A lighter, cheaper satellite
High-altitude pseudo-satellites are less expensive to launch than the conventional kind and safer too. Tim Dean reports.
Getting into space isn’t easy. Rocket launches are expensive and dangerous, and the greenhouse gas emissions that go with them are excessive. There has to be a better way.
The August test flight of Airbus’s new Zephyr was a serene affair. To launch the unmanned aircraft – which despite its 23-metre wingspan weighs only 53 kilos – a handful of people lifted it off the ground and threw it gently into the wind. As its propellers took over, they watched it silently soar into the sunrise.
Airbus hopes the vehicle – dubbed a high-altitude pseudo-satellite, or HAP – could do the job of satellites without the perils and expense of rocket launches. Soon there might be thousands gazing down upon us from the lofty reaches of the Earth’s atmosphere.
A HAP is essentially a super lightweight drone equipped with solar cells and batteries. The idea is the HAP will soar at an altitude of around 21 kilometres, above the weather but far below the 200-kilometre altitude of the lowest satellites.
The HAP gains altitude during the day using power from its solar panels to drive two tiny electric motors. By night it saves battery power by descending gradually, reaching 15 kilometres or so by morning. In principle, a HAP could remain aloft this way for months, and all for a fraction of the cost of a satellite.
“HAPs are strong where satellites struggle,” says Jens Federhen at Airbus, who leads the work on Zephyr – named after Zephyrus, the Greek god of the refreshing west wind.
“The main strength of HAPs is what we call ‘affordable local persistence’, that is, their ability to provide satellite-like services to limited areas on the ground at low cost, permanently and without interruption.”
The latest Zephyr 7 prototype looks ungainly, with its giant ultra-thin wings, but it is graceful in the skies. Its paper thin coating of highly efficient solar cells charge lithium-sulfur batteries that can store nearly twice the energy of a typical smartphone battery, weight for weight.
'Airbus hopes the Zephyr will provide an inexpensive alternative to satellite surveillance.'
Zephyr 7 recently completed a test flight lasting 11 days, which was impressive because the flight was conducted in winter, with shorter days in which to keep the batteries charged. The Zephyr record for remaining aloft is two weeks. As battery technology improves, Federhen expects that could extend to months.
The trick, he says, is keeping the weight of the Zephyr down to a minimum. “Our engineers look for weight reductions everywhere on the aircraft because it is always easier to save 10 grams in 10 places than to save 100 grams in one place.”
Airbus hopes the Zephyr will provide an inexpensive alternative to satellite surveillance. Satellites can only observe whatever happens to lie below them as they travel, but the Zephyr can keep an electronic eye trained on a particular location for days on end. And because it’s far closer to the surface than a satellite even in low-Earth orbit, the optics don’t need to be as large or complex to produce clear pictures.
It could also be used for communications by extending cellular networks into areas too remote to build cell phone towers – hence Google’s interest in HAPs.
A HAP can also easily touch down for maintenance or upgrades – try doing that with a satellite. Multiple HAPs would be significantly cheaper than a single satellite – which can cost $100 million just to get into orbit.
Airbus isn’t the only company developing HAPs. The US Defense Advanced Research Projects Agency gave Boeing $89 million in 2012 to develop the SolarEagle. This is a much larger craft with a wingspan of 120 metres. Its solar panels and fuel cells might keep it aloft for years.
Meanwhile Federhen and his team are working on the Zephyr 8. For those of us who love the drama of rocket launches, it may be time to reset expectations and learn to enjoy the serenity of hand propelled take off.
Paper-thin solar cells, capable of producing a kilowatt of power for every kilogram of panel, coat the Zephyr’s wings. The cells capture enough solar energy to keep the aircraft aloft for weeks at a time.
In a bid to keep the Zephyr’s weight to the absolute minimum, even landing gear has been sacrificed. The aircraft is so light, and flies so slowly, it can safely touch down on its belly.
The Zephyr’s 22.5-metre wing contains the lithium-sulfur batteries that store the energy captured by the solar panels. The swept-back, curled-down wing tips reduce drag, helping the batteries last longer.
Zephyr 7 specifications
Manufacturer: Airbus Defence and Space
Role: high-altitude pseudo-satellite (HAP)
Construction: carbon fibre
Weight: 53 kilograms
Wingspan: 22.5 metres
Powerplant: 2x 450 Watt electric motors
Cruising speed: 55 km/h
Cruising altitude: 21,000 metres
Payload: 2.5 kilograms