Short take-off, low funding

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When General Dynamics rolled-out its F-111A Tactical Fighter (TFX) prototype in 1964, the-then-US defence secretary Robert McNamara proclaimed it as "...the greatest single step forward in combat aircraft to occur in several decades". In the event, the "Aardvark", as it was nicknamed, turned out to be a "turkey" in most of the roles predicted by McNamara.

Three decades on, much of the rhetoric surrounding the US Air Force/Marine Corps/Navy Joint Advanced Strike Technology (JAST) programme is uncomfortably reminiscent of McNamara's claims for the TFX.

As with the F-111, the JAST has to be able to fulfil a number of dissimilar missions, with the added encumbrance of the basic design having to encompass advanced short take-off and vertical-landing (ASTOVL) technology.

The JAST and ASTOVL programmes have been parceled together, not necessarily because of any obvious mission or technological rationale, but at the behest of the US Department of Defense's (DoD) political masters. In 1994, the US Congress merged the Advanced Research Projects Agency's (ARPA) ASTOVL programme with the JAST project. Since the DoD's watershed "Bottom-Up Review" (BUR) of defence programmes in 1993, JAST has become the "only game in town" for US combat aircraft manufacturers - and survival of the ASTOVL project is now wrapped up intrinsically with this programme.

Worldwide, the ASTOVL concept remains predominantly the concern of maritime users, despite the Royal Air Force's fondness for its British Aerospace Harrier GR7. Both the USAF and the USN are concerned primarily with conventional take-off and landing (CTOL) configurations, although they do not rule out ASTOVL variants.

The enormity of the questions surrounding the JAST programme has not gone unnoticed by the US Defense Science Board Task Force, which has reported: "The Air Force, Navy and Marine Corps present diverse needs...These diverse requirements are difficult to reconcile in a multi-service vehicle".

These "diverse requirements", cover the USN's "first-day-survivable stand-alone strike fighter", intended to replace both, the cancelled General Dynamics/McDonnell Douglas (MDC) A-12 and eventually, the MDC F-18E/F. The USAF wants the JAST to replace its so-called, "multi-role sortie generator," the Lockheed F-16, while the US Marine Corps, is looking for a short take-off and landing (STOVL) successor, to the MDC AV-8B.

OUTSIDE THE USA

On current projections, the JAST programme will be the only substantial, ongoing ASTOVL development project in either the West or the East. Development of Russia's successor to the Yakovlev Yak-38 Forger - the Yak-41 Freestyle - looks to have been abandoned, with at least one prototype now languishing under canvas at Yakovlev's site in Leningradsky Prospekt in Central Moscow.

With funding for defence heavily curtailed, the Russian navy appears to be concentrating on guarding its conventional take-off and landing aircraft in the shape of the Sukhoi Su-33 (Su-27K) and its associated carrier, the Kuznetsov. All of its Kiev-class aircraft carriers have been taken out of service.

While Russia has, to all intents and purposes, dropped out of a STOVL programme, other countries - for example Thailand and Japan - are looking to acquire such a capability. Also in that part of the world, China has intentions with regard to STOVL, but, as yet, those intentions are unclear.

The UK, both at a service and industrial level, has aspirations to continue with ASTOVL programmes. Since the mid-1980s, these have been tied to US coat tails in the form of numerous agreements and memoranda of understanding.

Both the RAF and the Royal Navy ostensibly have ASTOVL requirements in place, to replace the Harrier GR7 and Sea Harrier F/A2.

While the RN's requirement to replace the Sea Harriers is at the top of its priority list, the RAF's plans for a GR7 successor remain more vague. The Eurofighter EF2000 and a Panavia Tornado GR4 follow-on to meet the Staff Target (Air) 425 requirement for a future offensive aircraft (FOA) are taking precedence.

The RN is in favour of the JAST/ASTOVL concept to meet its F/A2 replacement needs, and its requirement resembles closely that of the USMC.

The RAF's position is less certain. There are those within the service who remain to be convinced that, while the JAST/ASTOVL concept will meet GR7 needs, it will not fulfil the full ST(A)425 requirement.

BAe, long the partner of McDonnell Douglas (MDC) on the AV-8 Harrier family, is also ambivalent about the JAST/ASTOVL meeting ST(A)425 requirements. While BAe is partnered with MDC for the JAST/ASTOVL platform, it also has aspirations to pursue a "balanced stealth deep-strike penetrator aircraft" to meet the FOA requirement, possibly with Dassault as a partner.

