Unable to isolate the problem, the decision was made to separate from the Agena to analyze the situation. Scott returned control of the Agena to the ground as Armstrong struggled to steady the spacecraft enough to undock. Scott hit the undocking button on Armstrong’s command and then gave a long burst of the thrusters to back away from the Agena.

After the two spacecraft had separated, the Gemini’s roll rate accelerated confirming Armstrong’s suspicions that the problem was with Gemini. As the spacecraft reached a roll rate of one revolution per second three minutes after separation, the astronauts began to shut down the OAMS system and activate the reentry module’s reentry control system (RCS) to get the spacecraft under control. As Armstrong began to slow the rolling spacecraft as ground controllers attempted to contact Gemini 8, he switched off one of the two redundant sets of RCS thrusters to conserve its propellant – this system would be needed for attitude control during reentry. Armstrong carefully reactivated the OAMS thrusters and discovered that the number 8 thruster had failed in a stuck-on mode due to a short in its electrical control system. While the crew was finally able to regain control of their spacecraft, mission rules dictated an early return home because of the low propellant levels and potential leakage issues with the reentry module’s RCS thrusters after they had been activated in flight. Scott’s EVA was cancelled and additional dockings with the Agena were out of the question.

With the situation in orbit under control for the moment, the flight director made the decision about 7½ hours into the mission to bring Gemini 8 back during the seventh revolution for a landing in a preplanned contingency recovery zone in the Pacific 800 kilometers east of Okinawa. This would give the crews on the ground and in orbit ample time to make the needed preparations. With this decision, the USS Leonard F. Mason, a Gearing-class destroyer assigned to this zone, changed course and increased to full speed to reach Gemini’s expected landing point as recovery planes from Okinawa were dispatched.

As Gemini 8 made its final pass over the Indian Ocean almost three hours after the thruster malfunction, it automatically fired its four solid retrorockets to begin its early return home. Gemini 8 successfully landed only 2.6 kilometers from its intended aim point at 10:22 PM EST. The total mission length was 10 hours, 41 minutes and 25 seconds – only 15% of that planned. Gemini 8 was quickly joined by USAF pararescuemen who secured the spacecraft only 45 minutes after splashdown. Three hours later, the Mason finally arrived to recover the astronauts and their spacecraft. Armstrong and Scott were now safe and on their way to Okinawa and then back to the States.

Despite the problems which cut the mission short, the primary objective of rendezvousing and docking with the Agena had been successfully accomplished as were several secondary objectives. Meeting the other objectives including those of Scott’s cancelled EVA would have to wait until the upcoming Gemini 9 mission to be launched in June (see “The Angry Alligator & The Snake: The Mission of Gemini 9“). Efforts were made to address the ongoing thruster issues which had affected the earlier Gemini missions including the addition of a master switch to shut down the OAMS system to address an improbable repeat of the Gemini 8 thruster failure. Despite the set back, the successful docking of Gemini 8 brought NASA one step closer to reaching the Moon.

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Related Video

Here is an excellent NASA documentary from 1966 about the Gemini 8 mission entitled “Gemini VIII, This is Houston Flight”.

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Related Reading

“The Unflown Mission of Gemini 6”, Drew Ex Machina, October 25, 2015 [Post]

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

“Rendezvous in Space: The Gemini 6 Launch Abort”, Drew Ex Machina, December 12, 2015 [Post]

“Rendezvous in Space: Gemini 6 and 7”, Drew Ex Machina, December 15, 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 8 Press Kit”, NASA Press Release 66-52, March 11, 1966