The stratosphere has long been a challenging environment for aviation. But new designs could usher in a golden age of stratospheric flight.
I wrote about some amazing people, flying machines and balloons for BBC Future. Have a read in full below – or the original here!
It is June 2022, and a flying machine that looks like a cross between a prehistoric beast and a spaceship is about to take off. Named the Zephyr 8, it has long spindly wings the length of an airliner’s. Together with its small, thin body and head, these make it resemble a pterodactyl. Its shimmering tinfoil-like solar panels and lightweight skeletal frame are more like something you’d see on a craft meant for space…
It takes five or six people to lift the Zephyr. When it’s ready to launch, they run across the runway holding the craft above them. The machine’s two small propellers turn frantically before the plane begins its slow climb up into the cloudless sky to 60,000ft (18,300m) or 70,000ft (21,300m) – a relentless ascent into the stratosphere that can take 10 hours.
Its mission for the US Army is a secret, but clearly on its manufacturer’s mind is the desire to shatter a few records, particularly that for the longest flight duration for any type of airplane, which has stood for 63 years. In 1959 two men flew a four-seat Cessna light aircraft for 64 days, 22 hours and 19 minutes, refuelling in-flight from a truck.
British aviation pioneer Chris Kelleher designed the first Zephyr in 2002. His vision was of an uncrewed aircraft capable of “eternal flight” in the stratosphere. He foresaw that solar power and lightweight materials would lead to aircraft capable of staying aloft for months, or even years. The Zephyr S is the first production model.
The stratosphere is the second layer of our atmosphere. It begins around 33,000ft (10,000m) and ends at around 160,000ft (48,800m). If an aircraft can fly above 50,000ft (15,150m), it can fly above the turbulent weather that we experience closer to the ground, in the troposphere. The problem is that that high the air is very thin, making flying – and breathing – a challenge.
The long, spindly wings of the Zephyr help keep it aloft in the thin air of the stratosphere (Credit: Zephyr)
For a long time, there was only one option if you wanted to explore the stratosphere, and that was a balloon. Balloons could climb to the ceiling of the world, where there is too little oxygen for wings, or air-breathing engines. The problem then was staying alive at those altitudes, and a good number of balloonists failed trying.
In 1931, humanity finally reached the stratosphere, with one balloonist achieving a height of 52,000ft (15,800m) in a pressurised gondola attached to a hydrogen-filled balloon. Two years later, Jeannette Piccard became the first woman to reach the stratosphere, with an ascent to 57,600ft (17,600m).
From the 1950s it was the turn of expensive, state-financed and top-secret spy planes like the U-2, the SR-71, and recently the RQ-170 drone. Now the stratosphere is also home to weather balloons, amateur high-altitude balloonists, Chinese spy balloons and marketing stunts. A group of Cornish schoolchildren used a weather balloon to lift a Cornish pasty to an incredible 116,410ft (35,500m). It returned, frozen.
Yet the age of exploration isn’t over. The pressurised Windward Performance Perlan 2 glider set a new altitude record of 73,800ft (23,500m) in September 2018. It flew higher than any glider has ever flown, and even the maximum record altitude of a U-2 spy plane, using the waves created by the Andes mountain peaks to lift the glider all the way up to the stratosphere.
The challenge for their designers is to find the sweet spot of having an aircraft that is light and strong enough to stay up at those altitudes for long enough – Robert Kraus
The British-built Aalto Zephyr (the company was recently spun-out of Airbus) is one of a new type of flying machine designed to reconquer the stratosphere through eternal flight. Aircraft that, when combined with miniaturisation of components and powerful new computer models of the atmosphere, give humanity the possibility of a near-permanent presence at these high altitudes for the first time.
Known as Haps (high altitude pseudo-satellites), these autonomous, super-lightweight aircraft range from solar-powered gliders to solar-powered silver zeppelins.
Their jobs include providing 4G or 5G phone coverage and internet service after a disaster, spotting forest fires, and tracking the movement of enemy forces during wartime. All the while, they can do it better, more cheaply, faster, and more flexibly than satellites.
Technological improvements, especially in the development of lightweight materials, solar panels, and battery technology, make the “long endurance” part of the brief a reality.
The Zephyr can take up to 10 days to climb to its operational height (Credit: Zephyr)
“The challenge for their designers,” says Robert “Bob” Kraus, dean of the John D Odegard School of Aerospace Sciences at the University of Dakota, “is to find the sweet spot of having an aircraft that is light and strong enough to stay up at those altitudes for long enough, can haul enough payload to be useful to paying customers, and can survive the ascent – and descent – through the troposphere.”
Even in Low Earth orbit, micro-satellites orbit the Earth around 340 miles (547km) higher than an aircraft such as the Zephyr. This means there is still a slight latency – a delay to communication that can impede high-speed broadband communication. There are also some fundamental limitations in the use of satellites for remote sensing in terms of resolution, speed and flexibility that make the case even stronger for the use of Haps.
Some are now beginning to take physical shape.
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Inside a large cylinder-like structure in Roswell, New Mexico, for instance, is a large silver zeppelin Haps. It is another “eternal” aircraft. Its aluminium parts have been replaced with carbon. A Goodyear Blimp built to the same size would be 12 times heavier.
