Agena target vehicle
|Country of origin||United States|
|Launch mass||18,030–18,100 pounds (8,180–8,210 kg)|
|Dry mass||4,012–4,085 pounds (1,820–1,853 kg)|
|First launch||October 25, 1965|
|Last launch||November 11, 1966|
|Derivatives||Augmented Target Docking Adapter|
The Agena Target Vehicle (ATV), also known as Gemini-Agena Target Vehicle (GATV) was an unmanned spacecraft used by NASA during its Gemini program to develop and practice orbital space rendezvous and docking techniques, and to perform large orbital changes, in preparation for the Apollo program lunar missions. The spacecraft was based on Lockheed Aircraft's Agena-D upper stage rocket, fitted with a docking target manufactured by McDonnell Aircraft. The combined spacecraft was a 26-foot (7.92 m)-long cylinder with a diameter of 5 feet (1.52 m), placed into low Earth orbit with the Atlas-Agena launch vehicle. It carried approximately 14,021 to 14,054 pounds (6,360 to 6,375 kg) of propellant and gas at launch, and had a gross mass at orbital insertion of 7,117 to 7,271 pounds (3,228 to 3,298 kg).
The ATV for Gemini 6 failed on launch on October 25, 1965, which led NASA to develop the Augmented Target Docking Adapter (ATDA) as a backup, a smaller spacecraft consisting of the docking target fitted with an attitude control propulsion system, but lacking the Agena orbital change rocket. The ATDA was used once on Gemini 9A after a second ATV launch failure on May 17, 1966, but failed as a docking target because its launch shroud failed to separate.
Each ATV consisted of an Agena-D derivative upper rocket stage built by Lockheed Aircraft and a docking adapter built by McDonnell Aircraft. The Agena was launched from Cape Kennedy's Launch Complex 14 on top of an Atlas booster built by the Convair division of General Dynamics. The Agena first burn would occur shortly after shroud jettison and separation from the Atlas over the Atlantic Ocean. Over Ascension Island, a second burn would place the Agena into a low circular orbit.
The McDonnell Gemini spacecraft would then be launched from Launch Complex 19, as soon as 90 minutes later. Both countdowns would proceed in parallel and required close synchronization. The Gemini would rendezvous and dock with the Agena as soon as Gemini's first orbit toward the end of the program.
The Gemini astronauts would then fly the combined spacecraft in a stabilized mode and perform a number of experiments:
- Using the Agena's attitude control system to stabilize the combination, which saved the Gemini's propellants
- Extra-vehicular activity to perform practice work on a tool panel. This required installing handrails on later flights to prevent excessive astronaut exertion.
- Refiring the Agena engine to raise the spacecraft's apogee. Gemini 11 reached a record of 739.2 nautical miles (1,369.0 km). The modified Bell 8247 engine was qualified for up to 15 restarts.
- Undocking, unreeling a 50-foot (15 m) nylon tether between the capsule and the Agena and flying in a "dumbbell" configuration with the Agena below the astronauts, to check the gravitational effect on the formation stability in uncontrolled mode. This technique is now known as gravity-gradient stabilization.
- Using a similar tether and a few thruster bursts to rotate the two craft around each other as an early test of artificial gravity.
- After rendezvous with its own ATV, Gemini 10 performed a second rendezvous with the ATV from Gemini 8.
After the Gemini capsule separated for the last time, the Agena remained in orbit for a short time and was used to verify the command system.[clarification needed]
The first Gemini-Agena Target Vehicle (GATV) was launched on October 25, 1965, while the Gemini 6 astronauts were waiting on the pad. While the Atlas performed normally, the Agena's engine exploded during orbital injection. Since the rendezvous and docking was the primary objective, the Gemini 6 mission was scrubbed, and replaced with the alternate mission Gemini 6A, which rendezvoused (but could not dock) with Gemini 7 in December.
An investigation into the failure concluded that it was most likely caused by design modifications to the GATV versus a standard Agena D stage. The Agena D was designed to have its engine restarted just once while the GATV would need to be restarted five times. While a standard Agena D pumped oxidizer into the combustion chamber first and then followed with the fuel, the GATV was modified to do the reverse because the normal start method had a tendency to leak oxidizer. While this would not be a problem for the Agena D with its single restart, the multi-restart GATV would eventually lose all of its oxidizer before the stage's operating life (which would last weeks instead of hours) could be completed. Unfortunately, pumping the fuel into the combustion chamber first caused the engine to backfire and rupture from mechanical shock. It was found out that Lockheed engineers did not properly test the GATV to root out this problem (it had been tested at a simulated altitude of 21 miles up when actual Agena engine start would occur at around 75 miles up). The solution to the problem was switching back to the normal oxidizer-first engine start and also testing the GATV in appropriate conditions. Bell Aerosystems, the manufacturer of the Agena's engine, were also instructed to perform further ground-level tests.
