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69-454: On some missions, the payload was built directly into the Agena, which provided it with electric power, communications and three-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 an Ascent Agena . The Agena was upgraded twice from

138-633: A continuous sweeping motion that is desirable for fields and particles instruments, as well as some optical scanning instruments, but they may require complicated systems to de-spin antennas or optical instruments that must be pointed at targets for science observations or communications with Earth. Three-axis controlled craft can point optical instruments and antennas without having to de-spin them, but they may have to carry out special rotating maneuvers to best utilize their fields and particle instruments. If thrusters are used for routine stabilization, optical observations such as imaging must be designed knowing that

207-541: 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. As of 2014, the Agena-D is the most-launched US upper stage. 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

276-402: A ground station. The attitude control algorithms are written and implemented based on requirement for a particular attitude maneuver. Asides the implementation of passive attitude control such as the gravity-gradient stabilization , most spacecraft make use of active control which exhibits a typical attitude control loop. The design of the control algorithm depends on the actuator to be used for

345-440: A long-duration mission by producing control moments without fuel expenditure. For example, Mariner 10 adjusted its attitude using its solar cells and antennas as small solar sails. In orbit, a spacecraft with one axis much longer than the other two will spontaneously orient so that its long axis points at the planet's center of mass. This system has the virtue of needing no active control system or expenditure of fuel. The effect

414-491: A minimum of three reaction wheels must be used, with additional units providing single failure protection. See Euler angles . These are rotors spun at constant speed, mounted on gimbals to provide attitude control. Although a CMG provides control about the two axes orthogonal to the gyro spin axis, triaxial control still requires two units. A CMG is a bit more expensive in terms of cost and mass, because gimbals and their drive motors must be provided. The maximum torque (but not

483-426: A phenomenon known as Gimbal lock . A rotation matrix, on the other hand, provides a full description of the attitude at the expense of requiring nine values instead of three. The use of a rotation matrix can lead to increased computational expense and they can be more difficult to work with. Quaternions offer a decent compromise in that they do not suffer from gimbal lock and only require four values to fully describe

552-514: Is a device that senses the direction to the Sun . This can be as simple as some solar cells and shades, or as complex as a steerable telescope , depending on mission requirements. An Earth sensor is a device that senses the direction to Earth . It is usually an infrared camera ; nowadays the main method to detect attitude is the star tracker , but Earth sensors are still integrated in satellites for their low cost and reliability. A star tracker

621-517: Is aerodynamic stabilization. This is achieved using a drag gradient, as demonstrated on the Get Away Special Passive Attitude Control Satellite (GASPACS) technology demonstration. In low Earth orbit, the force due to drag is many orders of magnitude more dominant than the force imparted due to gravity gradients. When a satellite is utilizing aerodynamic passive attitude control, air molecules from

690-426: Is an optical device that measures the position(s) of star (s) using photocell (s) or a camera. It uses magnitude of brightness and spectral type to identify and then calculate the relative position of stars around it. A magnetometer is a device that senses magnetic field strength and, when used in a three-axis triad, magnetic field direction. As a spacecraft navigational aid, sensed field strength and direction

759-506: Is based on the measurement of the rate of change of body-fixed magnetometer signals. where m {\displaystyle m} is the commanded magnetic dipole moment of the magnetic torquer and K {\displaystyle K} is the proportional gain and B ˙ {\displaystyle {\dot {B}}} is the rate of change of the Earth's magnetic field. Spacecraft attitude determination

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828-440: Is caused by a tidal force . The upper end of the vehicle feels less gravitational pull than the lower end. This provides a restoring torque whenever the long axis is not co-linear with the direction of gravity. Unless some means of damping is provided, the spacecraft will oscillate about the local vertical. Sometimes tethers are used to connect two parts of a satellite, to increase the stabilizing torque. A problem with such tethers