“We started small, with a 9ft (2.7m) version that couldn’t lift its own weight, so we had to attach it to a balloon just to start generating data,” says Mikkel Vestergaard, founder of Sceye, the materials science company behind the stratospheric zeppelin. “Then in November 2020, we opened the hangar doors for the first time to a 270ft (82m) version.”
By May 2021, the zeppelin had made it to its target altitude of 65,000ft (19,800m) at an average rate of 1,000ft (305m) a minute, and this year alone they have six flights. What really sets the zeppelin apart is the payload it can carry. While the Zephyr can lift around 5kg (11lb), the zeppelin can already lift more than 100kg (220lb). Future versions could lift 300kg (660lb), or a communication array to broadcast broadband direct to a large number of people. Broadband services can be provided by zeppelin instead of masts.
Once you get to the stratosphere, there’s no point in staying unless you can turn that into a business – Mikkel Vestergaard
So far Sceye has run two flights for broadband-to-smartphone. This September will see its first demonstration flight for methane leaks monitoring. (Read about the growing fleet of satellites identifying methane leaks from space)
“We’re very big,” he laughs. “We’re very visible. And so we work best at home in a friendly territory, to connect people, to help people.
“At the end of the day, once you get to the stratosphere, there’s no point in staying unless you can turn that into a business.”
In the past, balloons such as this design from Urban Sky, have been designers’ preferred method to reach the stratosphere (Credit: Urban Sky)
Some of the balloons used to carry humans to the stratosphere have been the size of a football stadium. Not so Urban Sky‘s. Inflated on the ground, these reusable microballoons are the size of a car, with a payload attached underneath.
As the images show, they offer the high-res aerial imagery needed to monitor the aftermath of wildfires, and to even image entire cities, which insurance companies or city planners might need.
“We can literally launch from the back of the pick-up truck,” says Jared Leidich, co-founder and chief technology officer of Urban Sky. “We figure out from the weather models where we would need to launch a balloon from to have it passively float over its target, and drive there. Twenty to 30 years ago the models were not good enough to do this.”
But climatologists are starting to worry about the impact all this increased activity may have on the stratosphere. Water vapour is present in minute amounts. So, any increase in its use due to the use of hydrogen powered aircraft may have a disproportionate affect, especially contributing to the destruction of ozone layer.
You must consider how you might disturb the stratosphere if you suddenly start building a big aircraft fleet that will fly up there every day – Michaela I Hegglin
“Pollutants will remain in the stratosphere much longer because atmospheric mixing is a lot slower,” says Michaela I Hegglin, a professor in atmospheric chemistry at the University of Reading in the UK and a member of the Forschungszentrum Juelich in Germany.
“So, you must consider how you might disturb the stratosphere if you suddenly start building a big aircraft fleet that will fly up there every day,” says Hegglin.
At around 4am on 19 August 2022, the Zephyr finally succumbed to gravity after an incredible three-month flight of 35,000 miles (56,300km) at around 70,000ft (21,300m) and over 64 days aloft – only hours away from breaking the 1959 endurance record.
The Sceye zeppelin has already reached its target altitude of 65,000ft (19,800m) (Credit: Sceye)
“The flight was a huge success,” says Chris McLaughlin of the Aalto Haps programme. “It flew all the way down to South America on a mission and all the way back again. And the US Army pronounced themselves happy, but we are not allowed to open up on what they did with it.”
Its success was due to the singular focus of Aalto’s engineers at Farnborough in the UK, says McLaughlin. “One of the great skills of the team in Farnborough is they are obsessive about reducing weight. How do we reduce the weight of that wiring loom? Is there a better piece of carbon that we can put in place to strengthen this and at the same time reduce weight?”
The eight-to-10 hours the Zephyr takes to ascend to the stratosphere means that the aircraft needs a very stable weather environment to fly in. “We are planning to build several ‘Aalto ports’ around the world in countries with very good, stable weather at different times of the year,” says McLaughlin. “So we’ll always be able to get access to the stratosphere, and that was all part of Chris Kelleher’s original concept.”
The predicted increase in traffic may lead to demands for stratospheric traffic and environmental controls
In April 2023, Aalto signed a contract for a Zephyr trial flight over Japan with a mobile communications payload. The Zephyr is expected to enter commercial service by the end of 2024.
In their bid to reconquer the stratosphere, start-ups such as Aalto, Sceye and Urban Sky may be overcoming the technological hurdles to “eternal flight”, but they are going to face an increasing number of regulatory ones. The recent ill-fated flight of the Chinese spy balloon over the USA has highlighted that the stratosphere is national, not international, airspace, and the predicted increase in traffic may lead to demands for stratospheric traffic and environmental controls.
“It’s a fine natural balance that you have there. And you don’t want to mess around with it,” warns Hegglin.
The lure of flying in the stratosphere, however, may be hard to resist.
“The stratosphere is mostly empty right now,” says Leidich. “So we’re still working out the basic physics… This is sort of level-zero mathematical understanding of the area, which makes it fun.”