Stephanie Thomas

Robust Attitude Control Systems Design for Solar Sails, Part 1: Propellantless Primary ACS

A robust attitude control system (ACS) architecture is developed for near-term solar sail missions. The proposed sailcraft ACS consists of a propellantless primary ACS and a microthruster-based secondary ACS. The primary ACS employs two ballast masses running along mast lanyards for pitch/yaw trimming and thrust vector control. It also employs roll stabilizer bars at the mast tips for quadrant tilt control. The secondary ACS utilizes lightweight pulsed plasma thruster (PPT) modules mounted at the mast tips. Such a microPPT-based secondary ACS can be employed for attitude recovery maneuvers from various off-nominal conditions, including tumbling, that cannot be handled by the propellantless primary ACS. The overall simplicity, effectiveness, and robustness of the proposed ACS architecture are demonstrated for a solar sail flight validation experiment of a 40-m sailcraft proposed in a 1600-km, dawn-dusk sun-synchronous (DDSS) orbit. The proposed ACS architecture will be applicable with minimal modifications to a wide range of future solar sail flight missions with varying requirements and mission complexity. An overview of the state-of-the-art microPPT technology, PPT requirements for solar sails, and pulse-modulated control design and simulation results are presented in the companion paper (Part 2).

Control-Moment Gyroscopes for Joint Actuation: A New Paradigm in Space Robotics

Manned spacecraft will require maintenance robots to inspect and repair components of the spacecraft that are accessible only from the outside. This paper presents a design of a novel free-flying maintenance robot (known as a MaintenanceBot.) The MaintenanceBot uses Control Moment Gyros (CMGs) for manipulator arm and attitude control. This architecture provides high authority control in a compact low power package. Relative position and attitude determination is accomplished with an RF system supplemented by a vision system at close range. When not docked to the manned vehicle (which must be done periodically to refuel and recharge batteries or when the manned vehicle performs orbit changes) the MaintenanceBots fly in formation using a cold gas thruster system and formation flying algorithms that permit dozens of MaintenanceBots to coordinate their positions. The use of CMGs is a prominent feature of this design. An array of CMGs can exchange angular momentum with the spacecraft body to effect attitude changes, as long as certain mathematical singularities in the actuator Jacobian are avoided. The proposed maintenance robot benefits dramatically from the dynamics and control of a multibody robotic arm whose joints are driven by CMGs. In addition to high power efficiency, another advantage of this concept is that spacecraft appendages actuated by CMGs can be considered reactionless, in the sense that careful manipulation of the CMG gimbal angles can virtually eliminate moments applied to the spacecraft body. This paper provides a preliminary design of the MaintenanceBot. Analysis of the formation flying and close maneuver control systems is included. Simulation results for a typical operation is provided.

Robust Attitude Control Systems Design for Solar Sails, Part 2: MicroPPT-based Secondary ACS

A secondary attitude control system (ACS) of utilizing tip-mounted, lightweight pulsed plasma thruster (PPT) is developed for near-term solar sails. Such a secondary ACS can be employed for attitude recovery maneuvers from various offnominal conditions, including tumbling, that cannot be handled by a propellantless primary ACS (described in the companion paper). The microPPT-based ACS can also be employed for the spin stabilization of sailcraft as well as as a backup to the conventional ACS of sail carrier spacecraft prior to sail deployment as well as during pre-flight sail checkout operation. An overview of the state-of-the-art microPPT technology, PPT performance requirements for solar sails, and pulse-modulated PPT control design and simulation results are presented in this paper.

Avoidance Maneuver Planning Incorporating Station-Keeping Constraints and Automatic Relaxation

Space debris is a growing concern for the sustained operation of our satellites. The population in space is continually increasing, both on a gradual basis as new satellites are placed on orbit and in sudden bursts, as evidenced with the recent collision between the Iridium and inactive Cosmos spacecraft. The problem is most severe in densely populated orbit regimes, where many operational satellites face a sustained presence of close-orbiting objects. In general, the frequent occurrence of potential collisions with debris will have a negative impact on mission performance in two important ways. First, repeated avoidance maneuvers diminish fuel and thus reduce mission life. Second, excursions from the nominal orbit during avoidance maneuvers may violate mission requirements or payload constraints. It is therefore important to consider both fuel minimization and station-keeping objectives in the avoidance planning problem. In this paper, we formulate the avoidance maneuver planning problem as a linear prog...