897-401: Is compared to a map of Earth's magnetic field stored in the memory of an on-board or ground-based guidance computer. If spacecraft position is known then attitude can be inferred. Attitude cannot be measured directly by any single measurement, and so must be calculated (or estimated ) from a set of measurements (often using different sensors). This can be done either statically (calculating

966-406: Is most common reacts to an error signal (deviation) based on attitude as follows where T c {\displaystyle T_{c}} is the control torque, e {\displaystyle e} is the attitude deviation signal, and K p , K i , K d {\displaystyle K_{\text{p}},K_{\text{i}},K_{\text{d}}} are

1035-412: Is named after Isaac Newton . As with every SI unit named for a person, its symbol starts with an upper case letter (N), but when written in full, it follows the rules for capitalisation of a common noun ; i.e., newton becomes capitalised at the beginning of a sentence and in titles but is otherwise in lower case. The connection to Newton comes from Newton's second law of motion , which states that

1104-483: Is placed in space. (For some applications such as in robotics and computer vision, it is customary to combine position and attitude together into a single description known as Pose .) Attitude can be described using a variety of methods; however, the most common are Rotation matrices , Quaternions , and Euler angles . While Euler angles are oftentimes the most straightforward representation to visualize, they can cause problems for highly-maneuverable systems because of

1173-418: Is that meteoroids as small as a grain of sand can part them. Coils or (on very small satellites) permanent magnets exert a moment against the local magnetic field. This method works only where there is a magnetic field against which to react. One classic field "coil" is actually in the form of a conductive tether in a planetary magnetic field. Such a conductive tether can also generate electrical power, at

1242-617: Is the process of determining the orientation of a spacecraft (vehicle or satellite). It is a pre-requisite for spacecraft attitude control. A variety of sensors are utilized for relative and absolute attitude determination. Many sensors generate outputs that reflect the rate of change in attitude. These require a known initial attitude, or external information to use them to determine attitude. Many of this class of sensor have some noise, leading to inaccuracies if not corrected by absolute attitude sensors. Gyroscopes are devices that sense rotation in three-dimensional space without reliance on

1311-505: Is the unit of force in the International System of Units (SI) . Expressed in terms of SI base units , it is 1 kg⋅m/s , the force that accelerates a mass of one kilogram at one metre per second squared. The unit is named after Isaac Newton in recognition of his work on classical mechanics , specifically his second law of motion . A newton is defined as 1 kg⋅m/s (it is a named derived unit defined in terms of

1380-638: The Convair B-58 Hustler bomber. Until 1959, the Agena was also known as the Discoverer Vehicle or Bell Hustler. Attitude control of the horizontal flying Agena was provided by an inertial reference package with three gyroscopes , two horizon sensors , and micro-jets using a nitrogen-freon mixture of cold gas. 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

1449-591: The SI base units ). One newton is, therefore, the force needed to accelerate one kilogram of mass at the rate of one metre per second squared in the direction of the applied force. The units "metre per second squared" can be understood as measuring a rate of change in velocity per unit of time, i.e. an increase in velocity by one metre per second every second. In 1946, the General Conference on Weights and Measures (CGPM) Resolution 2 standardized

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1518-520: The celestial sphere , certain fields, and nearby objects, etc. Controlling vehicle attitude requires actuators to apply the torques needed to orient the vehicle to a desired attitude, and algorithms to command the actuators based on the current attitude and specification of a desired attitude. Before and during attitude control can be performed, spacecraft attitude determination must be performed, which requires sensors for absolute or relative measurement. The broader integrated field that studies

1587-500: The Agenas have remained in orbit long after they were needed, they've had time to break-up resulting in more orbital debris. They are thought to have exploded due to residual propellant igniting. Related lists Three-axis stabilization Spacecraft attitude control is the process of controlling the orientation of a spacecraft (vehicle or satellite) with respect to an inertial frame of reference or another entity such as

