Aiming high

Solar-powered aircraft designed to remain airborne for months at a time are being explored by several developers. Mark Broadbent outlines one manufacturer’s ambitions for its HAWK30.

The HAWK3k0 lifts off from Edwards Air Force Base in California on its first flight in September 2019.
Carla Thomas/NASA

A relatively new acronym in the aerospace sector is HAPS, which stands for high-altitude platform station or high-altitude pseudo satellite (the terms are synonymous). The International Telecommunication Union Radio Regulations defines a HAPS as any system able to operate at an altitude of around 65,000ft (19,812m) “at a specified, nominal, fixed point relative to the Earth.”

This classification covers a range of different platforms designed to fly at a very high altitude for extended periods of time, from manned and unmanned fixed-wing aircraft to airships and

The HAPS definition also refers to a new generation of emerging solar-powered fixed-wing designs. Despite their rather fragile appearance, these machines are very much at the sharp end of technology. With solar cells mounted across their structures to charge eff icient lithium-ion batteries, these HAPS are designed to fly at 65,000ft or higher and remain airborne for up to six months at a time.

How big is the HAWK30?

There are several solar-powered HAPS systems in development, with some having flown and others due to undertake test flights soon (see panel on page 71, A Class of High Flyers)

The HAPSMobile HAWK30 is perhaps one of the most striking examples of these new systems. The aircraft, which has ten engines, first flew last September from the NASA Armstrong Flight Research Centre at Edwards Air Force Base, California.It completed a second 90-minute test flight two months later, during which it successfully passed two dozen test points. These included negotiating 180o turns and assessing avionics, electrical power and engine performance.

Although solar aircraft have been around for decades, the HAWK30 and other solar-powered HAPS in development have emerged in the last few years due to advances in electric power generation and battery technology that make such systems commercially feasible rather than one-off research curiosities.

High-altitude persistence

The main advantage of all HAPS systems, manned or unmanned, conventionally fuelled or solar-powered, is their ability to provide a constant stream of data for long periods of time from whatever sensors the aircraft’s operator has installed – a high level of ‘persistence’ to use the jargon.

Space satellites are the usual means of achieving persistent wide-area surveillance and communications, but the laws of orbital mechanics restrict their passes over specific locations to certain times.

Although they fly at higher altitudes than conventional aircraft, HAPS systems are not similarly restricted because they remain well within Earth’s atmosphere (the boundary with outer space is widely reckoned to be the Kármán Line, 62 miles/100km or 327, 360ft/99, 979m up.

This gives operators the flexibility to direct them to points of interest at any time. There is also the advantage of being able to operate unimpeded by disruptive weather and busy air traff ic corridors far below. These factors, combined with the high-altitude persistence and relatively low costs compared with satellites, inevitably means HAPS have caught the interest of militaries due to their potential in intelligence, surveillance and reconnaissance.

The US Army and Lockheed Martin researched a solar-powered HAPS called the HALE-D during the 2000s. Meanwhile, the UK Ministry of Defence ordered three examples of the Zephyr in 2018, two of which crashed in test flights in Australia in separate accidents in 2019.

Effective relay

Aside from their military potential, HAPS have utility for civil applications such as climate research, mapping, geospatial surveys and monitoring of the weather.

Some organisations, including Aurora Flight Sciences and Ordnance Survey, have designed their systems primarily for these tasks.

Communications relay presents another opportunity for HAPS in the commercial domain. It is for this role that the HAWK30 was designed. Indeed, the aircraft’s developer, HAPSMobile, is majority-owned by the Japanese telecommunications provider, SoftBank.

With a wingspan of 256ft (78m), the HAWK30 is wider than an Airbus A380 super jumbo is long.
Carla Thomas/NASA

Effective telecommunications relay requires antennas to be positioned high enough above the ground to avoid rising terrain or buildings that would otherwise disrupt transmission. According to SoftBank, signals from the typical 131-164ft (40-50m) height of a radio tower will travel around 62 miles (100km) if they are not blocked.

The higher the tower the further the signal will travel, but obviously there is a practical limit as to how tall radio masts can be. A HAPS system installed with radio communications payloads will get around this issue, because flying at such a high altitude will enable it to transmit over a far wider area than could be achieved from a ground tower. SoftBank claims a broadband wireless access network base station operated from the HAWK30’s optimum 65,000ft cruising altitude will relay to an area 124 miles (200km) in diameter, giving it twice the coverage of a ground tower.

The company claims HAPS systems will make stable phone and internet coverage possible in difficult terrain such as mountains or deserts, or in developing countries where the communications infrastructure is either poor or non-existent. It says HAPS could also prove useful following natural disasters such as earthquakes, when terrestrial base stations may be damaged and knocked out of operation.

Faster communications

Visually, the HAWK30 resembles the Pathfinder, Pathfinder Plus and Helios created for the NASA Pathfinder initiative that researched solar-electric technologies in the late 1990s and early 2000s. Those aircraft were all produced by the US-based unmanned aerial vehicles specialist AeroVironment, which is a partner in HAPSMobile and built the first HAWK30 at its California factory.

