Mass manufacturing and stealth aircraft have never mixed well. Hundreds of thousands of parts must align at tolerances measured to the thousandths of an inch. A structural misalignment no wider than a few human hairs is enough to make an aircraft shine like a lighthouse in the electromagnetic spectrum.

In the elite club of stealth aircraft manufacturers, Lockheed Martin set the output record six years ago by averaging two F-22 Raptor deliveries per month, then topped that four years later as the F-35 Lightning II production rate reached three per month.

If Lockheed’s order projections are realised, however, the F-35 must become the stealth fighter equivalent of the Ford Model T in less than four years. That is when monthly output at Lockheed’s mile-long factory in Fort Worth, Texas, is supposed to reach a peak of 17 F-35s in 2019. Lockheed has built non-stealthy fighters faster in the past – the same factory built 33 F-16s in October 1981 – but the four-year goal for monthly F-35 deliveries is nearly seven times higher than any stealth aircraft programme has ever achieved.

With the scale of the challenge at full-rate production well understood, Lockheed designed the F-35 final assembly line to be different from the outset. In the two critical final assembly functions – aligning and mating the fuselage sections and wings and then installing the engine and control surfaces – the F-35 production system departs from Lockheed’s past practice on the F-16 and F-22.

But some of the plans for F-35 manufacturing have evolved and changed as the potential reality of full-rate production draws nearer.

A prime example is the fate of a now-discarded plan to install a continuously moving final assembly line. Emulating a Toyota car factory, Lockheed’s concept proposed an automated track that would advance F-35s down the line at a steady clip of 1.22m (4ft) per hour. Workers would keep pace as they attached the fighter’s control surfaces, installed the Pratt & Whitney F135 engine and performed systems tests.

In a 2004 news release announcing the moving assembly line studies, Lockheed officials said such an innovation, if implemented, could save $300 million over the life of the programme.

"Affordability is the cornerstone on which the JSF programme is built, and we're beginning to see how a continuous moving assembly line could help us meet our commitment to keep costs low," said Tom Burbage, Lockheed’s then-executive vice-president and F-35 programme general manager. "We are in the process of weighing the up-front investments against the long-term returns. So far, we like what we see."

However, Lockheed’s internal position has changed within the last three years, says Don Kinard, a Lockheed senior technical fellow charged with developing the F-35 “fighter production system”.

“We studied [a continuously moving line] for years,” Kinard said in a recent interview. “We did a lot of analysis and we figured that 95% of the benefit of a moving line could be captured with a pulse line. The moving line brought additional complexity to it, and the complexity and the cost of going from a pulsed line to a moving line was determined to be not worth the advantage.”

The F-35’s last stages of assembly will still be very different to those on the F-16 line, for example. Lockheed parks the F-16 after fuselage and wing mating in an assembly bay, where it does not move until the aircraft is ready to enter the paint hangar.

By contrast, Lockheed adopted a “pulsed” line for the F-35, with a flow-to-takt-time assembly model. In such a model, components flow through assembly positions all the way through the supply chain at intervals aligned with the monthly delivery rate.

Another split with Lockheed tradition on the F-35 line is the station where the four main fuselage sections are joined with the wings. It is here where all the major structural components are merged from a global supply chain. Lockheed makes the forward fuselage section and completes 60% of the wings from a parallel assembly line in Fort Worth. Alenia Aermacchi builds the remaining 40% of the wings, while Northrop Grumman and Turkey’s TAI split responsibility for building the centre fuselage. Lockheed’s plant in Marietta, Georgia makes the centre wing assembly, and BAE Systems builds the aft fuselage.

There are three major mate stations in the F-35’s structure. The 270 mate joint – measuring 270in from the aircraft’s nose – joins Lockheed’s forward fuselage to the Northrop/TAI centre fuselage. The 425 mate joint connects the centre fuselage with Lockheed’s centre wing assembly, and the 556 mate joins the centre wing to BAE’s aft fuselage.

Each section has to be mated precisely or there is a risk of misalignments in the structure giving away the F-35’s profile on radar. On top of this complexity, there are three major variants of the F-35 – each with unique structures and systems. As Lockheed was conceiving of the final assembly process, it was quickly obvious that a sophisticated tooling system was necessary for mating the major structures of each of the three variants.

Lockheed’s electronic mating and alignment system (EMAS) was developed to solve that problem. The EMAS replaces the large tooling towers that would have been required to mate each variant. Instead, a platform is erected, and all four major sections are lifted by crane into an F-35-shaped space in the middle to be mated. Each of the sections are digitally mapped and mated synthetically in the EMAS software. This simulation of the mating process is used to predict the number and thickness of the required shims and filler. Too much or too little can cause a stealth-degrading misalignment.

The four sections are initially joined together and inspected to make sure the thickness of the shims is accurate. Then, the EMAS pulls the three mate joints apart so workers can make corrections. The sections are then rejoined to validate the corrections. Finally, the EMAS pulls the sections apart one last time so workers can apply sealants and protective coatings on the interior of the structures.

The labour-intensive process represents the majority of the F-35 mating work, but the EMAS makes it possible to mate all three variants using a common platform, rather than individual tooling towers.

However, the EMAS was also an assembly bottleneck in the early years of F-35 production.

The first few lots of low-rate initial production were slowed by delays that ran deep into the supply chain. However, the joint programme office required Lockheed to keep the flow-to-takt system running, even if major assemblies were missing large numbers of parts. By the time these components reached the EMAS stage, Lockheed’s production system had broken down.

“Travelled work at mate isn’t a good thing. We did it to scrunch the schedule, but … out of station work is really chaos for us,” Kinard says. “Remember, we’re in flow – so if I don’t do it in this station it moves to the next station, or the next station. And that means there’s mechanics there that might not be familiar with [the part]. We want to have the part delivered and consumed in that position, all the work done and moved to the next position.”

Those problems have largely been resolved as Lockheed stabilised at a steady rate of three F-35s per month since 2013. F-35s previously arrived at the EMAS station only 30% complete, but now enter the mate routinely at more than 99% complete, Kinard adds.

"The only real travelled work we have right now anywhere in the factory is really in final assembly," he says. "It’s moving typically within one position from final assembly to another position in final assembly".