In the fourth of our series, we look at combat aircraft evolution - does the way forward lie in UAVs, manned aircraft - or both?

One hundred years after the Wright brothers made the first manned, powered flight, debate rages over whether the latest generation of combat aircraft will be the last to have a human crew. Post-war analysis of the air war over Iraq is likely to add to the debate, as experts assess the success of pre-planned precision strikes.

The manned versus unmanned debate is not new. In 1957, UK defence minister Duncan Sandys cancelled almost every UK combat aircraft programme in favour of missiles. The controversial decision halted development of the Avro 730 supersonic bomber, Fairey Delta 3 long-range missile-armed bomber interceptor, and the Saunders-Roe SR177 mixed-power fighter. The English Electric Lightning survived to become the mainstay of UK air defences for decades to follow.

The UK combat aircraft industry survived, only to receive another near-fatal blow in 1965, with cancellation of the BACTSR2 strike aircraft and Hawker Siddeley P1154 supersonic vertical/short take-off and landing fighter. The villain this time was budget cuts. The result, ultimately, was to bring the UK together with Germany and Italy on the Panavia Tornado strike fighter, which paved the way for today's four-nation Eurofighter Typhoon.

In 1957, long-range air-breathing missiles then under development, such as the Northrop B-62 Snark, North American B-64 Navajo and B-77 Hound Dog, looked every bit as promising as the Boeing X-45 and Northrop X-47 unmanned combat air vehicles (UCAV) of today. In the end, they proved irrelevant in combat terms, but paved the way for one of today's most potent unmanned weapons, the Raytheon BGM-109 Tomahawk cruise missile.

The way forward

The latest round of the manned versus unmanned debate centres on whether autonomous UCAVs are the future, and whether the Lockheed Martin F-35 Joint Strike Fighter (JSF) will be the last manned combat aircraft developed in the USA. Not surprisingly, JSF competition loser Boeing is leading the pro-UCAV charge, as it faces the prospect of exiting the manned fighter business when production of the F-15 and F/A-18 ends within the next decade. Equally unsurprising is Lockheed Martin's advocacy of manned fighters, with its F-16 continuing to sell and the F-35 planned to be in production beyond 2030.

Northrop Grumman has its feet firmly in both camps, as a partner on JSF and a leader in unmanned air vehicles (UAVs). As developer of the B-2 bomber, the company is considered a leading candidate to develop the USAir Force's planned long-range strike aircraft - which could be manned or unmanned. In Europe, meanwhile, industry attention is focused on production of the latest generation of fighters, the Dassault Rafale, Eurofighter Typhoon and Saab/BAE Systems Gripen. As a result, it sees UCAVs as adjuncts to manned aircraft, rather than replacements.

This time round, the fate of an entire industry does not rest on the outcome of the manned versus unmanned debate. Nor is the choice to be made as stark. Most of the raft of new technologies emerging from laboratories are applicable equally to manned and unmanned combat aircraft. The choice of how these technologies will be applied is likely to be made programme by programme, and country by country. And affordability is likely to be the driver.

Advances in sensor development should benefit both fighters and UCAVs. The new generation of active-array radars is the enabling technology for a range of new surveillance, reconnaissance and targeting capabilities. Active electronically scanned arrays generating "inertialess" beams enable essentially simultaneous operation in air-to-air and air-to-ground modes. The arrays, made up of hundreds of solid-state transmit/receive modules, promise unprecedented reliability. Performance degrades "gracefully" as modules fail, with manufacturers claiming the airframe will be retired before their radar needs repairing.

The days of a fuzzy blip on a cathode ray tube are long gone, and radar is now an imaging sensor. Today, synthetic-aperture radar (SAR) processing produces high-resolution ground maps in fighter cockpits. Development of foliage-penetration (Fopen) SAR able to locate targets under trees is under way. Next up is the demonstration of interferometric SAR (InSAR), intended to generate high-resolution digital terrain-elevation data in real time, while tracking moving targets, enabling GPS-guided weapons to be targeted with greater precision. Work is now beginning on active-array laser radars (ladars), promising higher-resolution imaging.

Imaging the future

Advanced sensors like Fopen and InSAR are likely to be applied first to UAVs that are able to provide persistent surveillance over the battlefield. But the images will be sent direct to the cockpit via datalink as fighters become part of the battlefield network. Ultimately, if they prove affordable, such imaging capabilities are expected to be incorporated into fighter radars, giving the crew the capability and flexibility to locate and identify unplanned targets.

