Skip to comments.Held aloft by hydrogen
Posted on 09/20/2005 4:28:19 PM PDT by Zuben Elgenubi
Held aloft by hydrogen
From The Economist print edition
Aviation: An unmanned aircraft powered by hydrogen has taken to the air for the first time. Such aircraft could have both military and civilian uses
BLAME it on the Hindenburg. Ever since the hydrogen-filled German airship exploded in 1937, hydrogen and aviation have not had much to do with each other. But a new aircraft, which made its first flight in May, could change that. Global Observer, the world's first liquid-hydrogen powered, unmanned-aerial vehicle (UAV), is not an airship, but an aeroplane that uses hydrogen as a fuel. It was built by AeroVironment, a firm based in Monrovia, California, which announced at a UAV conference in June that Global Observer had made its first flight above the desert sands of Yuma, Arizona.
Having pioneered solar-powered flight in the 1990s with America's space agency, NASA, AeroVironment has since refocused its efforts towards fuelled UAVs. Solar-powered UAVs promise potentially unlimited hours in the sky, but they have limitations. At latitudes above 45°, and during the winter, the sun is too weak to keep the aircraft flying. Liquid hydrogen is an attractive fuel for a high-altitude, long-endurance vehicle, since it has around three times as much energy per unit of mass as conventional jet fuel. The difficulty is keeping it liquid, since hydrogen boils at -253°C. Global Observer has a special insulated storage tank that can hold enough liquid hydrogen to keep the plane aloft for 24 hours. The hydrogen is combined with oxygen from the air in a fuel cell, producing electricity that drives the aircraft's propellers and powers its onboard systems. The aircraft is a scaled-down version of a much larger aircraft that would be able to stay aloft for more than a week.
Such an aircraft would have a number of uses as a low-cost alternative to satellites. Equipped with wireless-broadband relays, imaging equipment or monitoring devices, it could be used for surveillance, weather monitoring and telecommunications. In 2002, AeroVironment demonstrated the use of a solar-powered UAV as a mobile-phone relay and a TV broadcasting platform, in conjunction with NEC, Toshiba and other Japanese firms.
Ted Wierzbanowski, AeroVironment's managing director, admits that many people were sceptical about the switch to hydrogen. Building the prototype was the only way to put the hydrogen bogeyman to bed, he says. We think this is a big paradigm shift in the aerospace and space industry. The prototype aircraft has a 15-metre wingspan, but he expects two production versions, with 46-metre and 76-metre wingspans. Loitering at altitudes of between 18,000 and 21,000 metres, they would work in threes: as one plane started to run out of fuel, another would take off, to ensure continuous broadband or TV coverage.
But several challenges remain. One of the biggest is reliability, according to John Del Frate, the former manager of NASA's Environmental Research Aircraft and Sensor Technology project. Most commercial aircraft have to be inspected every 100 or 200 hours, he says. In one flight of these hydrogen and solar-powered vehicles, we'll easily exceed that amount. Worse, the aircraft must be able to withstand ultraviolet radiation, freezing temperatures and high winds, says Mr Del Frate, though AeroVironment claims to have addressed these problems.
Then there is the small matter of convincing potential buyers, such as Colonel Patrick Rhodes, the commander in charge of America's Air Force Space Command Battlelab, which tests new technologies for the military. For the moment, he says, hydrogen-powered lighter-than-air and heavier-than-air vehicles that will provide significant persistence over an area are not there yet. But Major-General Douglas Fraser, director of air and space operations for Air Force Space Command, is optimistic that the new UAVs will eventually prove themselves.
The new aircraft also faces competition from rival technologies, such as the Stratellite, a helium-filled airship being developed as a communications relay by GlobeTel Communications, a firm based in Florida. Mr Wierzbanowski reckons his approach has lower running costs, but airships can lift larger payloads. So there may be room in the stratosphere for both technologies.
While liquid hydrogen as a fuel might have three times the energy content of hydrocarbon fuels per unit of weight, its density is one tenth of the hydrocarbons (0.07 vs. 0.7g/ml) - i.e. the energy content per unit of volume is 30% that of hydrocarbon fuels. Add to it bulky cryogenic storage tanks - and the spatial requirements of liquid hydrogen as a fuel become problematic, unless the amounts used are very large, like in space shuttle.
If an aircraft were not fueled until immediately before deployment, could not the vaporisation of fuel as it is removed from the tank be used to keep the rest of it cool?
You are soooooooo bad!!!
But, I must give "props" to the manatee.
I like Hydrogen as a fuel. You combine it with other Hydrogen atoms to make Helium.
Oh wait. the greenies don't like that method.
Theoretically yes - but hydrogen heat of vaporization is not high, and its boiling temperature is very low (i.e. you have large heat gradient from outside air and large heat flows) - which means that you will still need to drastically curtail heat flow into your liquid hydrogen tank, or the natural evaporation will keep it cold during the few minutes it will take to evaporate dry. If you tank is of gigantic size, then it becomes less important (surface/volume ratio drops; heat flow is through the surface, and you'll have a lot of volume to evaporate). Also, without thermal insulation you will immediately start condensing atmospheric moisture and will be all covered with heavy hoarfrost (now, what was your takeoff weight?).
Can you blame them? It makes their voices sound like Munchkins.
OH, the humanity!!!!!
Density and cryo storage are the reasons why fuel cells are more practical. Extracting a hydrogen atoms from alcohol or gasoline is way more practical than compressing Hydrogen into the weight and volume necessary to propel a vehicle.
I haven't looked in a while but I thought peroxide plus hydrazine had a nice energy density as well as no carbon. Only problem is it's toxicity to humans (not really a concern for greenies since it's combustion artifacts are very green).
It's a simple question of weight ratios. A 5 oz aluminum bird can't carry a 2 pound hydrogen coconut.
LOL! (Cleaning coffee off screen and keyboard.)
LOL! I almost sh_t myself. :D
Man that is sick! Mind if I steal a copy? Blackbird.
Suppose two birds carried the coconut together...
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