Little technical information about the HAWK30 and its solar technology has been released by HAPSMobile beyond basics such as the wingspan and cruise speed. However, SoftBank has said the aircraft’s large size makes it necessary for it to have lots of solar cells mounted on its structure to send high-powered radio waves and generate the power required to keep it flying. Exactly how many cells has not been disclosed.

Faster communication is a key benefit of HAPS systems. Round trip time (RTT) is the telecommunications industry’s way of measuring the period taken to transmit a radio signal and receive a reply. The typical RTT is 400 milliseconds for signals from a satellite in geostationary orbit (typically located 22,369 miles/36,000km away from Earth) or 18 milliseconds from a satellite in low Earth orbit (745 miles/1,200km).

AeroVironment, which previously built the Pathfinder and Helios solar-powered aircraft, is a partner in HAPSMobile.

SoftBank claims the RTT on a HAWK30 flying in the stratosphere will be just 0. milliseconds. The company adds that because signals weaken as they travel across greater distances of free space, the transmissions from the HAWK30 will be around one million times stronger than from a satellite – and therefore better quality – because the aircraft is much closer to Earth.

Softbank says it plans to “make the HAWK30 communications network less susceptible to radio wave interference, and to ensure smooth handover between the HAWK30 and terrestrial base stations. The ‘30’ in the HAWK30 name is a reference to the fact that the system is optimised to fly at a latitude of plus or minus 30o from the equator, a position where it will be able to fly for a full year without landing.

There are, of course, higher-latitude areas of the world, such as within the Arctic Circle, where there is little daylight during the winter months, which might seem to be a restriction on the its solar power and endurance. HAPSMobile says it is working on a follow-on model, the HAWK50, to fly plus or minus 50o from the equator to provide a stable communications service for these regions.

SoftBank has also said the HAWK30 will be able to carry payloads other than those directly supporting communications relay, such as fixed wireless equipment and onboard cameras for remote monitoring services.

Forming new alliances

Following the two test flights from Edwards in 2019, the plan was for the HAWK30 to be transported to the Hawaiian island of Lanai, from where HAPSMobile is intending to launch further test flights into the stratosphere by the end of March 2020.

The company’s longer-term goal is to achieve mass production and commercialisation of the HAWK30 by 2023. To fulfil this aim, it has formed strategic alliances with a number of tech companies.

One of such partnerships is with Loon, owned by Google’s parent company, Alphabet. Loon already has a presence in stratospheric communications with a network of high-altitude balloons providing internet connectivity.

However, these are at the mercy of stratospheric air currents, so a high-flying fixed-wing platform such as the HAWK30 could provide a more reliable base station by providing targeted delivery to specific areas. According to HAPSMobile, the work with Loon is intended to “make significant breakthroughs in the stratosphere business” by improving bandwidth utilisation, integrating base stations, looking at aircraft management and collecting data on stratospheric flying.

Designed to fly at 65,000ft, the HAWK30 will be used in communications relay.

A separate alliance will see HAPSMobile join forces with Facebook on a project in which the HAWK30 will carry payloads of 60 GHz ‘fixed wireless’ technology to “spread ground-breaking stratospheric internet solutions around the world. (Facebook abandoned work on its own solar HAPS, a system called Aquila, in 2018.

Various hurdles remain before the new breed of high-flying HAPS like the HAWK30 start beaming down connectivity. Concerns over privacy are likely to be raised by civil liberties groups and, on a practical level, authorisations from aviation authorities will be required. As a 2019 blog post by SoftBank acknowledged, there are also simply still lots of regions where the ground infrastructure to support the internet is lacking, creating practical issues with hardware, speed and bandwidth.Other issues affecting development are insufficient power supplies and high operating costs.

None of this is deflecting HAPSMobile from its goal. Noting that approximately half of the world’s population has yet to access the internet, Hidebumi Kitahara, SoftBank’s vice president, head of Global Business Strategy Division, Technology Unit, believes that HAPS and the HAWK30 will “close the digital divide and build the foundation for an equal, information-based society.

A class of high-flyers

The HAPSMobile HAWK30 is one of several solar-powered fixed-wing HAPS systems under development. Some designs have flown; others are poised to launch.

The Airbus Zephyr (originally developed in the UK by QinetiQ) set a new endurance record in 2018 by staying airborne for 25 days, 23 hours and 57 minutes. Test and development work on the Zephyr continues.

Other solar HAPS include the Aurora Flight Sciences’ Odysseus, BAE Systems’ PHASA-35, the Ordnance Survey A3 and the Skydweller. The first test flights of the Odysseus and PHASA-35 are due to take place this year. Announced in November 2019, the Skydweller, under development by a US/Spanish start-up of the same name and backed by the aerospace giant Leonardo, is an evolution of Solar Impulse 2, the first electric aircraft to circumnavigate the globe.

None of these systems will quite match the size of the HAWK30.At 243ft (74m) wide, the Odysseus will be slightly shorter than the 256ft-wide (78m) HAWK30. The Skydweller will be even shorter, while the Solar Impulse 2, on which it is based, has a 206ft (63m) wingspan. All these aircraft, however, will dwarf the 114ft (35m) PHASA-35 and 124ft (38m) A3.