In July 2018, Airbus Defense & Space announced that its Zephyr S drone had flown for 25 days straight during testing over Arizona. The record flight came two years after the British military ordered three copies of the high-flying unmanned aerial vehicle for surveillance missions.

On Aug. 17, 2016, the British ministry of defense confirmed it would purchase a third Airbus Defense & Space Zephyr S high-altitude pseudo-satellite in addition to two previously confirmed in February 2016.

High-altitude pseudo-satellites — HAPS, for short — are super-lightweight unmanned aircraft that can linger around the stratosphere for weeks on end and act as localized satellites.

Perhaps Britain’s purchase should come as no surprise — Zephyr is British-born and -bred.

Until March 2013, the project belonged to Qinetiq, the private-sector offshoot of the British Defense Evaluation and Research Agency. But it wasn’t always a defense project.

(A version of this story first appeared in WarIsBoring back in 2016 here.)

Beginning with the 15-pound proof-of-concept Zephyr 2, the aircraft was designed not only for free flight, but also to feathered tethered to something else. At first this was the Clifton suspension bridge in Bristol in England’s southwest, but Qinetiq planned to attach its successor — Zephyr 3 — to the Qinetiq 1 balloon attempting to break the 1961 manned balloon flight altitude record of 113,740 feet.

With a 39-foot wingspan and a weight of just 33 pounds— around the same as a toddler — Zephyr 3 could fly at an altitude of 132,000 feet, more than enough to ensure that it would stay aloft even at the balloon’s record-breaking height. Its mylar-skinned carbon-composite wings carried solar panels along their length to power its five bespoke one-kilowatt motors designed at Britain’s Newcastle University.

Zephyr would circle the balloon at 155 miles per hour along a 1,476-foot tether and provide images of the balloon’s ascent. The balloon’s pilots had a radio control panel to adjust Zephyr’s flight. The balloon’s gondola was open to the atmosphere, so the pilots had to wear spacesuits. As a result, the control panel contained a series of large buttons that looked more like an arcade beat-’em-up than an aircraft remote control.

On Sept. 3, 2003, Qinetiq 1 and Zephyr 3 sat aboard the R/V Triton, a trimaran technology demonstrator built for the Royal Navy’s Future Surface Combatant frigate concept—similar to the U.S. Navy’s Littoral Combat Ship. As the launch crew inflated the 1,250-foot envelope, a tear ripped through the skin, forcing Qinetiq to abandon the flight. Zephyr 3 might have missed its chance for a launch, but the project continued on.

High altitude, long endurance:

Stratospheric flight has long been a goal for aircraft engineers. The best known high-altitude aircraft — Lockheed’s U-2 — can operate up to 70,000 feet or so and weather balloons typically top out before 100,000 feet.

For scientists, this region of the atmosphere is a great place to measure cosmic radiation and collect space dust. For disaster-relief missions or in remote areas without existing infrastructure, these strato-planes could also act as high-flying cellphone base stations.

The military applications are quite obvious. Aircraft such as Zephyr can relay imagery and signals intelligence to commanders on the ground at a fraction of the cost and risk of launching and operating a satellite in orbit. Additionally, satellites transit target areas twice a day — meaning non-continuous coverage.

High-altitude aircraft with long loiter times can stay on target for far longer than satellites and can redeploy over new targets with minimal hassle and forethought.

This category of aircraft is better known as “high-altitude, long-endurance” — HALE. Remotely-piloted aircraft systems advocacy group UVS International once defined HALE drones as strategic platforms with a maximum flight altitude of between 50,000 and 70,000 feet and endurance between 24 and 48 hours. With aerial refueling options and particularly solar power, however, HALE aircraft have been pushing the endurance boundaries.

In 2003, at the time of Qinetiq 1’s scrapped mission, HALE aircraft were a nascent technology — but not unheard-of.

The downing of Gary Powers’ CIA-owned U-2 in 1960 and the destruction of a U.S. military EC-121 spy plane over North Korea in 1969 spurred Cold War interest in unmanned high-altitude flight. Early efforts in the mid-1960s, such as the Lockheed D-21 and Ryan SPA 147 series of drones, provided high-speed reconnaissance and communications intelligence capabilities, but lacked the ability to linger over a target.

NASA’s environmental science mission was the primary driver in the lightweight, solar-powered approach to HALE. The AeroVironment Pathfinder used its 98.4-foot wingspan and eight motors to climb to an altitude of 67,350 feet — a record for solar aircraft. With some modifications to its solar cells and an increase in wingspan to 121 feet, the Pathfinder-Plus reached an altitude of 80,201 feet the following year.

