The stratosphere, a layer of our atmosphere often considered the edge of space, has long presented a formidable challenge to aviation. For decades, this high-altitude domain seemed out of reach for conventional aircraft. However, recent advancements in technology and innovative aircraft designs are heralding a potential golden age for stratospheric flight. But can planes truly fly in the stratosphere, and what does the future hold for aviation in this extreme environment?
In June 2022, the aviation world witnessed the launch of the Zephyr S, an aircraft resembling a futuristic pterodactyl, embarking on a mission that underscored the growing feasibility of stratospheric operations. With its immense, slender wings and lightweight structure adorned with shimmering solar panels, the Zephyr S, developed by Aalto HAPS, is designed for sustained flight in the stratosphere, typically between 60,000ft (18,300m) and 70,000ft (21,300m). This ambitious project, commissioned by the US Army, aimed to push the boundaries of flight endurance, echoing the spirit of record-breaking flights of the past, such as the 64-day Cessna flight in 1959.
A silver Sceye zeppelin is positioned outside its hangar, showcasing its large size and unique design for stratospheric flight.
The vision of “eternal flight” in the stratosphere, pioneered by British aviation expert Chris Kelleher, is becoming increasingly tangible. Kelleher’s initial Zephyr concept in 2002 envisioned unmanned aircraft leveraging solar power and lightweight materials for months- or even years-long missions aloft. The Zephyr S represents the realization of this vision, marking the first production model of such stratospheric aircraft.
The Stratosphere: A New Frontier for Aviation
The stratosphere, the second layer of Earth’s atmosphere, begins approximately at 33,000ft (10,000m) and extends up to around 160,000ft (48,800m). Flying above 50,000ft (15,150m) offers a significant advantage: escaping the turbulent weather patterns of the troposphere below. However, the trade-off is the significantly thinner air at these altitudes, posing substantial challenges for both aircraft propulsion and human survival.
Historically, balloons were the primary means of reaching the stratosphere. These buoyant giants could ascend to altitudes where air density is too low for conventional wings or air-breathing engines. Yet, sustaining life at these altitudes presented extreme risks, with early balloonists facing perilous conditions.
In 1931, humanity first ventured into the stratosphere when a balloonist reached 52,000ft (15,800m) in a pressurized gondola. Jeannette Piccard followed in 1933, becoming the first woman in the stratosphere, reaching 57,600ft (17,600m). The era of stratospheric exploration then shifted towards state-sponsored spy aircraft like the U-2 and SR-71 during the Cold War, and more recently, drones like the RQ-170. Today, the stratosphere is also populated by weather balloons, amateur high-altitude balloonists, and even creative stunts, like Cornish schoolchildren sending a pasty to 116,410ft (35,500m) using a weather balloon.
The Zephyr aircraft showcases its long and slender wings, a design crucial for maintaining lift in the thin air of the stratosphere.
The spirit of exploration continues with projects like the Windward Performance Perlan 2 glider, which set a glider altitude record of 73,800ft (23,500m) in 2018, surpassing even the U-2 spy plane’s maximum altitude. This glider utilized mountain waves generated by the Andes to ascend to stratospheric heights, demonstrating innovative techniques for high-altitude flight.
High Altitude Pseudo-Satellites (HAPS): A New Era of Stratospheric Aircraft
The Aalto Zephyr, originating from Airbus and now an independent entity, represents a new class of aircraft known as High Altitude Pseudo-Satellites (HAPS). These autonomous, ultra-lightweight aircraft, ranging from solar-powered gliders to silver zeppelins, are designed to establish a near-permanent presence in the stratosphere.
HAPS are poised to revolutionize various sectors. They offer the potential to deliver 4G and 5G cellular coverage and internet access, especially vital in disaster-stricken areas. They can also play a crucial role in environmental monitoring, such as detecting forest fires, and in military applications like tracking enemy movements. Compared to traditional satellites, HAPS offer advantages in cost-effectiveness, deployment speed, and operational flexibility.
Technological advancements, particularly in lightweight materials, solar panel efficiency, and battery energy density, are making long-duration stratospheric flight a reality.
