Hypersonic flight
Based on Wikipedia: Hypersonic flight
In August 2021, China launched something that made Pentagon officials very nervous. A vehicle shot into low-Earth orbit, circled the entire planet, then dropped back into the atmosphere and maneuvered toward a target—missing by only about two dozen miles. China insisted it was merely a spacecraft. American defense analysts weren't so sure.
Welcome to the new arms race, one measured not in megatons but in Mach numbers.
What Makes Flight "Hypersonic"?
The word sounds impressive, but what does it actually mean? Hypersonic flight is any flight faster than Mach 5—that's five times the speed of sound, roughly 3,800 miles per hour at sea level. To put that in perspective, a commercial airliner cruises at about Mach 0.85. A bullet fired from a rifle travels at around Mach 2 or 3. At Mach 5, you could travel from New York to London in under an hour.
But hypersonic isn't just "really fast supersonic." Something fundamentally different happens to air at these speeds.
When you push through the atmosphere at Mach 5 and beyond, the air molecules themselves start to break apart. Oxygen molecules split into individual oxygen atoms. At higher speeds and temperatures—above 3,700 degrees Celsius—even the strong bonds holding nitrogen molecules together start to snap. The air around your vehicle becomes a soup of reactive particles, a plasma that glows and burns and creates entirely new chemical compounds as it flows.
This isn't just a physics curiosity. It means the rules change. The equations that work perfectly well for designing supersonic jets become increasingly unreliable. Heat loads become extreme. Materials that shrug off ordinary aerodynamic heating begin to melt, ablate, or chemically react with the plasma streaming past them.
The First Humans to Go Hypersonic
The first manufactured object to punch through this barrier was cobbled together from spare parts. In February 1949, engineers at White Sands bolted a small WAC Corporal rocket on top of a captured German V-2 missile. They called it the Bumper rocket—not the most glamorous name for a historic vehicle. When they lit the fuse, the two-stage rocket screamed to 8,290 kilometers per hour, roughly Mach 6.7.
The vehicle didn't survive. It burned up on reentry, leaving only charred fragments. But it proved hypersonic flight was possible.
The first human to experience hypersonic speed was Yuri Gagarin, in April 1961, during his single orbit around Earth. His Vostok capsule had to survive reentry at hypersonic velocities—not through any clever aerodynamic design, but simply by absorbing the punishment through an ablative heat shield that burned away, carrying the heat with it.
A month later, Alan Shepard became the second person to go hypersonic, though his flight was suborbital—up and down, a brief arc over the Atlantic. His Mercury capsule punched back into the atmosphere above Mach 5, subjecting him to forces that would have killed an unprotected human.
But these were passengers, strapped into capsules, along for the ride. The first person to actually fly a hypersonic vehicle—to control it, to feel it respond to inputs—was Robert White, a test pilot with the calm demeanor that the job demanded. In November 1961, he pushed the X-15 research aircraft past Mach 6.
The X-15: Flying on the Edge of Space
The X-15 remains one of the most remarkable aircraft ever built. It looked less like an airplane and more like a missile with stubby wings and a cockpit bolted on. Which, in a sense, it was.
The aircraft couldn't take off on its own. A B-52 bomber would carry it to altitude, then drop it like a bomb. The pilot would ignite the rocket engine and ride a column of thrust toward the edge of space. At its fastest, in October 1967, an X-15 reached Mach 6.7—over 4,500 miles per hour.
At these speeds, the leading edges of the aircraft glowed cherry red. The friction of air molecules slamming into the vehicle at such velocities converted kinetic energy into heat with brutal efficiency. The X-15's skin was made of a nickel-chrome alloy called Inconel X, chosen specifically because it maintained its strength at temperatures that would soften ordinary steel.
The aircraft flew 199 times between 1959 and 1968. Thirteen of those flights exceeded 50 miles in altitude—high enough that by U.S. Air Force standards, the pilots earned astronaut wings. The program taught engineers things about hypersonic flight that wind tunnels simply couldn't reveal.
The Physics of Going Very, Very Fast
To understand why hypersonic flight is so challenging, you need to think about what happens to air when something slams into it at five times the speed of sound.
Imagine the nose of a hypersonic vehicle. Air molecules ahead of it can't get out of the way fast enough—the vehicle is approaching faster than the air's own pressure waves can warn neighboring molecules to move. So the air piles up, compressed violently into a shock wave.
