After an astounding 64 days aloft and a travelling a total more than 30,000nm, a British-built solar-powered UAV crashed just hours before it was due to break the ultimate world endurance record. TIM ROBINSON reports.
On 15 June, a spindly-looking flying machine was launched into the still desert air at Yuma, Arizona and began a glacially slow climb to the stratosphere, its two small propellers whirring to claw into the sky.
The aircraft was the British-built solar-powered Airbus Zephyr UAV – one of a new breed of HAPS (high altitude, pseudo-satellites) – a new category of UAVs that are aiming for zero-emission, ultra-long- endurance flight as a kind of terrestrial satellite – able to loiter in the stratosphere for weeks or months at a time to monitor borders, watch shipping, relay communications or conduct atmospheric science.
Amazingly, for those tracking its progress, some three months later, the Zephyr 8 (callsign ZULU 82) was still aloft having covered more than 30,000nm or more than one lap of the Earth at altitudes of 60,000-70,000ft. The 25m wingspan UAV had already smashed its previous endurance world record for UAVs of 26 days set in 2018, flown in international airspace to Belize and back and was closing in on the ultimate flight endurance record of 64 days, 22 hours, 19 minutes, and 5 seconds set in 1959 by two pilots in a Cessna 172 advertising a Nevada casino.
Sadly, it was not meant to be. On 19 August around 0400UTC, only hours before it was to break this six-decade-old record, the UAV was observed on ADS-B tracking sites encountering difficulties and descending rapidly, before contact was ultimately lost. Its last recorded altitude was 15,000ft – way lower than its operational cruising altitude of 60-70,000ft.
In a statement, Airbus Defence and Space said: “Following 64 days of stratospheric flight and the completion of numerous mission objectives, Zephyr experienced circumstances that ended its current flight", adding “Our teams are currently analyzing more than 1,500 hours of stratospheric mission data. The valuable experience from this prototype’s ultra-long endurance flight has proved to be a positive step toward the Army’s high-altitude platform goals.”
Flight demonstration
Zephyr 8 weighs less than 75kg, despite having a wingspan of 25 metres. (Airbus)
The Airbus Zephyr was conducting a secretive flight test demonstration flight on behalf of the US Army Futures Command’s Assured Positioning, Navigation and Timing/Space (APNT/Space) Cross-Functional Team (CFT). This had seen the UAV transit international airspace over the Gulf of Mexico to Belize, where it conducted some trials, before returning to orbit high over Arizona. (Notably, Belize is the location of the British Army’s jungle warfare school).
In a statement, the US Army said: “Our team is working hard to gather and analyze important data following the unexpected termination of this flight,” said Michael Monteleone, Director of the APNT/Space CFT. “Despite this event, the Army and its partners have gleaned invaluable data and increased knowledge on the endurance, efficiency, and station-keeping abilities of high-altitude UAS platforms. That knowledge will allow us to continue to advance requirements for reliable, modernized stratospheric capabilities to our soldiers.”
The US Army Futures Command also revealed that a follow-on demonstrations with Zephyr including a flight over the Pacific, originally scheduled for this year would now be shifted to next year saying “Given the amount of data that was generated during the 64-day flight and the time required to analyse it, as well as the need to investigate the events that led to the termination, further flight demonstrations have been postponed until 2023.”
What are HAPS?
Flying at 70,000ft, HAPS can loiter for days on end to read communications at the fraction of the cost of a satellite. (Airbus)
One might ask, why the US military, with its access to its multiple satellites, spyplanes and existing UAVs is interested in a slow, fragile solar-powered UAV from Britain? The answer is in the unique, niche capabilities that HAPS offer. Cheaper than most satellites, they can loiter at very high altitude over the same area or region, providing persistent unblinking surveillance or communications coverage for weeks or months at a time Unlike satellites with predictable and fixed orbits that take lots of fuel to change, HAPS can also be dynamically retasked to move their gaze to other areas, or travel thousands of miles (slowly) to take on a new mission – making them unpredictable where they might appear next. In addition, while this flight was publicly tracked by ADS-B, with the transponder off and operating at 60-70,000ft, UAVs like Zephyr might be thought of as semi-stealth platforms thanks to their composite construction and glider-like configuration. Finally, unlike satellites, HAPS can return to Earth and have instruments and sensors swapped out for newer models and thus be constantly upgraded.
