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Thursday, April 1, 2004

Airships: Making a Comeback

In this era of jet aircraft, rockets and satellites, it is the slow-moving airship that is gaining renewed interest for homeland security and other missions. Airships are well suited to carry large radar antennas and laser relay mirrors.

George Marsh

It was once safe to say that airships were relegated to aviation history. These slow-moving behemoths were, after all, virtually written off after the 1930 crash of the British-built R101 near Beauvais, in France, and the fiery Hindenburg disaster in 1937 at Lakehurst, N.J. However, airships now appear poised for resurgence.

Several countries are actively developing airships for surveillance, communications relay and missile defense roles. National needs are driving this renewed interest in airships, which long have used helium rather than flammable hydrogen for buoyancy. New technologies have further encouraged this interest. They include:

  • Low-weight structures and propulsion,

  • Solar power, and

  • The high power-to-weight ratio of modern electronics.

Lighter-than-air (LTA) platforms are being developed for use at both stratosphere-brushing altitudes and ground- or ocean-hugging levels. It's possible that the market could one day justify a breed of LTA avionics adapted to the characteristics of these new, yet old, platforms.

Protecting Borders

The United States, with its emphasis on homeland defense and security, is taking a serious look at high-altitude airships (HAAs) for continuous surveillance. Presently envisaged are HAAs, holding station some 70,000 feet above the Earth's surface for up to a year at a time to provide early warning of ballistic-missile and other threats. According to the North American Aerospace Defense Command (NORAD), a stratospheric platform system (SPS) of just 11 HAAs stationed around the U.S. coastline could provide overlapping radar coverage of all maritime and border approaches. Experience since the 1980s with radar-bearing aerostats supports NORAD's confidence. Tethered at some 15,000 feet, these balloons have helped disrupt drug traffic across southern U.S. borders. Some military planners even propose that enormous load-carrying HAAs could heft powerful kinetic energy weapons into the upper atmosphere.

Military planners hope that an HAA-based response to the post-9/11 era would be more affordable than satellites, AWACS (airborne warning and control system) aircraft, and unmanned air vehicle (UAV) alternatives. Using HAAs would avoid the high cost of launching satellites, as well as of payloads that must be engineered to withstand the g forces, shock and vibration of a rocket launch. HAAs would be stationed in an atmospheric band about 13 miles (21 km) above the Earth's surface. They would be positioned between powerful jet streams below and strong stratospheric winds above. The height of this benign "sweet spot" varies according to geographic location, but once there, airships should be able to hold station with modest power expenditures. Unlike a satellite, an HAA can return to base for maintenance or payload changing.

The UK, Canada, Korea and Japan also are actively working on HAAs. In the UK, Lindstrand Balloons Ltd. recently won a European Space Agency (ESA) contract to develop a stratospheric airship. Meanwhile, the Advanced Technologies Group (ATG) has high aspirations for its Sky Cat family of hybrid airships, i.e., ones using aerodynamic, as well as buoyant lift. Korea has a three-phase program under way to develop a surveillance aerostat. And in Japan, Fuji Heavy Industries is developing a communications LTA platform.

Lockheed-Led Team

Arguably the most ambitious program is under way at Lockheed Martin's Maritime Systems and Sensors' base in Akron, Ohio. Thanks to a Department of Defense (DoD) requirement for an HAA advanced concept technology demonstration (ACTD), a buoyant giant that is 495 feet (150 meters) long and 160 feet (49 meters) in diameter may soon take shape. It would be built in the enormous Akron Airdock, home of the veteran airship maker, Goodyear Aerospace Corp., which Lockheed now owns. NORAD is sponsoring the ACTD program; the Missile Defense Agency (MDA) is the program manager, and the U.S. Army Space and Missile Defense Command Battle Lab is the operational manager.

