Agina: I am the earliest rocket upper stage and also a multi-talented person in aerospace

Agena is the earliest rocket upper stage and an important power source for the launch and orbital operation of early U.S. military satellites. Agena has many variants, and without an encyclopedic description, people often cannot figure out the family composition. However, this also allows people to understand why such a thing as an upper stage was created.

Agena is used to drive the manned space station target vehicle

Agina, the versatile

Agena's military designation is RM-81, and it was originally developed by Lockheed for the WS-117L reconnaissance satellite project. Because electronic technology was not well developed at the time, photographic reconnaissance satellites had to carry film capsules to send back to Earth after filming, so the satellites themselves were quite large and heavy, and it was impossible to send them into orbit relying solely on the power of the launch vehicle.

During the model development process, the U.S. Air Force found that it was unrealistic to put various reconnaissance purposes into one WS-117L project. So WS-117L was split into "Samos" and "Corona" (later renamed "Discoverer") for photo reconnaissance, and "Midas" for missile warning. However, Agena was retained and took on the upper stage mission of all these satellites. Later, it was also used in NASA's Gemini manned spacecraft rendezvous and docking technology demonstration, serving as the propulsion module of the target aircraft. The Agena upper stage was adapted to the Atlas, Thor and Titan IIIB rockets, and was once considered for the space shuttle. Various types of Agena were launched from February 28, 1959 to February 1987, with a total of 365 launches. Only 33 of them served NASA, and the others served military satellites.

In some missions, the satellite is installed directly in front of the standard bulkhead of Agena, which provides power, communication and three-axis stable attitude control. However, some satellites are separated from Agena after launch and take on their own flight missions.

Early Agena upper stages were used to launch reconnaissance satellites.

Over time, Agena underwent two upgrades to support heavier and more complex satellites, such as the Corona reconnaissance satellite with multiple film bays and larger cameras.

The name Agena was determined by the Defense Advanced Research Projects Agency (later known as DARPA, then known as ARPA). Previously, it was called "Beta Centauri", the tenth brightest star in the night sky. However, ARPA suggested using another name for the star, "Agena", because it would "ignite in the sky", which was in line with Lockheed's tradition of naming products after stellar phenomena.

Characteristics of Agena

Agena has a diameter of 1.5 meters and has three-axis stabilization, which is extremely necessary for image reconnaissance. Agena uses the XLR81 engine developed by Bell Aerosystems and uses the familiar unsymmetrical dimethylhydrazine and nitric acid as propellants. These two fuels can not only be stored at room temperature and pressure, but also can self-ignite, without the need for an ignition system, simplifying the engine design. Therefore, the XLR81 engine can be restarted multiple times in orbit under the control of ground radio commands.

This engine was originally used in the Cornish B-58 supersonic bomber. This supersonic bomber developed in the 1950s could carry an MB-1C pod containing nuclear warheads. However, the MB-1C design was not very successful, so Cornish planned to improve it by installing a rocket engine at the back to turn it into a nuclear rocket. This plan was of course unlikely to succeed, but the XLR81 engine was successfully developed. The specific model used in the Agena A is the Bell 8048 (XLR81-BA-5) engine, which can generate 69 kilonewtons of thrust in two minutes.

Bell 8048 engine

Bell 8048 engine for Agena upper stage

The aluminum alloy structure of XLR81 is quite famous. The throat and nozzle of the rocket engine adopt the so-called regenerative cooling mode, but because of the parabolic shape of the throat, it is very difficult to drill holes for fuel here, after all, the drill bit is straight. Therefore, engineers at Bell Aerosystems solved the problem by arranging the cooling channels in the shape of a "one-piece circular hyperboloid".

Agena's yaw attitude measurement is achieved by an inertial reference system, which includes three gyroscopes, two level sensors and nitrogen-freon mixture micro-jets for cooling. Pitch and roll attitude measurement is achieved by two sealed integrating gyro units. The rate gyro unit determines the yaw error by sensing the orbital rate. The pitch and roll gyro errors are corrected by the horizon sensor, which is referenced by the sun and star sensors. This gives Agena a high pointing stability, allowing the Corona satellite's camera to achieve better ground resolution.

Because Agena is designed to maintain a fixed orientation in space while orbiting the Earth, a passive thermal control system was designed.

