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Unified Structures


Using composites to replace assemblies of smaller components with large integrated structures can reduce costs

George N. Bullen
Principal Engineer and Technical Expert
Advanced Programs and Technology Development
Northrop Grumman Corp.
El Segundo, CA 


The use of composite material as a primary structural material for the construction of aircraft and spacecraft has significantly increased in the last two decades. Northrop's B2, Boeing's 787, and Lockheed's F35 use composite materials extensively in their airframes and components. In addition, NASA's newest launch vehicle, ARES I, uses composite materials in one of its main structural components.

Until recently, composite materials have been used to replace metal parts on a one-to-one basis, and these materials have gained the nickname "Black Metal." Replacing the metal used to make components such as skins with composite increased the challenges involved in fastening components together. Much of the substructure—bulkheads, for example—remained metallic. Therefore a drill bit penetrated the soft composite skin before drilling the metal subcomponent. As a result, challenges arose when harder material in the form of a chip was drawn through the softer composite material. The variability of stacked material types with varying thicknesses drove costs higher, because of the increased complexity imposed on the assembly process by composites.Successful launch of NASA's MLAS vehicle. It met or exceeded design criteria for the test vehicle.

Fastener preparation (drilling and countersinking) and fastener installation represent the largest process operation involved in airframe assembly. Each component is critical to the success of the process, and therefore must be tightly controlled. In addition, a complex system and supply chain of standard tools, training, tool and equipment maintenance, inspection, and other ancillary components and operations are necessary to properly ensure the successful installation of each fastener. Drilling, countersinking, and fastener installation also drive 80% of the cost of quality, and 80% of lost-time injuries.

The initial approach to mitigating such a large contributor to cost, schedule, quality, and safety was to introduce methods and technology like automated drilling and countersinking into the production assembly lines. The technique was to leverage NC control and a precision machine approach to increase repeatability and drill rates, reduce mechanic fatigue, and eliminate a large number of the small tools used by mechanics that needed to be maintained.

Recently, the approach has focused on increasing the size of components to create a unified structure to build larger assemblies without fasteners, or with greatly reduced use of fasteners. Composite materials have aided in the move to unified structure.

There are advantages to this approach. Unified structure to reduce assembly cost and increase quality is not new. In the 1980s, the automotive industry, led by Toyota, began combining many pieces into larger and fewer components through a process called "Implosion." The automotive industry found that larger, single-piece components reduced assembly steps, cost, and complexity, and improved product quality.

"Implosion" of a segmented structure into a single unitized structure.

In aerospace, the use of composite materials can enable many pieces assembled with fasteners to be combined into a single large piece that incorporates all the design and strength benefits of the assembled components. The larger bonded or single-piece fabricated component reduces labor at the fabrication and assembly level, while eliminating or significantly reducing the number of fasteners required and the concomitant hole preparation. Other advantages include reduced mass, elimination of longitudinal joints, integration of assembly and separation joints, and reduced minimum gage penalty. Part counts are reduced, as well as supply-chain complexities and assembly operations.

Other advantages are realized upstream in the manufacturing process, when investments in capital equipment and contract tooling are significantly reduced or eliminated. When segmented structure is fabricated, all the components must be made, trimmed, and held in position to the tolerances necessary for assembly to meet the final assembly's requirements. NC machines for trimming, tools for holding and positioning, and inspection machines such as CMMs must be purchased, installed, operated, and maintained. The increased precision necessary to build modern aircraft has driven up the cost and complexity of these investments. Also, the cost and complexity of acquiring machines and tools for producing precision aerospace components prohibits facility flexibility, and limits factory reconfiguration due to the foundations, isolation pads, and structures needed to sustain operations.

These and many other issues have driven major aerospace companies to evaluate and assess the use of composites as a means of combining segmented parts into larger and larger unified structures.

Challenges come with the territory, however. With all of the apparent advantages, the drive to mitigate the inhibitors to large integrated structure has been the focus of considerable research investment by university, government, and industry.

One of the main focus areas is autoclaving. The autoclave has been identified for years as a necessary evil for the manufacture of composite materials used to construct aerospace parts.

Autoclaves are large and expensive, but needed to cure aerospace-grade composite materials. As engineers identify parts that can transition from segmented structure to unified structure, autoclaves that can accommodate the larger-size structures have become prohibitively expensive, with long lead times required to procure, build, and install the equipment. Autoclave operating cost also incentivizes the effort to develop out-of-autoclave materials and processes to eliminate the need for these capital investments, which are expensive to acquire and operate.

Two main thrusts by autoclave users seek to eliminate the autoclave for curing aerospace-quality parts. One is the development of an out-of-autoclave cure process for autoclave-cure materials. The other is the continued development of materials that use the liquid-resin infusion process, and cure at room temperature.