Recommended: Forget the F-35: The Tempest Could Be the Future

Recommended: Why No Commander Wants to Take On a Spike Missile

Recommended: What Will the Sixth-Generation Jet Fighter Look Like?

NASA and DARPA’s Helios followed up on Pathfinder’s success. Carrying fuel cells to store the energy needed to continue flying through the night, the aircraft could stay aloft for months at a time at an altitude between 50,000 and 70,000 feet. But just months before Qinetiq 1’s failed launch in 2003, Helios broke apart and crashed into the Pacific Ocean.

America and its allies were now fully engaged in the War on Terror, boosting military requirements for long-endurance missions. In 2002, Northrop Grumman’s RQ-4 Global Hawk became the benchmark for military unmanned HALE aircraft. In 2014, a U.S. Air Force Block 40 Global Hawk flew for 34.3 hours without refueling, an unofficial record.

Global Hawk is nevertheless constrained by its fuel capacity. It’s also limited to a service ceiling of 65,000 feet, above which the lower air density can cause the turbofan engine to flame out.

Solar-powered aircraft such as the Zephyr are slower and inherently less robust than high-speed jet aircraft are, but they make up for it in altitude and endurance — as Zephyr’s Farnborough-based team set out to prove.

Zephyr breaks records:

There was little news of Zephyr 3 after the failed balloon launch, but in February 2005 its successor, Zephyr 4, surfaced in the Woomera Prohibited Area — an unfathomably large test range sometimes called Australia’s Area 51. The British ministry of defense was tight-lipped — a good sign that the project had moved toward military applications.

Zephyr 4 was an incremental improvement over its predecessor. With some improvements to harden the design against the rigors of its high-altitude environment, the new model weighed in at 37 pounds with no change to its wingspan. Its method of deployment was the same — a helium balloon carried the Zephyr to 30,000 feet.

Zephyr 4 flew for just one hour but the test flight gave the Qinetiq team some key figures on power usage and operating ceilings that would be essential if they were to achieve overnight endurance.

After Zephyr 4, there were two Zephyr 5 prototypes — both of which could be hand-launched. Zephyr’s previous tests had shown it was far more capable of flying in the dense air towards the ground than its engineers had anticipated. Strong winds and inclement weather were still a danger to the long and gangly aircraft, but being able to fly from the ground made testing far less complicated than being lifted by a balloon.

The only differences between the two prototypes were in their power supplies and, consequently, their respective weights. Zephyr 5-2 carried non-rechargeable batteries weighing 55 pounds. Zephyr 5-1 carried both solar panels and a battery for a total weight of 68 pounds. This allowed the team to test two different weights — but only the rechargeable Zephyr 5-1 had any future as a long-endurance aircraft.

In December 2005, both Zephyr 5 aircraft flew low-altitude, launch-and-descent test flights at White Sands Missile Range in New Mexico for four and six hours, respectively. In July 2006, the aircraft returned to White Sands, where Zephyr 5-1 flew for 18 hours — seven of which were overnight — and reached 36,000 feet. The team finally had the basics down and could now concentrate on pushing the design’s capabilities further.

The Qinetiq team had lofty goals. ““At present long endurance is measured in terms of hours. Ultimately we are thinking in terms of months,” Paul Davey, Zephyr’s development director, said in a July 2006 news release. “The current development program has the potential to extend Zephyr’’s mission endurance to around three months, which could force a wholesale change to the way in which the industry thinks about UAV operations.”

Zephyr 6 increased the wingspan to 59 feet but managed to reduce the weight to around 66 pounds. The power system was overhauled to include paper-thick amorphous silicon solar arrays powering lithium-sulphur batteries. The improvements spoke for themselves.

At White Sands in July 2007, Zephyr flew for 54 hours up to a height of 58,355 feet. It then conducted a second flight of 33 hours and 43 minutes up to a maximum altitude of 52,247 feet.

By now it wasn’t just the British who were interested in Zephyr — the U.S. Department of Defense funded the project as part of the Joint Capability Technology Demonstration Program. This deal also saw Qinetiq acting as key technology partner with Boeing for DARPA’s Vulture — and later, SolarEagle — program, which sought to keep an aircraft in the air for five years.

DARPA de-funded the program in 2012, but Boeing’s designs included the same high-altitude motors used on the Zephyr.

