Too high for planes, too low for satellites. Until now. The stratosphere is about to get busy. Often called the ‘ignore-o-sphere’, the slice of Earth’s atmosphere sandwiched between 63,000 feet and the edge of space at 300,000 feet, has presented almost insurmountable challenges to aircraft designers.
The stratosphere is almost a no-fly zone. The air is too thin to support flight by most conventional aircraft but it is too dense for satellites to stay in orbit. Rockets blast through it to reach orbit and weather balloons drift at the mercy of the wind but only a handful of exotic aircraft ever operated in it including the U2 and the SR-71 Blackbird.
However, it is attracting new interest. The military are looking for cheaper alternatives to surveillance and communications satellites. Commercial organisations want to launch base stations in the sky to provide internet and cell phone coverage in remote areas.
New technologies, especially in fabrics, batteries, fuel cells and photo-voltaic cells, have allowed a new generation of engineers to dream up high altitude airships, unmanned planes and even stratospheric soccer balls.
What does this mean for us on the ground? Broadband internet in the outback, cell phones for Africa, missile defence, better pollution monitoring, better communications for soldiers and, perhaps, one day, holidays at 140,000 feet.
The high frontier
“Near-space has been a cultural blind spot – too high up for aircraft, but too low for satellites,” says Gen. John Jumper, USAF Chief of Staff. However, much of the renewed interest in the stratosphere has come from the military. “The ability to persist over a single geographical area is the holy grail of any kind of communications device,” says Col. Patrick Rhodes, Commander of the USAF’s Space Battle Lab. Today “we have to go out to multi-gazillion dollar geostationary satellites to achieve that.” However, this technology isn’t going to be cheap and it isn’t going to come overnight. The Global Hawk is flying today in limited numbers, but the rest of the vehicles on this page won’t get airborne for years or even decades.
Buck Rogers Blimp
Lockheed Martin’s helium-filled High Altitude Airship will measure 500 feet long and have a volume of 5.2 million cubic feet. Keeping the behemoth aloft for six to twelve months requires breakthroughs in fabric design for the skin and fuel cells and thin-film photovoltaics for power. “It’s a difficult problem otherwise we’d have done it 40 years ago,” says Lockheed’s Ron Browning.
The company has been working on the design for the Missile Defence Agency and a construction contract is expected later this year. A prototype might fly as early as 2008. If it is successful, the MDA plans to launch a dozen production models to guard the coasts of America against ship-launched cruise missiles.
Holiday at 140,000 feet
“We’re the slowest space program in history,” says John Powell, founder and CEO of JP Aerospace. The Sacramento, CA company is small in size but big in ambition. Powell likes slow. Slow is good. The company is the unfolding of a high school ambition to reach orbit by airship and the program has seen 86 test flights to date. He reckons it’ll take another seven years (and a cool $200m) to achieve orbit.
Speed is important in another way. Instead of today’s minutes-long rocket powered blast-offs, Powell envisages something like an ocean voyage to space. An ‘Ascender’ airship (a prototype has already flown) will carry stratonauts to a lighter-than-air ‘Dark Sky Station’ at 140,000 feet where they will transship to a mile-wide airship that will accelerate them to orbit over the course of a week. He jokes that a problem during the ascent will not require a split second abort decision. Instead the crew will sit down and discuss it over lunch. The Dark Sky Station will eventually house 50 people in shirtsleeve comfort and Powell predicts that it will be a popular tourist destination.
Around the world by football
Hokan Colting, CEO of 21st Century Airships, believes that high altitude airships should be spherical. “The cigar shapes work well at ground level but not much R&D has gone into airships in the last 50-odd years,” he says. In favor of round airships: helium expansion means that a balloon will expand over 50 times as it rises from the ground to 60,000 feet. A round shape means that the weight of the payload is always evenly distributed. It also gives the smallest surface area for a given volume, which reduces leakage. Most important: it looks like a soccer ball.
His company has already built the largest soccer ball in the world, a spherical airship prototype. At 57 feet in diameter it is a demonstration model for a much bigger airship that he hopes to build ready to circumnavigate the world next year. The projected record-breaker will be 120 feet in diameter. A 747 would fit inside the shell. He hopes that the soccer ball blazon will ease airspace restrictions and attract sponsorship: “it’s the most popular sport in the world,” he says.