1656-437: The Earth's upper atmosphere strike the satellite in such a way that the center of pressure remains behind the center of mass, similar to how the feathers on an arrow stabilize the arrow. GASPACS utilized a 1 m inflatable 'AeroBoom', which extended behind the satellite, creating a stabilizing torque along the satellite's velocity vector. Control algorithms are computer programs that receive data from vehicle sensors and derive

1725-461: The PID controller parameters. A simple implementation of this can be the application of the proportional control for nadir pointing making use of either momentum or reaction wheels as actuators. Based on the change in momentum of the wheels, the control law can be defined in 3-axes x, y, z as This control algorithm also affects momentum dumping. Another important and common control algorithm involves

1794-458: The SI definition of the newton: 1 kg⋅m/s . At average gravity on Earth (conventionally, g n {\displaystyle g_{\text{n}}} = 9.806 65  m/s ), a kilogram mass exerts a force of about 9.81 N. Large forces may be expressed in kilonewtons (kN), where 1 kN = 1000 N . For example, the tractive effort of a Class Y steam train locomotive and

1863-475: The Sun so they can provide electrical power to the spacecraft. Cassini ' s main engine nozzles were steerable. Knowing where to point a solar panel, or scan platform, or a nozzle — that is, how to articulate it — requires knowledge of the spacecraft's attitude. Because a single subsystem keeps track of the spacecraft's attitude, the Sun's location, and Earth's location, it can compute the proper direction to point

1932-634: The aeronautical field, such as: This class of sensors sense the position or orientation of fields, objects or other phenomena outside the spacecraft. A horizon sensor is an optical instrument that detects light from the 'limb' of Earth's atmosphere, i.e., at the horizon. Thermal infrared sensing is often used, which senses the comparative warmth of the atmosphere, compared to the much colder cosmic background . This sensor provides orientation with respect to Earth about two orthogonal axes. It tends to be less precise than sensors based on stellar observation. Sometimes referred to as an Earth sensor. Similar to

2001-448: The angular rate is not estimated directly, but rather the measured angular rate from the gyro is used directly to propagate the rotational dynamics forward in time. This is valid for most applications as gyros are typically far more precise than one's knowledge of disturbance torques acting on the system (which is required for precise estimation of the angular rate). For some sensors and applications (such as spacecraft using magnetometers)

2070-463: The appendages. It logically falls to the same subsystem – the Attitude and Articulation Control Subsystem (AACS), then, to manage both attitude and articulation. The name AACS may even be carried over to a spacecraft even if it has no appendages to articulate. Attitude is part of the description of how an object is placed in the space it occupies. Attitude and position fully describe how an object

2139-448: The appropriate commands to the actuators to rotate the vehicle to the desired attitude. The algorithms range from very simple, e.g. proportional control , to complex nonlinear estimators or many in-between types, depending on mission requirements. Typically, the attitude control algorithms are part of the software running on the computer hardware, which receives commands from the ground and formats vehicle data telemetry for transmission to

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2208-655: The attitude using only the measurements currently available), or through the use of a statistical filter (most commonly, the Kalman filter ) that statistically combine previous attitude estimates with current sensor measurements to obtain an optimal estimate of the current attitude. Static attitude estimation methods are solutions to Wahba's problem . Many solutions have been proposed, notably Davenport's q-method, QUEST, TRIAD, and singular value decomposition . Crassidis, John L., and John L. Junkins.. Chapman and Hall/CRC, 2004. Kalman filtering can be used to sequentially estimate

2277-555: The attitude, as well as the angular rate. Because attitude dynamics (combination of rigid body dynamics and attitude kinematics) are non-linear, a linear Kalman filter is not sufficient. Because attitude dynamics is not very non-linear, the Extended Kalman filter is usually sufficient (however Crassidis and Markely demonstrated that the Unscented Kalman filter could be used, and can provide benefits in cases where

