Solar Sailcraft of the First Generation - Technology Development

Based on the experience gained with the ground deployment demonstration of a 20 m x 20 m solar sail, performed in December 1999 at DLR, the future development of solar sails has been investigated recently by DLR on behalf of ESA within the PROPULSION 2000 – PHASE II study. The paper highlights some of the findings and gives recommendations for near-term actions. Solar sails are large and light-weight, generally deployable or inflatable, space structures that reflect solar radiation and thereby utilize the freely available solar radiation pressure (SRP) for propellant-free space propulsion. They have a characteristic length of several tens to hundreds of meters and specific masses of several grams to tens of grams per square-meter. Typically, the reflecting material consists of aluminum coated thin plastic films with a thickness of a few micrometers. The considered system design displays a three axes stabilized square sail with diagonal deployable or inflatable booms for tensioning the thin film. In order to derive the necessary technology development, the paper identifies minimum performance requirements for near-term solar sailcraft missions, using results from recent hardware developments and mission analyses, including trajectory simulation and optimization. A characteristic acceleration of 0.1 to 0.2 mm/s seems to represent a ‘lower bound’ for useful acceleration levels within typical planetary missions. The DLR/ESA ground demonstration cannot achieve this required characteristic acceleration. Thus, to improve the performance, it seems necessary to go to larger and thinner sails, and to modify the design of the deployment module. A technology development roadmap on subsystem-level is outlined, represented by different sails with increasing performance. In-orbit deployment demonstration missions are proposed for low Earth orbit with an altitude of about 350 km, so that automatic de-orbiting takes place after the mission due to atmospheric friction. Other missions are proposed for deep space (C3 ≥ 0 km/s), requiring a launcher that injects the sailcraft into an Earth escape trajectory. INTRODUCTION AND GENERAL CONCEPT Solar sails are large and light-weight, generally deployable or inflatable space structures that reflect solar radiation, thereby utilizing the freely available radiation pressure for propellant-free space propulsion. Solar sails provide a wide range of opportunities for low-cost interplanetary missions with large ∆V-requirements, many of which may be difficult or even impossible with other types of spacecraft. With the exception of rotating configurations, which are difficult to control and steer, a typical system design features a three axes stabilized square sail with diagonal deployable or inflatable booms for tensioning the coated thin film (e.g. aluminized PEN or Kapton). Attitude control could be achieved, e.g., with gas thrusters (especially for the deployment phase and as back-up), with control flaps at the boom tips or with a steerable control mast (see description below). Our baseline design is taken from the ODISSEE (Orbital Demonstration of an Innovative Solar Sail driven Expandable structure Experiment) proposal [Leipold et al. 1999a]. The sailcraft comprises a central deployment module for the sail and the four CFRP (Carbon Fiber Reinforced Plastics) booms that diagonally support the four sail segments, a micro-spacecraft – which is the “payload” to be transported – and a deployable control mast (see Figures 1 and 2). The control mast connects the micro-spacecraft with the sail structure and is attached to the deployment module via a 2-degreeof-freedom actuator gimbal, which allows to rotate 1 54th International Astronautical Congress of the International Astronautical Federation, the International Academy of Astronautics, and the International Institute of Space Law 29 September 3 October 2003, Bremen, Germany IAC-03-S.6.03 Copyright © 2003 by the author(s). Published by the American Institute of Aeronautics and Astronautics, Inc., with permission. Released to IAF/IAA/AIAA to publish in all forms. the control mast and the attached micro-spacecraft with respect to the sail. This way, the center of mass (CM) can be offset from the center of pressure (CP) and, using light pressure as an external force, a torque can be generated to control the sail attitude. This attitude control concept was originally proposed by [Angrilli et al. 1990]. The length of the control mast would be optimized to achieve the required angular acceleration levels (for a 10 m control mast and a (50 m) sail, a 90° turn takes typically about 10 minutes at 1 AU). As an additional option, an active attitude control system with gas thrusters may be installed in the microspacecraft. Figure 1: DLR design for a free-flying three-axis stabilized sailcraft with deployed control mast