Recapping the fly-off between Boeing's X-32 and Lockheed's X-35

Back in 2000, the most valuable fighter fly-offs in modern times took place between the Boeing X-32 and Lockheed X-35, as Gerard Keijsper explains.

It does not seem so long ago, but it is almost 30 years since the Defense Advanced Research Projects Agency (DARPA) issued a Request for Proposals for the Advanced Short Take Off and Vertical Landing (ASTOVL) aircraft technology demonstration program in 1992. The aim was to build a supersonic STOVL fighter with an empty weight of 24,000lb, similar to the F/A-18C Hornet, to keep the weight and cost in check.

Financial constraints

The USMC was having trouble getting political support for the Bell Boeing V-22 Osprey, with then-US Vice President Dick Cheney one of its fiercest critics, and they could not afford to fund another high-profile program on their own. USAF Brig Gen George Muellner, then Deputy Chief of Staff for Requirements at Air Combat Command, Langley Air Force Base (AFB), received a presentation from Lockheed/DARPA about a replacement for the F-16, but with more range. This was the kind of range the USAF craved, even if it didn’t have the funding to mount another project, owing to the cost of the F-22A Raptor.

The financial situation was tight because of the peace dividend – the Berlin Wall had fallen just a couple of years earlier – and no one wanted to put budgetary pressure on the F-22 program. DARPA pointed out that the USMC had a similar problem, but there could be co-operation if the extra fuel tank was replaced by a lift fan to offer the supersonic STOVL capability desired by the service. The USAF showed interest, asking DARPA to provide the technology by the end of the 1990s.

STOVL testing

Many agencies had tried to develop a supersonic STOVL fighter and the history books are full of examples of experimental failures, including the Hawker Siddeley P.1154; Dassault Balzac V; EWR VJ-101 (tilt engines and lift engines); Rockwell International XFV-12A and British Aerospace (BAe) P.1216, but problems such as hot gas ingestion, suck-down forces, and the fountain forces made them unsuccessful. Hot gas ingestion, the transition from conventional flight to hover and managing to lift its own weight at the same time had proven to be the three biggest problems in developing a successful supersonic STOVL fighter. For this reason, DARPA assigned an experimental ’X’ designation for the prototype: the X-32.

General Dynamics had been testing a full-scale E-7 ejector propulsion model in NASA’s 80ft x 120ft wind tunnel but, in the end, it offered a fan-in-wing propulsion concept similar to the Ryan XV-5A. McDonnell Douglas went in with an offer for both a SDLF (shaft driven lift fan) and GDLF (gas driven lift fan) propulsion system. Northrop offered a similar propulsion solution as the Russian Yakovlev Yak-38: the lift + lift-cruise system. Boeing offered the Harrier propulsion system called ’direct lift’.

The Lockheed and McDonnell Douglas designs could remove the lift fan for the conventional fighter and Northrop could do the same with the lift engine. Boeing didn’t have this advantage, so selected a delta wing as these could hold a larger fuel volume. A requirement set by DARPA was that a full-scale model had to be developed to prove the design could carry its weight in the STOVL flight regime to prevent a repeat of the Rockwell International XVF-12A, which could not lift its own weight.

In March 1993, Lockheed’s SDLF propulsion system and McDonnell Douglas’ GDLF system were selected, with General Dynamics, Boeing and Northrop losing out. Boeing rallied their US Senate representatives, arguing that the sole proven concept had been rejected. Through the Senate, Boeing got an agreement with DARPA to continue but they had to fund 50% of the costs themselves; Northrop continued with its own money.

Lockheed was already in talks with General Dynamics over acquiring the Fort Worth division, with the tipping point coming when General Dynamics Fort Worth missed out on a possible F-16 replacement and was subsequently sold to Lockheed. Two years before the Advanced Tactical Aircraft (ATA) program, General Dynamics’ A-12 Avenger II project had been cancelled. The F-16 production was expected to come to a halt soon as the USAF had opted to stop buying them. With General Dynamics losing out on this DARPA contract, it was expected that Fort Worth could soon be out of business.

McDonnell Douglas was co-operating with BAe in an exclusive deal on the GDLF, with a full-scale model prepared for testing. McDonnell Douglas had selected the General Electric YF120 for their GDLF.

The dream team

McDonnell Douglas and BAe teamed up with Northrop Grumman to form the dream team, as all western operational STOVL and recent naval STOVL experience was now under one roof.

