WHEN American TV networks decided to broadcast the Baseball World Series to Cuba for the first time, Sputnik was still two years off and the island wasn鈥檛 within the range of any broadcasting tower. So engineers installed antennas on a DC-3 plane, set it circling over the Straits of Florida and relayed the broadcast to Havana. Cuban fans watched as their local hero, left fielder Sandy Amoros, made a brilliant running catch to give the underdog Brooklyn Dodgers victory over the mighty New York Yankees in the 1955 series.
Forty-five years on, nobody uses planes to broadcast television pictures any more. And why should they? Communications satellites orbiting 22 000 miles above the equator do the job perfectly well. Planes, by comparison, seem useless. They can stay airborne for only a limited time. They have to fly fast to generate lift, which burns copious amounts of fuel. Worst of all, they are easily thrown off course by winds and clouds, so their communications gear swings about wildly instead of pointing precisely at customers on the ground.
But after June 2003 that could all change, thanks to a remote-controlled plane called Helios that can fly for months at a time on nothing more than sunshine and water. Helios is a vast, flimsy-looking wing, 74 metres across with eight propellers and five small landing pods strung along it like bunting. Yet it will be capable of flying at 60 000 feet (18 kilometres), well beyond the boundary between the inner atmosphere and the stratosphere. At that altitude there is no commercial air traffic, the jet stream鈥檚 ferocious winds all but stop and billowing thunderheads give way to wispy halos of harmless ice particles. By flying in circles at bicycle speed, Helios could 鈥減ark鈥� above a city. Fitted out with communications gear it would become an 鈥渁tmospheric satellite鈥�, capable of beaming e-mail, computer games, Web pages and television programmes to densely populated areas below.
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The idea of atmospheric satellites is nothing new. For years, aerospace engineers have been proposing them as a solution to the problems of communicating via high-orbit satellites. Until recently, the technology hasn鈥檛 been in place to make them a viable option. But now, thanks to NASA and its private sector partner AeroVironment of Simi Valley, California, Helios may be on the verge of solving the problem. And it may lead to unexpected spin-offs. For in Helios, they seem to have created a machine capable of flying forever.
For the time being, NASA and AeroVironment are developing Helios primarily as a competitor to communications satellites. According to NASA, the plane could trump satellites in all sorts of ways. Signals from satellites spread out like cones, which, by the time they reach Earth, cover thousands of square miles of uninhabited oceans, lakes and deserts. Planes would have a much smaller communications footprint. 鈥淲ith Helios, you can focus coverage to the area of interest, instead of most of the uninhabited world,鈥� says John DelFrate, the Helios programme manager at NASA鈥檚 Dryden Flight Research Center in California. That鈥檚 going to be a big advantage as demand for broadband communication capacity explodes.
What鈥檚 more, by loitering just 60 000 feet above a city instead of sitting 22 000 miles above the equator, planes avoid the annoying signal delays that result from bouncing signals to and from space. Craft like Helios can also fly home to make repairs or to install new communications equipment. Best of all, planes are cheap. Launching a high-orbit geosynchronous satellite can cost a billion dollars. A fleet of five Helios planes could cost as little as $10 million.
The idea sounds almost too good to be true and some critics say it is. First and foremost, if something went wrong, there would be no pilot aboard to save the day. 鈥淲hat are the chances of getting Federal Aviation Administration approval to fly an unmanned plane over New York City?鈥� asks Sherwin Beck, a remote sensing specialist at NASA鈥檚 Langley Research Center in Virginia. 鈥淕ood luck. If you lose a vehicle, you鈥檒l spend the rest of your career filling out papers.鈥� What鈥檚 more, flying at 60 000 feet is extremely difficult because the air is only 20 per cent as dense as at sea level. Other stratospheric aircraft, such as the American SR-71 spy plane, conquer that problem by going extremely fast. But in order to behave like a stationary satellite Helios has to fly very slowly-30 kilometres an hour at most.
Aerospace engineer Kirk Flittie, who is AeroVironment鈥檚 Helios programme manager, says these problems can be solved, and his company has laid out an aggressive test programme to prove it. A Helios prototype flew six flights over Edwards Air Force Base in California last November and December, reaching an altitude of 3000 feet, proving that Helios could take off, circle and land by changing the speed of its propellers.