The main source of power for the early Agena was silver zinc peroxide batteries, and solar arrays were installed on Agena from the early 1960s. The telemetry and control communication functions are implemented by an S-band transponder, which can receive ground command sequences (image motion compensation, change of attitude, etc.), store them and execute them one by one.

In the early days, the Agena upper stage was not standardized and was customized for each launch.

Model family

1. Agena A

The Agena A was the first model of the Agena family, and was launched into polar orbit using Thor and Atlas rockets from Vandenberg Air Force Base Station 75 and Cape Canaveral Launch Complex 1. In addition, it was launched twice from Cape Canaveral Launch Complex 14 using Atlas rockets. Eighteen Agena A missions were launched between 1959 and 1961, all for the Discoverer, Midas, and Samos reconnaissance satellite programs.

2. Agena B

In 1960, Lockheed introduced the improved Agena B. It used the Bell 8081 engine, which could produce 71 kN of thrust. This upper stage had the ability to ignite multiple times in orbit, carried more fuel, and the engine could work for 4 minutes in total. Agena B served the Samos-E, Samos-F and Midas early warning satellites, the Ranger lunar probe, and the Mariner planetary probe. The first flight of Agena B was the launch of Discoverer 16 on October 26, 1960, but it was unsuccessful. It was not until Midas 3 on July 12, 1961 that Agena B got the opportunity to fly again. The last flight of Agena B was the launch of the OGO3 satellite on June 7, 1966. A total of 75 were launched.

Agena B upper stage

3. Agena D

Agena D was proposed by Lawrence Edwards, an engineering executive at Lockheed, in 1962. Each Agena before this was customized individually. Edwards suggested standardizing the basic configuration of Agena and adding additional functions according to the payload requirements. At that time, the Pentagon also hoped that Agena could be used in conjunction with the Titan rocket. Because the failure rates of the other two rockets at that time - Thor and Atlas - were unacceptably high. This prompted people to propose improving the standardization level of launch vehicles to improve reliability.

After some research work, the standardization of Agena D includes the following:

The general configuration of Agena D includes four subsystems: ① main guidance, beacon, power and telemetry equipment; ② a standard payload control console; ③ a rear rack located above the engine for inserting optional equipment such as solar panels and small satellites; ④ Bell Aerosystems' engine, which can be restarted 16 times in space. Agena D has a diameter of 1.5 meters, a length of 6.3 meters, and a battery capacity of 19,500 watt-hours.

As of 2014, the Agena D is the most frequently launched upper stage in the United States, with 269 Agena Ds launched since the first launch of Keyhole 4-7 on June 28, 1963. Lockheed built a dedicated production line for this purpose, producing 40 per year. Edwards was in charge of engineering design for several years until the USAF declared the Agena D operational and the technical status was frozen. By the time it was retired, the Agena D had a reliability of over 95%.

The last flight of the Thor-Agena combination was in 1972, when it launched a Keyhole 4B satellite. In 1978, the Atlas-Agena D, using a refurbished Atlas F rocket, launched the Seasat satellite.

After that, Agena D was also used to launch the Keyhole-7 and Keyhole-8 reconnaissance satellites, three Mariner Venus probes, and two Mariner Mars probes. On February 12, 1987, the last Agena D was launched by a Titan IIIB rocket, sending the last SDS-1 satellite into orbit.

Agena target vehicle

The Agena target vehicle was based on the Agena D, with additional equipment to support the Gemini program's rendezvous and docking activities. It was equipped with a Bell Aerospace 8247 engine that could be fired up to 15 times in space. In actual missions, when the Gemini spacecraft docked with the target vehicle, the Agena's engines started, pushing the combination to a higher orbit and then back. During the Gemini 11 mission, the combination reached an elliptical orbit with an apogee of 1,375 kilometers, setting a record for manned space flight until Apollo 8 (the first manned mission to the moon) surpassed this record.

In the early 1970s, Lockheed studied the possibility of installing the Agena in the cargo bay of the space shuttle to carry payloads into orbit. Given the large diameter of the cargo bay of the space shuttle, Lockheed designed an Agena C with an increased diameter, but it was never built. After that, a fully modernized Agena 2000 was designed to be used on a lightweight version of the Atlas V rocket. With the cancellation of this rocket, the Agena 2000 also lost its chance to be launched.