The McDonnell Douglas team abandoned the GDLF concept, much to the chagrin of NASA, and adopted the lift + lift cruise propulsion system that Northrop had pursued. The dream team did not bother with a full-scale model for the lift + lift system as they reasoned they were developing a proven propulsion capable of lifting its own weight. At the same time as changing propulsion systems, it changed its propulsion provider, as they now wanted to offer a lower risk and a proven engine: the Pratt & Whitney F119. Both Boeing and Lockheed had selected the F119 engine from the outset and, as such, there was never a competitive engine offered for the JSF.

Joint Strike Fighter

The DARPA program was eventually absorbed by the Joint Advanced Strike Technology (JAST) program, which meant the two downselected contractors would get the chance to build a demonstrator aircraft. As a result, the X-35 designation was added. The fact that the demonstrators were to be built under JAST meant they would become more than simply technology development – they would be an actual aircraft demonstration. To reflect this, the name was changed to the Joint Strike Fighter (JSF) program.

Both Boeing and Lockheed Martin were downselected on November 16, 1996, and received a contract to build and test two prototypes on what was to become the JSF. McDonnell Douglas supplied the fighter powerhouse, the legacy of the F-4 Phantom II, F-15 Eagle and the F-18 Hornet, but had no other work in the future as the F-22 and the JSF would replace the entire US Inventory of fighter aircraft. Indeed, no new fighter designs were expected for another 30 years.

McDonnell Douglas had once been a major company on the civil side of aviation, but that had been diminished by competition from Boeing and Airbus. So, soon after, McDonnell Douglas was sold to Boeing. BAe and Northrop Grumman decided to join Lockheed.

The prototypes (or concept demonstration aircraft, as they were officially called) had to prove that the design would be stealthy, could replace the Harrier by landing vertically, have a short take-off run, and be launched from a conventional aircraft carrier while having a fighter performance equal to the F-16. In short, they should fulfil all the requirements of the three US armed services and many exports were expected to follow automatically.

The Lockheed effort

The task at hand was not an easy one but Lockheed’s Advanced Development Company – better known as Skunk Works – was on the job with innovative proposals. For the lift concept, new SDLF technology from Skunk Works was met with some optimism as it seemed to tackle most of the issues. The SDLF design was refined by engineer Paul Bevilaqua’s team and patented in 1993. To reduce cost, the initial design was borrowed from the F-22.

Three reasons why Lockheed pursued SDLF

1. The extra weight incurred as a result of the lift fan and the shaft was insignificant compared to the extra thrust gained by this new technology.

2. As the lift fan used no combustion, the exhaust was colder, making the ground environment better suited for operations.

3. STOVL thrust requirements can be disconnected from the main engine, so that the engine can be conventionally sized to the requirement of up-and-away thrust that’s needed to achieve transonic acceleration.

The Boeing effort

Boeing was not accepted by DARPA as it was not offering “new technology”. Its propulsion unit was a modern version of the Harrier system. Boeing reasoned that their offer was less risky as it had already proven itself and, as such, only had to solve the hot gas ingestion problem. The propulsion system selected required that the engine was located in the centre of the aircraft, close to the centre of gravity, making the design stubby looking.

A British company called Fairey Hydraulics Ltd (FHL) was responsible for a new development known as the Attitude Control System (ACS). This took care of the conversion from conventional wing-borne flight to jet flight. The ACS was formed by eight variable-area nozzles: two pitch nozzles positioned in the nose and two in the tail, along with two yaw nozzles on the tail and two roll nozzles in the wingtips.

The jet screen – an exhaust of cooler air that prevented hot air from going forward – located between the lift nozzles and the two pitch nozzles in the nose provided additional pitch control when the jet screen was moved. The system was activated by a switch on the instrument panel that would see the doors of the lift system opened simultaneously. A built-in test check of the ACS actuators then took place, before the flight controls kicked in. It was a complicated process.

To complete the conversion, the pilot had to push the flow switch control button on the throttle. The ACS had to close the main exhaust, so that thrust would be diverted to the vectoring nozzles at the centre of the aircraft. Once the flow switch had been operated, the X-32B was in STOVL mode, after which deceleration was controlled by vectoring the nozzles.

Maiden flights

The Boeing X-32A CTOL took off on its first flight on September 18, 2000, from Palmdale, piloted by Fred Knox, making it the first of the JSF concept demonstrator aircraft (CDA) to fly. Boeing was flying for about a month without competition, resolving small technical problems and learning that they would have to make do without aerial refuelling.

