Graham Warwick/ATLANTA
A CORNER OF THE massive assembly building at Lockheed Martin's Marietta, Georgia, plant has been resurfaced, repainted and surrounded by a high white fence. Behind this fence, preparations are well under way for final assembly of the first Lockheed Martin/Boeing F-22, aircraft 4001.
Originally, the stealthy F-22 was to be assembled inside an ultra-secure, building-within-a-building, but this proved to be too expensive, says manufacturing programme manager Randy Simpson. Instead, the nine flight-test aircraft will take shape in the relative openness of a fenced enclosure just a few metres from Lockheed Martin's C-130J assembly line.
Already, the forward fuselage is taking shape. The aft fuselage and wing sections are scheduled to arrive by rail from Boeing in Seattle, Washington, and the mid-fuselage from Lockheed Martin in Fort Worth, Texas, during the third quarter of this year. The first F-22, is scheduled to be flown, at Marietta in May 1997.
LOCATION SPECIFIC
The F-22 is being manufactured at three locations and each section has unique requirements: from the demand for the Seattle-built aft fuselage to withstand high temperatures, to the need to maintain tight control of the external lines of the Marietta-built forward fuselage.
Materials selection and manufacturing processes have been determined by the different design requirements on each section. In the case of the aft fuselage, the dominant factor is heat, Simpson says, as the major structural components - the booms to which both the horizontal and vertical tails are attached - lie alongside the nozzles.
As a result, 55% of the aft-fuselage structure is of heat-resistant titanium. The complex mid-fuselage section is 30% titanium, 30% aluminum and 30% composite, while the forward fuselage is 50% aluminum and 50% composite. The wing structure is almost 50% titanium, with composites making up most of the rest.
New manufacturing processes have been introduced to make the best use of the materials selected. In the aft fuselage, the titanium tail-booms are being assembled using electron-beam welding. The booms are built up from forward and aft sections produced by subcontractor Aerojet by robotically welding integrally stiffened titanium-isogrid panels to machined-titanium forward, middle and end frames.
Simpson says that a good weld requires "very exacting machining tolerances at the joints." The thickness of the structure, which must carry the engine, as well as tail, loads, adds to the difficulty of the task. Results so far have been excellent, he says, with the welding process proving to be "very repeatable". In production F-22s, the complete booms will be welded, eliminating the present mechanical joints.
According to Simpson, Boeing's assembly work on the aft fuselage will be limited to joining the boom sections, adding frames and brackets, attaching lower skins and installing systems. After assembly, the aft fuselage will be lifted out of the tool and transferred to an automated drilling cell, where the upper skins will be drilled and fastened robotically.
WING REPLACEMENTS
The drilling cell installed by Boeing for its contract to produce replacement composite wings for US Navy Grumman A-6s will be used for F-22 wing manufacture - a Boeing responsibility. "The wing is largely carbonfibre and well suited to automated drilling," says Simpson.
An assembled wing will be located vertically, and a gantry robot used to drill through skin and substrate in one operation. The F-22's combination of composite skins and titanium spars has required the development of drill designs which prevent titanium chips scoring the composite material and reducing fatigue life, he says.
While the main wing spars are machined from titanium forgings, the intermediate spars are composite. These "sine-wave" spars, are produced by sub-contractor Dow-UT using resin transfer moulding (RTM). Spars up to 4.5m long, are produced by stacking dry fibres, in a preform die and then injecting the resin.
RTM is used to produce some 325 composite components throughout the F-22. Simpson cites two advantages of the process - dimensional control and recurring cost. "The non-recurring expense is a bit higher, but we can set up, inject and cure in a fraction of the time it takes to lay up and cure [a component]," he says.
Dimensional control is critical on the F-22 - and not just to maintain tight tolerances on the external lines to minimise radar cross-section. "We have cut the margin for fit-up to the lowest possible level. We cannot afford any slop in the joints, as this affects fatigue life," Simpson says. "We are looking for a perfect fit, with minimum shimming. A better fit means less stress, less wear and tear, for a higher fatigue life and lower weight," he adds.