In the USA, the survival of JAST is widely viewed as a vital source of technology for future tactical aircraft and as a budgetary lifeline for defence and aerospace into the next century. The JAST programme took its shape from the Pentagon's BUR and in part, was a recognition that, the old ways of "business as usual" were no longer tenable. The affordability of maintaining an adequate tactical air capability was thrust to the forefront of procurement concerns.

The JAST approach has, from the outset, been aimed at providing a technology base, to provide transition to engineering and manufacturing development (EMD) for a family of next generation strike aircraft expected to be fielded around 2010.

The JAST project is aimed at producing affordable, long-term, replacements for the USAF's Lockheed F-16, the Navy's F-18 and Northrop Grumman F-14 and the USMC's AV-8B Harrier. Formally launched in 1993, JAST has become the focus for a radically revised approach to combat-aircraft procurement.

In making key appointments to the project, the JAST office describes the previous generation of programme officials as "...war-fighters and technologists". The new teams are using these concepts to combine the best, and most affordable, technologies, to form the building blocks for future strike systems. "The JAST programme is therefore not a technology-development programme, but rather a technology-transition programme," says the JAST office. To some, however, the distinction between the two may seem vague.

The first 12 contracts, worth $10.5 million, were awarded in May 1994 for strike-warfare concept studies. In December 1994, following the critical merger with the ASTOVL programme, a much bigger spread of contracts, worth more than $140 million, was awarded. These covered weapon systems, avionics, air-vehicle structures and materials, propulsion concepts/components, and modeling, simulation and analyses.

Early in 1995, the JAST programme was shown to be of high priority when the USAF issued a budget request for $151 million in fiscal year 1996. Although this was a trifling amount compared to the $2.14 billion requested for the Lockheed Martin/Boeing F-22 fighter, it is still almost double the $85 million 1994 funding level and overtakes the joint primary aircraft training system (JPATS) to enter the USAF's top five priorities.

The sheer size of the JAST requirement has also become apparent. The USAF needs 1,874 aircraft, the USMC some 642 and the USN around 300. Additional aircraft for allied powers - particularly for the RAF and the Royal Navy - mean that total JAST-derived aircraft sales could easily exceed 3,000.

ASTOVL PROGRAMMES

Boeing and Lockheed Martin are in discussion over joining forces on JAST. McDonnell Douglas and Northrop Grumman are already strongly linked through a JAST teaming arrangement shared with BAe. It is not yet known if an ASTOVL link between the latter two US companies will be developed.

Boeing is developing a delta-shaped aircraft similar to the Northrop M2-F2 lifting body. A direct-lift-propulsion concept is used for the STOVL role. "With a compact shape like that, the STOVL penalty is very small and it gives us a simple transition from jet-borne to wing-borne flight," says Mark Burgess, analysis and integration manager of Boeing Defense & Space's ASTOVL effort.

"We prefer a direct-lift solution, because it offers a lot of advantages to the [shared airframe] concept. Pratt & Whitney with their F119 teamed up with us in March 1994, as did Rolls-Royce, which has 30 years experience of direct-lift systems," says Burgess. Rolls-Royce will only be involved in the STOVL version and not the CTOL.

The Boeing aircraft, is described, by the JAST Programme Manager, Brig Gen. George Muellner, as the most, interesting design in the competition. It consists of a one-piece wing, with twin vertical endplates canted inwards, to provide stability and reduce infrared signature. The plates can be moveable in-flight.

"We were driven by the need for volumetric efficiency and a compact design. This led us to a lighter weight, which is a first-order driver on cost. We're working the hell out of the weight requirement - the empty weight has to be less than 24,000lb [11,000kg] and [our design] is substantially less. It's also got to have a spot factor [size on the deck of an aircraft carrier relative to the F-18] of less than 1.0. Cost, in turn, was a major design driver on vertical landing capability," says Burgess.

"We recognised the need for a large internal volume for fuel and payload. Vertical lift is a constraint for us, not like those with lift fans. But, on the other hand, direct lift does provide more agility and manoeuvrability."

Boeing is building a 94%-scale powered model, to be tested at a purpose-made site in Seattle. "We will start tests in June," says director of advanced tactical programmes Mickey Michellich, who adds that the purpose-made test site was constructed "...because of the problems we ran into noise-wise at NASA".