2346-413: The attitude. Attitude control can be obtained by several mechanisms, including: Vernier thrusters are the most common actuators, as they may be used for station keeping as well. Thrusters must be organized as a system to provide stabilization about all three axes, and at least two thrusters are generally used in each axis to provide torque as a couple in order to prevent imparting a translation to

2415-531: 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 the Titan rocket. This proposal originated in late 1962 when mounting frustration over the high failure rate of Thor and Atlas-Agena prompted

2484-464: The chamber throat made for a difficult gun-drilling problem, which Bell Aerosystems engineers solved by arranging the cooling channels in a "One-Sheeted Circular Hyperboloid " shape, allowing machinists to gun-drill straight cooling channels through the curved surfaces of the combustion chamber. The engine was derived from the XLR-81 propulsion unit for the canceled rocket-propelled nuclear warhead pod of

2553-538: The combination of sensors, actuators and algorithms is called guidance, navigation and control , which also involves non-attitude concepts, such as position determination and navigation . A spacecraft's attitude must typically be stabilized and controlled for a variety of reasons. It is often needed so that the spacecraft high-gain antenna may be accurately pointed to Earth for communications, so that onboard experiments may accomplish precise pointing for accurate collection and subsequent interpretation of data, so that

2622-582: The concept of detumbling, which is attenuating the angular momentum of the spacecraft. The need to detumble the spacecraft arises from the uncontrollable state after release from the launch vehicle. Most spacecraft in low Earth orbit (LEO) makes use of magnetic detumbling concept which utilizes the effect of the Earth's magnetic field . The control algorithm is called the B-Dot controller and relies on magnetic coils or torque rods as control actuators. The control law

2691-428: The direction opposite to that required to re-orient the vehicle. Because momentum wheels make up a small fraction of the spacecraft's mass and are computer controlled, they give precise control. Momentum wheels are generally suspended on magnetic bearings to avoid bearing friction and breakdown problems. Spacecraft Reaction wheels often use mechanical ball bearings. To maintain orientation in three dimensional space

2760-561: The expense of orbital decay . Conversely, by inducing a counter-current, using solar cell power, the orbit may be raised. Due to massive variability in Earth's magnetic field from an ideal radial field, control laws based on torques coupling to this field will be highly non-linear. Moreover, only two-axis control is available at any given time meaning that a vehicle reorient may be necessary to null all rates. Three main types of passive attitude control exist for satellites. The first one uses gravity gradient, and it leads to four stable states with

2829-471: The force exerted on an object is directly proportional to the acceleration hence acquired by that object, thus: F = m a , {\displaystyle F=ma,} where m {\displaystyle m} represents the mass of the object undergoing an acceleration a {\displaystyle a} . When using the SI unit of mass, the kilogram (kg), and SI units for distance metre (m), and time, second (s) we arrive at

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2898-490: The fuel, and inhibited red fuming nitric acid (IRFNA) as the oxidizer. This is a hypergolic fuel/oxidizer combination, and as such, it does not need an ignition system. This rocket engine could be restarted multiple times in orbit, by radio command, and it frequently was. The engine was notable for its unusual aluminum construction. The regeneratively-cooled channels that cooled the throat and nozzle were formed from straight gun drill formed channels. The parabolic shape of

2967-406: The heating and cooling effects of sunlight and shadow may be used intelligently for thermal control, and also for guidance: short propulsive maneuvers must be executed in the right direction. Attitude control of spacecraft is maintained using one of two principal approaches: There are advantages and disadvantages to both spin stabilization and three-axis stabilization. Spin-stabilized craft provide

3036-411: The horizon sensors, which were later supplemented by Sun and star trackers . This enabled the Agena to accommodate the higher pointing stability required for better ground resolution imaging with the improved Corona cameras. As the Agena was designed to hold a fixed orientation in space while orbiting Earth, a passive thermal control system was devised. The main source of the Agena's electrical power

