ALBERT

Description: In the absence of gravitational pull, the major design considerations for large space structures are stiffness for controllability, and transient dynamic loadings (as opposed to the traditional static load associated with earth-based structures). Because of the absence of gravitational loading, space structures can be designed to be significantly lighter than their counterparts on Earth. For example, the Space Shuttle manipulator arm is capable of moving and positioning a 60,000 lb payload, yet weighs less than 1,000 lbs. A recent design for the Space Station which had a total weight of about 500,000 lbs. used a primary loadcarrying keel beam which weighed less than 10,000 lbs. For many large space structures designs it is quite common for the load-carrying structure to have a mass fraction on the order of one or two percent of the total spacecraft mass. This significant weight reduction for large space structures is commonly accompanied by very low natural frequencies. These low frequencies cause an unprecedented level of operational complexity for mission applications which require a high level of positioning and control accuracy. This control problem is currently the subject of considerable research directed towards reducing the flexibility problem. In addition, however, the small mass fraction typically results in structures which are quite unforgiving to inadvertent high loadings. In other words, the structures are 'fragile.' In order to deal with the fragility issue CSC developed a load-limiting concept for space truss structures. This concept is aimed at limiting the levels of load which can occur in a large space structure during the construction process as well as during subsequent operations. Currently, the approach for dealing with large loadings is to make the structure larger. The impact this has on construction is significant. The larger structures are more difficult to package in the launch vehicle, and in fact in some instances the concept must be changed from a deployable truss to an erectable truss to permit packaging. The new load-limiting concept is aimed at permitting the use in large space structures of smaller trusses with a high level of strength robustness, in order to simplify the construction process. To date several analyses conducted on the concept have demonstrated its feasibility, and an experiment is currently being designed to demonstrate its operation.

Description: The primary characteristics of the 5-meter erectable truss is presented, which was baselined for the Space Station. The relatively large 5-meter truss dimension was chosen to provide a deep beam for high bending stiffness yet provide convenient mounting locations for space shuttle cargo bay size payloads which are approx. 14.5 ft (4.4 m) in diameter. Truss nodes and quick attachment erectable joints are described which provide for evolutionary three dimensional growth and for simple maintenance and repair. A mobile remote manipulator system is described which is provided to assist in station construction and maintenance. A discussion is also presented of the construction of the Space Station and the associated extravehicular active (EVA) time.

Description: Previously cited in issue 19, p. 2997, Accession no. A82-38972

Description: The types of equipment and structures that will be required to construct very large spacecraft in space are discussed. One of the basic issues that must be resolved is the appropriate mix of humans and machines in the construction process. While the use of robots offers the potential for reducing the number of extra-vehicular activity (EVA) hours required for particular construction operations, the availability of humans greatly increases the reliability of complex construction tasks. A hybrid system is described which makes the best use of man and machine to provide a highly reliable and versatile construction approach. Such a system will provide an efficient method for constructing large spacecraft until fully automated, robotic devices can be perfected. Details are given on an extensive ground test program which was designed to evaluate and demonstrate large spacecraft construction. A discussion is presented on the use of the Space Station Freedom, or an appropriate derivative, as a construction facility. Finally, a construction scenario and assembly timelines are presented for constructing a 20-meter-diameter high precision reflector.

Description: There is currently considerable interest in low-cost, lightweight, compactly packageable deployable elements for various future missions involving small spacecraft. These elements must also have a simple and reliable deployment scheme and possess zero or very small free-play. Although most small spacecraft do not experience large disturbances, very low stiffness appendages or free-play can couple with even small disturbances and lead to unacceptably large attitude errors which may involve the introduction of a flexible-body control system. A class of structures referred to as 'rigidized structures' offers significant promise in providing deployable elements that will meet these needs for small spacecraft. The purpose of this paper is to introduce several rigidizable concepts and to develop a design methodology which permits a rational comparison of these elements to be made with alternate concepts.

Description: Manuel Stein went to work for NACA (National Advisory Committee for Aeronautics) in 1944 and left in 1988. His research contributions spanned five decades of extremely defining times for the aerospace industry. Problems arising from the analysis and design of efficient thin plate and shell aerospace structures have stimulated research over the past half century. The primary structural technology drivers during Dr. Stein's career included 1940's aluminum aircraft, 1950's jet aircraft, 1960's launch vehicles and advanced spacecraft, 1970's reusable launch vehicles and commercial aircraft, and 1980's composite aircraft. Dr. Stein's research was driven by these areas and he made lasting contributions for each. Dr. Stein's research can be characterized by a judicious mixture of physical insight into the problem, understanding of the basic mechanisms, mathematical modeling of the observed phenomena, and extraordinary analytical and numerical solution methodologies of the resulting mathematical models. This paper summarizes Dr. Stein's life and his contributions to the technical community.

Description: A preliminary design study is presented of a mobile crane suitable for conducting remote, automated construction operations on planetary surfaces. A cursory study was made of earth based mobile cranes and the needs for major improvements were identified. Current earth based cranes have a single cable supporting the payload, and precision positioning is accomplished by the use of construction workers controlling the payload by the use of tethers. For remote, autonomous operations on planetary surfaces it will be necessary to perform the precision operations without the use of humans. To accomplish this the payload must be stabilized relative to the crane. One approach for accomplishing this is to suspend the payload on multiple cable. A 3-cable suspension system crane concept is discussed. An analysis of the natural frequency of the system is presented which verifies the legitimacy of the concept.

Description: Preliminary results are presented of studies being conducted by NASA on the construction of very large spacecraft. The various approaches are discussed for constructing spacecraft and their relative merits. It is observed that the Space Station Freedom has all of the basic design characteristics to permit its growth into an in-space construction facility for very large spacecraft. Also it is noted that if disturbances from construction operations are intolerable to other Space Station experiments, a co-orbiting construction facility could be built using previously developed Space Station truss hardware and systems. A discussion is also presented of a new PATHFINDER research initiative on on-orbit construction. This research effort is aimed at developing construction methods for very large spacecraft and includes the development of a 100 meter long space crane.

Description: An aerobrake structural concept is introduced which consists of two primary components: (1) a lightweight erectable tetrahedral support truss; and (2) sandwich hexagonal heatshield panels which, when attached to the truss, form a continuous impermeable aerobraking surface. Generic finite element models and a general analysis procedure to design tetrahedral truss/hexagonal heatshield panel aerobrakes is developed, and values of the aerobrake design parameters which minimize mass and packaging volume for a 120-foot-diameter aerobrake are determined. Sensitivity of the aerobrake design to variations in design parameters is also assessed. The results show that a 120-foot-diameter aerobrake is viable using the concept presented (i.e., the aerobrake mass is less than or equal to 15 percent of the payload spacecraft mass). Minimizing the aerobrake mass (by increasing the number of rings in the support truss) however, leads to aerobrakes with the highest part count.

Description: The status and some recent developments of continuum modeling for large repetitive lattice structures are summarized. Discussion focuses on a number of aspects including definition of an effective substitute continuum; characterization of the continuum model; and the different approaches for generating the properties of the continuum, namely, the constitutive matrix, the matrix of mass densities, and the matrix of thermal coefficients. Also, a simple approach is presented for generating the continuum properties. The approach can be used to generate analytic and/or numerical values of the continuum properties.