Helios is now back at AeroVironment鈥檚 Simi Valley factory preparing for more ambitious test flights. In the middle of next year, it will take off from Edwards and climb slowly into the stratosphere, settling briefly at 100 000 feet over a dry lake bed before flying back to Earth. If it succeeds, it will prove two things: that Helios can survive the ride through the blustery lower atmosphere, and that it can generate sufficient lift to fly in the ultra-thin stratosphere.
AeroVironment engineers are confident of success. As they point out, Helios is just a larger version of their solar-powered Pathfinder plane (New 杏吧原创, 17 June 1995, p 29), which broke the 80 000-feet (24-kilometre) barrier in 1998. Like Pathfinder, Helios has to climb through the weather-ravaged troposphere using just solar-powered propellers. Pathfinder鈥檚 carbon-composite wing spar and cross-beams survived this potentially rough ride two years ago, as did its solar cells. Not only that, but its flight computer proved capable of controlling the craft autonomously.
The next big test will come in June 2003, when Helios attempts to cruise at 60 000 feet for four days and four nights without refuelling. That would satisfy NASA, but Flittie says the goal is to fly for six months or even longer. No plane has ever achieved anything close to that. 鈥淭his essentially has to be the most reliable plane ever built,鈥� he says.
For long-duration flight, Helios will rely on a device never before tested on an aeroplane. 鈥淲e have to take everything we learned in Pathfinder and add an energy storage system,鈥� says Flittie. Storing energy is critical because Helios鈥檚 propellers have to turn non-stop for months at a time, even at night when the solar cells are idle.
Up all night
Chemical batteries are too heavy for the job. A lithium polymer battery like the one in a palmtop computer, for instance, would have to weigh 600 kilograms to store the energy Helios will need each night. NASA and AeroVironment have set a weight limit of 922 kilograms for the entire plane, which includes 100 kilograms for satellite gear. Only 90 kilograms has been allocated for energy storage. Any more and Helios would have to fly faster than its optimum speed just to stay airborne.
To solve the energy problem, NASA and AeroVironment are sponsoring a competition between two American energy storage companies, Lynntech of College Station, Texas, and Giner of Waltham, Massachusetts. Both are working on lightweight, water-based fuel cell systems that harness the reaction between hydrogen and oxygen to generate electricity. Wire up the fuel cell鈥檚 two terminals and you can deliver electricity to the propellers all night long.
During the day, Helios will generate electrical power using long strips of solar cells taped to the top of its translucent wings. The cells are double-sided, so as well as soaking up direct sunlight, they can absorb light reflected back from the clouds below. Most of the solar energy will go to the propellers, but some will be diverted to an electrolyser-a stack of sponge-like polymer membranes containing 20 litres of water. This device uses the energy to split the water into hydrogen and oxygen gases, which are then separated, pressurised and stored for later use.
At night, the hydrogen and oxygen are fed to a fuel cell where they recombine to regenerate water and release the stored energy (see Diagram). In theory, the cycle is closed and the plane should never run out of water. But preventing leaks is a big challenge. All existing fuel cells, such as those on the space shuttle, eventually use up their store of fuel and have to be recharged. 鈥淲e don鈥檛 have the luxury of pulling up to a filling station. We鈥檙e in service six months a year,鈥� says DelFrate.
Although DelFrate thinks it may be possible to sneak a reserve tank on board, the goal is zero leakage. 鈥淎 level of leakage acceptable for something on the ground might be a couple of hundredths of a per cent of fuel or oxidiser,鈥� says Alan Cisar, an inorganic chemist who leads the AeroVironment fuel cell work at Lynntech. 鈥淗ere it鈥檚 a closed system operating for months. If you have even a trace leakage, it means you鈥檒l run out of gas.鈥�
Preventing leaks is made even harder by the frigid conditions of the stratosphere. If the water inside the fuel cell were to freeze, it would expand and crack the intricate plumbing. And there is no room inside Helios for a heater or even insulation. But DelFrate doesn鈥檛 think they鈥檙e necessary anyway. 鈥淭he fuel cell process produces a fair amount of heat. If we judiciously use that heat, we think we can prevent the problem,鈥� he says.
Not only that, but each fuel cell/electrolyser-there will be two on board- must fit inside one of Helios鈥檚 landing-gear pods, which are just 1.5 metres high and 60 centimetres wide. Shrinking the cells is tricky because it means pressurising the gases to 2.7 megapascals (27 times atmospheric pressure) to keep the storage tanks down to a manageable size.