3105-496: The initial estimate is poor). Multiple methods have been proposed, however the Multiplicative Extended Kalman Filter (MEKF) is by far the most common approach. This approach utilizes the multiplicative formulation of the error quaternion, which allows for the unity constraint on the quaternion to be better handled. It is also common to use a technique known as dynamic model replacement, where

3174-471: The last SDS-1 satellite. The Agena 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 of Project Gemini . It was equipped with a Bell Aerospace Model 8247 engine, which was qualified for up to 15 restarts. On later missions, the Agena's engine was fired while the Gemini spacecraft was docked, in order to boost

3243-453: The long axis (axis with smallest moment of inertia) pointing towards Earth. As this system has four stable states, if the satellite has a preferred orientation, e.g. a camera pointed at the planet, some way to flip the satellite and its tether end-for-end is needed. The second passive system orients the satellite along Earth's magnetic field thanks to a magnet. These purely passive attitude control systems have limited pointing accuracy, because

3312-646: The maximum angular momentum change) exerted by a CMG is greater than for a momentum wheel, making it better suited to large spacecraft. A major drawback is the additional complexity, which increases the number of failure points. For this reason, the International Space Station uses a set of four CMGs to provide dual failure tolerance. Small solar sails (devices that produce thrust as a reaction force induced by reflecting incident light) may be used to make small attitude control and velocity adjustments. This application can save large amounts of fuel on

3381-408: The observation of external objects. Classically, a gyroscope consists of a spinning mass, but there are also " ring laser gyros " utilizing coherent light reflected around a closed path. Another type of "gyro" is a hemispherical resonator gyro where a crystal cup shaped like a wine glass can be driven into oscillation just as a wine glass "sings" as a finger is rubbed around its rim. The orientation of

3450-609: The opposing direction if a new orientation is to be held. Thruster systems have been used on most crewed space vehicles, including Vostok , Mercury , Gemini , Apollo , Soyuz , and the Space Shuttle . To minimize the fuel limitation on mission duration, auxiliary attitude control systems may be used to reduce vehicle rotation to lower levels, such as small ion thrusters that accelerate ionized gases electrically to extreme velocities, using power from solar cells. Momentum wheels are electric motor driven rotors made to spin in

3519-509: 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's Advanced Research Projects Agency for the star Beta Centauri , also known as Agena, because this upper stage would "ignite in the sky". This followed Lockheed 's tradition of naming products for stellar phenomena. The final launch

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3588-450: The oscillation is fixed in inertial space, so measuring the orientation of the oscillation relative to the spacecraft can be used to sense the motion of the spacecraft with respect to inertial space. Motion reference units are a kind of inertial measurement unit with single- or multi-axis motion sensors. They utilize MEMS gyroscopes . Some multi-axis MRUs are capable of measuring roll, pitch, yaw and heave . They have applications outside

3657-713: The precise location must also be known. While pose estimation can be employed, for spacecraft it is usually sufficient to estimate the position (via Orbit determination ) separate from the attitude estimation. For terrestrial vehicles and spacecraft operating near the Earth, the advent of Satellite navigation systems allows for precise position knowledge to be obtained easily. This problem becomes more complicated for deep space vehicles, or terrestrial vehicles operating in Global Navigation Satellite System (GNSS) denied environments (see Navigation ). Kilonewton The newton (symbol: N )

3726-719: The spacecraft is always slowly rocking back and forth, and not always exactly predictably. Reaction wheels provide a much steadier spacecraft from which to make observations, but they add mass to the spacecraft, they have a limited mechanical lifetime, and they require frequent momentum desaturation maneuvers, which can perturb navigation solutions because of accelerations imparted by the use of thrusters. Many spacecraft have components that require articulation. Voyager and Galileo , for example, were designed with scan platforms for pointing optical instruments at their targets largely independently of spacecraft orientation. Many spacecraft, such as Mars orbiters, have solar panels that must track

3795-624: The spacecraft to a higher orbit, and to bring it back again. During the Gemini 11 mission, an elliptical orbit with an apogee of 1,375 kilometers (854 mi) was reached, which set an altitude record for crewed spaceflight that held until Apollo 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