On October 24, 2000, Boeing test pilot Dennis ’Irish’ O’Donoghue experienced an accident. After he noticed the X-32A’s brakes were not working, he was forced to land on a lakebed and let the aircraft slide to a standstill. This caused the X-32A to be grounded for three weeks. The same day saw Tom Morgenfeld taking the X-35A for its first flight from Palmdale to Edwards AFB, a distance of 35 miles, taking the aircraft to 10,000ft with the landing gear down. In a short period, Lockheed managed to qualify the X-35A for aerial refueling and ticked off all the test points one by one at impressive speed.

The Boeing X-32A rejoined the flight program one week before Lockheed completed the tests for the X-35A by performing a supersonic flight and achieving a 20º angle of attack (AoA) and an altitude of 34,000ft. Boeing, it seemed, had some catching up to do, but had the advantage of not needing to convert one of their test aircraft as Lockheed had done.

The X-35A was now being reconfigured so that it could function as the X-35B. It had a ‘drop-in’ cylinder with ballast inside it. When the X-35 was converted from a CTOL variant to a STOVL variant, the cylinder was removed and replaced with the lift fan, which had the same mass. The Boeing X-32 continued its flight test program at an aggressive pace.

The X-35C carried out its maiden flight on December 16, 2000, with Joe Sweeney at the controls. This aircraft featured more control surfaces and a reinforced structure to ensure it could cope with the stresses of carrier landings, which have a higher impact on the structure of the aircraft. After completing aerial refueling qualification, the X-35C was flown from Edwards AFB to the Lockheed Fort Worth division in Texas and, a day later, completed its transfer to Naval Air Station (NAS) Patuxent River in Maryland, becoming the first X-plane in history to complete a cross-continental flight. Both the X-32A and the X-35C performed up-and-away flight tests, many field carrier landing practices (FCLPs), bolters and wave offs before completing demonstration flights.

Major milestones

On December 29, 2000, a major milestone was passed by adapting the X-35A to the X-35B with the lift fan installed. The cockpit was equipped for this part of the flight test program with a new control unit situated beside the throttle and the stick, known as a thrust vector lever (TVL).

It was by operating the TVL in the X-35B, that the pilot could vector the exhaust of the main engine from 22º to 95º and, similarly, vector the thrust of the lift fan nozzle through a range of 34ºs to 95º.

The business end of the program arrived with the first flight of the X-32B on March 29, 2001. The jet flew some conversion flights at Edwards, getting the speed down to 140 knots, before transferring to NAS Patuxent River.

As NAS Patuxent River was at sea level, things would be easier here for Boeing. The aircraft was technically adjusted, which meant the landing gear doors and the inlet cowl were removed from the jet to reduce weight. The fear that the aircraft was too heavy was a real one, but Boeing pointed out that their preferred weapon system concept (PWSC) was lighter and would not incur this problem.

Boeing did not want to repeat the embarrassment of the old experimental STOVL planes that could not lift their weight. On June 23, 2001, the X-35B took off from the ground vertically for the first time, with BAE Systems pilot, Simon Hargreaves, at the controls. The capability of hovering and vertical take-off and landing was proven at Palmdale.

As soon as the X-35B was at Edwards AFB the testing was geared to transitioning from normal flight to hovering. The Boeing X-32B made its first vertical landing with O’Donoghue in control. Amid the demonstration flights, the X-32B encountered the problem of hot gas ingestion during vertical landings, causing a pop stall on one occasion.

Making aviation history

USMC test pilot, Maj Art Tomassetti, made aviation history on July 20, 2001, when he became the first pilot to complete a short take-off using the lift fan. He then proceeded to level flight and achieved supersonic speed. After completing the supersonic dash, he landed the aircraft back vertically, performing exactly the mission profile it was anticipated the PWSC would have to fulfil. To prove a point that it was no accident, Lockheed repeated the feat again on July 26, 2001, this time with Hargreaves at the controls.

Two days later, on the last flight test day and in answer to Lockheed Martin's X-mission, Boeing X-32B – flown by USMC Maj Jeff Karnes – executed a short take off, transitioned into conventional flight, broke the sound barrier, then transitioned back to the STOVL mode in such a manner that he was still airborne due to aerodynamical forces, then made a low-speed landing. A vertical landing was not risked. For this flight, the inlet cowl and landing gear doors were reinstalled.

Both the X-32B and the X-35B had exceeded the weight limit set by DARPA. Lockheed Martin had proven that the X-35B had excess power in hover, whereas Boeing was on the edge of its capability. For future growth capability, the decision to select the Lockheed Martin F-35 was a no-brainer. The lift fan was successful, but the F-35 became significantly heavier than the originally intended 24,000lb, resulting in a substantial cost increase.