The Defense Department hosted the next test flight in 2008. Between July 26 and 31, Zephyr flew for 82 hours and 37 minutes — three and a half days — at the U.S. Army’s Yuma Proving Ground in Arizona. Controlled by autopilot and satellite link, Zephyr carried a six-pound U.S. government dual-channel air-ground VHF radio up to 62,000 feet.

This was a new record for an unmanned aircraft, but because no official from the record-keeping Fédération Aéronautique Internationale was present to witness the flight, it wasn’t official. But Zephyr had stomped Global Hawk’s own, unofficial 34-hour record into the ground — and the team was far from done.

“We think Zephyr is very close to an operational system — within the next two years is what we’re aiming for,” chief engineer Chris Kelleher — who sadly died in the summer of 2015 — told the BBC after the Arizona test flight. “We have one more step of improvements. We trying to design a robust and reliable system that will really sit up there for months. And we want to push the performance.”

While Zephyr 6 continued testing with the U.S. Naval Air Warfare Center, Qinetiq began work on Zephyr 7.

This iteration was, until very recently, the current prototype. It could fly more than a month with a daytime altitude of 65,000 feet — and 45,000 feet at night. At 73.8 feet, it was 50-percent larger than its predecessor — and weighed 110 pounds.

This increased weight was in part due to it carrying more batteries than Zephyr 6 did. The new model also carried a downward-looking camera and a forward-looking optical/infrared camera to help operators fly the aircraft from the ground.

After being launched by hand, a pilot at a container-based ground station remotely controlled the plane until it reached the minimum altitude for the autopilot to kick in. Once aloft, the autopilot could navigate between GPS waypoints or, when necessary, the ground station operator could take manual control.

The aerodynamic profile of Zephyr had also changed. The wings now had the inverted V-shape of a cartoon seagull, and its tail featured a high T-shaped configuration.

The Qinetiq team returned to Yuma Proving Ground with the new model in July 2010 where it once again smashed the endurance record. The aircraft managed more than 14 days of continuous flight — 336 hours and 22 minutes — at a maximum altitude of 70,740 feet. This time an official from the Fédération Aéronautique Internationale was there to witness the flight, putting the flight into the record books.

This unmanned airplane endurance record stands as of August 2016.

From this point, the record-breaking was over. In December 2010, Qinetiq started pitching Zephyr as a war-ready military surveillance and communications platform, but in reality it was still very much in a prototype form.

In 2013, Qinetiq divested the project to EADS Astrium which then became Airbus Defense & Space later that year. The Zephyr teams at Qinetiq and Airbus have since pushed forward with a truly production-ready model.

Technical marvel:

Zephyr represents an incredible achievement. Operating at a region of the atmosphere where most aircraft attempt to spend as little time as possible, Zephyr is subject to all sorts of engineering challenges.

At its operating ceiling, Zephyr flies high above the wind and weather, but it’s extremely vulnerable during its ascent and descent through the windy troposphere — the region of the atmosphere we inhabit.

During testing, Zephyr was only launched in perfect weather conditions— although Qinetiq press releases repeatedly played up the ground temperature and storms around the test days. But that is not to downplay the effects of the desert temperatures it faced on the ground.

From around 40º Celsius at launch, Zephyr’s ambient temperature would fall 2º Celsius per 1,000 feet. But the aircraft mostly flies in and around the ozone layer, which absorbs the sun’s ultraviolet light and emits the energy as heat. As a result, the ambient air temperature actually begins to rise once Zephyr ascends past 65,000 feet.

Once Zephyr gets out of the dense tropospheric air and into the stratosphere, it also has to increase its power usage to maintain propulsion.

When night falls, the aircraft must descend to 45,000 feet to conserve power by flying in more dense air. Here the temperature falls to as low as -80º Celsius. This extreme cold can cause components to become brittle, affect electronics and cause metal components to contract.

The engineers chose to insulate and heat components to protect them, even though that increased Zephyr’s weight. But given these extreme temperatures, it isn’t possible to fully protect each component from thermally-generated expansion and contraction.

Cosmic rays — energetic particles emanating from space — are an additional threat to the platform’s electronics. Zephyr operates right in the peak region for cosmic radiation. These rays can cause data corruption and critical electronic faults and it is unclear how much shielding Zephyr 7 carried to protect against this threat but it reportedly hasn’t suffered any ill effects … yet.

Zephyr operators will also face challenges resulting from its solar-power generation. The amount of solar energy available to any given surface area varies by latitude and date. The tropics offer the most energy, but the farther from the equator you go, the more significant the drop-off in available energy during winter.

