Short-haul it most certainly is not. Defence giant BAE Systems proudly boasted about the maiden flight of its Phasa-35 drone last week, flagging the prospect of the high-altitude solar-powered craft eventually staying aloft for a year.
Admittedly, its first flight at an Australian test centre was considerably shorter and lower.
“It lasted about an hour,” says Ian Muldowney, engineering director at BAE. “It was about testing that it flew.”
BAE is thinking big about the programme. Although Phasa-35 has a 115ft (35m) wingspan – the same as a Boeing 737 – it weighs just 330lb (150kg). About 10pc of this is payload. It’s never going to hold pilots, even if they could endure its long flights, instead relying on autonomous systems or ground control.
Phasa-35 is not going to set any speed records either. Its solar-powered electric motors drive two propellers that give a cruising speed of just 30 knots once it gets to its operating altitude of 65,000ft (almost 20,000m), which Muldowney says it takes about “two or three days to meander up to”. At night it drops to a lower altitude as there is no sun to power it, climbing back up to altitude when the light returns.
But speed and power are not the point of Phasa-35, the latest of a crop of what are known as “High Altitude Pseudo Satellites” – or HAPS.
Their aim is to provide a semi-permanent high-altitude base which can carry sensors that look down, monitoring vast swathes of ground from their lofty locations on the edge of space and flying above the weather. Another role is to carry communications gear, and act as mobile phone masts in the sky, providing coverage to areas without it. Flying at 65,000ft, a HAPS can cover an area 125 miles (200km) in diameter.
Miniaturisation of electronics means that relatively small payloads are not an issue. Communications kit and sensors that might have weighed tons a decade ago are now far lighter.
“Think of HAPS as satellites without wings,” adds Mr Muldowney.
They’re certainly cheaper than satellites. Nasa reckons it currently costs about $20,000 (£15,400) to launch a single kilogram into space, and that’s before the cost of building a satellite with all its ancillary systems. Budget a couple of hundred million to build a high-end spy satellite that weighs a ton or two.
HAPS offers similar – but not quite as good – capabilities at a lower price but with greater flexibility. Once a satellite is in orbit, it’s up there for good, either in a fixed orbit or able to be repositioned but requiring it to burn up its limited fuel during its average 15-year life.
HAPS can be easily repositioned using effectively free solar power, or landed and reloaded with different payloads for different missions. However, their long climbs to operating heights mean that flights have to be planned, while weather must also be taken into account.
Although they don’t have the endurance of satellites, these lightweight aircraft can significantly outlast manned vehicles. Airbus’s Zephyr – of a similar size to BAE’s vehicle but which first flew more than five years ago – holds the world record for the longest unrefuelled flight, staying aloft for 25 days and nights.
When it comes to cost, Mr Muldowney says he will “sell you a Phasa-35 for a few million, none of the technology in the airframe is classified” once testing and certification is completed in 2022.
“But that’s not the point,” he adds. “We’re looking at offering them to commercial customers who need them for specific purposes.”
This could be for surveying land or crops, mapping or restoring communications in disaster-hit areas. Advances in the capabilities of digital sensors mean that applications for the information they collect are limited only by human imagination.
For military purposes, it is easy to see the attraction. HAPS can be tasked to spy on a region, their altitude meaning they can stay out of the reach of most threats – and even if they are in danger, there’s no risk to human life.
Sir Brian Burridge, chief executive of the Royal Aeronautical Society and a former RAF Air Chief Marshal, adds: “They offer persistent surveillance in a way that you cannot with a conventional aircraft with limited endurance – these are pretty much fire and forget assets.” BAE and Airbus are not alone in pursing the technology. In the commercial sphere Google and Facebook looked into HAPS as a way of offering broadband in remote areas, but have since scaled back their interest. Google is looking at balloon technology, while Facebook wrapped its work into Airbus’s.
Japan’s Softbank is funding US company AeroVironment’s work on a much larger HAPS called Hawk 30. This 255ft wingspan, 10-propeller aircraft is intended to work as a telecommunications hub.
In the US, Boeing is looking at a similar-sized aircraft with a payload of 55lb (25kg), though this has yet to fly.
Sir Brian says that he “can see the attraction of HAPS to the UK and European mind” where military budgets are less able to cope with the costs of satellites than in the US.
Independent aerospace analyst Howard Wheeldon agrees. With China rapidly developing its space capabilities, a dependence on satellites might even become a vulnerability.
“Satellites could be vulnerable and are expensive, time consuming and hard to replace,” he says. “We are much better at realising this than the US.”