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Ballooning eyes: Future is innovative air ships for low cost operations
Aerostat radars work as a type of micro-satellite between the ground and the orbit. Once it is placed in the middle of atmosphere it can track everything that comes within its range. It is proven that putting aerostat radars in the sky is cost effective, yet it can enhance operational awareness by many folds when normal radar requires plenty of operational parameters to accomplish mission requirement. But still some military commanders fear that in case of bad weather and night operations it may not be as effective as electronic radars which can be handy for greater operational flexibility.

As the military technology makes rapid progress in enhancing operational flexibility, the cost effectiveness of a weapon or platform rules the day and attracts attention of the military planners who always look for cost cutting without compromising the operational readiness of their forces.

Thus, aerostat radars are emerging as a choice of new generation military surveillance platform which can deliver accurate and timely data without involving much operational costs.   

The era of aerostat radars began in the 70s and died shortly after due to clash of ideas and various project unfeasibility but now it has bounced back once again as most militaries are struggling to keep their eyes afloat in an environment of cost cutting and low budgetary allocation for routine surveillance and reconnaissance missions.

Many countries are now using aerostat radars as first line of surveillance. India which has bought Israeli aerostat radars EL/M-2083 four years ago is planning to induct more such platforms which can have a surveillance range up to 500 km and have radio interception capabilities of greater magnitude.

The aerostat radars have proved their efficiency in many occasions. One such example is when Kuwaiti Air Defence Brigade first noticed massive Iraqi troops are coming towards the border in 1990 in a surprise move by Saddam Hussein they launched emergency evacuation of their leaders to safer places.  

Similarly, India is now thinking of buying more and more such platforms after the Mumbai attack to keep a close watch on the sea route to India and policing the vast coastlines to prevent any such terror attacks.

US taking lead

The US DoD is showing interest in two categories of airships - those that can carry large cargo at low altitude, exemplified by the Defense Advanced Research Projects Agency (DARPA) Walrus program, and those that can operate in high-altitude low- wind conditions and remain on station for long periods of time. The configuration of the Skunks Works ship indicates it is the former-a hybrid heavy-load carrier.

The interest is across the services and the notional applications are diverse, ranging from logistics - delivery of an integrated fighting unit within theater, for example - to sensor, communications and even laser-weapon relay platforms.

But airships are not getting popular despite having potentiality. Major unresolved issues could derail the airship dream, such as their traditional delicate ground handling, and possibly prohibitive economics and vulnerability. These issues have been debated endlessly on paper, and now Lockheed Martin, a prime airship proponent, is investing to seek real answers.

A hybrid airship derives most of its lift by being filled with a lighter-than-air gas such as helium. Overall, it is heavier than air and gains the final 20 per cent or so of lift by flying like an aircraft, but with slow takeoff and landing speeds that allow operations from short unprepared strips.

New projects

The Skunk Works made the first flight of its P-791 test-bed recently at its facility on the Palmdale Air Force Plant 42 airport. The manned flight was about a 5-minute circuit around the airport in the morning and appeared to be successful. The company did not announce or want to discuss the flight.

The P-791 is not part of a government contract, but rather an independent research and development project by the Skunk Works to better understand airship capabilities and technologies, such as materials, a company official says. However, it may also be a quarter-scale prototype of a heavy-lifter.

To gain more span to act like a wing, the P-791 is three pressurized lobes joined together. An observer of the first flight says it was about the size of three Fuji blimps blended together. The Fuji blimp, a Skyship 600 model, is 206 ft long. That suggests the P-791 would have a gross lift of roughly 3-5 tons.

The craft performed very tight 360-degree turns while taxiing. It made a brief takeoff roll, climbed to a low altitude, made a few banks-including a long sweeping turn-then came back and landed.

The speed of the testbed was estimated at about 20 kt. A full-scale version would be able to go much faster over 100 kt. Lockheed Martin has long proposed a large transport airship, at one time called the Aerocraft.

Hybrid airships have a long history. The Aereon Corporation in New Jersey started experiments in the late 1950s, but they were small scale. The company tested the ‘deltoid aero-body’ shape, also called a deltoid pumpkinseed, with a 1,200-lb manned demonstrator in 1970-71.

That was followed by several studies funded by the military at less than $1 million. In the UK, the Advanced Technologies Group built a 40 ft long unmanned SkyKitten hybrid airship and flew it in 2000. Nothing in the field has progressed to the size or apparent sophistication of the Skunk Works test-bed.

The P-791 uses four air cushions as landing gear, located on the outer lobes. Taxiing the vehicle could be like flying a hovercraft, except one with greater exposure to winds. An advantage of the air cushions is they could be reversed to suck the aircraft onto the ground to resist winds for cargo operations.