A series of three sub-scale wind tunnel and component tests was completed at the end of 1994. Testing of jet effects on a model in a NASA 4 x 6m wind tunnel was also completed in mid-January 1995. Tests scheduled for the second quarter of 1995 include measurement of aerodynamic forces and moments at transonic speeds at NASA's Langley Research Center. Transonic and supersonic inlet tests are due in the third quarter of this year. Inlet tests at low-speed and static conditions follow in the first quarter of 1996. Boeing has also requested use of the 24 x 36m wind tunnel at NASA Ames for the first quarter of 1996.

"One of the things that R-R has done for us is to bring considerable hot-gas re-ingestion expertise. In tests so far, we're showing an inlet-temperature rise during landing, but we're currently running at levels well within those of the Harrier and we're keeping a close watch on that," adds Burgess.

LOCKHEED OFFERS LIFT FAN

Lockheed Martin, Pratt & Whitney, Allison and its new owner, Rolls-Royce, are teamed to offer an aircraft with a shaft-driven lift-fan system in its ASTOVL configuration.

"In 1986, we weren't even looking at shaft-driven lift fans. We had RALS [remote augmented lift system], tandem fan and lift-plus-lift/cruise, etc, but NASA Ames came to the Skunk Works and said that it would prefer to look at a low signature VTOL vehicle," says Lockheed Martin "Skunk Works" advanced-development programme manager Grant Carichner. This was for the then-classified USMC Thunder Cat project.

"Paul Bevilaqua [Skunk Works ASTOVL programme manager] and I spent three weeks in a room going over every propulsion system ever used, to get STOVL performance. Out of that came the idea to use a shaft-driven fan."

The size, wing loading and sweep were derived from the design-mission objectives. "The requirement for a Mach 1.4 super-cruise meant we had to look for something different than direct lift. With the big fan you need for V/STOL this simply prevents super-cruise. Now we've learned to spell ASTOVL and the fact that the 'V' is further back means less is required of the engine [because the V now refers to vertical landing rather than vertical take-off]," says Carichner.

Lockheed Martin says that the common P&W F119 core increases the concept's "affordability" and makes it more compact and modular, with a "favourable thrust balance." Operationally, the engine was chosen because of its "substantial thrust-augmentation capability".

Lockheed Martin says that the idea of a shaft-driven fan comes from General Electric's conversion, during the 1950s, of the J79 turbojet into the CJ805 aft-fan turbofan. "Although that engine didn't really catch on, it showed you could take an existing engine and extract power from the core stream and send it to work somewhere else," the company says.

Not unexpectedly, perhaps, Lockheed Martin's concept resembles the F-22. Similarities include the reduced-signature engine inlets and fore-body shaping, trapezoidal wings and twin, outward-canted, vertical fins. "The only things that have changed since 1988 are the forward inlet and fore-body shaping," the company says.

The lift-fan module is located aft of the cockpit. The fan itself takes air in through a round inlet in the fuselage spine, which is enclosed, for cruise beneath simple hinged doors. Power for the fan is taken directly off the central axis of the F119 through a drive shaft and connected via a multi-disc "wet" clutch. The Allison-built clutch is either engaged or disengaged, and does not provide varying degrees of gearing. The 2,500kW (25,000shp) unit was developed from the Thunder Cat programme. Underneath the fan module is a lift-fan nozzle, fitted with a yaw vane.

The core engine is a derivation of the F119, called the SE611A, and feeds a Rolls-Royce-built roll-duct system on either side in ASTOVL configuration. R-R is also making the deflection valve in the lift nozzle. The engine's main nozzle is fitted with a "swirl augmentor" and an "optimised" exhaust, for low signature.

"The system has almost serendipitous characteristics, particularly in the ground environment, where you are trying to deal with that familiar oxymoron - more thrust but at low core temperatures. With this system, you're taking hundreds of degrees out of the gas stream, which cuts the aft-post [exhaust stream] temperature considerably, and the front post is lower anyway [because it is compressed, ambient air]. Plus, the front post is more critical from an ingestion, weapons and personnel point of view. The other nice thing is the transfer of thrust forward and aft. You don't have to add it here and subtract it there, so it's good from a flight-control characteristics perspective."

Lockheed Martin has had a busy 1994, having completed tests of a sub-scale lift-fan nozzle, the vectoring lift/cruise nozzle, fan off-take and roll nozzle and a inlet-fore-body model. Tests of a jet-effects interactions model were also completed during the year.

Lockheed Martin has built an 86%-scale model, which is due to be tested at NASA Ames in September 1995. The airframe is scaled around a P&W F100-229 engine," because we couldn't get an F119. They're too hard to come by, so we had to settle for an F100." The fan rig, however, comes directly from an F119. "The geometry is almost perfect," claims the company.