Decentralized Formation Flying Control in a Multiple-Team Hierarchy

Abstract: In recent years, formation flying has been recognized as an enabling technology for a variety of mission concepts in both the scientific and defense arenas. Examples of developing missions at NASA include magnetospheric multiscale (MMS), solar imaging radio array (SIRA), and terrestrial planet finder (TPF). For each of these missions, a multiple satellite approach is required in order to accomplish the large‐scale geometries imposed by the science objectives. In addition, the paradigm shift of using a multiple satellite cluster rather than a large, monolithic spacecraft has also been motivated by the expected benefits of increased robustness, greater flexibility, and reduced cost. However, the operational costs of monitoring and commanding a fleet of close‐orbiting satellites is likely to be unreasonable unless the onboard software is sufficiently autonomous, robust, and scalable to large clusters. This paper presents the prototype of a system that addresses these objectives—a decentralized guidance and control system that is distributed across spacecraft using a multiple team framework. The objective is to divide large clusters into teams of “manageable” size, so that the communication and computation demands driven by N decentralized units are related to the number of satellites in a team rather than the entire cluster. The system is designed to provide a high level of autonomy, to support clusters with large numbers of satellites, to enable the number of spacecraft in the cluster to change post‐launch, and to provide for on‐orbit software modification. The distributed guidance and control system will be implemented in an object‐oriented style using a messaging architecture for networking and threaded applications (MANTA). In this architecture, tasks may be remotely added, removed, or replaced post launch to increase mission flexibility and robustness. This built‐in adaptability will allow software modifications to be made on‐orbit in a robust manner. The prototype system, which is implemented in Matlab, emulates the object‐oriented and message‐passing features of the MANTA software. In this paper, the multiple team organization of the cluster is described, and the modular software architecture is presented. The relative dynamics in eccentric reference orbits is reviewed, and families of periodic, relative trajectories are identified, expressed as sets of static geometric parameters. The guidance law design is presented, and an example reconfiguration scenario is used to illustrate the distributed process of assigning geometric goals to the cluster. Next, a decentralized maneuver planning approach is presented that utilizes linear‐programming methods to enact reconfiguration and coarse formation keeping maneuvers. Finally, a method for performing online collision avoidance is discussed, and an example is provided to gauge its performance.

Propellantless AOCS Design for a 160-m, 450-kg Sailcraft of the Solar Polar Imager Mission

An attitude and orbit control system (AOCS) is developed for a 160-m, 450-kg solar sail spacecraft of the Solar Polar Imager (SPI) mission. The SPI mission is one of several SunEarth Connections solar sail roadmap missions currently envisioned by NASA. A reference SPI sailcraft consists of a 160-m, 150-kg square solar sail, a 250-kg spacecraft bus, and 50-kg science payloads, The 160-m reference sailcraft has a nominal solar thrust force of 160 mN (at 1 AU), an uncertain center-of-mass/center-of-pressure offset of ±0. 4m ,and a characteristic acceleration of 0.35 mm/s 2 . The solar sail is to be deployed after being placed into an earth escaping orbit by a conventional launch vehicle such as a Delta II. The SPI sailcraft first spirals inwards from 1 AU to a heliocentric circular orbit at 0.48 AU, followed by a cranking orbit phase to achieve a science mission orbit at a 75-deg inclination, over a total sailing time of 6.6 yr. The solar sail will be jettisoned after achieving the science mission orbit. This paper focuses on the solar sailing phase of the SPI mission, with emphasis on the design of a reference AOCS consisting of a propellantless primary ACS and a microthruster-based secondary (optional) ACS. The primary ACS employs trim control masses running along mast lanyards for pitch/yaw control together with roll stabilizer bars at the mast tips for quadrant tilt (roll) control. The robustness and effectiveness of such a propellantless primary ACS would be enhanced by the secondary ACS which employs tip-mounted, lightweight pulsed plasma thrusters (PPTs). The microPPT-based ACS is mainly intended for attitude recovery maneuvers from off-nominal conditions. A relatively fast, 70-deg pitch reorientation within 3 hrs every half orbit during the orbit cranking phase is shown to be feasible, with the primary ACS, for possible solar observations even during the 5-yr cranking orbit phase.

Realistic Solar Sail Thrust

Before a solar sail will be used as a primary propulsion system for a space flight mission, many technical areas must be developed further. One of these areas concerns understanding the propulsion performance of a realistic solar sail well enough so that solar sail orbits can be confidently predicted to meet defined mission requirements. This paper identifies major contributors to solar sail thrust uncertainty, and analyzes the most significant ones to provide a better understanding of thrust generation by a “realistic” solar sail. Performance of representative “realistic” sailcraft are compared to similar “ideal” and “non-ideal” sailcraft to illustrate the differences.

