The two British projects to develop solar-powered HAPS (high altitude pseudo-satellites, alternatively known as high altitude platform stations) appear to be leading the field now that most similar efforts in the U.S. and elsewhere have foundered. However, despite a development history stretching back over 20 years, the Airbus Zephyr and the BAE Systems PHASA-35 still have not achieved operational status.
Both efforts stem from pioneering work done by QinetiQ here at Farnborough from 2001 onwards. Airbus Defence and Space bought the Zephyr project from QinetiQ in 2013. However, some of the engineers left to form their own company, Prismatic, and develop a rival design named PHASA-35. PHASA stands for persistent high altitude solar aircraft, and 35 denotes its wingspan. BAE Systems bought Prismatic in 2019.
As the designs have matured, they have benefited from advances in solar cell and battery technology, composite structures, and the miniaturization of potential payloads. Both are designed to fly at 60,000 to 65,000 feet for months at a time. The latest Zephyr-S version weighs about 75 kg, has a wingspan of 25 meters, and carries a payload that varies according to the performance required, according to Airbus. The PHASA-35 weighs twice as much and carries a payload of 15 kg. According to Phil Varty, BAE’s head of business development for PHASA-35, its payload is double that offered by the Zephyr. He told AIN that the larger size and payload of the PHASA-35 “hits the sweet spot.” Airbus plans a larger Zephyr with a similar all-up weight.
Airbus has envisioned no fewer than 12 applications in the “see and sense” category, using a variety of sensors including EO, IR, and hyperspectral imaging, radar imaging, laser imaging detection and ranging (LIDAR), passive radio frequency monitoring (eg for SIGINT), and the maritime auto identification system (AIS). But communications connectivity currently appears to be the most promising market. Compared with satellites, the HAPS are immeasurably cheaper to build and offer flexibility of coverage, with no fixed orbit. According to Varty, no communications satellite in geostationary orbit can handle 4G or 5G, and they also suffer from a latency problem. Of course, a HAPS also can return to the ground for maintenance and upgrades and swap payloads if required.
Last January, Airbus signed an MoU with Nippon Telegraph and Telephone Corp (NTT), plus Docomo, Japan’s leading mobile phone operator, and Japanese broadcast satellite specialist SKY Perfect JSAT. They will study a wideband connectivity ecosystem that would employ the Zephyr as well as satellites. The trio noted that a HAPS can interconnect to the nearest terrestrial network gateway and extend the reach of existing mobile services directly to end-user devices, providing service options including rural, emergency, and maritime connectivity.
However, BAE’s Varty says that defense communications, rather than commercial, is the main market. In particular, BAE Systems eyes the U.S. market and has talked of “a wide Department of Defense and federal agency customer base.” The first flight of the PHASA-35 in Australia in February 2020 occurred under the sponsorship of the UK Defence Science Technology Laboratory (DSTL) and Australia’s Defence Science and Technology Group. Earlier, in 2015, Airbus signed a demonstration contract with the UK Ministry of Defence for communication applications, using three “production standard” Zephyr-Ss built in 2017-18. But in 2019, two of them crashed while flying from Wyndham in Western Australia, where Airbus had established a launch site the previous year. It said that it chose the location “due to its largely unrestricted airspace and reliable weather.” Jana Rosenmann, head of unmanned systems at Airbus, called the site “our gateway to the stratosphere and the main flight base for Zephyr going forward.” Airbus had previously performed test flights from a desert base in Arizona in the U.S.
The crashes illustrated a major challenge to HAPS operations—the vulnerability of their fragile structure to adverse weather conditions, especially during climb and descent through the troposphere. When coupled with the need to coordinate with air traffic control (ATC), it’s not surprising that both projects have chosen remote areas to conduct their flights. Regular operations of PHASA-35, as well as Zephyr, will happen in unpopulated areas as well; even a seven-day transit to a desired operational area is negligible within the context of multi-month endurance.
At present, air vehicles flying above 60,000 feet do not need to coordinate with ATC. However, the FAA has published a proposal for traffic management in the Upper Class E airspace within which HAPS operate, including shared situational awareness to aid separation.
Operating in the stratosphere brings its own problems: intense solar radiation and UV, low pressure that can cause arcing problems in electrical components, and very low temperatures that vary markedly between day and night. The batteries on a HAPS must allow for enough charging during the daytime to power the vehicles all night. According to Varty, the best conditions exist within 20 degrees of the equator, where there is a lot of solar energy and the hours of daylight versus nighttime do not vary much by season.
The Zephyr has collected considerably more flying experience than the PHASA-35. As long ago as 2010, the Zephyr 7 set a then-world endurance record for an unmanned aerial vehicle: 14 days, 22 minutes. Here at Farnborough, in 2018, Airbus invited showgoers to follow a test flight over Arizona of the first Zephyr S in Arizona in real time.
Last September, Airbus completed another test campaign in Arizona, which achieved 36 days of stratospheric operation over two flights and a world altitude record for this UAV class of 76,100 feet. The Zephyr also conducted four low-level flights, so its flying hours to date have totaled 2,435. Airbus said that the flights demonstrated multiple payload integrations as well as precise stratospheric maneuverability and station-keeping. Despite the earlier crashes, the UK Ministry of Defence remains enthusiastic. Officials said that the campaign marked significant progress and called Zephyr “an important program within UK Strategic Command.”
The PHASA-35 has flown only once, in 2020 at low level. BAE Systems planned to fly it again in the U.S. in 2021, this time to altitude, but Varty said that the weather conditions were unusually unfavorable, with the jetstream farther south than usual. However, the company has flown a quarter-scale vehicle at Warton in the UK, and in Spain. Those operations also involved low-level flights, but Varty told AIN that because the wing has the same Reynolds number as the full-scale vehicle, it would perform the same at high altitude. The company has also done ground tests on the avionics and flight control system, and cold-soak tests for 72 hours on various subsystems. It also has tested a communications sensor payload provided by DSTL and a datalink. It expects to fly to altitude this year.
In the U.S., government-funded solar-energy projects such as Pathfinder, Helios, Solar Eagle, and Vulture led nowhere. Several solar-powered airship developments, including HiSentinel, HALE-D, and ISIS, failed. (More recently, a Thales and Alenia combine has been developing a similar concept, which has made limited progress and is scheduled to fly next year).
Facebook and Google have funded solar-powered HAPS with the ambition of filling the gaps in worldwide internet coverage. Both projects have foundered. Google then turned to high-altitude balloons in Project Loon, which demonstrated some utility in remote locations and as a substitute for ground base stations damaged in the Puerto Rico earthquake in 2017. The scheme required massive computational power to accurately model the winds in the upper atmosphere so that the positions of the balloons could be adjusted through control of the gas volume. But they still experienced unwanted drift, and Google has ended the project.
That leaves AeroVironment, which has worked on solar-powered aircraft for 40 years, mostly with funding from NASA. Its current project, called the Sunglider, employs a much larger and more complex airframe than that of the Zephyr or PHASA-35, with a 78-meter wingspan and 10 propellers. The company has focused its efforts on commercial telecommunications, in a joint venture with majority Japanese partner SoftBank. “We look forward to maintaining our momentum toward aircraft certification and commercialization,” the company said.