3864-450: The spacecraft will oscillate around energy minima. This drawback is overcome by adding damper, which can be hysteretic materials or a viscous damper. The viscous damper is a small can or tank of fluid mounted in the spacecraft, possibly with internal baffles to increase internal friction. Friction within the damper will gradually convert oscillation energy into heat dissipated within the viscous damper. A third form of passive attitude control

3933-508: The specific attitude maneuver although using a simple proportional–integral–derivative controller ( PID controller ) satisfies most control needs. The appropriate commands to the actuators are obtained based on error signals described as the difference between the measured and desired attitude. The error signals are commonly measured as euler angles (Φ, θ, Ψ), however an alternative to this could be described in terms of direction cosine matrix or error quaternions . The PID controller which

4002-564: 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. Its orbital configuration had

4071-497: 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 a Bell 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 launch KH-7 GAMBIT and KH-8 Gambit 3 reconnaissance satellites, three Mariner probes to Venus and

4140-444: The two Mariner space probes to Mars . 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 refurbished Atlas 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 a Titan IIIB rocket on February 12, 1987, carrying USA-21,

4209-556: The unit of force in the MKS system of units to be the amount needed to accelerate one kilogram of mass at the rate of one metre per second squared. In 1948, the 9th CGPM Resolution 7 adopted the name newton for this force. The MKS system then became the blueprint for today's SI system of units. The newton thus became the standard unit of force in the Système international d'unités (SI), or International System of Units . The newton

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4278-419: The vehicle. Their limitations are fuel usage, engine wear, and cycles of the control valves. The fuel efficiency of an attitude control system is determined by its specific impulse (proportional to exhaust velocity) and the smallest torque impulse it can provide (which determines how often the thrusters must fire to provide precise control). Thrusters must be fired in one direction to start rotation, and again in

4347-460: The way that a terrestrial gyrocompass uses a pendulum to sense local gravity and force its gyro into alignment with Earth's spin vector, and therefore point north, an orbital gyrocompass uses a horizon sensor to sense the direction to Earth's center, and a gyro to sense rotation about an axis normal to the orbit plane. Thus, the horizon sensor provides pitch and roll measurements, and the gyro provides yaw. See Tait-Bryan angles . A Sun sensor

4416-635: Was silver 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. 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 from Vandenberg Air Force Base Launch Complex 75 and Point Arguello Launch Complex 1 respectively. Two Atlas launches occurred from Launch Complex 14 at Cape Canaveral . The Agena-A

4485-483: Was launched atop Thor and Atlas rockets. It was equipped with a Bell 8081 engine, which could generate 71 kN of thrust with a burn time of 240 seconds, and be restarted in orbit. These launched the SAMOS-E , SAMOS-F (ELINT Ferret), and MIDAS (Missile Defense Alarm System) military early-warning satellites, Ranger lunar probes, Mariner planetary probes, OGO , and Nimbus satellites. Agena-B's first flight

4554-479: Was of an Agena D on February 12, 1987, configured as the upper stage of a Titan 34B . In all, 365 Agena vehicles were launched by NASA and the US Air Force. The Agena was 5.0 feet (1.5 m) in diameter, three-axis stabilized (for the benefit of the reconnaissance system cameras) and its Bell Aircraft XLR81 engine produced 16,000 lbs (71 kN) of thrust using unsymmetrical dimethylhydrazine (UDMH) as

4623-472: Was propelled by a Bell 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. During 1960, Lockheed introduced the improved Agena-B, which could be restarted in orbit and had longer propellant tanks for increased burn time. It

4692-612: Was proposed, but never built. The Agena-2000 was intended as a modernized Agena, and would have been used on the Atlas V Light Evolved Expendable Launch Vehicle . 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: Since many of

4761-408: 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 by Lockheed engineering executive Lawrence Edwards , who suggested standardizing

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