EMPHASIS ON THE short-take-off/vertical-landing (STOVL) variant of the US/UK Joint Strike Fighter (JSF) is increasing. Progress with the JSF concept-demonstration phase is leading programme and service officials to speculate that the STOVL variant planned for the US Marine Corps and the Royal Navy could find applications with the US Air Force and Navy. This could conceivably lead to abandonment of plans for a carrier-capable (CV) variant of the JSF, reducing costs for the near-3,000-aircraft programme.
Speaking in late April, at a JSF symposium organised by the American Helicopter Society, Air Force and Navy officials acknowledged interest in the STOVL variant (Flight International, 7-13 May). The USAF looks likely to buy at least 200 STOVL JSFs to replace Fairchild A-10s, while the Navy appears prepared to drop its 250 CV variants, and buy more JSFs - if the STOVL aircraft should prove to be able to meet its demanding requirements.
Increasing interest in the STOVL variant is the result of contractor work under way to reduce the penalties of commonality between the three planned JSF variants. Boeing is setting the pace, claiming that its direct-lift STOVL aircraft can meet the Navy's 1,100km (600nm)- range requirement while providing greater than 90% commonality with the USAF's conventional take-off-and-landing (CTOL) variant.
Lockheed Martin says that its STOVLJSF, with its shaft-driven lift fan, cannot meet the Navy's range specification without being a much larger aircraft than that which the Marine Corps and Royal Navy want. US Navy officials remain sceptical that the STOVL JSF can meet the requirements. Rear Adm Carlos Johnson, USN head of aviation plans and requirements, remains to be convinced that the STOVL aircraft can provide a 1,100km range and cautions that no decision on dropping the CV variant will be taken until after the competing concept-demonstrators have been flown in 2000.
CARRIER OVERHEAD
The benefits of dropping the CV variant are obvious. The carrier-capable aircraft is the most expensive of the three JSF versions, with a target unit-flyaway cost of $31-38 million, compared with $30-35 million for the STOVL variant and less than $28 million for the CTOL aircraft. The CV mission is the most demanding, calling for a "first-day-of-war" stealthy strike-fighter able to carry two 900kg joint stand-off weapons and two advanced medium-range air-to-air missiles (AMRAAMs) internally, over a combat radius of 1,100km.
The CTOL and STOVL aircraft carry two 450kg joint direct-attack munitions and two AMRAAMs internally. Differences between the variants are being narrowed, however, as contractors refine their designs. Standardising on 900kg-weapon capability will be proposed to the services in mid-May, says JSF programme director Rear Adm Craig Steidle. "Contractors say the weight penalty is not great," says USAF director, global power programmes, Brig Gen Bruce Carlson.
Weight is of concern for the Marine Corps, although Gen Bob Magnus, deputy chief of staff for aviation, admits that there are advantages to the bigger weapons bays, including the ability to carry two 450kg bombs or three AMRAAMs in each. The Royal Navy is also likely to favour the increased weapons-carriage flexibility.
Lockheed Martin's JSF design already includes 900kg-weapons capability in all three variants, says JSF programme manager Dave Wheaton. Boeing will have to revise its design, and air-vehicle team leader Dennis Muilenburg declines to comment on whether the STOVL variant will meet the Navy's 1,100km range requirement, carrying two 900kg bombs.
Steidle expects both the 900kg-weapon capability and an advanced gun to be included in an updated joint initial-requirements document, released later this year. This will be refined annually until the final joint operational-requirements document, on which the JSF engineering- and manufacturing-development (EMD) programme will be based, is issued in 1999. EMD bids will be submitted in 2000 and one team selected in 2001.
Performance of the radically different STOVL concept-demonstrators is expected to be critical to the EMD shortlist. In each case, the STOVL demonstrator will be the second aircraft to fly. Boeing and Lockheed Martin plan to fly the CTOL concept-demonstrator first, in March 1999, followed the STOVL aircraft two months later. The CTOL aircraft will be used to demonstrate "up-and-away" performance before being converted to the CV configuration to evaluate carrier suitability.