Lockheed Martin is leading a team engaged in Phase 2 of a three- (possibly four-) phase program. Phase 1 was a concept definition study undertaken by three groups: Team Lockheed Martin, Boeing's HAA One Team, and the Worldwide Aeros Corp. Completion of this phase last July was followed by a downselect to one team for Phase 2. The winner, Lockheed Martin, along with StratCom International and other partners, is in the midst of a nine-month, risk-reduction phase, in which the HAA's design has to be matured and offered for critical design review later this year. If the Missile Defense Agency and its principals are satisfied with the HAA concept's viability and projected costs, a contract may be awarded for the third phase.

Phase 3 would have a prototype HAA constructed and flown, probably from White Sands, N.M., by mid-2006. Contract value for this phase probably will be about $50 million, following some $40 million for Phase 2. A fourth phase, worth $9 million, would cover a period of user evaluation, extending to 2008. A fully operational system could follow by 2010.

The primary deliverable from Phase 3 (if awarded) should be an airship capable of climbing to 65,000 feet and staying aloft for one month, which is much greater than the endurance of contemporary UAVs. According to Allen Barber, vice president and general manager of Lockheed Martin's Akron operations, this HAA also would meet a primary DoD requirement of being able to loiter with a 4,000-pound (1,814-kg) multimission payload in quasi-geostationary orbit. The one-month endurance required of the prototype could be extended to a year for an operational variant. The prototype also will have to demonstrate required autonomous flight control and station-keeping, and its onboard power systems must be capable of generating up to 10 kilowatts (kW) of power for the payload and propulsion system.

"Its long time on station and the ability to carry different payloads will provide multimission capabilities not possible with other assets," Barber comments. "When launched, the HAA will commence a new era in flight."

The fact that LTA vehicles have been built in the 1,175-foot long, 325-foot wide and 211-foot high (358-by-99-by-64 meter) Akron Airdock since 1928 may have had some role in Lockheed's Phase 2 win. The company and its forebears have had a long history with airships. Having certificated the GZ-22 airship for the Federal Aviation Administration (FAA), Lockheed also understands issues involving the operation of an unmanned airship through controlled airspace and over populated areas. Its prototype would be propelled by four large, twin-bladed propellers-two on each side of the vehicle-driven by an electric motor.

Solar-Powered

ITN Energy Systems and Iowa Thin Film Technologies, associates in the HAA project, are developing self-contained, flexible, thin-film photovoltaic (PV) arrays for the Lockheed airship. The PV arrays convert solar energy into electrical power for the propulsion motors and payload. ITN Energy Systems is working on a light, flexible, copper-indium-gallium-diselenide (CIGS)-based film. Iowa Thin Film is employing a similar approach with slightly different materials. The company's PowerFilm products have monolithically integrated semiconductor panels deposited on a durable, flexible, paper-thin polymer substrate. PV arrays initially would be attached mechanically to the airship's upper hull, but in future models they could be integrated into the envelope fabric.

Solar-electric propulsion has been demonstrated on other stratospheric vehicles, notably NASA's Helios, a large ultra-light flying wing designed to loiter at up to 100,000 feet. Sixty-two thousand solar cells on the upper wing surface generated power for Helios' 14 propeller motors, but the aircraft-lost in June 2003 near the Hawaiian islands-had minimal payload capability. Solar-derived power for the Lockheed Martin HAA would be stored in low-weight, high-energy-density batteries initially. However, future vehicles could have regenerative fuel cell technology, probably based on work at the NASA-Glenn research facility in Cleveland.

Onboard Electronics

Controlling and evaluating an HAA's operations from a ground station calls for a comprehensive instrumentation and telemetry fit. The flight environment, airship stability and control, internal thermal and atmospheric environment, and power generation and management must be monitored. The payload environment-thermal, air composition, vibration, electromagnetic-also must be managed. Systems must establish the airship's buoyancy status, in case of helium leakage through the fabric envelope or the migration of air and water vapor into the LTA enclosure.

Commands to manage the HAA's primary systems will be directed to an onboard antenna and transceiver. Control and guidance systems will have to meet standards now being demanded of all UAVs required to navigate through controlled airspace, en route to and from their mission locations. This will necessitate both navigational and "see and avoid" elements.