At this shock wave, the air's velocity drops almost instantaneously. And when you take something moving at thousands of miles per hour and stop it abruptly, all that kinetic energy has to go somewhere. It goes into heat.
The stagnation point—the very tip of the nose, where the airflow velocity is momentarily zero—experiences the highest temperatures. But there's a counterintuitive twist: the air right at this point actually forms a kind of insulating cushion. The shock wave deflects most of the airflow around the vehicle, and the relatively stationary air at the stagnation point, while extremely hot, doesn't transfer as much heat as you might expect.
The real thermal assault happens at the boundary layer—the thin region where fast-moving air meets the vehicle's surface. Here, friction converts kinetic energy to heat through viscous dissipation. The boundary layer at hypersonic speeds becomes enormously thick compared to subsonic flight, and the temperatures within it can exceed the melting point of most metals.
When Air Stops Being Air
Here's where hypersonic physics gets truly strange. At temperatures above 2,000 Kelvin (about 1,730 degrees Celsius), oxygen molecules start to dissociate—the bonds holding two oxygen atoms together break, leaving individual, highly reactive oxygen radicals floating in the stream.
Heat things up more—above 4,000 Kelvin—and the even stronger bonds of nitrogen molecules begin to break too.
Now you have a plasma. Oxygen radicals and nitrogen radicals start recombining in new ways, creating compounds that didn't exist moments before. Nitric oxide forms. Some of this nitric oxide ionizes, shedding electrons to become electrically charged. The air around your hypersonic vehicle has become a reactive, glowing, electrically conductive plasma.
This creates problems beyond just thermal management. Radio waves don't pass through plasma very well. During the space shuttle's reentry, there was a communications blackout period when the plasma sheath around the vehicle blocked all signals. This same phenomenon affects any hypersonic vehicle, making guidance and communication during certain phases of flight extremely difficult.
Scramjets: Breathing Fire at Mach 5
If you want to fly hypersonic, you need propulsion. Rockets work, but they're inefficient—you have to carry both fuel and oxidizer, and the weight adds up quickly.
A jet engine is much better. It scoops up oxygen from the atmosphere, so you only need to carry fuel. But conventional jet engines have a problem: they slow the incoming air to subsonic speeds before combustion. At hypersonic velocities, compressing the air this much generates so much heat that the engine melts itself.
A ramjet improves on this. It uses the vehicle's forward motion to compress incoming air, with no moving parts to wear out. But ramjets still need to slow the air substantially before burning fuel, limiting them to around Mach 5.
The scramjet—which stands for "supersonic combustion ramjet"—solves this by doing something that sounds impossible. It burns fuel in a supersonic airstream. The air entering a scramjet never slows below the speed of sound. Fuel is injected into this screaming torrent and somehow ignites, burns, and produces thrust.
Making this work is fiendishly difficult. The air passes through the combustion chamber in milliseconds. Mixing fuel and air uniformly, igniting the mixture, and completing combustion before it exits the engine requires extraordinary precision in design.
The National Aeronautics and Space Administration's X-43A demonstrated scramjet propulsion in 2004, burning for just 10 seconds before gliding for 10 minutes. The Boeing X-51 Waverider did better in 2013, sustaining scramjet combustion for three and a half minutes while reaching Mach 5.1. These were proof-of-concept flights, but they demonstrated that sustained hypersonic air-breathing flight was achievable.
The Thin Air Problem
Hypersonic vehicles typically fly at high altitudes, where the air is thin. This helps with drag—less air means less resistance—but it creates a new set of physics headaches.
At sea level, air molecules are packed tightly together. The average distance a molecule travels before hitting another—called the mean free path—is about 68 nanometers, far smaller than any feature on an aircraft.
At 100 kilometers altitude, the mean free path stretches to about a foot. At this scale, the continuous-fluid assumptions that underpin traditional aerodynamics begin to fail. Air starts behaving more like a collection of individual particles bouncing off the vehicle surface rather than a smooth flowing medium.
Engineers quantify this with something called the Knudsen number—the ratio of the mean free path to a characteristic dimension of the vehicle. When the Knudsen number approaches one, especially around sharp features like the nose cone, standard aerodynamic equations become unreliable. You have to switch to kinetic theory, treating each air molecule as an individual projectile.
