Several months later, reports were leaked, and the world learned for the first time that a human being had flown faster than the speed of sound. Chuck Yeager had “broken the sound barrier,” reaching a velocity greater than “Mach” 1 — a term named for the Austrian physicist Ernst Mach — that refers to an aircraft traveling at the speed of sound at a set temperature and air pressure. 

Since that moment scientists and researchers around the world began work to expand the envelope of high-speed travel. With one barrier broken, the focus quickly shifted to the potential for reaching “hypersonic” speed — traveling greater than 5 times the speed of sound, at an altitude below 90 kilometers. At Mach 5 a flight from New York to LA would take just 30 minutes. The possibilities captivated the imagination of a generation. 

Flying at the speed of sound is difficult enough. But hypersonic travel is constrained by its own set of unique technical challenges. When traveling at 5 times the speed of sound air resistance generates copious amounts of heat, which requires active thermal protection for the aircraft. Some aircraft manage this heat with systems to regeneratively cool their fuselage in flight. But these mechanisms can in turn make the aircraft heavier, which makes reaching higher velocities more difficult.

And these high-velocity environments can be tough on the airframe of the aircraft, reducing stiffness and strength. Advancements in metallurgy over the past century have led to the engineering of special nickel alloys and advanced carbon composites designed specifically to handle such extreme environments.

In the United States, many of these challenges were first faced by the engineers and crew of the X-15, an experimental aircraft developed by North American Aviation and operated by NASA and the US Air Force. The X-15 was powered by a Reaction Motors XLR11 engine, the first liquid-fuel rocket engine developed in the United States specifically for use in an aircraft. This early engine used ethyl alcohol and liquid oxygen as propellants, and later generations would switch to anhydrous ammonia, liquid oxygen, and hydrogen peroxide. 

On October 3rd, 1967, almost exactly 20 years after Chuck Yeager’s historic flight, the X-15 set a record for the highest velocity achieved by a crewed aircraft — Mach 6.7. This record has never been broken. 

During its 9-years of operation, NASA’s X-15 was flown 199 times by 12 pilots, including Apollo 11 astronaut Neil Armstrong. Of these 12, 8 flew higher than 80 kilometers, (the US criteria for spaceflight,) which qualifies them as X-15 astronauts. 

Like many X-series aircraft, the X-15 was designed to be carried and drop-launched from the wing of a Northrop Grumman B-52. This mother ship would carry the X-15 to an altitude of 13.7 kilometers, where it was dropped while flying at 805 kilometers per hour.

In the late 1950s, the Lockheed Corporation was also developing the X-17 research rocket, designed to test the effects of high-mach atmospheric entry. Standing 12.3 meters tall, this three-stage rocket was a test vehicle for the development of nuclear-armed submarine ballistic missiles like the UGM-27 Polaris.

And much of the lessons of the X-15 and X-17 went into the design and development of the space shuttle, which would pass through hypersonic speeds as it slowed in re-entry to Earth’s atmosphere.

Now, half a century later, and after extensive efforts involving both public and private sectors, hypersonic flight research is again on the rise.

Aircraft like the X-15 were propelled by rocket engines and carried both their fuel and oxidizer. But engineers knew that using atmospheric oxygen to burn fuel could be critical for reducing the weight of the aircraft, and several parallel efforts were focused on “air-breathing” propulsion systems, called ramjets.

Propelling a jet aircraft requires high-pressure air to maintain the flow through the nozzle and generate thrust. Turbojet engines on fighter jets have compressors that produce the required pressure. Ramjets however work by “ramming” the external air into the engine using the forward speed of the vehicle. This eliminates the need for compressors, which simplifies the engine and saves weight — but with a caveat. Ramjets actually require supersonic speeds to work most efficiently. 

It’s an old idea. The development of ramjets was first proposed by French aerospace engineer René Lorin, who outlined the idea and its principles in 1913. Lorin was granted a patent for his invention but the technology was way behind the physics, and he was never able to build a prototype. Building on his research, numerous French aerospace engineers worked on theoretical concepts, but it wasn’t until 1926 when Andrew Carter from the United Kingdom proposed the first practical ramjet-like propulsion system designed specifically to enhance the range of artillery shells. Similar work was also carried out by Hungarian inventor Albert Fonó, however, his proposal to equip gun-launched projectiles with ramjets was rejected by the Austro-Hungarian Army.

