HIGH-ALTITUDE PSEUDO-SATELLITES (HAPS) – A QUICK REVIEW

At the recent Farnborough Airshow, my personal highlight was a visit to the Airbus Zephyr production facility (above), in the nearby Science Park. I wrote about this for AIN here and, as predicted, this fascinating solar-powered design has since claimed a new endurance record for an aircraft – more than 25 days.

The Zephyr project was started at Farnborough in 2001, and was sold to Airbus Defence & Space by QinetiQ in 2013. But apparently, satellite provider EADS Astrium (a predecessor of Airbus D & S) had already invested in the project, when it needed more money.

Meanwhile, the US government funded similar efforts, such as Pathfinder, Helios, Solar Eagle and Vulture. None led anywhere, but in the last few years there have been advances in in solar cell and battery technology, and miniaturisation of payloads. In the US, IT giants Facebook and Google both funded solar-powered HAPS in recent years, with the ambition of filling the gaps in worldwide internet coverage. Surprisingly, perhaps, both projects have foundered, but back in the UK, some former Zephyr pioneers have developed a rival design named PHASA (Persistent High-Altitude Solar Aircraft). BAE Systems has invested in their company, named Prismatic.

Questions remain. Is there an optimum wingspan/payload ratio? Are all the flutter problems solved? What is the best way to climb and descend these delicate airframes through turbulent air to the calmer conditions of the stratosphere? Can a ‘constellation’ of them be successfully connected by laser communications? Will their role as loitering imaging platforms be challenged by the proliferation of small satellites offering ever-decreasing revisit rates at much lower cost and much higher resolution than previous spaceborne imaging systems?

Solar-powered airships are an alternative, theoretically offering greater stability and payloads. Again, though, some US projects burned through a serious amount of taxpayer’s money in the last decade. Remember HiSentinel, HALE-D and ISIS, all now abandoned? There are fabric and thermal management issues. The only current airship project appears to be the Thales Alenia Stratobus, due to fly in 2021. However, the Italian Aerospace Research Centre (Italian acronym CIRA) believes that hybrid airship would be best, and it showed a model at Farnborough.

For the moment, Airbus is clearly leading the way, with a real customer for the current Zephyr S (the UK Ministry of Defence) and a larger-payload version (Zephyr T) coming next. Airbus has even designed a multi-mode radar imaging system for Zephyr-T, weighing just 20kg.

However, a more prosaic means of filling the internet communications gap is already in service. The high-altitude balloons of Google’s Project Loon have proved their worth over remote locations in Africa, New Zealand and elsewhere, and as emergency substitutes for damaged ground base stations during the Puerto Rican hurricane disaster last year. The project employs massive computational power to accurately model the winds in the upper atmosphere so that the positions of these balloons can be adjusted through control of the gas volume. Their payloads are solar-powered, but they don’t rely on batteries to stay at 65,000 feet.