The airframe has been fitted with the engine at Pratt & Whitney's West Palm Beach site in Florida. It will be returned to the "Skunk Works" in Palmdale, California, to be fitted with wings, undercarriage, instrumentation and flight-control systems "in a few months". The large-scale powered model will begin hover tests at NASA Ames's outside aerodynamic reference facility (OARF) in September. Transition testing follows in the first quarter of 1996.

MCDONNELL DOUGLAS FAVOURS GAS

MDC is teamed with BAe, General Electric and Rolls-Royce. Twenty BAe engineers are assigned to the programme at St Louis, Missouri. GE is providing the YF120 variable-cycle engine (which lost out to the P&W F119 in the ATF competition), and R-R is supplying most of the STOVL hardware.

"The real challenge was in meeting all the diverse mission requirements. They ranged from supersonic cruise to loiter. We figured the best way was a canard-wing configuration coupled with a lift fan driven by gas from the primary engine," says MDC ASTOVL team leader James Cupstid.

The design is aimed at a "take-off distance of within 300-500ft for an [HMS] Invincible-class ship, depending on weapons load". Design for good STOVL performance was influenced by several factors, including nozzle location to minimise re-ingestion and lift loss. Another driver was the prevention of "suck-down", where air is induced to flow over the airframe, reducing pressure below and literally "sucking" down the aircraft.

MDC is testing a fore-plane-equipped aircraft with twin vertical fins, which resembles a small version of the Northrop/MDC YF-23. The aircraft has folding-wing capability and internal and external weapons. A bifurcated inlet duct to the primary engine, is supplemented by auxiliary inlets for the lift system.

For STOVL operations, a clamshell diverter-valve in the jet-pipe blocks off the main nozzle and directs core and bypass air from the YF120 through aft-vectoring nozzles taken straight from an MDC/BAe AV-8B. Some air is also passed forwards to the remote fan by two ducts running fore and aft either side of the main inlet.

Core air, augmented by bleed from the low-pressure turbine and fan, forms two concentric streams, which swirl through an outer scroll-work, with six hollow vanes, before entering a hub turbine. This is formed from two stages of the F110 turbine and drives the fan itself, made from the front fan of an F110. Inlet-guide vanes are used to vary the thrust from the remote fan, while the flow is directed fore and aft by vanes.

The space taken up by the lift-fan module in the ASTOVL variant will be used for a fuel tank in the CTOL version. The CTOL variant will also be equipped with an air-refueling probe.

A 5.6% hot-gas ingestion (HGI) flow model has been tested at MDC. A 9.1% HGI model was tested in the NASA Lewis 9 x 15m wind tunnel in late 1993. A 7.1% model will be tested next. Testing of a dynamic HGI and thermal/acoustic model was done at Warton in the UK.

Other tests have been done on a 7.5% jet-induced-lift model, 12% low-speed powered model and a 13% low-speed-inlet model. "We were very pleased with the results, which exceeded our performance predictions in both auxiliary and primary inlet-flow mode," says Cupstid.

ASTOVL integration models tested include a 12.5% low-speed aerodynamic model and 13% fore-body model. "We took a 7.5% high- speed aerodynamic model to Bedford, UK and tested a 13% static diffuser model in the USA. A high-speed-inlet fore-body model was under test at NASA Ames in the first quarter of 1995.

"All tests lead to July 1996, when a full-scale powered model will be mounted in NASA Ames' 24.4 x 36.6m low-speed wind tunnel for transition tests. GE will be using off-the-shelf components for the lift fan and YF120 engine," says Cupstid. The model was made at St Louis in January and was due to be completed by late March. Testing of the propulsion system also took place in March at GE's Peebles test site, Ohio.

"The propulsion system will then be integrated and functional checkout will be completed in the Harrier Hush House [St Louis]. Then we'll truck it to Ames in June for installation in July," Cupstid says. It will later be moved to the OARF in October/November where the model will be lowered into ground effect, says Cupstid..

NORTHROP GRUMMAN IS DIRECT

The Northrop Grumman/P&W/R-R team is working under a "no cost" agreement with ARPA, under which the US Government provides no financial support. "But they will review us and give us test-facility support," says Northrop Grumman ASTOVL programme manager Grant Zwernemann.

