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60-471: The Advanced Cryogenic Evolved Stage ( ACES ) was a proposed liquid oxygen / liquid hydrogen upper-stage for use on a number of different launch vehicles produced by Boeing , Lockheed Martin , United Launch Alliance (ULA). During the last five years of the program, ACES was proposed for eventual use on the Vulcan space launch vehicle designed by the U.S. company United Launch Alliance. The ACES concept had

120-402: 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

180-440: 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

240-491: 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

300-426: 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

360-441: 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 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

420-416: Is cryogenic with a freezing point of 54.36 K (−218.79 °C; −361.82 °F) and a boiling point of 90.19 K (−182.96 °C; −297.33 °F) at 1 bar (15 psi). Liquid oxygen has an expansion ratio of 1:861 and because of this, it is used in some commercial and military aircraft as a transportable source of breathing oxygen. Because of its cryogenic nature, liquid oxygen can cause

480-511: 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

540-498: 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

600-426: 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

660-502: 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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720-440: 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

780-432: Is classified as an industrial gas and is widely used for industrial and medical purposes. Liquid oxygen is obtained from the oxygen found naturally in air by fractional distillation in a cryogenic air separation plant . Air forces have long recognized the strategic importance of liquid oxygen, both as an oxidizer and as a supply of gaseous oxygen for breathing in hospitals and high-altitude aircraft flights. In 1985,

840-401: 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

900-406: 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

960-483: 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

1020-418: 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

1080-402: 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 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

1140-617: 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

1200-437: 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

1260-458: 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

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1320-472: 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

1380-470: The USAF started a program of building its own oxygen-generation facilities at all major consumption bases. Liquid oxygen is the most common cryogenic liquid oxidizer propellant for spacecraft rocket applications, usually in combination with liquid hydrogen , kerosene or methane . Liquid oxygen was used in the first liquid fueled rocket . The World War II V-2 missile also used liquid oxygen under

1440-634: 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

1500-448: 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)

1560-458: 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

1620-448: 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

1680-651: 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

1740-540: 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

1800-413: 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

1860-574: 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

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1920-548: 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 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

1980-428: 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

2040-698: The event, the long- duration upper stage concept was never able to obtain top-line attention from any of the companies as it never won full development funding from the US government, the primary source of funding for the Delta , Atlas and Vulcan rockets. However, some aspects of changes planned for ACES, such as the larger tank-diameter, were eventually used in the Centaur V upper stage. Two Advanced Cryogenic Evolved Stage (ACES) concepts were originally developed in 2005 by both Boeing and Lockheed Martin . By 2010, ULA had inherited

2100-561: 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

2160-481: 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

2220-586: The intellectual property of both proposals, and the ACES concept had evolved into a new high-performance upper stage to be used on both Atlas V and Delta IV / Delta IV Heavy launch vehicles. Now called the Advanced Common Evolved Stage , ACES was proposed to be a lower-cost, more-capable and more-flexible upper stage that would supplement, and perhaps replace, the existing ULA Centaur and Delta Cryogenic Second Stage (DCSS) upper stages. This upper stage

2280-453: 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

2340-493: The materials it touches to become extremely brittle. Liquid oxygen is also a very powerful oxidizing agent: organic materials will burn rapidly and energetically in liquid oxygen. Further, if soaked in liquid oxygen , some materials such as coal briquettes, carbon black , etc., can detonate unpredictably from sources of ignition such as flames, sparks or impact from light blows. Petrochemicals , including asphalt , often exhibit this behavior. The tetraoxygen molecule (O 4 )

2400-635: 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

2460-718: The name A-Stoff and Sauerstoff . In the 1950s, during the Cold War both the United States' Redstone and Atlas rockets, and the Soviet R-7 Semyorka used liquid oxygen. Later, in the 1960s and 1970s, the ascent stages of the Apollo Saturn rockets , and the Space Shuttle main engines used liquid oxygen. As of 2024, many active rockets use liquid oxygen: Attitude control system Spacecraft attitude control

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2520-440: The nitrogen has evaporated from such a vessel, there is a risk that liquid oxygen remaining can react violently with organic material. Conversely, liquid nitrogen or liquid air can be oxygen-enriched by letting it stand in open air; atmospheric oxygen dissolves in it, while nitrogen evaporates preferentially. The surface tension of liquid oxygen at its normal pressure boiling point is 13.2 dyn/cm. In commerce, liquid oxygen

