Spy Blimps and Heavy Lifters
The latest thing in airships.
- By Ben Iannotta
- Air & Space magazine, September 2007
(Page 2 of 4)
The Army contract specifies that the SA-90 must demonstrate its usefulness by flying at 18,000 to 20,000 feet for up to 24 hours. Aluminum propellers, 18 feet in diameter, will provide maneuvering. Hovering is easy, but engineers want to see if the sphere can fly at 55 mph to keep up with special operations units on the ground. Of course the spheres will never cut through the air as easily as cigars, so engineers are working on a way to compensate. According to company program manager Rick Osmun, Sierra Nevada hopes to use a 10-foot-diameter prototype to show that a cone-shaped “aero tail” attached to the rear of the SA-90 will reduce drag the way the taper of a sailboat’s stern increases speed.
Though the sphere would evade shoulder-fired rockets, a miniature moon hovering over the battlefield could be an easy target for enemy aircraft. Plans call for camouflaging the spheres “air-superiority gray” like U.S. Air Force fighters.
Today, most airship designers have ambitions to reach the stratosphere, 60,000 feet up, which is about 10 times the current average blimp’s maximum altitude. In addition to being safely above commercial air traffic and the winds of the jetstream, the altitude would give customers the ability to stare continuously at the same patch of ground.
Satellites do that now from what is called geostationary orbit, 22,000 miles above Earth. At that altitude, the satellites’ orbit matches the speed of Earth’s rotation. (At a lower altitude, the satellite would orbit faster.) Satellites maintain their orbit around Earth through a balance of two opposing effects: Earth’s gravity and centrifugal force. An airship would balance the force of gravity with aerodynamic lift, buoyancy, or a combination of the two; it could provide 24-hour coverage of a single section of ground but, at an altitude of only 11 miles, with a much closer view than a satellite offers.
New technologies may finally bring the stratosphere within reach. To go to that altitude, airships would need to carry so much helium they’d have to be gigantic; one concept under study by the Pentagon’s Defense Advanced Research Projects Agency could hold a 15-story building. The resulting design challenge sounds like a logic problem: “If you’re bigger, you have more drag, and if you have more drag, you need more propulsion, and if you need more propulsion, you need more energy,” says Ron Browning, director of marketing at Lockheed Martin Defense and Surveillance Systems in Akron, Ohio, a rubber manufacturing center with a long history of balloon and airship fabrication.
Lockheed Martin’s Akron unit might have been a shoo-in to crack the stratosphere were it not for a matter of money. In 2005, the U.S. Missile Defense Agency awarded Lockheed $149 million for construction of a 400-foot-long prototype called the High Altitude Airship. By 2009 a demonstration version was to fly to 60,000 feet and carry a 500-pound payload. An operational version would carry thousands of pounds of sensors to spot hard-to-find cruise missiles.
This year, the Missile Defense Agency announced that budget cuts required it to “eliminate funding for the High Altitude Airship” beyond fiscal year 2007. Lockheed managers are now lobbying members of Congress to restore funding. “We don’t think this program is going to be a dead end,” Browning says. “There are too many positives with it.”
A key to the High Altitude Airship is the hull material. It has to be light so it doesn’t drag the ship down, but it also must be strong to handle the pressure of the helium. (At 60,000 feet, the density of the air is only six percent what it is at sea level, and that low pressure would produce enormous helium expansion.) And because the goal is “persistent” coverage for weeks on end, the material can’t be porous or the helium would escape.
At stratospheric altitudes, there is less wind, but keeping the airship in place will still require a propulsion system. Standard engines are no good because as they burned fuel, the craft would get lighter and slowly rise until the expanding helium split its seams.
Though the prototype will use lithium ion batteries, Lockheed decided that the operational ship should run on solar power. The top and sides of the craft would be covered with photovoltaic cells to convert sunlight into electricity. Extra energy would be stored during the day to keep the craft’s two propellers churning at night.
“The challenge is getting through that first diurnal cycle,” Browning says. “That has not been accomplished before.”