Augmented Target Docking Adapter
After the failure of the first GATV, NASA commissioned McDonnell to develop a backup docking target minus the Lockheed Agena rocket, the Augmented Target Docking Adapter (ATDA). This consisted of the Gemini docking collar and an attitude control propulsion system based on the Gemini Reentry Control System. The ATDA was 10.9 feet (3.3 m) long, with a mass of 1,750 pounds (794 kg). A few questions were raised about the compatibility of the ATDA with the Atlas booster, since it had much lower mass than the GATV, potentially throwing off the launch vehicle's aerodynamics and calibrated settings. However, Convair assured McDonnell that it would pose no technical issues with the booster.
A second GATV launch failure occurred on May 17, 1966, as Gemini 9 astronauts Tom Stafford and Eugene Cernan sat on their pad awaiting launch. The Atlas-Agena lifted smoothly into a cloudy sky, vanishing from view around T+50 seconds. Shortly before Booster Engine Cutoff (BECO), the guidance control officer announced that he had lost contact with the booster.
Telemetry indicated that vernier and sustainer cutoff occurred at T+300 seconds and Agena staging had taken place. The Agena continued transmitting signals until T+436 seconds, when all telemetry ceased. However, the Atlas's right booster engine had gimbaled hard to right at around T+120 seconds, flipping the launch vehicle 216° around headed back towards Cape Kennedy. This rotation had made it impossible for ground guidance to lock on. Radar stations in the Bahamas tracked it heading north and descending. Atlas engine cutoff and Agena staging occurred on schedule, and both vehicles plunged into the Atlantic Ocean 107 nautical miles (198 km) downrange. The Agena engine did not activate since the proper altitude and velocity had not been attained, preventing the guidance system from sending the start command. While the exact cause of the engine gimbal control was not found, telemetry indicated that a short-to-ground occurred in the circuit for the servoamplifier output command signal, which may have been caused by cryogenic leakage in the thrust section.
While Convair accepted responsibility for the launch failure, Lockheed engineers expressed concern about telemetry data that indicated a servo failure in the Agena, leading to doubts as to whether the stage would have still operated properly if the Atlas hadn't malfunctioned. However, the true cause of failure surfaced when the Air Force released film taken by tracking cameras at Melbourne Beach, Florida, which showed the Atlas pitching over and heading downward. It was then determined that the Agena's servo malfunction was caused by passing through the Atlas's ionized exhaust trail.
The Gemini 9A modified mission launch was rescheduled for June 1, 1966, using the ATDA. However, the shroud that protected the docking adapter during launch failed to separate, due to lanyards being incorrectly secured with adhesive tape. Gemini 9A was launched on June 3, and when in orbit, the crew observed that the shroud of the ATDA had partially opened and was described by Stafford as "looking like an angry alligator". Docking was not possible, but the rendezvous maneuver was practiced instead. 
|Target||Gemini mission||Launched||Reentered||NSSDC ID||Gross mass in orbit||Comments|
|GATV-5002||Gemini 6||October 25, 1965
|October 25, 1965
|GEM6T||N/A||Atlas-Agena exploded during launch.
Gemini 6A achieved first rendezvous with Gemini 7 instead.
|GATV-5003||Gemini 8||March 16, 1966
|September 15, 1967||1966-019A||7,117 pounds (3,228 kg)||Achieved first docking, but mission soon aborted due to stuck Gemini thruster.
ATV later used as secondary target on Gemini 10.
|GATV-5004||Gemini 9||May 17, 1966
|May 17, 1966
|GEM9TA||N/A||Failed to orbit.|
|ATDA No. 02186||Gemini 9A||June 1, 1966
|June 11, 1966||1966-046A||1,750 pounds (794 kg)||No Agena rocket. Successful rendezvous, but no docking due to shroud separation failure.|
|GATV-5005||Gemini 10||July 18, 1966
|December 29, 1966||1966-065A||7,236 pounds (3,282 kg)||Boosted Gemini 10 to 412-nautical-mile (763 km) apogee.|
|GATV-5006||Gemini 11||September 12, 1966
|December 30, 1966||1966-080A||7,271 pounds (3,298 kg)||Boosted Gemini 11 to record 739.2-nautical-mile (1,369.0 km) apogee.
First demonstration of artificial gravity created in microgravity.
|GATV-5001A||Gemini 12||November 11, 1966
|December 23, 1966||1966-103A||7,117 pounds (3,228 kg)||No apogee boost due to defective Agena engine. Performed tether experiment.|
- Lockheed Missiles & Space Company (1972-02-25). "Shuttle/Agena study. Volume 1: Executive summary". NASA.
- Gemini 6 Target: NASA Space Science Data Coordinated Archive
- Or "Atlas Target Docking Adapter", Encyclopedia Astronautica
- "Gemini 9 Target A" (Version 4.0.25 ed.). NASA. August 16, 2013.
- "Gemini 9A" (Version 4.0.25 ed.). NASA. August 16, 2013. Retrieved October 30, 2013.
- "Gemini 9 Target B" (Version 4.0.25 ed.). NASA. August 16, 2013. Retrieved October 30, 2013.