This means that Zephyr is limited in where it can operate, depending on the season. It’s unlikely that Zephyr will be patrolling Britain for months on end outside summer, for example. But that doesn’t mean it’s entirely incapable of operating during winter. In August 2014, Zephyr 7 flew for more than 11 days in an undisclosed southern hemisphere location in order to show that it could operate for long periods despite winter’s longer nights.

Besides the importance of Zephyr’s atmospheric and climatic requirements, it suffers several structural limitations. It has to be extremely lightweight with very long wings. The weight of the payload, motors, batteries and solar panels are evenly distributed along the wings in the upper atmosphere, but down on the ground, these wings droop under their own weight. Zephyr operators will have to be very careful when launching and stowing the aircraft or risk stress fractures — or worse.

In addition, the lightweight requirement necessitates design compromises. Each component must provide a strong power-to-mass ratio. For Zephyr 6, at least, its amorphous silicon solar cells featured photovoltaic layers that were micrometers in thickness. This made them around 10-percent efficient compared to thicker commercial cells.

Its lithium-sulphur batteries, although lighter than lithium-ion, lagged behind in efficiency. They required constant heating to 20º Celsius in order to maintain their efficiency and also needed to be housed in fire-suppressing containers in order to prevent self-combustion catastrophes.

Ultimately, Zephyr’s major impediment is its limited payload capacity. Zephyr 6 could carry a measly six pounds compared to Global Hawk’s 2,000 pounds or AeroVironment’s Global Observer’s 400 pounds. All these concerns about weight are necessary to ensure that Zephyr can not only fly, but also perform its required missions — but it seems unlikely that Zephyr’s packages could ever reach the efficacy of its competitors.

Its payloads must not only be carried up to altitude, but also must be powered by its solar panels. This suggests that existing jet-based payloads will not be entirely suited to Zephyr operations.

Zephyr is the closest we have to an eternal-vigilance aircraft, but it can’t meet all of its operators’ information, surveillance, target acquisition and reconnaissance needs. It will fill a small part of a commander’s toolbox, at least until battery and camera technologies allow for less strain on the fragile balance that keeps it aloft.

British Zephyrs:

The British military expected to receive its three Zephyrs in 2017 under a £13-million contract. Britain stated its intent to purchase Zephyr in its National Security Strategy and Strategic Defense and Security Review 2015, after which then-prime minster David Cameron told Parliamentthat “British-designed unmanned aircraft will fly at the very edge of the earth’s atmosphere and allow us to observe our adversaries for weeks on end, providing critical intelligence for our forces.”

Despite the Ministry of Defense’s silence on the actual purpose of its Zephyr purchase, the Strategic Defense and Security Review offered a major hint. “[British Special Forces] will have the information they need, including through our investment in advanced high-altitude surveillance aircraft,” the review stated.

In 2916 British Special Forces were spotted training Syrian rebels. A platform such as Zephyr flying out of Cyprus — where the U-2 currently operates — could provide always-on signals and imagery to commanders back home, as well as provide current intelligence- and data-support to troops on the ground.

Zephyr 8 — or Zephyr S as it will be known for its production run — has an 82-foot wingspan, but it’s also 30-percent lighter despite carrying 50 percent more batteries. This leaves more space for its 11-pound payload.

Zephyr S probably represents the evolution that was necessary for the platform to pick up production contracts. Zephyr S carries high-definition optical/infrared cameras, narrow-band mobile communications with a 100-megabits-per-second broadcast capacity. Its NIIRS 6 camera can produce imagery with a resolution of up to 15 centimenter for objects on the ground.

At the Farnborough Airshow in July 2016, Airbus announced the Zephyr T — a larger, twin-tail design with a wingspan of 108 feet, capable of carrying a 44-pound payload on its larger 136.6-pound air frame. Airbus is planning a full-scale build in 2018 before the aircraft becomes operational the following year.

Zephyr T is a shot across the bow of Northrop Grumman’s Triton — the naval variant of the Global Hawk. It will carry a maritime radar twinned with a synthetic aperture radar to detect surface ships. This positions it as a long-endurance maritime and border-patrol aircraft or anti-piracy surveillance aircraft — both of which would appeal to Britain’s current naval roles at home and abroad.

Zephyr’s biggest test will be convincing other nations that it’s worthy of procurement. The United States clearly has a long-standing interest in the project, so an American order — in either a military or civilian capacity — is not unthinkable. Another option might be Japan, who has been procuring surveillance and patrol aircraft to secure its contested seas with China.