Aircraft that can sense and control the airflow around them; fighters that can defeat missiles with a laser shield - these are just two emerging technologies that could revolutionise aerospace.
Every few years a technology emerges that profoundly reshapes the industry. Sometimes they burst on to the scene, like the 1980s' introduction of stealth with the Lockheed F-117, but always they are the result of decades of research and development.
Today, technologies are being incubated in laboratories or prepared for flight testing that could change aircraft fundamentally. Among these are high-energy lasers and micro-electromechanical systems (MEMS).
Lasers are nothing new. They are used every day in light shows and eye surgery. Aerospace uses them to align structures and illuminate targets. But new applications are emerging with development of powerful solid-state lasers.
The USA has been working on high-energy lasers for 30 years, but only now can put such a device in an aircraft - and a large one at that. In 2003, the US Air Force plans to shoot down a theatre ballistic missile with the Airborne Laser (ABL), a modified Boeing 747-400 carrying a megawatt-class laser.
If the demonstration is successful, the USAF will field a fleet of ABLs for ballistic missile defence. The ABL is one of a family of high-energy laser weapons now under development. The other weapons are the ground-based Tactical High Energy Laser and the Space Based Laser.
All three programmes use chemical lasers developed by TRW. The reason, says TRW's laser directorate manager John Waypa, is that chemical lasers are the most powerful available and the most efficient - 25% compared with 5% for a solid-state laser - which makes handling the heat produced more manageable.
Storing energy
"If you have a megawatt-class weapon laser, and it's only 20-25% efficient, you have a tremendous amount of heat to deal with," he says. "To put out a megawatt, you have to generate four to six times that amount of energy. You have to handle the rest of the energy that does not go out [in the laser beam], and the easy way to do that is to store it in chemical bonds."
The relative efficiency of a chemical laser also makes it feasible to generate the power required on board an aircraft. "The solid-state lasers in use now are 5% efficient. If we could build a megawatt-class system, we would need 20mW power, and that is totally impractical," Waypa says.
What finally made an airborne laser viable was the development of uncooled optics for beam shaping and steering, he says. "Even with a 99.999% reflective surface, with a megawatt-class laser the remaining 0.001% is still a tremendous amount of energy. The optics have to be water cooled and that is heavy and induces vibration."
Uncooled silicon optics were developed in the late 1980s, under the "Star Wars" Strategic Defense Initiative. "The impact was tremendous," Waypa says, halving the weight of a space-based laser. As a result, uncooled optics are used in all three of the high-energy lasers TRW is developing.
The silicon optics are "exquisitely" reflective, he says. "We've put one more nine in there and reduced the problem by an order of magnitude." The mirror substrate does not absorb energy, but transmits it to a heat dump. "Uncooled optics are the single leap in technology that has made high-energy lasers viable," Waypa says.
A chemical oxygen-iodine laser was selected for the Boeing ABL because its shorter, 1.3Ám wavelength allowed for smaller optics and, because it is less affected by atmospheric propagation, providing greater range. While each aircraft is expected to cost around $800 million, the US Air Force considers the ABL to be a good investment. Cost per shot is estimated at around $54,000, compared with over $100,000 for other weapons in the US theatre air and missile defence arsenal.
While large chemical lasers are the only weapons option for the foreseeable future, TRW is working on solid-state lasers that could be used on smaller aircraft. "Military applications require kilowatts of power, and that means diode-pumped solid-state lasers," Waypa says. Such devices are 10-15% efficient and could be developed to produce up to 10kW.
Perspex innovation
The route to directed-energy weapons on combat aircraft will be incremental, and begins with today's targeting lasers. The UK's Defence Evaluation Research Agency (DERA) has efforts under way to improve current systems and develop new applications.
One DERA innovation is the Perspex dye block, which is safer and lighter than the methanol-based dyes normally used in targeting lasers. The block, like liquid dye, has a life of about 1,000 'shots' (enough for 20s lasing in a targeting system), but can be rotated so that the pulse strikes a different face, giving longer life.
The agency is also developing three-colour lasers that produce a white light beam. Such systems can defeat the most common countermeasure, a filter opposite in colour to the laser - green light, red filter - that blocks the beam's path. With three colours a beam will pass through any filter. The aim initially is for a range of 3-5km (2-3 miles) for a comparatively low-powered laser.
One of DERA's main concentrations is on using lasers to defeat infrared-guided missiles - by confusion or destruction of the seeker. Directed infra-red countermeasures (DIRCM) systems are already under development, but solid-state lasers hold the key to miniaturising them for combat aircraft.
DERA plans to have a "fast jet" system ready within three to five years. The size problem will be overcome using conformal apertures or by combining the DIRCM with another electro-optical system. The UK agency foresees the DIRCM being combined in a single unit with an infrared search and track sensor, laser rangefinder/designator and possibly a laser-radar target identification system.
Live-fire tests of prototype laser-based DIRCMs suitable for tactical aircraft were conducted in the USAlate last year. These systems seduce missiles away from the aircraft, but as imaging infrared seekers become more common, more powerful lasers will be needed to burn out the optics and electronics. This will be the first step towards providing a lethal laser shield around the aircraft.
Military applications of solid-state lasers are likely to benefit from increased commercial use, particularly in fields such as laser machining. The aerospace industry is already adopting advances such as laser welding and laser direct manufacturing - the creation of complex three-dimensional parts from powered metal using a computer-controlled laser.