Space battle lab
As if throwing down the gauntlet to Roswell conspiricists, The US Air Force launched a series of a high altitude balloons this summer in the Arizona desert. In a project called Combat SkySat, the Air Force’s Space Battle Lab (cool name) hopes to demonstrate that off-the-shelf technology can be used to relay communications over long distances. The PRC148 radio used in the experiment has a normal range of 7-10 miles. By hoisting a pair of them into the stratosphere, they extended the range to 400 miles. In one experiment a forward air controller was able to talk to an A-10 pilot while he was still on the runway. This capability would be ideal for operations in Afghanistan where mountains routinely block radio communications.
Perpetual motion machine
The Mercator, an uncrewed plane with a 16m wingspan, will cruise the stratosphere for weeks or months at a stately 20-30 knots. Made out of carbon fibre, it is powered by solar cells. During the day they drive the two propellers and charge batteries for night operations. A 40 percent scale model is already flying on a weekly basis at low altitudes. The designers, a British firm named QinetiQ, also make satellites. The Mercator will offer similar capabilities at a fraction of the cost. “What we’re aiming for is perpetual self-sustaining flight,” QinetiQ’s Paul Davey.
File server in the sky
The Global Hawk is a veteran of the wars in Iraq and Afghanistan. Like the fabled U2, it can prowl the stratosphere taking images with infrared, electro-optical sensors or radar. Unlike the U2, it is unmanned and can stay aloft for more than 32 hours at a stretch. It is launched with a single mouse click and the plane carries out the entire flight autonomously.
A recent innovation by Northrop Grumman, makers of the Global Hawk, puts a file server and military radio on board. This allows front line grunts to call up images from the plane in real time using a PDA connected to a radio modem. “It’s just like MapQuest,” says the manufacturer.
Student blimp
Three Johns Hopkins undergraduates, Nick Keim, Benjamin Jackson and Mike Chin, built an 18-foot long prototype blimp. It never reached the stratosphere. In fact, it barely reached its design ceiling of 250 feet. However, it tested technologies that will be used by a much larger airship being developed by the university’s Applied Physics Lab. They plan to shoot the High-Altitude Reconnaissance Vehicle (HARVe) to 300,000 feet in a rocket and let it inflate as it descends to 100,000 feet. This cuts out the dangers of the ascent through the turbulent troposphere (that’s the bit of the atmosphere with weather in it). As one of the few people who have designed and built a blimp since the war, student Nick Keim is well-placed to offer advice to other companies: “Plan and test. Plan every single part. Test every single piece.”
Technology challenges
Design
The stratosphere pushes conventional aircraft design to the very limits. The thin atmosphere means that they need a large wingspan. This presents pilots with a tricky problem. For example, fly a U2 too fast (130 knots indicated) and the plane gets into Mach tuck and the wings fall off. Fly too slow (110 knots) and the plane stalls and the wings fall off. Pilots call it the coffin corner. The higher you go the narrower the range becomes until, eventually, conventional flight is impossible.
Human physiology
If it’s tough on planes, it’s even tougher on pilots. Medically and physiologically, you are actually in space above 63,000 feet. At this altitude a naked human would decompress: gas in the ears, sinuses, lungs and bowels would expand in a deeply unpleasant way. Then your skin would start to freeze in the ambient temperature of -55 to 63 degrees Centigrade. Water in your blood would begin to boil. Finally after about six seconds, your brain would stop working because of hypoxia. Death would occur about three minutes later. This means that you don’t have to worry about radiation; which is 70 times the level you get on the ground. No wonder most of the new stratospheric vehicles are not designed for human occupation.
Weather
But the weather is nice. Average winds are 15-25 knots. With the low pressure this is equates to 1-2 knots at sea level (except in some places like Afghanistan and Korea where mountain wave turbulence can make even the stratosphere sporty). And the view, looking twelve miles straight down, is fantastic providing you don’t suffer from vertigo.
Power systems
Advances in three separate areas make these vehicles possible. First, batteries. Driven by the cell phone and laptop market, lithium-ion batteries have become lighter and more capable. Second, power cell technology has improved at a similar rate, driven by car and other applications. Finally, solar cells have become much more efficient and lighter.
“If you go back 12 months you wouldn’t have had those technologies and we’re still on the edge,” says QinetiQ’s Andrew Rogoyski. The Mercator and High Altitude Airship prototypes can operate using today’s technology but they will need another 2-5 years’ development if they are to achieve their long-term aims of virtually unlimited endurance.