The Digital Thread - Key to F-35 Joint Strike Fighter Affordability

F-35s make their way through the moving assembly line at the F-35 production facility at Lockheed Martin Aeronautics in Fort Worth, TX. The moving line is designed to improve production quality and speed, and is the first ever for a modern fighter. The F-35 Lightning II is the newest addition to a historic legacy of military fighter aircraft from Lockheed Martin, whose corporate roots trace back through Curtis-Wright Aviation, Lockheed Aircraft Corporation, General Dynamics Fort Worth Division, Convair, and Consolidated Aircraft in the early 1900s. This line-up includes notables such as the F-22, F-117, F-16, and F-111. The F-35 itself is the result of a number of Department of Defense (DoD) developmental programs, and began with the premise that industry could develop a versatile platform able to satisfy the diverse requirements for U.S. warfighters in the Air Force, Navy, and Marines, as well as for our allies around the globe. The concept of a multi-variant – conventional takeoff and landing (CTOL) variant for the Air Force and international allies, a carrier variant (CV) for the Navy, and a short takeoff, vertical landing (STOVL) variant for the Marines, the United Kingdom, and Italy – is quickly becoming a reality as we close out the 19-aircraft System Development and Demonstration (SDD) program, and are well into aircraft assembly for the low-rate initial production (LRIP) programs, including LRIP 1 (2 aircraft), LRIP 2 (12 aircraft), and LRIP 3 (17 aircraft). LRIP 4 (32 aircraft) will begin assembly this year, and long-lead funding for LRIP 5 (42 aircraft) is under way. As of this article, a total of 43 aircraft are currently in production with a new F-35 starting assembly every 15 calendar days.

The Lightning II is entering the transition to full-rate production phase between now and 2016, which includes ramping existing factories – Pratt and Whitney (engines) in Connecticut, BAE Systems in the U.K. (empennage and aft fuselages), Northrop Grumman in California (center fuselages), Lockheed Martin Palmdale (control surfaces and edges), and Lockheed Martin Fort Worth (wings, forward fuselages, mate, final assembly, and delivery) from one aircraft per month to one aircraft per day. In addition, the international production facilities in partner countries are starting. Those facilities include TAI in Turkey for the center fuselages, Alenia in Italy for the wings, Kongsberg Defence and Aerospace in Norway for conventional edges, and Terma in Denmark for horizontal tails, conventional edges, and composite parts. One of the strengths of the F-35 production philosophy is that it has leveraged design, manufacturing, and production talent across the world in pursuit of the most cost-effective technologies. Many other countries have expressed interest in participating in the F-35 program as customers for the completed jets, as well as bringing their manufacturing talents to support the overall Lightning II global objectives.

The F-35 Lightning II design takes clear advantage of a common digital thread technology that has allowed unprecedented engineering and manufacturing connections, facilitating dramatically expanded automation for military fighter manufacturing and assembly. Digital thread implies that 3D exact solid models from engineering design are used directly by manufacturing for NC programming, coordinate measurement machines (CMM) inspections, and tooling (which are also 3D solid models). These same models are also used by Autonomic Logistics Global Sustainment (ALGS) for training and maintenance development (tech order graphics). To date, this digital thread has resulted in unprecedented first-time part fit, reductions in tool rework, and significantly less than legacy engineering changes resulting from data reconfiguration by suppliers. Finally, this 3D digital thread data is contained in a common system, Product Data Manager (PDM), and seamlessly used by partners and suppliers around the world.

The F-35 is approximately one-third titanium, one-third aluminum, and one-third composite by weight, varying somewhat based on requirements between F-35 variants. For example, the F-35C CV has significantly more titanium than the F-35B STOVL model because of the larger wing and carrier-landing loads. Supply Chain Management has been working for years with suppliers of raw materials and fabricators to lock in capacity to ensure stability in the F-35 supply chain. There are multiple sources of titanium and aluminum qualified right now for most applications, and the process of converting from titanium blocker forgings to titanium close form forgings to reduce the buy to fly material requirements for this expensive material is occurring. Transition planning is also under way for single source materials, such as the aluminum forging alloy and forgings produced by Alcoa for the large wing bulkheads.