Managing the blizzard of information available from on- and off-board sensors is the greatest challenge facing cockpit designers. Adding direct in-flight control of UAVs and UCAVs, their sensors and weapons, will only complicate the task. A clear future direction is provided by the JSF, with its distributed infrared sensors providing a 360¡ view around the aircraft, presented on the helmet-mounted display so that the pilot can look in any direction - even through the cockpit floor - and see navigation and targeting information. This is the first step towards a totally synthetic cockpit environment, produced by fusing sensor information with onboard databases, which will allow the crew to fly and fight regardless of visibility.

As sensor and battle management assume greater importance, flying the aircraft is likely to become less a task for the crew and more a function of automation. Here the work being done to enable autonomous UCAV operations is key. The same knowledge-based software tools that allow ground controllers to plan UCAV missions will help crews of manned aircraft replan missions in flight as new target information becomes available.

Digital fly-by-wire flight controls have transformed the capability of combat aircraft, enabling aerodynamically less-than-ideal stealth designs to be made both agile and safe. Work on tailless fighter designs, including flight tests of Boeing's manoeuvrable X-36 and stealthy Bird of Prey, paved the way for the UCAV.

The tailless X-45 is the first aircraft to use fluidic thrust-vectoring. This injects air at key locations within the nozzle to provide multi-axis thrust vectoring. A low-observable, conformal fluidic nozzle is lightweight, low cost, and has no moving parts.

Future combat aircraft, manned or unmanned, are likely to use innovative flight controls. The F-35 JSF and the X-45 UCAV use power-by-wire electrohydromechanical actuators, but they move conventional flight control surfaces. The wing of the future is likely to be covered with arrays of micro-actuators that modify the airflow to achieve the same control effect as ailerons and elevons. The airframe structure itself will be actively controlled, for example to modify wing twist. Eventually the wing planform, including sweep and span, may change in flight using morphing aircraft technology.

Embedded sensors

Electronics will be embedded in "smart" skins so that they can act as conformal antennas for communications and electronic warfare. Embedded sensors will monitor airframe health, allowing the vehicle management system to reconfigure aircraft systems and flight controls autonomously to compensate for battle damage. Smart skins are also considered key to active stealth technology, which would enable the aircraft to manage and minimise its radar and visual signatures, the latter possibly using "camouflage" that adapts, chameleon-like, to the ambient environment.

Defensive and offensive directed-energy weapons are already under development, and are likely to be built into future combat aircraft whether manned or unmanned. The first step is the directed infrared countermeasures (DIRCM) system, which uses a turret-mounted lamp or laser to jam incoming heat-seeking missiles. Large systems are being fitted to transport aircraft and helicopters and small turrets are under development for tactical aircraft.

As solid-state high-power laser technology advances, the DIRCM is expected to give way to more lethal defences. Work is already under way to laboratory-demonstrate a 25kW laser as the first step towards fielding a 100kW-class laser weapon capable of destroying missiles and ground targets. Issues of weight, power and cooling remain to be overcome, but the roadmap to airborne directed-energy weapons is becoming clear. Some reports suggest the USA is already able to field high-power microwave weapons that can knock out enemy electronics.

All this will take power, and a major thrust of military propulsion technology development is to maximise the energy extraction capability of future combat aircraft engines. This includes using the engine core as an electrical generator. The additional power is needed to drive all-electric engine accessories and aircraft subsystems, as well as lubrication-free magnetic bearings inside the engine, in addition to the sensors and weapons.

Greater affordability

While the direction of fighter engine development for the last six decades has been to improve performance, it is shifting towards greater affordability. Already fighter operators have the choice of taking advantage of increased temperature margins as higher thrust or longer life, and durability is assuming greater importance as operating costs come under attack. In the USA, the new Versatile Affordable Advanced Turbine Engines programme aims to increase affordability sixfold by 2010 and tenfold by 2020, compared to a 2000 technology baseline.

Affordability - of sensors, airframe, engines and weapon systems - will determine whether the latest debate on manned versus unmanned is decided in favour of UCAVs or crewed combat aircraft. Whether the JSF programme delivers on its promise of reversing the ever-upward trend in fighter costs will be key. The spectre of Sandys may not be haunting the fighter industry yet, but it is likely that, within the foreseeable future at least, air power will involve a mix of unmanned and manned combat aircraft.

Source: Flight International