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The chronicles of human flight have always been interwoven with our aspiration to explore the skies. From the myth of Icarus to the modern airline traveller, there has been an innate human desire to fly faster, further, and higher. One of the most captivating aspects of this journey is high-altitude flight. Its history is as thrilling as its future prospects.

Atmospheric Layers

Layers of the atmosphere taken onboard the International Space Station in 2007. (Photo credit: NASA)

At this point, doing a refresher on the different layers of the atmosphere could be helpful. Unless you’re on your way to space, most high-altitude flight occurs in the troposphere and stratosphere.

Troposphere. Known as the lower atmosphere, most weather occurs in this region. The troposphere begins at the Earth’s surface, but the height of the troposphere varies. It is 18-20 kilometres high at the equator, around 9 kilometres high at 50°N and 50°S, and around 6 kilometres high at the poles. The gases in the troposphere decrease with height, and the air becomes thinner. Therefore, the temperature in the troposphere also decreases with height to around -50°C at the tropopause.

Stratosphere. The stratosphere extends from 6 to 20 kilometres above the Earth’s surface to around 50 kilometres. This layer holds 19 percent of the atmosphere’s gases but holds very little water vapour. Astoundingly, stratospheric temperatures actually increase with height. Heat is produced in the process of the formation of Ozone, and this heat is responsible for temperature increases, from an average -50°C at the tropopause to a maximum of about -15°C at the top of the stratosphere.

Mesosphere, Thermosphere and Exosphere. The mesosphere extends from around 50 to 85 kilometres altitude. The thermosphere, also known as the upper atmosphere, extends from 85 to 600 kilometres. The Kármán line, at an altitude of 100 kilometres above mean sea level, is a proposed conventional boundary that demarcates where space begins. Finally, we get to the exosphere, which extends from about 600 to 10,000 kilometres above our planet.

The Balloon Age

The narrative of high-altitude flight began with hot air balloons. In 1783, the Montgolfier brothers in France achieved the first human flight in a balloon filled with hot air. Balloons would, for the next century, remain the primary means to reach significant altitudes. In the late 19th and early 20th centuries, daredevils and explorers used balloons to set altitude records, some even reaching the stratosphere. The current high-altitude balloon flight record, reaching 41,419 metres, was achieved in 2014 by American computer scientist Alan Eustace.

The Jet Age

X-15 “Flight 188” begins its acceleration to Mach 6.7 in 1967.  (Photo credit: U.S. Airforce)

After major advancements in aviation and rocketry during World War II, the technological stage was prepared to take things to the next level. At 14:31:49 local time on the 3rd October 1967, U.S. Air Force test pilot William John “Pete” Knight sat in his cockpit under the belly of his mothership B-52 Stratofortress. In one second, he would jettison, ignite his rocket, and begin an acceleration and climb to an altitude of 31.1 kilometres (102,100 feet) and become the fastest aviator in history, reaching a speed of 7,270 km/h (Mach 6.7). Flight 188 was one of 199 flights carried out by 12 pilots, including Neil Armstrong, for the North American X-15 experimental aircraft program. The aircraft didn’t only set speed records but altitude records too, it reached a height of 107.9 kilometres (354,200 feet) in August 1963. The aircraft was a defining airframe during a time when high-altitude flight was playing a critical role in understanding the upper atmosphere, and the design of future spacecraft which would need to pass through it.

Flight 188 held the crewed winged space plane speed record until it was broken by STS-1, the Space Shuttle Columbia on April 14, 1981. 188 is still a speed record for a non-orbital aircraft in the atmosphere under a powered crewed flight. This is a perfect example of how high-altitude flight becomes a very complicated state of affairs quite quickly when we consider all the different types of aircraft involved; balloons, gliders, fixed-wing internal combustion, and rocket planes make an appearance under different categories on the record lists.

High-altitude flight has always been intertwined with the pursuit of scientific knowledge. By ascending to great heights, researchers have been able to gather invaluable data on atmospheric conditions, weather patterns, and the behaviour of various phenomena. This wasn’t always necessarily with intent, either. Many discoveries have been made by accident, including the discovery of the jet stream.

The Military Angle

An ER-2 aircraft takes flight. (Photo credit: NASA)

Developing high-performance aircraft for military purposes has been another driving force behind high-altitude flight. Over the years, military strategists have recognised the advantages of operating at high altitudes, where aircraft can evade enemy radar and gain tactical superiority. This has led to the creation of advanced reconnaissance planes and missile systems capable of reaching extreme heights. The quest for dominance in the skies has fueled innovation and the continuous improvement of aircraft design and technology. One of the most famous examples of a military endeavour to fly high was the United States’ U-2 spy plane.  The aircraft was used during the Cold War over the Soviet Union, Cuba, Vietnam, and China and was designed to fly at 21.3 kilometres (70,000 feet), above interceptor aircraft, missile and radar technology of the time. This capability did not last long in typical arms race style, as adversaries improved their interceptor capabilities.

It is often the case that older military technology falls into a scientific role later in life. A prime example of this is the ER-2, NASA’s high-altitude research aircraft – the U-2’s gentle sister. NASA uses this aircraft for scientific experiments, including payload testing and instrument design for future satellite missions.