In reality, the HAA is an aerial truck, a large bubble of helium intended to haul aloft a sophisticated surveillance, communications or even weapons payload. Sitting out of harm's way in the upper reaches of the atmosphere, a sensor-typically commanding a 750-mile (1,207-km) diameter footprint-will "see" threats with higher resolution than is available from satellites in space. And they will use less power in the process. Communications equipment, both on board and on the ground, can likewise be less powerful than required for orbiting satellites, while the time lag that often mars communication via satellites will be much reduced.

HAA avionics will not be subject to the high radiation levels, severe thermal gradients, space debris impacts, and other hazards of spacecraft operation. Nor will they suffer the g forces, vibration, repeated takeoff and landing shocks-plus thermal and pressurization cycles-that avionics must endure on conventional aircraft.

On the other hand, payloads will face high ozone concentrations, low atmospheric pressure, ultraviolet and other radiation, plus strong diurnal (day/night) thermal cycling. Prolonged unmanned operation will call for exceptional integrity and reliability, and the equipment will have to be light, consume minimal power, and possibly tolerate power supply irregularities. HAAs will therefore need specific electronics. A particular requirement within the MDA-managed program is that the prototype craft should use components, structures and subsystems that will be scalable to an operational airship having a payload capacity five or six times that of the prototype.

Gimballed sensor packages, as with UAVs, will probably combine optical, infrared and radar elements, ideally with data fusion to provide an integrated representation of detected objects. Airships promise to be particularly suitable platforms for large radar antennas, however, making radar a likely system of first choice.

DARPA'S ISIS

Antennas could be almost as large as the airship's cross-sectional area, creating the potential for unprecedented power aperture. The Defense Advanced Research Projects Agency (DARPA) believes that unmanned stratospheric airships will permit the development of a new class of battlefield sensors, with better capabilities than any existing or planned sensor system. This is the stimulus behind the agency's Integrated Sensor Is Structure (ISIS) initiative to develop such capability.

DARPA sees the HAA's key elements as:

  • large, low-weight, low-power density, phased array antenna;

  • A system that supplies power to the radar and the airship's motors; and

  • The HAA platform, itself.

DARPA recognizes that considerable weight reduction and functional integration will be needed if payloads are not to exceed feasible airship lift budgets. It has solicited proposals for enabling research through a broad agency announcement. DARPA would like to see an HAA-borne ISIS radar that can detect and track large numbers of air and ground targets while simultaneously providing wideband communications. ISIS also should dynamically reallocate its resources to perform these activities in line with changing battlefield conditions. Antennas with a huge power aperture at high altitude could, DARPA calculates, detect, track and engage multiple low-flying airborne targets out to the radar horizon and a large number of dismounted troops and other ground targets moving at less than 1 meter (3.3 feet) per second. With grazing angles better than 3 degrees, ISIS could "see" out to 186 miles (300 km). The persistent nature of the surveillance would allow ISIS to develop detailed clutter maps, determine normal traffic patterns, and hence recognize unusual situations. Regions of high interest could be given special monitoring attention.

Companies applying for DARPA research funding are certainly encouraged to think "outside the box." DARPA reckons that a suitable large-aperture antenna would have to be of "areal" density-mass per unit of area-less than a 10th that of today's lightest space-based antennas, to stay within the airship's lift capability. This may be possible, given that the ISIS antenna does not need to be stowed and deployed, does not have to survive a rocket launch, and requires no radiation shielding. Since the ultra-large antenna probably will be quite flexible, dynamic calibration techniques may be needed to ensure that the radar beam is correctly formed and pointed in the desired direction. Use of silicon-germanium chips might permit more electronic integration, reducing the electronics to a single chip per array element.

DARPA points out that participating companies will need to apply every weight-saving trick in the book. Dual or multiple use of "mass" will be appropriate; for example, the support structure might also serve as an energy storage medium if fiber battery technology is used. Also, two radar band sensors could share the same aperture area. And sensor radiators and solar collectors could share the same area, possibly in the envelope skin.

