 Agena-D outfitted as a Gemini programTarget Vehicle | |
| Manufacturer | Lockheed Corporation |
|---|---|
| Country of origin | United States |
| Used on | |
| General characteristics | |
| Height | 248 inches (6.3 m) |
| Diameter | 60 inches (1.5 m) |
| Launch history | |
| Status | retired |
| Total launches | 365 |
| First flight | 1959 |
| Last flight | February 12, 1987 |
| RM-81 Agena | |
| Powered by | OneBell XLR-81 |
| Maximum thrust | 16,000 pounds-force (71 kN) |
| Specific impulse | 293 seconds (2.87 km/s) (vac) |
| Burn time | 120–265 s |
| Propellant | UDMH/Red fuming nitric acid |
TheRM-81 Agena (/ədʒinə/) was an American rocketupper stage andsatellite bus which was developed byLockheed Corporation initially for the canceled WS-117Lreconnaissance satellite program.[1] Following the division of WS-117L intoSAMOS andCorona for image intelligence, andMIDAS for early warning, the Agena was later used as an upper stage, and an integrated component, for several programs, includingCoronareconnaissance satellites and theAgena Target Vehicle used to demonstraterendezvous and docking duringProject Gemini. It was used as an upper stage on theAtlas,Thor,Thorad andTitan IIIB rockets, and considered for others including theSpace Shuttle[2] andAtlas V. A total of 365 Agena rockets were launched between February 28, 1959[3] and February 1987. Only 33 Agenas carried NASA payloads and the vast majority were for DoD programs.
On some missions, the payload was built directly into the Agena, which provided it with electric power, communications andthree-axis stabilization. Payload components were usually located ahead of the Agena's standard bulkhead. On missions where the payload was not built into the Agena, and instead separated after launch, the Agena was known as anAscent Agena. The Agena was upgraded twice from the original Agena A in order to support heavier and more sophisticated satellites, such as Corona spacecraft with multiple and more powerful cameras.
The Agena name was suggested by the Department of Defense'sAdvanced Research Projects Agency for the starBeta Centauri, also known as Agena, because this upper stage would "ignite in the sky". This followedLockheed's tradition of naming products for stellar phenomena.[4]
The final launch was of an Agena D on February 12, 1987, configured as the upper stage of aTitan 34B. In all, 365 Agena vehicles were launched by NASA and the US Air Force.[5]
The Agena was 5.0 feet (1.5 m) in diameter, three-axis stabilized (for the benefit of the reconnaissance system cameras) and itsBell AircraftXLR81 engine produced 16,000 lbs (71 kN) of thrust usingunsymmetrical dimethylhydrazine (UDMH) as the fuel, andinhibited red fuming nitric acid (IRFNA) as the oxidizer.[6] First versions used Type IIIA IRFNA with 14% ofnitrogen dioxide, while later versions switched to Type IV High Density Acid (HDA) with 44% of nitrogen dioxide. HDA is more corrosive, but has higher density and increased performance.[7] This is ahypergolic fuel/oxidizer combination, and as such, it does not need an ignition system. This rocket engine could be restarted multiple times in orbit, byradio command, and it frequently was. The engine was notable for its unusualaluminum construction. The regeneratively-cooled channels that cooled the throat and nozzle were formed from straightgun drill formed channels. The parabolic shape of the chamber throat made for a difficult gun-drilling problem, which Bell Aerosystems engineers solved by arranging the cooling channels in a "One-Sheeted CircularHyperboloid" shape, allowing machinists to gun-drill straight cooling channels through the curved surfaces of the combustion chamber.[citation needed] The engine was derived from theXLR-81 propulsion unit for the canceled rocket-propelled nuclear warhead pod of theConvairB-58 Hustler bomber. Until 1959, the Agena was also known as the Discoverer Vehicle or Bell Hustler.[3]
Attitude control of the horizontal flying Agena was provided by aninertial reference package with threegyroscopes, twohorizon sensors, andcold-gas thrusters using anitrogen-freon mixture.Pitch and roll were sensed by two hermetic integrating gyro units. A rate gyro unit determined yaw error by sensing orbital rate. Pitch and roll gyro errors were corrected from the horizon sensors, which were later supplemented bySun andstar trackers. This enabled the Agena to accommodate the higher pointing stability required for better ground resolution imaging with the improved Corona cameras.[1]
As the Agena was designed to hold a fixed orientation in space while orbiting Earth, a passive thermal control system was devised.[1]
The main source of the Agena's electrical power wassilver peroxide-zinc batteries, which from the early 1960s on were supplemented by solar arrays. An S-band transponder enabled the Agena to receive ground command sequences (image motion compensation, altered attitude, etc.), which could be stored for later execution.[1]
Three versions of the Agena were flown:
The Agena-A was the first type of Agena to be built. It was launched atop Thor and Atlas rockets, mostly into polar orbits fromVandenberg Air Force Base Launch Complex 75 andPoint ArguelloLaunch Complex 1 respectively. Two Atlas launches occurred fromLaunch Complex 14 atCape Canaveral.
The Agena-A was propelled by aBell 8048 (XLR-81-BA-5) engine, which could produce 69 kN (about 15,500 lbs) of thrust with a burn time of 120 seconds.
Twenty Agena-As were launched between 1959 and 1961, all of them for the Discoverer, MIDAS, and Samos programs.[8]
During 1960, Lockheed introduced the improved Agena-B, which could be restarted in orbit and had longer propellant tanks for increased burn time. It was launched atop Thor and Atlas rockets.
It was equipped with aBell 8081 engine, which could generate 71 kN of thrust with a burn time of 240 seconds, and be restarted in orbit.