2580-505: The objective to improve the on-orbit lifespan of current upper stages . In 2015, ULA announced conceptual plans to transition the Vulcan rocket to the ACES second stage, also referred to as Centaur Heavy, after 2024. Vulcan will initially launch with the Centaur V upper stage. Both Boeing and Lockheed Martin had separate but similar ACES concepts prior to the formation of the jointly-owned (50% Boeing/50% Lockheed Martin) ULA in late 2006. In

2640-408: 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

2700-547: 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

2760-450: 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

2820-626: The present dozens of hours to multiple days. As of April 2015, an internal combustion engine to be used to power the IVF system on ACES was to be produced by Roush Racing . In August 2016, ULA's President and CEO Tory Bruno said both Vulcan and ACES were intended to be human rated. One potential application for ACES was stated by ULA in 2010 to be the use of the longer endurance and the greater fuel capacity as propellant depot with in-space refueling capability to retrieve derelict objects for near-space clean up and deorbit . These new approaches offer

2880-868: 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 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

2940-719: 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

3000-450: 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

3060-508: 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

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3120-491: The stage. The design included producing power, maintaining stage attitude and keeping the propellant tanks autogenously pressurized . Using these fluids was designed to eliminate the need for hydrazine fuel, helium for pressurization, and nearly all batteries in the vehicle. IVF was designed by ULA to be optimal for depot operations, since only liquid hydrogen and liquid oxygen would need be transferred, and it could, if built, conceivably extend mission lifetimes from

3180-471: The technical prospect of markedly reducing the costs of beyond- low Earth Orbit object capture and deorbit with the implementation of a one-up/one-down launch license regime to Earth orbits. Liquid oxygen Liquid oxygen has a clear cyan color and is strongly paramagnetic : it can be suspended between the poles of a powerful horseshoe magnet . Liquid oxygen has a density of 1.141 kg/L (1.141 g/ml), slightly denser than liquid water, and

3240-419: 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

3300-460: 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

3360-563: Was expected by ULA to debut on the Vulcan launch vehicle no earlier than 2023 but in July 2015 the timeframe was clarified to not likely fly until 2024–2025. In 2018, ULA gave multiple presentations that again showed an ACES debut in 2023. In 2019 however, ULA said that while they still planned to develop ACES, they no longer had a specific date for when that would be. ACES was planned to use ULA's proprietary Integrated Vehicle Fluids (IVF) technology to significantly extend its lifetime in space. ACES

3420-528: Was first predicted in 1924 by Gilbert N. Lewis , who proposed it to explain why liquid oxygen defied Curie's law . Modern computer simulations indicate that, although there are no stable O 4 molecules in liquid oxygen, O 2 molecules do tend to associate in pairs with antiparallel spins , forming transient O 4 units. Liquid nitrogen has a lower boiling point at −196 °C (77 K) than oxygen's −183 °C (90 K), and vessels containing liquid nitrogen can condense oxygen from air: when most of

3480-443: Was intended to incorporate improved insulation for improved cryogenic storage and longer coast durations. In April 2015, the name of the stage was reverted to the original Advanced Cryogenic Evolved Stage name, as the new ULA Vulcan design would be the only first stage rocket intended to use ACES, beginning no earlier than 2023. In September 2020, ULA stated they were no longer actively developing ACES. As of April 2015, ACES

3540-491: Was planned to include common bulkhead propellant tanks with a diameter of 5.4 m (18 ft), capable of carrying 68,000 kg (150,000 lb) of propellant. In late 2017, ULA decided to bring the 5.4 m (18 ft) diameter and advanced insulation elements of the ACES upper stage forward. Under the new plan, Vulcan's upper stage is the Centaur V , with two LH2/LOX RL10 engines and no IVF or other ACES extended- duration technology as had been planned for ACES. ACES

3600-580: Was then expected to have the same tank diameter as Centaur V, but stretched (longer tank length), with the possible addition of two more RL10s and IVF. On 11 May 2018, United Launch Alliance (ULA) announced that the Aerojet Rocketdyne RL10 engine was selected for Centaur V, following a competitive procurement process. The IVF technology uses a lightweight internal combustion engine to use hydrogen and oxygen propellant boil-off (normally wasted when boil-off gases are vented to space) to operate

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