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The Agena Target Vehicle (ATV) 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.〔 ==Operations== Each ATV consisted of an Agena-D derivative upper rocket stage built by Lockheed and a docking adapter built by McDonnell. The Agena was launched from Cape Canaveral Air Force Station 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 . The modified Bell 8247 engine was qualified for up to 15 restarts. * Undocking, unreeling a 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. The first 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-Martin 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Agena target vehicle」の詳細全文を読む スポンサード リンク
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