Northrop Grumman is proposing a lift-plus-lift-cruise solution under which P&W provides a derivative of the F119 for lift/cruise while R-R provides a "current-technology" lift engine. "The design emphasises compact size, a single design approach and emphasis on current technology for low risk. We are being guided by affordability studies," Zwernemann says.

The concept offers four main advantages, according to the company. Firstly, it "...de-couples STOVL from CTOL...so we can do independent engine sizing to suit the different requirements from each. This enables us to size the lift/cruise engine for good 'up and away' performance without impacting on the vertical-lift side of things. It also provides good growth capability because each is completely separate," says Zwernemann.

Secondly, it allows better aerodynamic integration of the diverse requirements of vertical lift and transonic/supersonic flight, producing a "good supersonic aircraft." Thirdly, it provides manufacturing modularity.

Finally, it is a "proven" propulsion system, used on the Russian Yakovlev Yak-38 and Yak-141. "The only difference is we're using turbofans whereas the Russians used turbojets," Zwernemann says.

He does, however, acknowledge some drawbacks, including the extra development costs for a purpose-designed lift engine and the reliability and maintainability impact of a second engine. Another problem is that the exhaust is hotter than for lift fans (or even direct-lift units), leading to potential problems of hot-gas ingestion and a larger ground footprint.

In ASTOVL configuration, the lift engine is located behind the cockpit and fitted with an R-R-developed vectoring nozzle. In the CTOL variant, the lift-engine space would be used for additional fuel storage. The lift engine will be similar in size to the RB.162 developed by R-R in the 1960s and used as a booster engine in the HS.121 Trident 3B airliner. The 1990s version will have a high thrust-to-weight ratio and a maximum rating of around 6.6-8kN thrust. "It will be a simple engine with a short-duty cycle as it will only be used for a few minutes during takeoffs and landings," says Zwernemann.

The P&W F119 occupies the main engine bay around the aircraft's centre of gravity. Immediately aft of this is an exhaust module, fitted on each side with a vectoring nozzle, for the ASTOVL variant. In the CTOL version, the module would be replaced with a "spool section". An advanced vectoring nozzle exhaust is used on both variants.

The low-aspect-ratio wing of the Northrop Grumman design is small enough not to need a wing-fold. It also features a hammerhead leading-edge extension, which resembles a fore plane. "It's smaller than a canard and takes advantage of vortex roll-off from the forward leading edge. We've found it to be a powerful control feature," says Zwernemann. The aircraft has only a single vertical fin.

"The test programme is under way. It is emphasising sub-scale testing, because we do not have the funding for larger scale tests," he says. Hot-gas-ingestion flow-path tests have already been undertaken in a water tunnel, using coloured dye to represent the flow fields. Also completed are initial low-speed-aerodynamics and lift-engine-inlet wind tunnel tests. "We are currently on detailed low-speed aerodynamic testing and hot-gas ingestion, then we'll move onto hover and transition jet effects," says Zwernemann.

Further testing will include high-speed aerodynamics, inlet/airframe integration, aft-body drag and high-speed jet effects. "We used computational fluid dynamics, based on some of the B-2 work, to model external and internal flow-paths," he says.

NEAR-TERM CAPABILITY

While the JAST project, should it come to fruition, will be the focus of next-generation Western ASTOVL programmes, other countries are looking to acquire such a capability in the near term.

Thailand intends to purchase nine surplus Spanish navy AV-8S Matadors to equip its future light aircraft carrier, but is waiting for final approval from the US Government.

Spain has offered to sell the Thai navy seven AV-8s and two tandem-seat TAV-8As, which have been relegated to training with the delivery of the more advanced EAV-8Bs. Spain's AV-8Ss were originally obtained through the USA and Washington's permission is required to transfer the aircraft to Thailand. The Thai navy plans to use the AV-8S from its new 11,485t aircraft carrier HMTS Chakkrinareubet, scheduled for delivery in early 1998.

The AV-8s are considered to be an interim measure, the long-term aim being to order replacement AV-8B Harrier II Plus aircraft.

Japan's Maritime Self-Defence Force (JMSDF) has harboured a long-term wish to acquire a shipborne V/STOL aircraft, such as the Sea Harrier F/A.2 or the AV-8B. A purchase decision, however, has proven politically too difficult for the Japanese Government.

The JMSDF argues that, given its large fleet of frigates and destroyers and the country's total reliance on imported oil and other raw materials, a V/STOL/anti-submarine-warfare carrier is essential. The JMSDF is hopeful that this will be borne out in Japan's wide-sweeping review of its defence roles and capabilities.