Specialized Equipment
Unique to the F-35 is the use of highly accurate NC technology to machine composite skins and substructures to eliminate shimming and facilitate excellent control over the outer mold line of the aircraft. This specialized equipment relies on a solid gantry system, tight environmental temperature control, and calibration using laser compensation which corrects for machine gantry position with more accurate laser measurements. The cost of this sort of equipment, currently three to five times that of conventional NC machines, is being reduced as more companies incorporate the technology into their equipment. These cost reductions may eventually allow expansion of the technology to more of the airframe parts, resulting in reduced shimming at assembly in the future.

Epoxy and BMI composite materials are sole sourced (CYTEC), but there are several manufacturing sites to protect supply. Automated fiber placement is used on inlet, nacelle, and wing skins and although there is not currently sufficient capacity for fiber placement to support rate production, Supply Chain Management has been working with select suppliers to develop transition plans. A unique inspection technology is used for many of the Lightning II composite skins. The technology, originally developed by Lockheed Martin scientists, is called Laser Ultrasonics. Laser Ultrasonics inspection replaces traditional systems by using a laser beam to create a thermo-elastic event leading to an ultrasonic sound wave in the composite laminate. The sound wave return is then detected by another laser to measure the response of the material. This technology, recently acquired and now marketed by PaR Systems, is approximately 10 times faster than conventional water squirter systems and does not require the inspection head to closely follow the contour of the part, which eliminates expensive tooling. Another composite related technology invented by Lockheed Martin is being used to help control composite thickness. A laser radar system accurately measures the part thickness, determines where compensation plies need to be added to adjust the thickness, communicates this information with a Gerber ply cutting machine, and finally ties in with a Virtek laser projector to locate the compensation plies on the laminate. This approach has yielded thickness control similar to that of the highly accurate machining technologies previously discussed.

Laser technologies are also finding their way into new applications for the Lightning II. Lasers are now being used in F-35 final finishes to locate materials, masking areas, inspection areas, and decals to replace tooling. Laser measurements are used in conjunction with the digital thread to allow virtual mate in advance of physical mate to identify potential interference issues ahead of time. One of the more exciting developmental areas is the use of laser projection of fastener call-outs onto part surfaces to provide mechanics drawing fastener information for each individual hole. These projected call-outs substantially reduce the time it takes to decipher a drawing to obtain the same information. Optical projection approaches are also being pursued as they are generally lower cost than laser systems.

Digital thread technologies have also enabled the Lightning II to incorporate gantry drilling for about 80% of the exterior surface holes that are drilled, reamed, and countersunk in a single pass. Implementation of these automated drilling machines is cost effective because of the forecast F-35 aircraft volumes, as well as the resulting span reductions and quality improvements associated with automated drilling compared to manual drilling. In addition to gantry drilling, the F-35 program incorporates the robotic drilling in hard-to-reach areas, using metrology-guided drilling heads. Historically, robot drilling equipment has not been sufficiently accurate (hole position) nor stiff enough (hole quality) to drill in hard-to-reach areas, but robotic applications can substantially reduce system cost compared to gantry equipment and will allow the expansion of the number of holes that can be auto-drilled.

The increase in F-35 rates, the use of advanced technologies, international participation, and recent issues in other large transition-to-production programs spawned the creation of the F-35 Fighter Production System (FPS). Fundamentally, FPS is a systems-engineering approach to manufacturing execution and includes integration of design maturation, affordability, change management, tooling and facilities, supply chain development, production processes, and systems (quality, information technology, etc), as well as people and performance development. An integrated master schedule integrating all of the activities is key to the FPS, as is a well developed risk identification and mitigation process. The FPS is looking at future technologies that could benefit affordability and quality for the F-35 Lightning II, such as reducing manual finish task; automated sealant and liquid shim applications; lower-cost, highly accurate drilling and machining equipment; and laser and optical projection of work instructions. New technologies will have to buy their way onto the manufacturing floor, but the current projections of total F-35 production of 4,000 aircraft or more will support affordability-related implementations for many years to come. As we stand now in base camp for the climb up the rate-production mountain, the F-35 Lightning II is well-equipped to meet the current challenges and well-positioned to take advantage of emerging technologies to meet future goals

Lockheed Martin Aeronautics
Fort Worth, TX
lockheedmartin.com