David M. Murphy

Scalable Solar-Sail Subsystem Design Concept

A scalable solar-sail concept, which integrates recently developed gossamer coilable longeron mast technology, has been developed, providing simple reliable deployment and structural robustness with minimum weight. This sail system is also unique in that it is composed of tensioned membranes without the incorporation of catenaries. This simplification is made possible through a mathematical demonstration of the insignificance of structural wrinkles on propulsive effectiveness. The sail package is a mass-optimized propulsion subsystem that can be mounted to a general heritage spacecraft to provide continuous low-level thrust. The design baseline is a three-axis-stabilized four-quadrant 40-m-square sail with attitude controlled by gimbaling the spacecraft on an extended boom. Considerations for the baseline design definition and the resulting performance vs size are reviewed.

Demonstration of a 10-m Solar Sail System

The NASA In-Space Propulsion (ISP) program has been sponsoring system design development and hardware demonstration activities of solar sail technology over the past 16 months. Efforts to validate by test a moderate-scale (10-m) 1/4 symmetry ground demonstration sail system are nearly complete. Results of testing and analytical model validation of component and assembly functional, strength, stiffness, shape, and dynamic behavior are discussed.

The SCARLET light concentrating solar array

In recent years, spacecraft power levels have continued to grow, accelerating the evolution towards higher efficiency photovoltaic devices. Light concentrating arrays enable the cost-effective implementation of recently developed high-efficiency solar cells while providing high array efficiency, protection from space radiation effects and plasma interaction minimization for high voltage arrays. The line-focus concentrator concept delivers two added advantages: (1) low-cost mass production of the lens material; and (2) relaxation of precise array tracking requirements to only a single axis. New array designs emphasize lightweight, high stiffness, stow-ability, and ease of manufacture and assembly. In this paper, the authors address the current status of the SCARLET (solar concentrator array with refractive linear element technology) concentrator program with special emphasis on cost and mass performance trade-offs versus cell type and Sun tracking capability.

Solar-Sail Attitude Control Design for a Flight Validation Mission

This paper presents the solar-sail attitude control system design for a solar-sail flight validation mission proposed in a dawn–dusk sun-synchronous orbit. The proposed solar-sail attitude control system architecture consists of a propellantless primary attitude control system and a microthruster-based secondary attitude control system. The primary attitude control system employs two ballast masses running alongmast lanyards for pitch/yaw trim control and roll stabilizer bars at themast tips for roll control. The secondary attitude control system uses lightweight pulsed plasma thruster modules mounted at the mast tips. Such a microthruster-based secondary attitude control system can be employed for attitude recovery maneuvers from various off-nominal conditions, including tumbling, that cannot be handled by the propellantless primary attitude control system. The overall simplicity, effectiveness, and robustness of the proposed solar-sail attitude control system architecture are demonstrated for a sailflight validation mission employing a 40-m, 150-kg sailcraft in a 1600-kmdawn–dusk sun-synchronous orbit. The proposed solar-sail attitude control systemwill be applicable with minimal modifications to a wide range of future solar-sailing missions with varying requirements and mission complexity.

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).