Formations for Close-Orbiting Escort Vehicles

One concept for protecting and inspecting valuable space assets such as GPS satellites is the use of escort vehicles flying in formation with the asset. This paper presents a systematic analysis of nominal escort orbits and delta-V budgets for formation maintenance and asset inspection in LEO, GPS, and GEO orbits. Navigation accuracy and formation controller type are addressed as they affect the minimum size of the orbit and delta-V budget. The effects of differential drag, solar pressure, and the J2 gravitational harmonic are considered separately. Geometric constraints such as nadir-pointing payloads on the asset are also considered.The objectives of the escort vehicles are to maximize the protection space around the prime asset, maintain a safe separation distance from the prime and other escorts, avoid interference with the prime’s payloads, perform periodic inspections, and maximize the mission lifetime by minimizing the fuel consumption required for formation maintenance. Since the asset’s orbit is to be unchanged, the escort’s nominal orbits will be arrayed around the asset with the asset at the center of the formation. This is in contrast to a regular elliptical formation where all spacecraft are on the ellipse and will result in smaller separations for the same size ellipse. In order to achieve a large protection space and avoid the sensing/communication field of view of the prime, a passive formation is desired in which the escort orbits about the prime once each orbit, while oscillating back and forth in the cross-track direction. The cross-track amplitude involves orbital element differences which can potentially result in along-track drift due to the J2 perturbation. Selection of the element differences to avoid inclination differences serves to minimize the drift rate.The nominal escort geometry is an elliptical formation with a minimum separation distance of 1 km and a cross-track amplitude of about 0.707 km. The worst case secular drift rate is 13.2 m/hour, occurring at 42 degrees inclination in a 600 km LEO orbit. By defining the same formation with zero inclination difference and maximum difference in right ascension, the drift rate can be reduced to 2 mm/hour. For GEO orbits, even with the maximum inclination difference, the drift rate is less than 2.4 cm/hour. The drift rates scale linearly with the size of the escort’s relative ellipse. Delta-Vs to maintain these formations are on the order of 45-60 m/s/year. The contribution from J2 in LEO varies from less than 10 m/s to 350 m/s depending on the orbit parameters discussed above, but for many orbits can be compensated for with a semi-major axis difference. The contribution from drag (LEO only) is about 25 m/s. Solar pressure contributes about 30 and 43 m/s in LEO and GEO respectively. For both drag and solar pressure, the differential area was assumed to be 5 square meters, representative of a large space asset and a small escort vehicle.

Integrated Attitude and Orbit Control of an Interstellar Heliopause Probe

Solar sailing has been identified as a promising and enabling technology for future space missions; as such it is currently the object of a significant research effort within various space agencies, the academic world and industry. Active research and development activities have been performed by the European Space Agency (ESA) in the recent years to increase the technological readiness level of the key elements allowing the deployment and control of a spacecraft efficiently propelled by solar radiation pressure. A six-degree-of-freedom simulation environment has been developed to obtain the main results of the study and validate the attitude and orbit control concepts proposed for the Interstellar Heliopause Probe (IHP) mission is presented.

Architecture for Low-Power, High-Agility Multibody Control

Control-moment gyroscopes (CMGs) have been used for attitude control on spacecraft that require large torques. We propose them for use in multibody robotic systems where high agility (or dexterity) and low power are important design goals. Although many CMG arrangements are possible, our baseline concept includes a scissored pair of CMGs for each rotational joint. A scissored pair is an array of two single-gimbal CMGs with parallel gimbal axes and opposite angular velocities. The scissored-pair arrangement ensures that the sum of the CMGs’ angular momentum aligns with a single axis, like that of a reaction wheel, which drastically simplifies the control algorithms. This architecture offers singularity-free, reactionless slewing of a multi-joint robotic arm. We describe the implementation of this concept as a manipulator arm on a free-flying MaintenanceBot. When a joint is actuated, the torque comes not from a direct-drive motor that interacts with its neighboring body, but from manipulating the distribution of momentum among the CMGs and the body to which they are mounted. Perhaps the most important impact of reactionless CMG-based control is that it requires only 1%-10% the electrical power for a comparable RWA-based or joint-driven robotic system. This entirely new approach to actuating kinematic chains in orbit holds promise for highly effective in-situ construction and repair of satellites. The CMG-based MaintenanceBot architecture can radically outperform other systems in power for high-agility maneuvers. Furthermore, this architecture opens up a large trade space for power vs. agility, allowing highly effective MaintenanceBots to be incorporated for relatively low power-specific mass. Or, for a given mass, the MaintenanceBot can withstand more demanding operations for longer than competing architectures. A prior paper presented a preliminary design for the CMG-actuated MaintenanceBot. In this paper, we present new results from the incorporation of robot arm dynamics into the simulation and the development of algorithms (i.e. inverse kinematics) to position the arm.