Air pressure may also be the best way to spread landing loads into the inflatable structure. It is not clear if there are any devices, such as wheels, to keep the airship from sliding sideways when taxiing in crosswinds. The craft has a special towing system.

Ground handling is a major issue facing hybrid airships. Conventional lighter-than-aircraft require large ground crews and, because they are especially sensitive to winds on the ground, the airstrip is an area ripe for accidents.

Hybrids are only slightly heavier than air, and a hybrid must show large improvements in ground handling over a standard blimp to be successful. The P-791’s current limits are to remain in the hangar if winds are above 5 kt, and there is a 10-kt. limit for taxiing and flight.

The P-791 appears to have four propellers - two at the tail and two on the sides. The tail units appear to be able to pivot for yaw vectoring, and it’s unclear if the ones on the sides can move. There are four vectored propulsions used for ground handling, but it is not clear if these are the main propellers, or separate units perhaps connected with the air cushion system.

The rings around the motors may be shrouds for the propellers and rings for vectoring. Vectored thrust can be useful for lighter-than-air blimps, which lose conventional control authority as they approach zero airspeed while landing, but a hybrid airship lands with some airspeed that may keep the tail control surfaces effective. But for control during low-speed air cushion taxiing, vectoring would seem essential.

The P-791 appears similar to the proposed full-scale version of the British SkyKitten, called the SkyCat. They have similar overall shapes - though the Skunk Works design is wider - and similar propulsion layouts and both use air cushion landing gear. Perhaps the two programs have people in common.

Model

One of the Coalition Forces’ most valuable assets in its new age war in Afghanistan has a distinctively old school look about it. Aerostats-similar to blimps and hot air balloons that have been used by militaries for more than 100 years-equipped with high powered video surveillance capability.

An airship flies primarily by Archimedes’s principle, which describes the buoyancy of a body submerged in a denser fluid. That is, an airship operates more like a submarine than an airplane or a helicopter.

Those aircraft have to generate 100 percent of their lift from the flow of air over their wings or rotor blades. An airship, however, employs a lighter-than-air nonflammable gas such as helium to give it buoyancy. When the lifting gas displaces a volume of air that weighs more than the entire airship (including fuel and payload), the airship floats.

That resultant lift is what’s known as the airship’s static buoyancy. For instance, to lift 1 kilogram at sea level, the airship needs approximately 1 cubic meter of helium gas. Airships weigh considerably more than that, of course, the Skybus that recently flew in Maine tipped the scales at 1600 kg unfilled.

In appearance, the Skybus 80K bears the same oblong shape as the Goodyear Blimp, and it’s based on the same flight principles that have governed airships since the 1800s. But this airship, one of a number of commercial and military vehicles now under development, represents a distinct break from tradition.

Unlike their dirigible cousins of past centuries, these new vehicles are being designed to lift heavy payloads, remain aloft for weeks or even months at a time, and fly without pilots - all while expending far less energy than a conventional airplane or unmanned aerial vehicle.

The Predator UAV, for instance, can carry a payload of 340 kilograms on a typical mission of up to 40 hours. SAIC’s Skybus 1500E pilot-optional airship is being designed to carry a payload three times that size and stay aloft for up to 21 days.

The renewed investment in airships comes at a time when the energy footprint of all modes of transportation is being scrutinized. Some aviation visionaries now argue that one can not continue using exclusively petroleum-based fuels to power aircraft.

Such concerns have prompted new research into jet bio-fuels and energy-efficient jet engines. “We have also begun to understand that not every flight has to be made at eight-tenths the speed of sound. For certain tasks, airplanes just can not compete with airships,” said a Pentagon official.

Development

The Heli-Stat was an airship/helicopter hybrid built in New Jersey in 1986.

The Aereon was a hybrid aerostatic/aerodynamic craft built in the 1970s.

The Cyclocrane was a hybrid aerostatic/rotorcraft in which the entire airship envelope rotated along its longitudinal axis.

CL160 was a very large semi-rigid airship to be built in Germany by the start-up Cargolifter, but funding ran out in 2002 after a massive hangar was built. The hangar, built just outside Berlin, has since been converted into a resort called ‘Tropical Islands’.

In 2005, there was a short-lived project focused on long distance and heavy lift was the WALRUS HULA sponsored by the US DoD. The primary goal of the research program was to determine the feasibility of building an airship capable of carrying 500 short tons (450 metric tons) of payload a distance of 12,000 miles (20,000 km) and land on an unimproved location without the use of external ballast or ground equipment.

