http://blog.wired.com/cars/2008/12/boeing-has-seen.htmlBoeing is betting there’s a big future for hydrogen-fueled aircraft and is developing an unmanned plane that would use liquid hydrogen — the same stuff used in rockets — to reach altitudes topping 60,000 feet and stay there for days at a time.

The plane is one of several alt-fuel aircraft that Boeing, which earlier this year tested a hydrogen fuel cell airplane, is working on. Although it is still a long way from meeting such lofty expectations, Boeing believes a liquid-hydrogen plane, which would have reconnaissance and atmospheric research applications, could solve many of the challenges inherent in designing so-called high-altitude, long endurance (HALE) aircraft.

One of the biggest is the need to carry large quantities of fuel in the wings, which impacts lift because the

wings must be very large relative to fuselage size. Boeing says one possible way to get around this problem is to use solar power, but the technology is not yet up to the task. Airships are another option, but they are susceptible to crosswinds and can be difficult to handle in foul weather.

Such issues make liquid hydrogen a compelling option.

Boeing’s been working on the plane (seen above in a rendering) for more than a year and filed a patent on it in August. The plane uses an internal combustion engine developed by Ford and adapted to burn hydrogen. The company says the engine ran for nearly four days in a test chamber, including three days in conditions that simulated an altitude of 65,000 feet.

Liquid hydrogen (LH2) has long been used in rockets. It is used as a coolant to chill engine nozzles before being mixed with liquid oxygen or another oxidizer and burned as fuel. On the upside, liquid hydrogen is among the lightest fuels and most of its emissions are water vapor. On the downside, liquid hydrogen requires much more volume than other fuels to store the same amount of energy. It takes roughly four liters of LH2 to match the energy content of one liter of jet fuel.

Boeing’s answer to that problem, according to AVweb, is storing the hydrogen in a high-volume fuselage instead of the wings. That optimizes endurance, stability and performance without compromising fuel capacity.

Boeing suggests that the plane, which it believes could fly up to 10 days (longer than that at lower altitudes), could be used for border patrol and other surveillance activities, or as a flying cell phone tower in remote areas.