No Pilots Onboard

The Helios design was built on its predecessors, the NASA Pathfinder and NASA Centurion aircraft. (Photo credit: NASA)

It is important to acknowledge that high-altitude flight is not solely limited to crewed aircraft. Uncrewed balloons, gliders, and rocket planes have all played significant roles in exploring the upper atmosphere. Balloons, for instance, have provided scientists with a platform for conducting experiments and observations at otherwise inaccessible altitudes. NASA’s Long Duration Balloon is a prominent example of how balloons have played an important role in accessing the high atmosphere. Gliders have demonstrated their efficiency in soaring through the thin air at high altitudes, showcasing the potential for carbon neutral high altitude flight. As technology and vision have developed, electric aircraft are beginning to have their say too. The highest altitude achieved by an electric-powered aircraft was by the NASA Helios. In August 2001 it reached an altitude of 29.52 kilometres (96,863 feet).

Today, the challenges of high-altitude flight are as much about sustainability as they are about capability. With the increasing awareness of environmental challenges, there’s a focus on developing high-altitude flight technologies that are efficient and eco-friendly. Drones, for example, are now being used for research purposes in the stratosphere, offering a glimpse into the power of unmanned high-altitude exploration.

In the 1960s, things began at high speed with rocket planes, but now aircraft speeds are slowing down. Given the limited propulsion provided by batteries and electric motors, high-altitude flight of these aircraft must be slow, which comes with its own difficulties.

The HAPS Challenge

High altitude platform stations (HAPS) fall under two categories: aerodynamic, i.e. aeroplanes with wings using flight, or aerostatic, i.e. airships or balloons.  Airships and balloon efforts have issues regarding the dynamic nature of the atmosphere, and have limited capability to overcome wind, making the often primary objective of remaining at a fixed point difficult. Their main identified use is in communications, as they offer limited flexibility to change location quickly, which would be more useful for earth observation.

Aircraft have an advantage with respect to these limitations, but their success also has complications. The key challenge in designing an aerodynamic HAPS vehicle is being able to achieve the energy balance required for flight. That is, harvesting enough solar energy during the daylight hours to charge the batteries in preparation for flight through the night using the batteries alone. To do this requires a vehicle that is extremely efficient across the board. The vehicle needs to have low aerodynamic drag, extremely light structures, highly efficient motors and propellers, and efficient, lightweight solar cells and batteries.

2023 test flight of the Kea Atmos Mk1 in Canterbury, New Zealand

Solar-powered uncrewed aircraft have only become commercially achievable in the last few years as battery technology has improved to produce cost-effective batteries with the required energy density to perform the task. Some of the key difficulties in HAPS design are:

1. Energy Density of Solar Power. While solar power is a renewable and clean energy source, the energy density of sunlight is relatively low. The limited amount of energy that can be captured by solar panels necessitates the use of highly efficient solar cells to maximize power generation.
2. Weight and Power Constraints. HAPS aircraft need to be lightweight in order to achieve and maintain high altitudes. However, incorporating sufficient solar panels to generate the required power while keeping the weight within acceptable limits is challenging. It becomes a delicate balance between power generation and aircraft weight, as additional solar panels may lead to increased weight and decreased aerodynamic efficiency.
3. Aerodynamic Considerations. Achieving optimal aerodynamic efficiency is crucial for solar-powered HAPS aircraft, as any additional drag can reduce flight endurance and efficiency. In addition as the vehicle flies very slowly, atmospheric gust is of the same order of magnitude as the forward airspeed. This means the wing will see a significant change in the angle of attack due to gust. The vehicle must be aerodynamically designed to be less susceptible to gusts and have sufficient control power to correct the attitude of the vehicle. The flying qualities of the vehicle are usually fairly marginal based on designing the vehicle to meet the other aforementioned criteria of weight and aerodynamic efficiency.
4. Structural Considerations. HAPS aircraft must transcend through all layers of the atmosphere to reach the high altitudes in which they operate. The vehicle is exposed to various atmospheric conditions, including gusts, turbulence, strong winds, temperature extremes, and low air density. Designing an airframe capable of withstanding these conditions while remaining lightweight is mutually exclusive, and the engineering challenge is achieving the right compromise.
5. Autonomous Systems. The vehicle is designed to operate for months at a time and is controlled autonomously from a ground station. Developing an autopilot that can control a vehicle that is very elastic, owing to its lightweight construction, through a turbulent atmosphere is non-trivial.
6. System Complexity. Integrating solar panels, energy storage, propulsion, and control systems into a single aircraft requires sophisticated engineering and system integration. The design must ensure that all components work together seamlessly to maximize efficiency and flight endurance.
7. Operational and Regulatory Complexity. HAPS vehicles are a novel concept, and standards and regulations are at an immature level in terms of airspace regulations and certification requirements. The vehicle ideally would be operated over multiple countries, necessitating cooperation and coordination with the airspace regulator from each country.

The Future

As we look to the future, high-altitude flight will continue to play a crucial role in pushing the boundaries of human achievement. From advancements in hypersonic flight to the development of reusable space launch systems, the quest for higher altitudes will fuel innovation and shape the future of aerospace technology. We will likely see hybrid technologies play a role in propulsion and the continuation of space tourism ventures. How drones and AI will play a role in uncrewed flight is intriguing, while commercial airliner activities will likely strive to fly higher for more efficiency as technology advances.  In conclusion, the history of high-altitude flight is a testament to human ingenuity and our relentless pursuit of the unknown.

Standing on the cusp of unprecedented aerial innovations, we are reminded that the sky is not the limit. The journey of exploring high-altitude realms will continue, with a promise of blending the best of technology, sustainability, and adventure.