In 2005, two contractors, Lockheed-Martin and US Aeros Airships were each awarded approximately $3 million to do feasibility studies of designs for WALRUS. In late March of 2006, DARPA announced the termination of work on WALRUS after completion of the current Phase I contracts.

But the return of great airships remains uncertain. The DARPA Walrus was cancelled in 2006, though the agency did hand out some cash to Pasternak to demonstrate his COSH gear.

Design

Modern airship designers are targeting two pressing needs: intelligence, surveillance, and reconnaissance missions and the transporting of multi-ton payloads to locations unreachable by conventional transport.

For example, airships are ideal for continuously monitoring sites where improvised explosive devices or rocket launchers may be deployed. They also excel at scanning for distant airborne threats.

That is why, in June, the US Army awarded a US $517 million contract to Northrop Grumman and British firm Hybrid Air Vehicles to build three airships, each as long as a football field, to monitor trouble spots in Afghanistan.

Cargo airships, meanwhile, are especially attractive for places that have poor roads and for remote regions that have no roads at all.

The lifting gas is contained within the airship’s outer skin, a large fabric bag or envelope that is aerodynamic, lightweight, and rugged. Inside the envelope are one or more smaller bags, called ballonets, which hold ordinary air.

On the ground, electric fans pump air into the ballonets until the pressure of the helium surrounding the ballonets exceeds atmospheric pressure by a very slight margin of about 480 pascals.

The ballonets occupy between 25 and 50 percent of the airship’s total gas volume. Bleeding off a measured amount of air through valves in the ballonets provides room inside the envelope for the helium to expand as the ship rises.

As the airship ascends, the decreasing atmospheric pressure causes the helium inside the airship to expand steadily. Once all the air in the ballonets is gone, the airship cannot ascend higher without either bursting or venting its helium.

This point is known as the airship’s pressure altitude. To descend, the airship uses its electric fans to blow air back into the ballonets. This gas-management system must constantly keep the helium at a pressure that’s slightly higher than the surrounding atmosphere, to preserve the aerodynamic shape of the envelope.

If ascending and descending were all an airship did, this combination of gases and ballonets would be sufficient. But an airship also needs a certain amount of power and propulsion, to run the onboard navigation and communications systems and any electronics in the payload, and to maneuver to different locations.

Most current airships use traditional gasoline engines, but increasingly designers are looking at alternative power and propulsion systems. One idea is a regenerative system incorporating photovoltaic and fuel cells, in which hydrogen fuel cells produce water vapor.

The solar power could be used to separate the water back into its component gases, the hydrogen would then be fed back into the fuel cells.

Almost all airships flying today are of a non-rigid design, which means the ship’s shape comes only from the pressure of the gases inside. By contrast, the giant airships of the 1930s, the Hindenburg being the most iconic example, had rigid internal skeletons made of aluminum or wood.

Inside this cage were a dozen or more gas-filled lifting bags. Those days also saw the development of semi-rigid designs, which typically had a stout aluminum keel running lengthwise from the nose to the tail, providing a convenient mounting point for the individual gas cells and distributing the lift of each cell evenly.

The only semi-rigid airships flying today are the Zeppelin NT series, which began operations in the late 1990s and are used primarily for sightseeing and advertising. The SkyHook airship, a joint project of Boeing and the Canadian company SkyHook International, is a rotary-airship hybrid that will haul oil-drilling equipment in northern Canada.

Although non-rigid airships aren’t weighed down by an internal framework, they still have to support the gases, fabric, and other components, as well as any payload.

Obviously, the greater the airship’s weight, the larger the volume of lifting gas needed and the bigger the envelope size. As the size increases, so does the vehicle’s surface area and consequently the amount of aerodynamic drag during flight. These and other factors dictate the amount of power required to propel the airship through the sky.

Although people pilot most of today’s airships, the newer designs are increasingly pilot optional, meaning that a crew can fly them during tests or initial deployments and then quickly switch them to remote operation.

Several fully remotely operated airships are also in development. One of their chief uses right now is for battlefield surveillance. These airships carry various imagers and detectors to altitudes of 1500 to 5500 meters on missions lasting 24 hours or more.

Guardian Flight Systems, based in North Carolina, developed the pilot-optional Polar 400 for the US DoD. In the fully pilotless category is SAIC’s Skybus 80K airship, which so far has conducted more than 62 hours of flight tests in Maine.

To date, the Skybus 80K is the only unmanned airship to hold an experimental designation from the US Federal Aviation Administration. It has a gas volume of 2300 m3 (80 000 cubic feet) and is designed to carry a 230-kg surveillance payload as high as 3000 meters for up to 24 hours.