The Zephyr aircraft is depicted during its ascent, a process that can take up to ten days to reach its operational altitude in the stratosphere.
Robert “Bob” Kraus, Dean of the John D Odegard School of Aerospace Sciences at the University of Dakota, highlights the design challenges for HAPS. “The challenge for their designers,” Kraus explains, “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.”
Unlike satellites in Low Earth Orbit, which orbit approximately 340 miles (547km) above Earth, HAPS operate much closer, significantly reducing latency in communication. This proximity, coupled with enhanced resolution and flexibility compared to satellites, strengthens the case for HAPS in remote sensing and high-speed communication applications.
Emerging Stratospheric Aircraft Designs
Innovation in stratospheric aircraft is taking diverse forms. In Roswell, New Mexico, Sceye is developing a large silver zeppelin HAPS. By replacing aluminum with carbon fiber, Sceye has drastically reduced weight, making their zeppelin significantly lighter than a Goodyear Blimp of comparable size.
Mikkel Vestergaard, founder of Sceye, describes their development journey: “We started small, with a 9ft (2.7m) version… Then in November 2020, we opened the hangar doors for the first time to a 270ft (82m) version.” By May 2021, their zeppelin reached its target altitude of 65,000ft (19,800m), showcasing its rapid ascent capability. A key advantage of the Sceye zeppelin is its payload capacity, capable of carrying significantly more weight than the Zephyr, with future versions potentially lifting up to 300kg (660lb). This capacity makes it ideal for deploying broadband communication arrays to serve large populations. Sceye has already conducted flights for broadband-to-smartphone services and plans to demonstrate methane leak monitoring capabilities.
The Sceye zeppelin is shown, having successfully reached its target operational altitude of 65,000 feet (19,800m), demonstrating its capability for stratospheric flight.
Urban Sky offers another innovative approach with reusable microballoons. These car-sized balloons, launched from a simple pickup truck, provide high-resolution aerial imagery for applications like wildfire monitoring and urban planning. Jared Leidich, co-founder of Urban Sky, emphasizes the advancements in weather modeling that make their operations feasible: “Twenty to 30 years ago the models were not good enough to do this.”
An Urban Sky microballoon design is presented, representing the historical preference for balloons in stratospheric exploration before the advent of modern stratospheric planes.
Navigating the Challenges of Stratospheric Flight
Despite the exciting advancements, increased stratospheric activity raises environmental and regulatory concerns. Climatologists are beginning to assess the potential impact of increased air traffic in the stratosphere. Michaela I Hegglin, a professor in atmospheric chemistry at the University of Reading, cautions, “Pollutants will remain in the stratosphere much longer because atmospheric mixing is a lot slower.” She emphasizes the need to consider the potential disturbances from a large fleet of stratospheric aircraft operating daily.
The Zephyr S flight in August 2022, although ultimately succumbing to gravity after a remarkable three-month, 35,000-mile journey, underscored the progress and challenges. Chris McLaughlin of the Aalto Haps program hailed the flight as “a huge success,” highlighting the team’s focus on weight reduction as a key factor in their achievements. Aalto plans to establish “Aalto ports” in regions with stable weather conditions to ensure consistent access to the stratosphere, reflecting the original vision of Chris Kelleher.
However, the regulatory landscape for stratospheric flight is still evolving. The incident involving a Chinese spy balloon over the USA in early 2023 highlighted the complexities of airspace sovereignty in the stratosphere. As stratospheric traffic increases, demands for traffic management and environmental regulations are likely to grow. “It’s a fine natural balance that you have there. And you don’t want to mess around with it,” warns Hegglin.
The Future of Planes in the Stratosphere
The allure of the stratosphere remains strong. Start-ups like Aalto, Sceye, and Urban Sky are overcoming technological hurdles, paving the way for routine stratospheric flight. While regulatory and environmental challenges lie ahead, the potential benefits of stratospheric aviation are compelling.
As Jared Leidich aptly puts it, “The stratosphere is mostly empty right now. So we’re still working out the basic physics… This is sort of level-zero mathematical understanding of the area, which makes it fun.” The journey to fully harness the potential of stratospheric flight is just beginning, promising a new era of exploration and innovation in aviation.
