Phantom Eye, a hydrogen-powered demonstrator that will stay aloft at 65,000 feet for up to four days, is a key part of Phantom Works’ rapid prototyping initiative. Phantom Eye is being designed and built at Boeingfacilities in St. Louis and Seattle as well as Irvine and HuntingtonBeach in California. While the propulsion system is key to Phantom Eye’s success, the airframe is deceptively simple. Several composite materials technologies for the prototype are being used, allowing for low-cost manufacturing without compromising performance.