Waveriders and Boost-Glide Vehicles
Two main approaches have emerged for hypersonic flight: cruise missiles powered by scramjets, and boost-glide vehicles that ride their initial velocity like a stone skipping across water.
A waverider is designed to ride its own shock wave. The underside of the vehicle is shaped so that the shock wave, instead of dissipating uselessly, provides additional lift. The Boeing X-51 was a waverider, its flattened shape optimized to surf the pressure wave it created.
Boost-glide vehicles take a different approach. A rocket boosts them to hypersonic speed and high altitude, then they glide back down through the atmosphere, trading altitude for range. Without an engine running, they're harder to detect with infrared sensors. And because they fly in the atmosphere rather than following a ballistic arc through space, they can maneuver—dodging, weaving, changing course in ways that make interception extremely difficult.
China's XingKong-2, which translates poetically as "Starry Sky-2," first flew as a waverider in August 2018. Russia's Avangard is a boost-glide vehicle designed to sit atop intercontinental ballistic missiles, deploying during reentry to maneuver toward targets at speeds that conventional missile defenses cannot track.
The New Arms Race
The strategic appeal of hypersonic weapons is obvious. Traditional ballistic missiles follow predictable parabolic arcs—launch them, and sophisticated tracking systems can calculate where they'll land and attempt to intercept them. Hypersonic glide vehicles reenter the atmosphere and can turn, making their final destination unpredictable until very late in flight.
Russia has been particularly aggressive in developing these systems. The Avangard uses composite materials capable of withstanding temperatures up to 2,000 degrees Celsius—the original carbon fiber solution proved unreliable, so engineers developed new heat-resistant compounds. In December 2019, Russia fielded Avangard vehicles mounted on intercontinental ballistic missiles with the Yasnensky Missile Division.
The 3M22 Zircon is Russia's hypersonic anti-ship missile, designed to sink aircraft carriers before they can react. Tests over the White Sea demonstrated the system's capability, though Ukraine's claim that a Zircon was shot down in February 2024—apparently not even reaching Mach 3—suggests the technology may not be as mature as Russian claims indicate.
China's development has been even more ambitious. That orbital test in August 2021—the one that circled Earth before descending toward a target—demonstrated a capability the United States doesn't yet possess. The DF-27, tested in February 2023 according to leaked documents, covered 1,900 kilometers in just 12 minutes. That range puts Guam, and American aircraft carriers operating in the Pacific, within reach.
Perhaps most unsettlingly, Chinese researchers have begun running artificial intelligence simulations of hypersonic air combat. One scenario showed a Mach 11 aircraft outrunning a Mach 1.3 fighter while firing missiles backward at its pursuer. The fire control systems for such over-the-shoulder launches don't exist yet. But the fact that anyone is seriously modeling such scenarios suggests how rapidly the field is evolving.
America Plays Catch-Up
The United States, despite pioneering hypersonic research with the X-15 decades ago, has found itself trailing in the current competition. Around 2018, Russian and Chinese tests prompted a flurry of American programs.
The Common Hypersonic Glide Body, a joint Army-Navy project, successfully tested a prototype in March 2020. The Air Force's AGM-183 and the Army's Long-Range Hypersonic Weapon entered development. But progress has been uneven—the Air Force dropped out of the tri-service project in 2020, and a test in October 2021 failed when the booster malfunctioned.
Dr. Greg Zacharias, the Air Force's chief scientist, laid out an optimistic timeline: hypersonic weapons by the 2020s, hypersonic drones by the 2030s, and recoverable hypersonic drone aircraft by the 2040s. Whether these timelines prove realistic remains to be seen.
Defending against hypersonic weapons may be even harder than building them. Traditional missile defense relies on radar tracking and predictable trajectories. Hypersonic glide vehicles defeat both. Their lower altitude makes them harder to see until they're close, and their maneuverability makes their final target uncertain until the last moments.
Countermeasures will require fusing data from multiple sensor types—radar and infrared together—to capture the full signature of a hypersonic vehicle in the atmosphere. Space-based sensors will be essential for tracking launches and early flight paths. The entire defensive architecture built around intercepting ballistic missiles may need fundamental redesign.
The Commercial Horizon
Not everyone chasing hypersonic flight wants to build weapons. Several companies are developing hypersonic aircraft for civilian transport.
