Fighting fit

With three companies producing various and complex parts for the JSF’s system development and demonstration models, never has the need for perfection been greater. Stuart Nathan explains


Assembling the world’s most advanced fighter plane was never going to be an easy task. But at BAE Systems’ Samlesbury site, it is clear that one of the biggest challenges is ensuring that all the different parts fit together.

BAE is one of the three companies producing parts for the system development and demonstration (SDD) models of the Joint Strike Fighter (JSF), now officially designated F-35 and to be known as the Lightning II, along with Lockheed Martin and Northrop Grumman. Samlesbury is producing the aft fuselage and vertical and horizontal tailplanes for all 23 aircraft in the project’s SDD phase.

There are three versions of the JSF, all designed for different roles. The conventional take-off and landing (CTOL, designated F-35A) is designed to replace the F-15 and A-10 aircraft. The short take-off, vertical landing (STOVL, F-35B) may take over the role of the Harrier. Although there are substantial differences between these two variants to accommodate the swinging thrust system necessary for vertical landing in the STOVL version, the planes are described as ‘common’, with many parts identical on each.

The third version, designed for aircraft carriers (F-35CV) is a ‘cousin’ of the other two variants, with much larger wings and major differences in construction. BAE is making the aft fuselage and tail sections for all three, as well as the outboard wing sections for the carrier version.

Northrop Grummann, meanwhile, is making the centre fuselage, while Lockheed Martin is responsible for forward fuselage, wings, and the edges and control surfaces of the horizontal and vertical tail sections. And it’s vital that the pieces marry together perfectly with the other parts when it comes to final assembly at Lockheed’s Fort Worth facility.

Samlesbury delivered the finished parts for the first F-35, a CTOL version, late last year, and the first three components for the first STOVL aft fuselage have just gone into the jig at Samlesbury. It’s a complex construction, involving over 1,800 separate parts and a variety of different materials, said Mark Durning, head of airframe operations. Engineering tolerances are tight.

‘One of the things we had to demonstrate when bidding for the contract was that we could meet the tolerances,’ said Durning. ‘We have to drill the skins and sub-structure to accuracies of around five thousandths of an inch, and there are over 6,000 holes in the aft fuselage assembly alone.’

Another crucial test involved the different materials used. The main structure of the aft fuselage is in aluminium and titanium, but the section – like the rest of the plane – is covered in a carbon-fibre skin. This is also produced at Samlesbury in several sections, and these must butt up against each other with no discernable step.

Achieving these accuracies involves having the right equipment and co-ordinating resources. The JSF is made using what is called a ‘digital thread’ philosophy – a computer database based around CATIA software which links design and manufacturing across the entire project.

This threading approach isn’t new to BAE – like many of the technologies and techniques being used in the project, it was originally adopted for the Eurofighter Typhoon – but it has been further refined during the JSF build.

The thread allows all the teams and facilities involved to work together. In particular, it helps the production engineers get an early start in translating the designs into control routines for the machine tools that will be used in the construction phase.

This, said Durning, has led to a considerable improvement in the accuracy of machining. ‘in the past, when we were machining a part we always intended the second one off to be the one that went into a flyer -a plane intended for flight,’ he explained. ‘The first one off, we expected to have a few imperfections, so it might have gone into a static model.’ In fact, he added, traditionally the first part off wouldn’t even be made in the final material -a cheaper metal would be used, just to test the machines.

‘But that doesn’t happen any more -we can use the real material straight away. And in the case of the STOVL, the first part off has gone straight into the jig.’

BAE has invested heavily in machine tools for the project, acquiring five five-axis drilling and milling machines from German firm Dörries Scharmann Technologie. The metal and carbon fibre sections of the aft fuselage are drilled separately, said Durning, then married together -and all the holes, of course, have to line up perfectly. ‘Again, we developed this for work on the Typhoon, and have taken it further for the JSF,’ he said.

With so many different materials being used, and all of them expanding and contracting at different rates as the temperature varies, the environment within the manufacturing facilities must be kept under tight control. Samlesbury and Fort Worth are at a constant 20.8o C – a temperature designed for all the components to be married perfectly, and assembled and locked together.

Control is tight in the main body of the plant, with temperature sensors on regularly-spaces poles around each manufacturing zone feeding into a custom-designed air conditioning system. ‘With most temperature-controlled plants, the sensors are on the walls,’ said Durning. ‘But wall temperature really doesn’t matter. What’s crucial is how hot it is where you’re actually working, which is why our sensors are in the middle of the floor.’

The first STOVL aft fuselage is scheduled for delivery ‘at the end of the year,’ said Durning, with the tail parts to be finished soon after. The plant will then build four more STOVL units, followed by five CTOL parts, and then begin work on the carrier versions.

The first flight for the F-35 is scheduled for the autumn, and if all goes to plan, the STOVL parts currently in the jig at Samlesbury will have their first flight early in 2008.

What’s still uncertain, however, is whether BAE will be undertaking manufacturing on the JSF once it goes into full production. According to Durning, the company believes its experience in the SDD phase will stand it in good stead, but no contracts have yet been signed.