MEMS on the move
Commercial interest is fuelling rapid advances in another promising technology - micro-electromechanical systems. MEMS takes advantage of provide integrated-circuit manufacturing processes to produce microscopic machines. Ultimately it will be possible to place a sensor, computer and actuator on a chip to provide closed-loop control of the environment in which the device is embedded. For aerospace applications, the potential benefits are enormous.
The term "MEMS" was first coined in the late 1980s, after which the field exploded, says Dr Bill Tang, MEMS programme manager at the US Defence Advanced Research Projects Agency (DARPA). The first applications to emerge were automotive, including accelerometers to trigger airbag deployment.
This was the "killer application", Tang says, a commercially viable product that both improved performance and reduced cost.
Commercial applications are now expanding into the biomedical, communications and data storage markets. The devices promise to miniaturise diagnostic equipment and even administer drugs automatically. In wireless communications, MEMS mechanical filters will allow more channels to be packed into a given frequency, while MEMS memory devices will use individual atoms to store data. Both will be developed within the next 10 years, Tang believes, to meet the exponential demand for communications and data storage capacity.
Aerospace applications will emerge within the next five to 10 years, he predicts, beginning with the replacement of existing components using MEMS devices that are cheaper, smaller and more reliable. A likely early application is guidance systems, where MEMS will replace gyros and accelerometers, reducing cost while maintaining performance.
DARPA is also looking to MEMS technology to create a new generation of ultra-small satellites - called Picosats. "They can be as small as this," says Tang, holding up a postcard-sized box. Microscopic jets would be used for attitude control and the Picosat could carry MEMS antennas and other devices.
The next step will be to add new functions. "For example, we will be able to litter the airframe with sensors and understand the flow around an aircraft," Tang says. "We will be collect flow data in real time and use it to design a much better aircraft. Eventually we will put actuators around the aircraft and manipulate the flow."
MEMS are an enabler for another potential breakthrough technology - micro adaptive flow control (MAFC). This involves the control of unstable airflows over air vehicles, and allows large effects to be achieved with small inputs. Using MEMS actuators, Tang says, it will be possible to control turbulence to reduce drag or vortices to increase manoeuvrability.
"Traditionally aircraft designers stay away from unstable flow, while bugs and birds manage unsteady flows well," says DARPAMAFC programme manager Dr Rich Wlezien. "In insect flight the flow is always separated; with aircraft it is always attached. Insects dynamically manage unstable flow to generate lift."
The potential benefits of managing unstable flow are substantial. Under DARPA's MAFC programme, Bell Boeing is working on a wing flap modification for the V-22 tiltrotor that promises to increase vertical-lift payload by 30%. Tiny pulsing air jets on the leading edges of the deflected flaps will delay flow separation and reduce downwash drag.
Local blade suction
Under another MAFC project, Massachusetts Institute of Technology is developing an aspirated turbine-engine compressor that uses local suction on the blades to double the work output per stage. The aim is to reduce the number of compressor stages required from 12 to three, says Wlezien. Under the MEMS programme, Tang says, DARPA is funding work on devices able to withstand the harsh environment inside a jet engine.
Other projects include active control of the shockwave/boundary-layer interaction in an engine inlet, using an active porous surface which responds to the local position of the shockwave to provide pressure relief and eliminate the need for costly, heavy boundary-layer bleed systems. Sikorsky, meanwhile, is studying the use of synthetic jets to postpone retreating-blade stall in helicopter rotors, to increase speed and reduce noise.
A near-term application of adaptive flow control involves the Boeing C-17 transport's Pratt & Whitney F117 engine, says Wlezien. The current core-flow thrust reversers are expensive, but are needed to direct hot exhaust gases away from ground personnel when the aircraft is being loaded and unloaded with the engines running. Fluidic flow control promises to eliminate the need for reversers by mixing the hot core and cool fan flows more effectively.
Small-scale tests on a JT8D engine have been successful, and a full-scale ground test is scheduled for early 2001. Small amounts of bleed air are used to provide pulsed inputs which force a side-to-side instability in the core jet, causing the flow the mix more rapidly. "Relatively small inputs produce a big effect. It's a very elegant solution," he says.
MAFC and MEMS come together in DARPA's Micro Air Vehicle programme. This aims to demonstrate tiny unmanned aircraft - some conventional, some distinctly unconventional. The Microbat project has used MEMS micro-manufacturing processes to fabricate a titanium-reinforced membrane that emulates a bat's wing and "outperforms nature", says Wlezien.
Programmes like the Microbat are aimed at producing tiny flapping-wing unmanned air vehicles with the ability to fly, hover and perch, for surveillance and reconnaissance missions inside buildings. As such, they are unlikely to influence the design of mainstream aircraft, but they provide a showcase for the capabilities of both MAFC and MEMS technologies.
Flow control
Since the beginnings of aviation, wings have been shaped to prevent separated, unstable flow, resulting in the classical aerofoil shape. Adaptive flow control could change all that, Wlezien believes. "Now, the only way to keep flows attached is with a pressure gradient. If we have a totally different way to control attachment, we can go to totally different configurations. We have only begun to scratch the surface."
Previous efforts at flow control involved massive, costly suction. "Now we can be much more subtle, using small, low-power actuators distributed on the wing," he says. "The next step is distributed sensing and actuation, so we can sense and control the separation, the way a bird feels the flow about to separate from its wing."
Wlezien describes adaptive flow control as "one of the most exciting developments in aerodynamics" since the introduction of computational fluid dynamics. "It is changing all the rules on how to design aircraft." Industry was sceptical at first, he admits, but is now getting interested.
"The aircraft people are getting much more clever about using this. It is out of the laboratory and it is real."
Source: Flight International
