Advanced Grid Stiffened Composite Structures
Dateline: 05/25/98Guide Intro
This week's feature article was written by Dr. Steven Huybrechts of the Air Force Research Lab, Space Vehicles Directorate. Dr. Huybrechts invented the hybrid tooling method for manufacturing grid stiffened composites. In this article, he describes the history of grid stiffened structures and discusses some current applications.
AGS Structures
Although the necessity of using composite materials in the next generation of launch vehicles has become widely accepted, the specific structural type (monocoque, sandwich, skin-stringer, etc.) has yet to be determined. One possible candidate is the Advanced Grid Stiffened (AGS) structure, a rib-skin configuration evolved from early aluminum isogrid stiffening concepts. AGS structures show great promise for satisfying increased reliability and reduced cost requirements due to their adaptability to automated manufacturing. They also lend themselves to use in operational environments, due to their high strength, resistance to moisture absorption, and damage tolerance. Some AGS structures are shown in Figure 1 and Figure 2.
Figure 1. Flat isogrid panel. Isogrids are AGS structures whose ribs form equilateral triangles.
Figure 2. Conical AGS structure.
The McDonnell-Douglas Corporation (now part of The Boeing Company) holds the patent rights for development of the first aluminum isogrid, the earliest precursor of the modern AGS structure. This structure is machined from a single piece of aluminum stock and consists of a skin with stiffeners that form equilateral triangles. The stiffness behavior of an isogrid is isotropic within the plane of the structure, leading to the "iso" prefix. Despite being developed several decades ago, this structure is still used as the basis for most Titan and Delta launch vehicle weight critical structures. An example of an aluminum isogrid is shown in Figure 3.
Figure 3. Aluminum isogrid used in McDonnell-Douglas launch vehicle components.
In the late 1970s, with increasing interest in composite materials for aerospace applications, early attempts were made to fabricate composite isogrid structures. Composite materials are particularly well suited for this application as the typical stresses in an isogrid structure's ribs are highly directional along the rib length. The high directionality of composite materials allows for the majority of the material's stiffness and strength to be directed along this directional state of stress, leading, in many cases, to an order of magnitude increase in stiffener strength.
Early composite isogrid structures were manufactured for aerospace applications by government research groups in both the USA and the USSR, with most early American concepts being developed at the National Aeronautics and Space Administration (NASA). Several composite isogrid launch vehicle components were tested in the USSR, although the promise of light weight isogrids was never fully realized. Further research into composite isogrid fabrication continued sporadically in the USA at a number of universities and aerospace companies. Most aerospace industry work is still considered proprietary.
In the early 1990s, the Air Force Phillips Lab (now the Air Force Research Lab) began an effort that finally achieved high quality, light weight composite isogrid structures. These structures were manufactured using tooling made of silicon rubber and proved to have extremely high strength-weight ratios. While successful, the manufacturing method developed was very labor intensive and was limited primarily to cylindrical structures. A cylinder produced as part of this effort is shown in Figure 4. This cylinder weighed 3.2 kg (7 lbs) and was loaded axially with 17,500 kg (38,500 lbs) without experiencing any failure. The fabrication of this cylinder is shown in Figure 5 and used solid molded silicon rubber sheets wrapped around metallic mandrels.
Figure 4. Composite isogrid cylinder fabricated using solid silicon rubber tooling.
Figure 5. Early Composite Isogrid Tooling.
In the past few years there has been a renewed interest in isogrid structures leading to an abundance of new design ideas and manufacturing concepts. The traditional equilateral pattern, which leads to the name isogrid, has been abandoned in favor of stiffener patterns optimized to specified loading situations. Recognizing this change, these structures are increasingly referred to as composite grid structures or advanced grid stiffened (AGS) structures. Additionally, many new and innovative manufacturing methods have been created. Some notable examples are the SnapSat concept from Composite Optics Inc., the Tooling Reinforced Integral Grid (TRIG) concept from Stanford University, and the Hybrid Tooling concept from the Air Force Research Laboratory. These new methods promise to make affordable, high quality composite grid structures a reality.
Both the SnapSat and TRIG processes use automated methods, which assemble and bond previously cured laminates or tubes. Both of these processes are highly flexible and show great promise for spacecraft applications, which require stiff, high tolerance parts.
The Hybrid Tooling manufacturing method was developed specifically for launch vehicle applications, which are driven by high strength requirements. This solution uses expansion tooling inserts in a thermally stable base tool. This use of multiple tooling materials lead to the name "Hybrid Tooling". The combination of materials allows for precise control of lateral rib compaction, while maintaining process controllability. In addition, the inserts are easily applied to more complex shapes without the problems of groove alignment or helix angle control experience in solid silicon rubber tooling concept. An example of a Hybrid Tooling tool is shown in Figure 6. The Hybrid Tooling concept is proven to be compatible with filament winding and expected to be compatible with fiber placement.
The first AGS composite structure developed with this tooling was a payload shroud, which weighed only 37 kg (82 lbs). The existing aluminum shroud structure that this design replaced weighed 97 kg (212 lbs). In addition to this 61% reduction in weight, the AGS shroud program achieved an 88% reduction in manufacturing time, a 300% strength increase, and a 1000% stiffness increase over the existing aluminum design. This AGS shroud was successfully launched on 23 Feb 97, from Wallops Island, VA. This was the first successful flight of an AGS structure. This payload shroud structure is shown in Figure 7.
Figure 6. The Hybrid Tooling Concept.
Figure 7. AGS Payload Shroud
The Air Force Research Lab is currently engaged in the transition of this technology to an operational vehicle. A baseline fairing for the Minotaur launch vehicle is being designed and fabricated by Air Force Research Lab, The Boeing Company, Dynacs Engineering, Orbital Sciences Corporation and CSA Engineering. This fairing has an anticipated launch date of Jan 2000 and will incorporate most of the recent advances in AGS structures technology.
Guest Articles
Do you have a project, editorial subject, or other composites topic you would like to write about? Guest articles can be submitted to Barry Berenberg at composite.guide@about.com. You may want to send a description of your article before submitting the whole thing.
Previous Features