Both of these processes were combined in the fabrication and assembly of NASA's Max Launch Abort System (MLAS), which successfully launched late year from Wallops Island, VA. The multistage vehicle was composed of a boost skirt, coast skirt, and forward fairings that enclosed a crew module. All of the MLAS vehicle's three stages were fabricated in Gulfport, MS, using liquid-resin infusion shipbuilding methods and simple, low-cost tooling. The vehicle's fins were produced in Huntsville, AL, using an out-of-autoclave process. Final vehicle assembly was carried out at NASA's Wallops Island facility.

Much material and process development has been undertaken to eliminate—or significantly reduce—the need for autoclaves, but other challenges need to be addressed.

When the automotive industry transitioned to unified structure, customer satisfaction went up when rattles went down. But the side effect of unified body parts was increased repair cost. Before unification, a fender or bumper could be replaced if it was damaged. After unification, an entire quarter panel had to be replaced. Due to the cost and critical nature of aerospace parts, the cost of component repair is compounded in aerospace manufacture and maintainability.

Fixtures necessary to hold a segmented structure in precise position during the fastening process are large and complex.

Under current segmented-manufacturing processes, if a part is damaged the assembly line can continue producing while a "surge" occurs to replace the part, and install it downstream in the production flow. If a large piece of unified structure is damaged, everything stops until the part is replaced. The same is true for tooling. In segmented processes, tooling can be cycled in and out of the production line for maintenance or calibration in a controlled manner to sustain production flow. A unified structural tool taken off the line could stop production. The same is true in the field. Currently, segmented parts that are remove-and-replace (R&R) at the point of use can be replaced or repaired with small out-of-service time and cost. Large segmented parts are more costly, and could potentially shut down a vehicle for weeks or months.

With small parts, access to systems and subsystems by field support and maintenance crews is comparatively uncomplicated. A small part can be removed to repair damage to wiring, tubing, systems, and other hidden elements that might need repair, test, and evaluation.

Another inhibitor is system, subsystem, and vehicle installation and integration. When small pieces are combined into a large structure, the ability to install components and provide access to the workforce becomes a consideration. Today fighter aircraft are built from the inside out with the covers (the skins) applied after all systems are installed and connected. And with segmented assembly, the production line is spread out to provide room for many operations to occur, and to allow mechanics to perform their work. When segmented structure becomes unified, the confined workspace may slow the installation of systems and components.

The issues of unified structure damage and repair cost, and the challenges associated with access to unified structure, are being addressed by development of decision models that enhance the ability of engineering and manufacturing personnel to make decisions that leverage the advantages of unified structure.

NASA's Max Launch Abort System (MLAS) during installation on the launch stand at Wallops Island, VA.Here's an example: One of the models used in the decision for manufacturing breaks and unification of the MLAS vehicle was derived from the shipbuilding industry. Shipbuilders have made their products in large, unified-structure modular segments for years. Shipbuilders realized early on that, based on a number of criteria, some components had to be broken into smaller segments. A main consideration was how much had to be installed into the segment once it was produced. The more that went into the segment, the more consideration was given to segmentation to facilitate access. The emptier the subassembly, the larger it could be made, because it did not inhibit installation of subsequent components and parts.

Of course there are other considerations involved, such as the degree of unification, production breaks, and segmentation, but the model is primarily composed of a one through ten degree-of-difficulty decision model. The first step is to quantify the number of subsystems, and their complexity and installation difficulty on a scale of one to ten, with ten representing the highest degree of difficulty. Then, the decision model is overlaid onto a segmentation matrix to determine the size of the components to be produced, and where the production breaks would occur. Applying the process maximizes the unified structure benefits, while recognizing best installation-size limitations.

The benefits derived by using composite materials in aerospace products are widely recognized. Over the last two decades, the percentage of composite material used in aerospace vehicles, and the number of parts produced using composite material, have increased significantly. The use of composite parts is beginning to expand beyond its original application as a replacement for metal.

The cost of fabricating and assembling aircraft and spacecraft parts and assemblies has led to initiatives to leverage the attributes of composites to translate to larger unified structures. The advantage of unified structure for cost-and-weight reduction, as well as increased quality, incentivizes unified structural initiatives such as out-of-autoclave cure. Autoclaves, because they are costly to install and operate, have been among the prime targets for elimination to enable employment of large unified structures.

Challenges remain, among them material application rates, determining production breaks, and installation of systems and subsystems. The benefits derived from combining many parts into a few offer so many advantages that the drive for unified structure will continue. The challenges are many. For example, as structures grow, tests need to be performed to confirm that the processes used for layup translate well to larger structures.

Just as the automotive industry recognized two decades ago, reducing assembly steps through employment of unified structure reduces cost and increases quality. The move to translate this positive lesson from the automotive industry to aerospace and space structures should yield similar benefits.


This article was first published in the March 2010 edition of Manufacturing Engineering magazine. 

Published Date : 3/1/2010

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