More ambitious is the US Army’s Long Endurance Multi-Intelligence Vehicle. The LEMV will carry 1100-kg payload up to 6000 meters for as long as 21 days without refueling. Its first deployment is to be in Afghanistan in late 2011 or early 2012.

A number of defence companies considered vying for the LEMV contract. But few months ago, the five-year contract - one of the largest airship contracts to be awarded since World War II - went to Northrop Grumman and Hybrid Air Vehicles.

To operate in the thin atmosphere at such high altitudes for extended periods of time, an airship needs to be light (at least compared with lower-flying counterparts) and have an efficient propulsion system that can function with little or no oxygen.

Also essential is a design that minimizes aerodynamic drag, which is why high-altitude airships almost always have the familiar ellipsoidal shape.

Among the power sources being considered for high-altitude airships are electric motors coupled with lithium-ion batteries, hydrogen fuel cells, and flexible-film photovoltaic, which would blanket the upper parts of the airship’s huge surface. Any of these options would need to weigh less and be more efficient than standard engines.

Future programs

To fly even higher and longer with heavier sensor payloads is the ultimate goal of military leaders who see the modern airship as an unblinking, ever-present eye in the sky.

Under the DoD’s $149 million High Altitude Airship-program, Lockheed’s Maritime Systems & Sensors Division in Akron, Ohio, is now exploring ways to build an airship capable of carrying a 230-kg sensor package into the stratosphere, as much as 18 kilometers up, where it would remain for a month at a time.

At that altitude, one airship would be able to monitor a patch of ground 1200 km across. Just 11 of them could provide radar coverage of the coastal and southern borders of the continental United States, according to the North American Aerospace Defense Command.

If that sounds ambitious, consider the proposed high-altitude airship known as the Integrated Sensor Is Structure, or ISIS.

Under this $400 million program jointly funded by the DARPA and USAF, Lockheed’s Skunk Works is building an unmanned stratospheric airship powered by solar cells and fuel cells that would be capable of operating at 21 kilometers’ altitude for up to 10 years at a time. A one-third-scale prototype, itself longer than a football field, is scheduled to fly in 2013.

What makes ISIS unique is the integration of its mission sensors-a UHF radar for monitoring vehicles and soldiers on the ground and an X-band radar for tracking cruise missiles up to 600 km away-into the body of the airship.

According to Raytheon, which is building the radars, the radar antennas form a cylinder in the center of the airship. By integrating the sensor system into the structural supports, the design reduces the airship’s overall weight and adds structural stiffness.

Even so, the demands of a 10-year high-altitude mission mean that the full-scale ISIS will need to be made of extremely durable, yet lightweight materials - materials that may not yet exist.

In addition, its power system will need advanced photovoltaic and fuel cells capable of generating enough power to operate the radars, navigation system, communications gear, and the electric motors that will turn the airship’s giant propellers. A lot of extreme engineering is going into today’s airship designs.

While the upcoming stratospheric surveillance airships will carry relatively small payloads, some airships now in development will lift a great deal more - payloads of hundreds of tons, albeit at lower altitudes. That presents an entirely different set of challenges.

An airship designed to carry 50 metric tons of cargo would be hundreds of meters long and weigh tens of tons lying empty of helium on the factory floor. The sheer size would make its assembly a daunting task. These new vehicles would likely be built in smaller subsections that would later be joined together in immense hangars.

A more critical issue is how to compensate for the sudden increase in the airship’s static lift that occurs when a heavy payload is unloaded. The most straightforward remedy is to add onto the airship an amount of weight equal to the payload as the payload is removed.

Some heavy-lift designers are also developing hybrid vehicles. These incorporate the static lift of helium along with some form of dynamic lift, such as helicopter-style rotors or airplane-like wings.

In most of these designs, the helium is sufficient to lift the vehicle’s weight, while the dynamic lift is devoted to the payload’s weight. This produces an aircraft that is slightly heavier than air and so is much less buoyant during cargo unloading.

While these hybrids hold promise, they also have some inherent technical challenges. For one, the additional dynamic lift increases aerodynamic drag. To help with generating dynamic lift, they also typically have a flatter profile than conventional airships, but this shape gives them a higher ratio of envelope fabric to gas volume, increasing the airship’s empty weight.

Higher weight and drag, of course, mean more propulsive power and more fuel, both of which make the ship even heavier. And some hybrids employ multiple lobes in their design, which can create problems as the gases inside heat up from the sun’s rays.

Helium conducts heat six times as efficiently as air, so a multi-lobed hybrid may tend to list toward the side that is not exposed to the sun.