It is no wonder then, that HAPS such as the Airbus Zephyr, BAE Systems Phasa 35, Leonardo Skydweller and Stratospheric Platforms are attracting attention from militaries, commercial organisations or governmental entities for this capabilities – even though these are pushing the frontiers of aeronautics. Potential missions include ISR, border patrol, SIGINT, maritime surveillance, communications relay, 5G broadband node, Earth Observation, scientific research and many more.
However, they do come with limitations. Firstly, for solar-powered designs, the need for as much sunshine as possible means that extreme northern or southern latitudes are off limits. Second, as effectively very lightweight motorised, gliders they are also limited in the payload they can carry. This however, is less of a problem as sensor packages, cameras etc continue to be miniaturised – a trend that has also led to the explosion in small cubesat and nanosats. Finally, with their ultra-lightweight construction (Zephyr 8 weighs less than 75kg, despite a wingspan of 25m) calm weather conditions on launch and recovery are vital to enable it to spiral up to 70,000ft and higher above any storms and strong winds.
What went wrong?
The existing endurance record was set by Bob Timm and John Cook, who remained aloft for 64 days, 22 hours and 19 minutes in a Cessna 172 between December 1958 and February 1959. They refuelled by lowering a hook via a winch down to a fuel truck that would follow the aircraft on a straight stretch of road. The winch would then pull up a fuel hose from the truck, which would be used to fill the tanks in around three minutes.
It is obviously still unknown as to the cause and went wrong - with the official statement: “Zephyr experienced circumstances that ended its current flight” being deliberately vague. While ADS-B data cannot be fully relied upon for exact positioning, it did show that the incident, whatever it was happened at high altitude (50,000ft+) in the cruise phase of flight. Additionally, the ADS-B track earlier that morning showed that the drone seemed to be experiencing some form of headwinds over Arizona, with the airspeed dropping to a recorded 1-2kt at one point – far below its usual (slow) cruise speed of around 30kt. This data suggests that it may have hit some form of high-altitude turbulence or strong winds in the stratosphere that contributed to the lightweight and extremely fragile UAV breaking up in flight.
Another cause might be the aircraft reaching the limits of its lithium–sulphur batteries – which are one of the limiting factors as it increases its endurance. Though the aircraft uses solar power to fly in the daytime, during the night it must rely on stored power to fly – and over three months aloft this could have pushed the batteries beyond their limits. For an aircraft that can stay airborne for three months, in the chilly temperatures of 60-70,000ft, it is thus perhaps better to think in terms of equivalent satellite reliability for its components, rather than conventional aircraft.
This is now the third Zephyr UAV lost during testing, with two others crashing in 2019 in Australia – both at lower altitude and involving turbulence/adverse weather. That said – at this point the cause of the crash is this is pure speculation and we must await the findings of any accident report and investigation.
Summary
ZULU82 came within hours of breaking the ultimate endurance record. (Airbus)
In short, although it would have undoubtedly been heartbreaking for the flight test team to watch helplessly as ZULU82 disintegrated and fluttered back to Earth, just hours away from breaking a major world record in aeronautics, there are more than enough reasons to be positive about this latest flight that came so close to success after 64 days aloft.
Indeed, this flight achieved a number of firsts for Zephyr 8, including the first flight in international airspace, flight overwater, beyond line-of-sight control via SATCOM, from three different ground control centres (Huntsville, Alabama; Yuma, Arizona and Farnborough, UK.) and direct downlink of data while outside US airspace. This then was a key operational test of a HAPS system, showing how it would be used in a real-world scenario and a successful validation of the concept.
Indeed, the customer, the US Army seems pleased with the test, noting: “This experimentation successfully demonstrated Zephyr’s energy storage capacity, flight endurance, station-keeping and agile positioning abilities” in an official statement.
In short, while this is an undoubtedly a setback and frustrating for the flight test team to come so near to an absolute aeronautical record, it has to be remembered that Zephyr is pioneering an entirely new niche of zero-emission, long-endurance flight in the stratosphere that has only been previously explored by the SR-71, U-2, and high-altitude research balloons. In this, is truly breaking new frontiers in flight and certainly will be back.