MAXIMISE COMMONALITY
Boeing's approach has been to maximise commonality. The only external differences are those of the extended wingtips on the CTOL and CV variants, which increase span to 11m. There is no wing fold, but the CV variant has overwing vortex flaps to increase lift for carrier approaches. The STOVL variant has a 9.1m span to fit UK carriers without wingfold. All variants are 13.6m long.
The company, which has now teamed with McDonnell Douglas, plans to build its JSF in four modules. The aft body, with canted tails and two-dimensional thrust-vectoring nozzle, is common to all variants, as is the wing and forebody. The centre fuselage, which fits under the wing, is tailored to the different versions.
The engine is common to all three variants, with the direct-lift hardware added to the STOVL aircraft. This consists of a spool duct with deployable lift nozzles mounted immediately aft of the engine and upstream of the remote augmentor and vectoring nozzle. When the latter is closed, engine exhaust is diverted to the swivelling lift nozzles. Fan air is ducted to a jet screen forward of the nozzles. This generates lateral and longitudinal "fans" of cooler air, which prevent the hot engine-exhaust reaching the inlet. The jet screen also acts as a forward-lift post to help balance the aircraft, augmented by an aft-pitch nozzle, roll tubes in the wing and yaw nozzles in the tail. Pitch, roll and yaw nozzles are fed with engine-exhaust air.
Boeing's X-32 concept demonstrator will be equipped with the Pratt & Whitney SE614, a derivative of the F119 which powers the Lockheed Martin/Boeing F-22. The engine has a scaled-up fan driven by a low-pressure turbine with an additional stage. The high thrust and bypass ratio required for direct-lift STOVL benefits the CTOL and CV variants, says Muilenburg, by increasing speed, manoeuvrability and range. The lift-system hardware adds only 270kg, he adds.
The STOVL variant will "-have no trouble meeting the CV range requirement", he says, because of the fuel capacity of the large wing. Boeing's CTOL and STOVL JSFs will have an empty weight of around 10,000kg, an internal fuel capacity of more than 16,800kg and a payload exceeding 5,900kg. The CV variant will have an empty weight of around 10,900kg, a fuel capacity of more than 7,700kg and a payload exceeding 7,700kg. Maximum take-off weight of all variants is around 22,700kg.
Lockheed Martin has more differences between its JSF variants, but it believes that its approach offers lower risk. The CTOL and STOVL variants are externally almost identical. The 41.8m2 (450ft2) wing area was set by USAF sustained-manoeuvre specifications and the 10m span by USMC carrier requirements. The RN variant has a wing fold at 9.1m span. The CV version has the same wingbox, and a fold at 9.1m span, but new high-lift leading- and trailing-edges, and larger tips as well as bigger horizontal tail and rudders. The 50.2m2 wing area is set by carrier-approach requirements. Length is 15.5m for all variants.
Concept-demonstrator
The company's X-35 concept-demonstrator will be powered by P&W's SE611, another F119 derivative with scaled-up fan and additional turbine stage. In the CTOL and CV variants, the engine will be coupled with a low-observable axisymmetric nozzle. In the STOVLversion, engine thrust is diverted downwards by a swivelling "three-bearing" nozzle, while the powerplant drives a lift fan behind the cockpit. Fan air is diverted to roll ducts in the wing which provide lift and control.
Lockheed Martin's concept allows a smaller engine to be used for all variants, with power being extracted from the turbine in STOVL mode to shaft-drive the two-stage lift fan. Wheaton will not reveal the lift-system weight, but says that the combined engine and fan provide a greater thrust-to-weight ratio than a direct-lift system. The cooler lift-fan air also blocks engine exhaust from entering the inlets. The ground environment is "-good at the front, but still a challenge at the back", he says.
Steidle says that the propulsion system remains the greatest technical challenge for the JSF programme, particularly in achieving the thrust needed for STOVL at the desired durability levels. P&W hopes to begin ground testing the first JSF engines in February 1997, two months ahead of schedule. General Electric will run the core for its YF120-FX alternate engine within two years, with flight tests planned on the winning JSF during EMD.
Deliveries are planned to begin with the STOVL variant, with the USMC receiving the first JSF in 2007. If the aircraft fulfils its promise, then almost half the aircraft planned could be built as STOVL variants.
Source: Flight International