Controlling the F-22's weight is a "constant battle", requiring continual trade-offs between "...weight, performance, cost, and physics...we are challenging the laws of physics [in our efforts to reduce weight]", Simpson says. "We are within 1-2% of the right balance. We need to be close," he admits.
CONGESTED HEART
Assembly of the first mid-fuselage section has been under way at Lockheed Martin's Fort Worth, Texas, factory, since June 1995. "This is the heart of the aircraft," Simpson explains. "Most of the systems pass through or reside here. The mid-fuselage is a highly congested maze of wires, tubes, ducts and structure. A complicating feature is the inlet ducts, which weave their way through the mid-fuselage, curving around and over systems," he says.
Although single-piece filament-wound inlet tunnels were considered, the final design uses compound-curvature tooling to produce the composite ducts in inner and outer sections, with the number of joints minimised to reduce weight. The main reason for this is to improve access to the mid-fuselage during assembly.
The mid-fuselage is built in three modules. Each is assembled vertically, starting at the aft bulkhead and working from the inside out. First the frames are loaded, then the inboard duct skins, systems and outboard duct skins. An elevator in the inlet tunnel allows assembly workers to move up and down inside each module.
Under the modular-tooling concept, subassemblies are first built up on transportable "co-fixtures" which then plug into the major assembly jig. Duplication of the co-fixtures allows subassembly to proceed in parallel with assembly, and this approach enables more people to work on the aircraft simultaneously.
Completed modules will be turned horizontal for joining. The first mid-fuselage section, is scheduled to be joined in March, with installation and checkout of the systems, to be completed by August. The completed section will then be transported by truck to Lockheed Martin's Marietta, Georgia, plant for final assembly.
As delivered, the 5.2m-long mid-fuselage will weigh around 2,600kg and will contain some 5,000 structural parts, more than 100,000 fasteners and all or part of the aircraft's fuel, hydraulic, electrical, environmental-control and auxiliary-power systems.
According to Simpson, the original final-assembly concept called for the delivery of fully stuffed, functionally checked, sections. This has evolved to require partners to "stay out of certain areas" to facilitate joining. Instead, kits will be shipped with the sections to allow the wheel wells and weapons bays, for example, to be completed at Marietta.
The F-22 fuselage sections will overlap slightly, with forward-fuselage longerons running back through the mid-fuselage bulkhead, for example. This is the lowest-weight solution Simpson says, as a straight splice adds weight through the duplication, of bulkheads at the joint. A laser-guided alignment system will be used to mate the fuselage sections.
SURFACE CONTROL
Assembly of the first forward fuselage began at Lockheed Martin's Marietta plant in November 1995. This relatively small section of the aircraft is largely aluminum and composites. "About half the composite parts are RTM for dimensional control of the external surface," which has tight tolerances on steps and gaps in the composite skin, Simpson says.
Traditional fabricated-aluminum bulkheads, frames and racks have been eliminated and replaced by single-piece machined components. Five-axis machining of bulkheads results in major weight savings in the mid- and forward fuselage, while holding tolerances three times tighter than those achieved in the Lockheed Martin F-16.
Parts are machined, to the minimum web and flange thickness - under 1.5mm in some areas - in an effort to control weight, he says. As a result, some parts must be handled with increased care. "We've changed the way we transport parts, with individual packaging and a lot more protection and padding," Simpson reveals.
The avionics racks present perhaps the most exacting machining task, requiring tolerances as small as 0.025mm. Simpson describes the racks as "thermal-management systems of infinite variety". The rack for the F-22's "brain", the common integrated processor, for example, must duct cooling fluid through rows of plug-in electronic modules without leaking. "Nothing like this has been built before," he maintains.
Fabrication and assembly of the first F-22 is going well, Simpson says. "The computer-based three-dimensional design tools we are using on this programme prove themselves out when we put parts in the assembly jig and the parts fit," he says.
More than 50% of the first parts produced have proved usable on the first aircraft and more than 90% of the Marietta-produced components have passed first-article inspection the first time around. "This is an usually high first-yield percentage for these types of parts," the company says.
Source: Flight International
