Boeing · UAV · United States · Digital Age (2010–present)
Open in interactive gallery →The Boeing SolarEagle (also Vulture II) was a proposed American high-altitude pseudo-satellite (HAPS) UAV — Boeing Phantom Works's planned 5-year-flight-duration solar-electric aircraft for DARPA and USAF persistent-surveillance and communications-relay missions. Boeing developed the concept between 2010 and 2012 under the DARPA Vulture II programme, which was cancelled in 2012 before any flight article was built. Only paper studies and sub-scale demonstrators were completed.
SolarEagle would have been a giant solar-electric UAV with a wingspan of 122 m — longer than a Boeing 747's 64.4 m — a length of 21 m, a weight near 5,000 kg, and a payload of roughly 450 kg. Distributed electric motors driving multiple propellers were to be powered by about 60 kW of solar arrays mounted on the wings, tail, and nose. Cruise altitude was set at 27,000 m, above weather and commercial traffic. The design target was 5 continuous years aloft: the aircraft would never land, recharging its batteries from sunlight by day and cruising on battery reserves at night. Sensors were to include ELINT, SAR, communications relay, and electro-optical payloads, giving persistent coverage of a fixed area at near-orbital altitudes as a low-cost satellite alternative.
SolarEagle and DARPA Vulture II were cancelled in 2012 for three reasons. First, technological maturity: solar-cell efficiency and lithium-ion battery energy density did not yet support a 5-year-flight design. Second, cost: the programme estimate had grown to about $1 billion, and DARPA had shifted priorities to other unmanned programmes — the X-47B, RQ-170, and the planned MQ-25. Third, the rise of alternatives: improving commercial satellite imagery and later high-altitude programmes (Google's Loon, AeroVironment Solara, Airbus Zephyr) covered overlapping mission sets. The concept is now being revisited by several HAPS efforts — AeroVironment Sunglider, Airbus Zephyr, and Boeing's own follow-on designs — as gains in solar cells, batteries, and electronics push the 5-year target closer to reach.
The Boeing SolarEagle was a special drone that never got built. It was also called Vulture II. Boeing worked on it between 2010 and 2012 for a group called DARPA. The project was cancelled in 2012 before any real plane was made.
The SolarEagle would have been huge. Its wings stretched 122 meters wide — longer than a Boeing 747, which measures about 64 meters across. That means the SolarEagle's wings were nearly twice as wide as a jumbo jet's!
The drone ran on solar power. Panels on the wings, tail, and nose soaked up sunlight during the day. At night, it used stored battery power to keep flying. It had no engine burning fuel at all.
The goal was to fly for five whole years without landing. It would cruise very high — about 27 kilometers up. That is above all weather and above where passenger planes fly. From up there, it could relay radio signals and take spy photos like a low-cost satellite.
Only drawings and small test models were ever finished. The full-size SolarEagle was never flown. A similar idea was later explored by a plane called the Airbus Zephyr.
The project was cancelled in 2012 before a real aircraft was ever built. Only drawings and small-scale test models were completed. Sometimes big projects get stopped before they become real planes.
During the day, the solar panels charged big batteries on board. At night, the drone used that stored energy to keep its electric motors running. This way it could fly around the clock without stopping.
It was designed to fly very high and watch large areas on the ground. It could also relay radio and communications signals, acting like a cheap satellite. The American military wanted it for long missions that could last years.
A similar solar drone called the Airbus Zephyr has explored the same idea. It also flies very high on solar power for long periods. The SolarEagle concept helped inspire these kinds of high-flying solar drones.
Closer than in 2012, but not yet routinely achieved. The Airbus Zephyr S logged 64 days of continuous flight in August 2018 — a meaningful step toward year-long flight, but well short of SolarEagle's 5-year target. Three technologies remain limiting. First, solar-cell efficiency: today's commercial silicon arrays run near 25%, with multi-junction cells around 30%, while SolarEagle needed 35-40% to close its energy balance. Second, battery energy density: current Li-ion cells deliver about 250 Wh/kg, but night-time operation required closer to 500 Wh/kg, a figure that solid-state and Li-S chemistries are approaching in 2026. Third, structural durability: a 5-year unbroken flight imposes fatigue loads on lightweight composite structures that have not yet been demonstrated. Programmes such as Zephyr, Sunglider, and BAE Systems PHASA-35 are now targeting 12-month flights and can be read as the direct continuation of the SolarEagle concept.