On top of all this, DARPA wants a small logistics trail. A future regenerative power system utilizing fuel cells will help achieve this. ISIS must, the agency says, be deployable to stations where there are no nearby airports.

Relay Mirror

The high-altitude airship being designed for the Missile Defense Agency also could hoist complex optics, or "mirrors," to relay high-power laser energy from airborne or surface-based sources over the horizon, directing the beams at enemy ballistic or cruise missiles. The Air Force Research Lab's (AFRL's) Directed Energy Directorate is developing mirror relay technology, and plans an initial, ground-based feasibility test in 2005. The project requires complex optics and algorithms for acquiring, tracking and pointing the laser beam.

The job will require controlling the beam to "sub-arc-second" accuracy, says DonWashburn, AFRL's relay mirror program manager. "We've never done this relay problem before," he says, "where you have two apertures, and you're pointing them in different directions." The dynamics are difficult, as one aperture is trying to maintain line-of-sight contact with the laser source-possibly an airborne laser moving at 656 feet (200 meters) a second-while the other is pointing to a target. Another problem is how to maximize the power that can be projected from the energy source to the receive aperture on the relay platform, when dealing with atmospheric distortions.

Originally, the airship platform was viewed as a stepping stone to space-and the use of satellite-based relay mirrors. But the high-altitude airship turns out to be a good platform. At 70,000 feet and higher, the HAA is geostationary; it's also survivable and designed to kill missiles. Washburn believes the equipment eventually can be "light-weighted," to come under the HAA's 4,000-pound (1,814-kg) weight limit.

AFRL is building a subscale demonstrator, known as ARMS-for aerospace relay mirror system-to reduce risk and demonstrate critical technologies. In the mid-2005 experiment, the ARMS apparatus, suspended from a crane, will use two 29.6-inch (75-cm) apertures to redirect a low-power beam from a ground source to illuminate a target board.

Now Flying

In South Korea a high-altitude airship (HAA) already is flying. Like the United States, South Korea can justify a dose of high-altitude watchfulness and therefore is flight testing a technology demonstrator for an electrically powered stratospheric airship. The Korea Aerospace Research Institute (KARI) is aiming for a platform of similar size to that intended by Lockheed Martin, but decided to reduce risk by building a smaller-scale craft first. The Worldwide Aeros Corp., one of the three prime contractors that undertook Phase 1 of the U.S. program, designed and built the airship, which is 162 feet (50 meters) long and 40 feet (12 meters) in diameter. In Phase 2 of its four-year HAA program, KARI will construct the full-sized craft. The Koreans' main objective is to achieve effective wide area surveillance, but they also see potential non-military applications for HAAs, including commercial wireless communications and mapping.

Fred Edworthy, Worldwide Aeros' vice president-programs, tells Avionics Magazine that his company also has put together a command-and-control, station-keeping and telemetry package for the Korean reduced-scale HAA. This is based on its integrated subsystem's station-keeping technology (SKT), initially developed to be part of an all-digital flight control system for Worldwide Aeros' candidate for the U.S. Missile Defense Agency.

Within SKT, two modified GPS units, with vertical component processing and inertial sensor augmentation, determine the airship's position in three dimensions. A digital flight control computer makes the best determination of position from these sources and calculates the degree and direction of any displacement. It sends appropriate corrective signals to the controllers for the airship's electric motor drives, which then maneuver the airship back to its correct station. The system is engineered to minimize power use.

Worldwide Aeros says that airships, as well as constituting effective surveillance platforms, can fulfil real-time targeting, electronic warfare, battle management, and command-and-control roles. Potential civil applications include photographic survey, resource management, pollution and heat loss monitoring, urban planning and wildlife management. Future surveillance payloads will, it says, include such sensors as stabilized optical, infrared with long and short wavelengths, conventional and millimeter wave radar, plus airborne thematic mapper, ultraviolet scanner and data link systems.

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