It has been more than half a century since Chuck Yeager rocketed through the sound barrier and almost 40 years since the Hawker P1127, forerunner to the Harrier, first lifted off the runway. Nobody has yet been able to blend the two and field a supersonic short take-off and vertical landing (STOVL) fighter. This is set to change with the Joint Strike Fighter (JSF), set to make its debut early this millennium.
The JSF promises to deliver supersonic speed and a lot more. The programme is intended to produce a multi-faceted weapon system capable of meeting the conventional needs of the US Air Force and Navy, without being compromised operationally by the STOVL necessities of the US Marine Corps and UK Royal Air Force and Royal Navy. Added to these challenging goals are requirements to cloak the design for stealth and remain affordable.
To explain why supersonic STOVL is only now on the verge of becoming reality, it is worth reflecting on a developmental graveyard that stretches back four decades. It is littered with discarded and abandoned projects that have all fallen victim to a combination of political, financial and technical death blows.
First to stumble was the Hawker Siddeley P.1154 in 1965. This aircraft would have used plenum-chamber burning vectored thrust to make the jump to Mach 2. The concept of burning fuel in the fan nozzles was revisited 20 years later as part of the UK/US Advanced STOVL (ASTOVL) effort. But there remained the unresolved problem of the effect of hot exhaust gases on the ground environment in the hover.
The Rockwell XFV-12A followed in 1972. This reflected a different approach to supersonic STOVL by ducting engine exhaust, augmented with ambient air, out of full-span "ejector flaps" on the wing and canard. This ungainly demonstrator never mustered the power to leave the ground, let alone reach Mach 2. It was scuppered with US Navy plans for V/STOL carriers.
Ejector revival
General Dynamics later tried to revise the ejector concept by routing the exhaust to slots in the wing roots of a delta-wing ASTOVL design. "There were many variables, such as crosswind effect, that could not be controlled. The weight margins were too tight and would have been eaten up by weight growth," recalls Fran Ketter, former General Dynamics chief propulsion analysis and present-day Lockheed Martin JSF propulsion system integration manager.
By the early 1990s, the ASTOVL programme had metamorphosed into the much broader Common Affordable Lightweight Fighter (CALF). This marked a shift in emphasis towards finding one solution that served as a supersonic STOVL successor to the British Aerospace/McDonnell Douglas (MDC) Harrier II and conventional replacement for the Lockheed Martin F-16.
MDC and Lockheed Martin took different routes to thrust augmentation. MDC proposed a mixed-flow design, which in STOVL mode would have bled compressor air from the engine and ducted it forward to drive a lift fan. This addressed the critical STOVL issue of weight-to-power ratio by significantly boosting airflow during hover.
After CALF was rolled into the pre-JSF Joint Advanced Strike Technology programme in 1994, MDC replaced the gas-driven lift fan with a lift engine - a lift-plus-lift/cruise configuration pioneered by the Russians on the Yakovlev Yak-38. But MDC failed make the JSF play-off. While a drop-in dedicated lift engine appeared more conducive to a modular design, it pushed up STOVL weight and support costs.
Lockheed Martin, having earlier tinkered with a tandem-fan design, took the shaft-driven lift fan approach to thrust augmentation and went on to become one of two JSF finalists. "The engine is sized right for the up-and-away mission, taking you supersonic in a reasonable acceleration time, and it is not penalised by the STOVL requirement," says Ketter.
The Pratt & Whitney JSF119-611 powering Lockheed Martin's X-36 JSF concept demonstrators is a modification of the F119 in the Lockheed Martin/ Boeing F-22, incorporating a larger fan and low-pressure (LP) turbine. The latter is connected to a two-stage lift fan via a driveshaft and clutch. Lift fan vanes and a vectoring rear nozzle provide pitch and yaw control, while bleed air is ducted to wing "posts" to regulate roll. The lift fan's ambient-temperature exhaust acts as a dam preventing the hot engine exhaust flowing forward into the inlets.
Boeing was a late entry in the supersonic STOVL race, but after surveying all the available options, opted for Harrier-style direct lift. "We looked at different augmentation concepts to add airflow capacity, but we never found one to compete with direct lift. They tended to be too heavy and complex, and, with more moving parts, more things can go wrong," says Dennis Muilenburg, Boeing JSF weapon systems director.
Direct lift has its own major technical challenges, not the least of which is reconciling the Harrier problem of finding more thrust for speed and payload without burning a hole in the flightdeck or runway when landing vertically.
Boeing's engine for its X-32 JSF concept demonstrators is the JSF119-614, also based on an uprated F119 with larger fan and new LP turbine. In the STOVL mode the rear nozzle is closed and exhaust is redirected to two retractable thrust-vectoring nozzles. Control is provided by small pitch, yaw and roll nozzles fore and aft and in the wing. A jet screen prevents hot gases entering the inlet.
Weight saving
There is no STOVL thrust augmentation in Boeing's design, making every kilogramme that can be shaved off the empty weight count. The JSF operational requirement stipulates the aircraft must able to return and land vertically with a minimum 1,800kg (4,000lb) "bringback" load of fuel and stores. Boeing has had to work hard to make up a bringback shortfall with weight-saving measures, including clipping the STOVL version's wing by 0.77m (2.5ft) and slicing 0.46m off the fuselage.
Weight margins are less of a concern for Lockheed Martin, with its STOVL propulsion system generating 60% more thrust and 160% more airflow than if the engine was employed in direct-lift mode, thanks to the 18,000lb-thrust (80kN) lift fan. Boeing says Lockheed Martin's lift fan, clutch and driveshaft weigh considerably more than the 320kg its direct-lift system adds to the engine.
"It does add weight-and we pay a penalty during up-and-away, but the engine is big enough to accommodate that," claims Ketter. "When that weight becomes a concern, in hover, the thrust benefit we get, divided by the weight penalty, gives us a benefit ratio in the order of three to one. We 're getting the augmentation and we don't think we're paying much for it," he says.
Developing and demonstrating a workable supersonic STOVL concept is only part of the battle. It must also be integrated into a low-observable airframe equally capable of performing conventional take-off and landings on land and at sea. The JSF teams, however, have new enabling technologies that the P1127 designers could only dream of.
"STOVL is not really new technology, but the way we're implementing it is. We will have basically built the aircraft on the computer before we build it in real life. We're flying the aircraft in our simulator to a very high fidelity. The expectation is that when we get to flight test, it is going to fly as advertised," says Muilenburg.
Boeing, having opted for direct lift, has positioned its engine Harrier-style close to the aircraft's centre of gravity. This results in a comparatively compact aircraft, which Boeing contends is better from an integration standpoint and provides good manoeuvrability. Lockheed Martin, on the other hand, has put its engine at the rear, balanced by the lift fan behind the cockpit.
His point underscores the key to the JSF's success. The Harrier was designed around a vertical lift/landing capability, with payload/range and speed being secondary considerations. The JSF, in contrast, is intended to be a supersonic multi-role fighter with the inherent flexibility for STOVL operation.
This represents a critical shift in STOVL philosophy, which hitherto has confined the Harrier to niche applications with a handful of air arms. If the JSF lives up to its promises, the aircraft could open the door to a much broader acceptance of vertical flight this century.
Source: Flight International