Helios engineers are not the first to tackle high-pressure cells. Nuclear submarines use electrolysers operating at well over 10 megapascals to strip out oxygen from sea water. But submarine systems are not as heavily constrained by weight or size, so engineers can stack the membranes between heavy metal plates. Helios doesn鈥檛 have this luxury. 鈥淚n our system, the whole object is to get the weight down,鈥� says Cisar. That means the membranes will have to be sandwiched between lightweight carbon-composite end plates. Cisar says that Lynntech ran a self-contained stack at 2.7 megapascals for several days last April. But exactly how they achieved this he will not reveal, for fear of tipping off engineers at rivals Giner, where officials declined even to discuss their work.
Cisar admits that the stack only produced about half the required power, and still relied on metal end plates. Still, everyone was ecstatic that the membranes did not blow up or leak, because when hydrogen and oxygen meet, they ignite easily. In fact, during the high-pressure lab test, engineers surrounded the cell stack with cement walls and sandbags. 鈥淲hen you鈥檝e got gases like that in significant quantities at high pressure, if something went wrong there are lots of ways it could ignite,鈥� Cisar says.
Lynntech鈥檚 next step is to test a full-sized prototype inside an altitude chamber at National Technical Systems鈥� Rye Canyon facility in California, to mimic the pressures, temperatures and winds of the stratosphere. But before that can happen there is plenty of work to do to overcome the problems of operating at high pressure. Since the gases are pressurised, a leak could get out of hand quickly-and these planes will fly without a pilot over densely populated cities.
That鈥檚 why the engineers are planning emergency procedures to assure the Federal Aviation Administration that the planes will not explode and rain debris on cities. Helios will be fitted with sensors to detect leaks, and safety valves to vent gases and turn off the flow. The whole system could be shut down with emergency switches, Cisar says, and Helios would then glide gently back to Earth. The plane also has an emergency parachute.
If they succeed in plugging leaks and keeping the craft鈥檚 water supply from freezing, engineers at NASA and AeroVironment believe they will come closer than anybody to achieving everlasting flight. 鈥淎nything that can fly for months without landing or taking on fuel is pretty much flying continuously,鈥� Flittie says. Cisar, who reckons his fuel cell could run for years without blowing, agrees. 鈥淢aybe five years from now, with a long track record of six-month flights, someone will say, `Let鈥檚 go for a year with Helios鈥�,鈥� he says. 鈥淭hen, who knows?鈥�
Race to the skies
Helios isn鈥檛 the only 鈥渁tmospheric satellite鈥� in development, and it may not be the first to succeed. Scaled Composites, an aerospace technology company based in Mojave, California, has built a piloted plane called Proteus that is on course to beat Helios by at least two years.
Like Helios, Proteus is designed to fly in the stratosphere. A proof-of-concept version has already clocked up hundreds of hours of high-altitude flight, including a journey from California to Paris in 1998. But unlike Helios, Proteus isn鈥檛 designed for long endurance flight. Its current limit is about ten hours, so its developers envisage fleets of planes flying overlapping shifts of eight hours to provide continuous communications coverage.
Proteus differs from Helios in other respects too. Whereas Helios is remote controlled, Proteus is a piloted plane with a crew of two. It is powered by standard jet engines instead of propellers, and flies much faster. Helios鈥檚 optimum speed is a leisurely 30 kilometres an hour. Proteus鈥檚 ground speed is a more conventional 440 km/h.
Even so, the plane is highly innovative. To achieve a ten-hour flight, it has to be lightweight and fuel efficient. Engineers decided on a plastic foam core, less dense even than balsa wood. The plane鈥檚 carbon-composite 鈥渟kin鈥� is just 5 micrometres thick in some places, so a careless technician could step right through the wing. But the payoff is incredible energy efficiency. Proteus burns just 190 litres of jet fuel an hour, a sixth as much as a corporate jet of similar size.
If everything goes to plan, Proteus could be in commercial operation by the middle of 2001. Angel Technologies, a start-up company in St Louis, Missouri, is scouting for funds to buy a fleet of 100 planes for $7.6 million each. Angel鈥檚 goal is to install communications antennas beneath their fuselages and fly them over major US cities to provide broadband Internet services. The planes would fly in a wide, left-banking circle with their hardware pointing at customers on the ground. Marc Arnold, chief executive of Angel, says that Proteus鈥檚 big advantage over Helios is that it is piloted and should easily get approval from the Federal Aviation Administration. But he accepts he has a battle on his hands. 鈥淭his is the new, sub-space race,鈥� he says.
If everything goes to plan, Proteus could be in commercial operation by 2001, two years ahead of Helios.
