Battle of the X-Planes

Dr Duncan L Copp

May 2003

Picture the scene. You and your friends decide you'd like a hamburger. You find a well-established burger joint and roll up to the counter. You're after a burger with all the trimmings, but no bacon, oh and just one slice of cheese, and yes, extra mayo. Your friends know exactly what they want too – and they're equally fussy. 'No problem,' says the chef, turning to a highly efficient, cost effective streamlined production line. Within minutes he produces three tailored meals based around the common ingredient. And three satisfied customers take their seats and eat.

That tailoring of the product to suit each individual's order would be straight forward enough, but try applying it to the construction of a new 21st century fighter aircraft and you've a considerable technological challenge on your hands. Such were the wishes of the Pentagon when forced to accept that planes designed to win the Cold War were now aged and needed replacing. This technological challenge is called the Joint Strike Fighter (JSF). The plan is to produce a new fighter plane that replaces not one but four different US military aircraft that are currently in service.

The requirements for the JSF are formidable – a single pilot fighter plane with near precision all-weather strike capability, carrying a variety of air-to-surface weapons, and able to defend itself in a dog fight. Furthermore, the JSF must meet or exceed the needs of the modern warfare environment for a multi-service (US Air Force, US Marines and US Navy), multi-national customer (the UK are partners in the JSF project and are likely to order up to 150). And the aircraft must cost a fraction of the manufacturing and maintenance price of current serving aircraft. A tall order indeed.

The US's top two military aircraft manufacturers, Lockheed Martin and Boeing, responded to the challenge, entering into an aggressive aircraft design competition – the biggest corporate clash of Titans for years. The bounty for the contract was high. Very high. Current estimates are that the Pentagon will acquire some 3000 JSFs, at a total value of US$200 billion over the lifetime of the program.

Boeing submitted its X-32 design (X standing for experimental). Lockheed Martin offered the X-35. The contract proved so complex that some 250 US Defence Department personnel worked on evaluating the two entries. In October 2001, the winner was announced – the contract would go to Lockheed Martin.

Three of a kind

The idea behind the JSF is to have three variants of a single aircraft – the F-35. The F-35 is a hybrid of the F-22 Raptor fighter, a previous invention emerging from the infamous Skunk Works, Lockheed Martin's development hangers (their equivalent of the Bat Cave). The three variants are the F-35A, F-35B and F-35C. At first glance they are difficult to distinguish. Each variant shares the basic stubby airframe (made of titanium, aluminium and composites), is similar in shape and size (approximately 17 metres in length with a 10 metre wingspan) and is powered by the same single Pratt and Whitney JSF119-611 engine (delivering 40,000 lb of thrust). But under their skins lie distinctly different machines, tailored to meet the demanding requirements of three very different military environments.

For the Air Force – the F-35A

Designed with the Air Force in mind, the F-35A will take off and land on a conventional runway, like the current F-16s and A-10s it will replace. In action, the F-35A will complement the F-22 Raptor fighter, of which it is a hybrid. It's a plane that bristles with armoury, equipped with an internal 27mm gun mounted on the left of the craft, two internal weapons bays each capable of carrying a 2000 lb smart bomb, and an air-to-air radar-guided missile. An efficient radar-absorbing stealth skin results in the aircraft emitting an echo equivalent to a sparrow. All this, along with a top speed of twice that of sound and a combat flight radius of over 500 miles, makes the F-35A a formidable ground assault aircraft.

For the Marines – the F-35B

The Marines, on the other hand, need something a little different. Operating from short-runway landing ships, their requirements are for an aircraft capable of a short take-off and vertical landing (STOVL, for short), as well as being able to make a dash to supersonic speeds – the first military aircraft to possess these two abilities. Enter the F-35B. The F-35B will replace the famous subsonic British-designed AV-8B Harrier jump jets. The F-35 airframe has been modified to accommodate a propulsion system that allows the F-35 to hover like the Harrier. The Harrier hovers by redirecting hot air from the engines via pivoting ducts, providing a downward thrust that lifts the plane vertically off the ground. The F-35B needs a total of 40,000 lb of thrust to hover, but only half of that thrust is produced in the same way as the Harrier. How the F-35B produces the remaining thrust for hovering is the real innovation.