Hermeus, based in Atlanta, demonstrated something remarkable in November 2022: an engine that transitions from turbojet operation to ramjet operation without any rocket boost in between. This continuous operation across speed regimes has been a holy grail of hypersonic propulsion—if you can accelerate smoothly from standstill to Mach 5 and beyond, you can imagine practical hypersonic airliners.
Venus Aerospace and AstroMechanica are pursuing similar goals. The physics argument for hypersonic passenger travel is intriguing: yes, you're burning fuel fast, but you're also covering distance fast. If you can climb to thin air quickly and cruise in a regime where drag is lower, the net energy per mile might actually be competitive with slower aircraft.
Proponents claim that hypersonic transport could actually use less total energy than conventional flight while cutting journey times from hours to minutes. New York to Tokyo in ninety minutes. Los Angeles to Sydney in two hours. The tyranny of distance, reduced to an inconvenience.
The Stratolaunch Approach
One company is taking a decidedly unconventional path. Stratolaunch built the world's largest airplane—the Roc, with a wingspan of 385 feet, wider than a football field is long—specifically to carry hypersonic test vehicles to altitude before release.
This air-launch approach solves several problems. The test vehicle doesn't need to waste fuel climbing through the thick lower atmosphere. It can start its hypersonic run from an already-optimal altitude. And if something goes wrong, the mother ship can return safely to base.
The Roc has been conducting hypersonic test flights, providing a flexible platform for iterating quickly on designs. In a field where a single test flight might cost hundreds of millions of dollars using traditional launch methods, having a reusable carrier aircraft offers enormous advantages.
Materials at the Edge
Behind all the headlines about missiles and records lurks a quieter revolution in materials science. Nothing about hypersonic flight works without materials that can survive conditions that would destroy ordinary metals.
At Mach 5 and above, leading edges experience temperatures exceeding 1,000 degrees Celsius. At Mach 10, you're looking at 2,000 degrees or more. Few materials can maintain structural integrity at these temperatures while also being light enough to fly.
Carbon-carbon composites—the same material used in Formula 1 brake discs—can handle extreme heat but tend to oxidize rapidly in the atmosphere. Ceramics are heat-resistant but brittle. Refractory metals like tungsten and rhenium can take the temperature but are extremely heavy.
The search is on for materials that combine the best properties: high-temperature strength, oxidation resistance, low density, and manufacturability. Ultra-high-temperature ceramics, ceramic matrix composites, and novel alloys are all under investigation. Russia's struggles with Avangard's heat shield—the switch from carbon fiber to new composites—illustrate how critical and difficult this materials challenge remains.
Generating Power from Plasma
Chinese researchers have found an unexpected use for all that high-temperature plasma. Using shock waves in a detonation chamber, they compressed ionized argon plasma moving at Mach 14 and directed it through magnetohydrodynamic generators.
Magnetohydrodynamic generation, often abbreviated MHD, works by passing an electrically conductive fluid through a magnetic field. The moving charges generate a current—the same principle as a conventional generator, but without any moving mechanical parts. The Chinese experiments demonstrated current pulses that could theoretically scale to gigawatt levels given enough argon gas.
Whether this has practical applications remains to be seen. But it illustrates how the extreme conditions of hypersonic flight can enable physics that's impossible at ordinary speeds.
Looking Forward
We stand at an inflection point. Hypersonic technology is transitioning from laboratory curiosity to deployable system. Within this decade, multiple countries will field hypersonic weapons. Within the next decade, civilian hypersonic aircraft may enter service.
The strategic implications are profound. Defense architectures built over decades assume ballistic threats following predictable paths. Hypersonic glide vehicles invalidate those assumptions. The offense-defense balance that has shaped nuclear strategy since the Cold War may be shifting in ways we don't yet fully understand.
The commercial implications are equally vast. If hypersonic travel becomes practical, the world shrinks dramatically. Business decisions, family visits, emergency responses—all would operate on different timescales. Whether the economics and environmental costs prove acceptable remains an open question.
What's certain is that we're leaving the era when Mach 5 was an exotic achievement reached only by experimental aircraft and returning spacecraft. Hypersonic flight is becoming, if not routine, then at least achievable. The physics hasn't changed since the X-15 pilots pushed through those invisible barriers in the 1960s. But our ability to engineer solutions to those physics problems—materials that survive, engines that function, controls that respond—continues to advance.
The age of hypersonic flight isn't coming. It's here.