These launched theSAMOS-E,SAMOS-F (ELINT Ferret), andMIDAS (Missile Defense Alarm System) military early-warning satellites,Ranger lunar probes,Mariner planetary probes,OGO, and Nimbus satellites. Agena-B's first flight was the (unsuccessful) launch of Discoverer 16 on October 26, 1960. The Agena-B took months to be ready for Atlas launches and did not fly on that booster until Midas 3 on July 12, 1961. The final Agena-B flight was the launch of OGO 3 on June 7, 1966. A total of 76 were launched.
The Agena D was the result of a proposal byLockheed engineering executiveLawrence Edwards, who suggested standardizing the basic Agena configuration (up to this point, each Agena was custom-built for both the payload and the launch vehicle it was used with), and adding additional features depending on payload requirements, and a requirement from the Pentagon that the Agena be made compatible with theTitan rocket. This proposal originated in late 1962 when mounting frustration over the high failure rate of Thor and Atlas-Agena prompted the suggestion that greater standardization of launch vehicles would improve reliability. David N. Spires summarizes the standardization as follows:
The Agena D's common configuration included four usable modules containing the major guidance, beacon, power, and telemetry equipment, a standard payload console, and a rear rack above the engine for plug-in installation of optional gear-like solar panels, "piggyback" subsatellites, and an optional Bell Aerosystems engine that could be restarted in space as many as sixteen times.[9]
Its orbital configuration had a diameter of 60 in (1.5 m) and a length of 248 in (6.3 m), and provided 19,500 Wh of electrical power from batteries.[10] As of 2014, the Agena-D is the most-launched US upper stage.[11] A special production line was set up to turn out 40 Agena-D spacecraft per year. Edwards remained responsible for the engineering for several years, until the Air Force declared the Agena-D as operational and froze its design. By the time of its retirement, the reliability of the Agena-D exceeded 95 percent.
It was launched atop Atlas, Thor, Thorad and Titan IIIB rockets. It was equipped with aBell 8096 engine, which could generate 71 kN of thrust with a burn time of 265 seconds.
The first Agena-D launch was of KH-4 #7 on June 28, 1963, and a total of 269 Agena-Ds were launched. The Agena-D was used to launchKH-7 GAMBIT andKH-8 Gambit 3 reconnaissance satellites, three Mariner probes toVenus and the twoMarinerspace probes toMars.
Thor-Agena flew for the last time in 1972 when it launched a KH-4B satellite. The last Atlas-Agena used an Agena D stage atop a refurbishedAtlas F missile to launch Seasat in 1978. Twelve more Agenas were launched on Titan vehicles through 1987 before the stage was completely retired.
The final Agena-D upper stage was launched on aTitan IIIB rocket on February 12, 1987, carrying USA-21, the lastSDS-1 satellite.
TheAgena Target Vehicle was based around the Agena-D, with equipment fitted to support use as a rendezvous and docking target for missions conducted as part ofProject Gemini. It was equipped with aBell Aerospace Model 8247 engine, which was qualified for up to 15 restarts.[2] On later missions, the Agena's engine was fired while the Gemini spacecraft was docked, in order to boost the spacecraft to a higher orbit, and to bring it back again. During theGemini 11 mission, an elliptical orbit with anapogee of 1,375 kilometers (854 mi) was reached, which set analtitude record for crewed spaceflight that held untilApollo 8, the first crewed mission to the Moon, exceeded it.
In the early 1970s Lockheed studied the use of Agena as a payload booster in the Space Shuttle payload bay.An Agena-C with an increased diameter was proposed, but never built.
The Agena-2000 was intended as a modernized Agena, and would have been used on theAtlas V LightEvolved Expendable Launch Vehicle.[12] The Atlas V Light was cancelled in favor of standardizing the Medium configuration, and as a result the Agena-2000 was never built.
The first launch of an Agena-D was Agena Flight #64 on 27 June 1962. By the end of 1967, a total of 162 Agena-D had been launched:[13]
| Launches | Success | Failure | Cumulative | ||
|---|---|---|---|---|---|
| Agena | Booster | ||||
| 1962 | 8 | 8 | – | – | 8 |
| 1963 | 23 | 18 | 3 | 2 | 31 |
| 1964 | 35 | 32 | 3 | – | 66 |
| 1965 | 29 | 26 | 1 | 2 | 95 |
| 1966 | 39 | 36 | 2 | 1 | 134 |
| 1967 | 28 | 26 | 1 | 1 | 162 |
| 1968 | 21 | 20 | – | 1 | 183 |
| 1969 | 17 | 17 | – | – | 200 |
| 1970 | 14 | 14 | – | – | 214 |
| 1971 | 12 | 10 | ? | ? | 226 |
| 1972 | 8 | 6 | ? | ? | 234 |
| 1973 | 5 | 4 | ? | ? | 239 |
| 1974 | 3 | 3 | – | – | 242 |
| 1975 | 4 | 4 | – | – | 246 |
| 1976 | 4 | 4 | – | – | 250 |
| 1977 | 4 | 4 | – | – | 254 |
| 1978 | 4 | 4 | – | – | 258 |
| 1979 | 1 | 1 | – | – | 259 |
| 1980 | 1 | 1 | – | – | 260 |
| 1981 | 2 | 2? | – | – | 262 |
| 1982 | 1 | 1 | – | – | 263 |
| 1983 | 2 | 2 | – | – | 265 |
| 1984 | 2 | 2 | – | – | 267 |
| 1985 | 1 | 1 | – | – | 268 |
| 1987 | 1 | 1 | – | – | 269 |
| Total | 269 | 245? | 10? | 6? | – |
Since many of the Agenas have remained in orbit long after they were needed, they've had time to break-up resulting in more orbital debris.[14] They are thought to have exploded due to residual propellant igniting.[15]
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