The uplifting secret is a lifting fan, designed and built by Rolls Royce. The fan is made of a pair of counter-rotating turbines. The turbines spin when a shaft, powered by the aircraft's main engine, engages with the fan by means of a gear. By using a coupling gear system, the amount of torque (turning force) applied to the fan can be controlled, so the fan's rotation and hence thrust is variable. The thrust from the rotating fan is not produced by expelling hot engine exhaust. Instead the fan uses cool ambient air passed through a pair of doors that open directly behind the cockpit. Using cool air is a smart move for a number of reasons. Since the engine is not reliant on producing all the air needed to hover (by means of expelling hot exhaust, as is the case in the Harrier) it has less work to do, and so can operate at a lower temperature. Lower temperatures mean two things. One is a reduction in heat stress. The other is that aircraft expelling cooler air are more difficult to detect with heat-seeking anti-aircraft missiles. By using the new Rolls Royce lifting fan, the safety of the F-35B, both in flight and battle, is improved.

For the Navy – the F-35C

The final variant in the JSF family is designed to meet the needs of the US Navy. The F-35C must cope with low speed aircraft carrier approaches and catapult launches and landings. As with any aircraft there is a trade-off between velocity and lift – drop your velocity and you'll need to increase your lift, or you risk stalling. To create more lift at lower speeds a greater wing surface area is needed. This is achieved on the F-35C by larger leading-edge flaps and state-of-the-art foldable wingtip sections. The aircraft also has larger wing and tail control surfaces, allowing greater control as the plane takes off and, more crucially, as it comes into land. Control is something you appreciate when trying to land a fighter on the small landing deck of a moving ship! The larger wings have another benefit – the ability to store more fuel, so allowing a heavier cargo and a greater operational range.

Economic production

When it came to awarding the contract, the design of the world's most sophisticated fighter aircraft was only half the issue. The winning company also had to convince the US government that it could produce the JSF in the most economical manner. This required some radical thinking. While both Boeing and Lockheed Martin developed suitable aircraft, it was Lockheed Martin's proposed production line and maintenance proposals that clinched them the most lucrative deal in the history of military aviation.

The JSF assembly line will employ what's known as a 'mixed model' production. The three F-35 variants won't roll off three production lines – just one, much like our hamburger analogy. Since each aircraft will share around 80% of the same parts, the construction line will have 'reduced tooling' – the same tools can be used to make the same parts for the three different aircraft. This will greatly reduce assembly costs. In the past, time and money would have been spent bolting and unbolting tools from multi-production lines before the next part of assembly could take place.

Ease of maintenance

Maintenance of the F-35 aircraft will also be streamlined. Each plane will implement a 'prognostics and health management' system, a sort of automated medical check every time the aircraft flies. Computers will monitor the performance of many different components within the plane and update the time when a part is predicted to fail. With this information, engineers can fix or replace a part just before it fails, keeping the aircraft flying and the costs to a minimum. In all, current estimates are the F-35 variants will cost 40-50% less to operate and support than comparable aircraft – a saving of billions of dollars over the lifetime of the program.

As of spring 2003, the flight tests of the F-35A, B and C have proved highly successful, with the prototypes breaking numerous aviation records. Barring any unforeseen engineering problems or shift in economics, the F-35s will enter military service in 2010 off a production line Henry Ford would have been proud of.

Find out more

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Websites

Boeing – Phantom Works
www.boeing.com/phantom
Website of Boeing's top-secret innovations complex, Phantom Works.

Lockheed-Martin – Joint Strike Fighter
www.lmaeronautics.com/products/combat_air/x-35/index.html
The Lockheed-Martin website, with information on the requirements, team, design and testing of their Joint Strike Fighter programme. Includes images of the aircraft.

BAE Systems
www.baesystems.com
Website of the UK-based company in partnership with Lockheed-Martin in the Joint Strike Fighter programme competition.

McDonnell Douglas
www.boeing.com/history/mdc
A brief history of McDonnel Douglas, bought out by Boeing after failing to make it to the final round of the Joint Strike Fighter programme.

RAF – Joint Strike Fighter
www.raf.mod.uk/equipment/jsf.html
RAF site with Joint Strike Fighter programme information and images.

RAF – Typhoon
www.raf.mod.uk/equipment/typhoon.html
RAF site with Typhoon Eurofighter information and images.

Ministry of Defence
www.mod.uk/dpa/projects/jca.htm
MOD site detailing the UK's involvement with the Joint Strike Fighter.

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