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A liquid-hydrogen-powered unmanned spy plane from Boeing’s Phantom Works had a very successful test flight earlier this week, climbing a mile and a half into the sky.

If eventually deployed, this unmanned aerial system would give the United States state-of-the-art intelligence, surveillance and reconnaissance with its cutting-edge capabilities: The drone can fly a whopping four days without refueling.

Testing was conducted at NASA's Dryden Flight Research Center at Edwards Air Force Base in California on Monday. During the flight, Phantom Eye flew higher than 8,000 feet at a cruising speed of 62 knots, and it stayed in the air for 66 minutes.

The current Phantom Eye is intended to carry a 450-pound payload while operating for up to four days at altitudes of up to 65,000 feet.

Phantom Eye is propeller-driven and weighs 9,800 pounds. With its advanced propulsion system, using hydrogen-powered internal combustion engines, it could provide an advantage monitoring large areas.

Typical payloads for Phantom Eye would include multiple sensor packages for monitoring, tracking and communications. It could provide an “eye in the sky” for surveillance, search and rescue, disaster relief and many other purposes.

Last Flight

Phantom Eye completed its first autonomous flight on the first of June last year at NASA Dryden.

Flying at an altitude of 4,080 feet and staying in the air for twenty-eight minutes – the second flight has achieved more than twice the flight duration in less than a year.

At this point, Phantom Eye reached a cruising speed of 62 knots and this was accomplished using Boeing’s COMC2 software.

Following the first flight, Boeing upgraded both the aircraft's software and hardware such as the landing gear. While the first landing was a bit bumpy with landing gear taking a bit of damage, this time the landing was spot on.

Reflecting the company’s belief in its potential, Boeing has chosen to self-fund the unmanned air vehicle’s development.

From a green perspective, Phantom Eye’s tech liquid hydrogen propulsion has extraordinary fuel economy.

Boeing says its only by-product is water – quite a remarkable innovation.

Boeing is also exploring beyond Phantom Eye into other HALE concepts based on its technologies. One project the company is investigating would stay aloft for a week or more while carrying payloads more than an astonishing 2,000 pounds.

Ballet dancer turned defense specialist Allison Barrie has traveled around the world covering the military, terrorism, weapons advancements and life on the front line. You can reach her at wargames@foxnews.com or follow her on Twitter @Allison_Barrie.