Validation of a Scalable Solar Sailcraft System

A T THE start of 2003, ABLE Engineering (now part of ATK Space Systems and Sensors), in parallel with other activities [1] also under the purview of the In-Space Propulsion (ISP) projects office at NASA Marshall Space Flight Center (MSFC), began developing scalable analytical tools and advanced design technologies for a solar sail system, which led to two follow-on phases for system ground demonstrator (SGD) development and validation. These efforts, led by ATK, were performed with the assistance of the Systems Technology Group of SRS Technologies (the sail assembly provider), the NASA Langley Research Center (LARC) for sail shape and dynamics modeling and test execution, Arizona State University (ASU) for attitude-control modeling, Princeton Satellite Systems (PSS) for sailcraft control software, and the NASA Marshall Space Flight Center (MSFC) space environmental effects (SEE) laboratory (materials characterization and life evaluation). In the first phase of the program (six months), activities were focused on design and analysis refinement of the initial sail system concept [2] and refinement of plans for hardware development and demonstration [3] in phases 2 and 3. The phase 2 effort encompassed design, fabrication, and validation through a series of component and system tests of a quadrant (one sail and two masts) of a 10-m system. Validation activities culminated with the demonstration of deployment, sail shape, and system dynamics measurement [4] in a vacuum at the LARC in April of 2004 [5]. Analytical correlation activities demonstrated that gossamer mast, sail subassemblies, and system behavior are predictable [6]. In phase 3, a larger and more complete sail system was designed, fabricated, and demonstrated [7], first in ambient conditions at ATK in Goleta, CA (ATK-Goleta) and later at a large vacuum-chamber facility in April and May of 2005. The 20-m sailcraft hardware represents a flightworthy full (four sails, four masts) system that includes a boom for offsetting instruments well away from the plane of the sail, a dual-purposed mechanism for deployment and attitude control, tie-down and release hardware, solar panels, and launch vehicle interfaces, which were integrated in a carbon-composite central assembly that also functions as a bus chassis. Further descriptions of the hardware are provided herein, along with a review of some of the critical design developments important to the evolution and success of the SGD efforts. Lessons learned in the development of subsystem hardware on a quadrant of a 10-m sail system, referred to herein as the 10-m quadrant, were integrated into design and analysis activities supporting the fabrication and testing of the full sail system. The evolution and integration of additional subsystems included in the 20-m sailcraft are described. Results of 20-m sailcraft system validation activities are emphasized, followed by sail-material life-testing results, discussion of scaled performance, and concluding remarks.

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.

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.

POWERSAIL: THE CHALLENGES OF LARGE, PLANAR, SURFACE STRUCTURES FOR SPACE APPLICATIONS

Deployable surface structures, a subset of gossamer structures, are a near-term solution for producing large- area surfaces in orbit. These surfaces have a variety of applications, high stowed packing efficiency, and mass properties that increase flexibility in the overall system design. ABLE Engineerings SquareRigger, a major component of the Air Force Research Laboratory (AFRL) PowerSail program, is the first modular, large- scale, planar deployable surface structure intended for space applications. In developing and studying the basic module of this class of structure, a number of new challenges have arisen in ground testing and modeling. These challenges have led to changes in the modeling and testing procedures, resulting in an approach that closely ties finite element modeling with ground testing of modules and components. The approach is intended to generate increasingly accurate simulations of on- orbit behavior of surface structures unable to be completely tested on the ground.

MicroPPT-Based Secondary/Backup ACS for a 160-m, 450-kg Solar Sail Spacecraft

Solar sail tip-mounted, lightweight pulsed plasma thrusters (PPTs) are proposed for a secondary (or backup) attitude control system (ACS) of a 160-m, 450-kg solar sail spacecraft of the Solar Polar Imager (SPI) mission. A propellantless primary ACS of the SPI sailcraft 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 of such a propellantless primary ACS would be further enhanced by a secondary ACS utilizing tip-mounted, lightweight PPTs. The microPPT-based ACS is intended mainly for attitude recovery maneuvers from various off-nominal conditions that cannot be reliably handled by the propellantless primary ACS. However, it can also be employed for: i) the checkout or standby mode prior to and during sail deployment, ii) the post-deployment transition mode (prior to the propellantless primary ACS mode operation), iii) the solar sailing cruise mode of a “trimmed” sailcraft, and iv) the spin-stabilized, sun-pointing, safe mode. Although a conventional bus ACS is required for the SPI mission as the sail is jettisoned at the start of its science mission phase, the microPPT-based ACS option promises greater redundancy and robustness for the SPI mission. For other sailing missions, where the sail is never jettisoned, this secondary ACS provides a lower-cost, lower-mass propulsion for deployment control and greater redundancy than any traditional reaction-jet control system. This paper presents an overview nf the state--of-the--art microPIjT technology, the design requirements of microPPTs for solar sail attitude control, and the preliminary ACS design and simulation results.