The actual construction and testing of ramjets didn’t begin until the mid-1930s in France, Germany, and the Soviet Union. French engineer René Leduc ground tested a ramjet up to Mach 0.9. By 1938, work on full-scale ramjet-powered aircraft had begun, and ground tests were conducted up to Mach 2.5. In Germany, Dr. Wolf Trommsdroff led a successful effort to develop artillery shells powered by ramjets. Tests conducted in the 1940s saw the shells accelerating up to Mach 4.2. And the first operational ramjet-powered missile took the form of the V1 “buzz bomb.”

In the Soviet Union, Boris Stechkin began ground testing ramjet components put to Mach 2 in the 1930s, before the war ended his research.

In the US, ramjet development picked up in 1940 when aeronautics engineer Henry J. E. Reid joined hands with Roy Edward Marquardt in the UK, whose primary work focused on ramjets and their military applications. Their efforts continued after the war and augmented the development of the BOMARC, Talos, and the UK’s Bloodhound anti-air missiles.

As the technology matured, the advantages of ramjets led the US Navy to set up the “Bumblebee” program, in collaboration with John Hopkins University’s Applied Physics Laboratory. This program led to the development of surface-launched and air-launched ramjet-powered missiles.

Ramjets have been shown to work efficiently at Mach 3, but at higher speeds, their efficiency starts to drop, so modifications were required to force the continued combustion of supersonic airflow. Engines of this type became known as supersonic-combustion ramjets — or “scramjets.”

Interest in scramjet engines began in the late 1950s with the US, Canada, UK, France, Germany, and Russia all conducting a plethora of theoretical studies on significant performance gains at a post-Mach 5 velocity. The US Navy started prioritizing hypersonic propulsion development in the form of the External Ramjet (ERJ) program. In 1958 the ERJ demonstrated a net-positive thrust at Mach 5 for the first time. Following this early success, the Navy started an exploratory development program to demonstrate the technology necessary for a scramjet-powered missile, which came to be known as the Supersonic Combustion Ramjet Missile (SCRAM).

SCRAM was predicted to have a 350-mile range flying at Mach 7.5. While it underwent considerable development and ground testing it never proceeded into flight testing, and was ultimately canceled in 1977 due to mounting technical complexities. 

A scramjet engine was also envisioned for the Rockwell X-30, a spaceplane technology demonstrator developed under the National Aero-Space Plane (NASP) program. The goal of the NASP was to create a single-stage-to-orbit (SSTO) spacecraft, and later a passenger spaceliner to potentially replace the Space Shuttle. Backed by NASA and the US Department of Defense, comprising DARPA, Air Force, Navy, and the Strategic Defense Initiative Offices, the program called for a scramjet-based aircraft capable of achieving Mach 8. In 1986, contracts were awarded to Rockwell, McDonnell Douglas, and General Dynamics to develop the hypersonic SSTO vehicle. Rocketdyne and Pratt & Whitney were awarded $175 million to develop the propulsion system.

Despite steady progress in structural and propulsion technology, the X-30 was plagued by added military requirements and unsolved issues with environmental control systems, safety equipment, and human ratings. As development progressed into the early 1990s the X-30 was larger, heavier, behind schedule, and over budget. It was ultimately canceled in 1993.

But learnings from the X-30 and decades of scramjet propulsion development on other projects culminated in NASA’s experimental X-43. The X-43A aircraft successfully flew at hypersonic velocity and set the current record for the fastest uncrewed jet-powered aircraft — clocking in at Mach 9.6. Developed under NASA’s Hyper-X program, and managed by the Langley Research Center and Dryden Flight Research Center at Edwards, California, the X-43 program involved Boeing, Orbital Sciences Corporation, General Applied Science Laboratory, and Micro Craft Inc, and was designed to demonstrate a scramjet’s ability to achieve hypersonic speed. 

Because the X-43 scramjet engine could only operate at speeds of Mach 4.5 or higher, it was mounted to a Pegasus rocket, which boosted the aircraft to the required base velocity. Together called the “stack” by project members, the Pegasus and X-43 were drop launched by a Boeing B-52 Stratofortress bomber.

Following the success of the X-43 the emphasis for NASA became sustained hypersonic flight, and development shifted to the X-51 “WaveRider.”

Developed in cooperation with DARPA, Boeing, and Pratt & Whitney Rocketdyne, the X-51 “WaverRider” was designed to fly at Mach 5 at an altitude of 21,000 meters. Similar to the X-43, the WaveRider was drop-launched by a B-52, and was initially propelled by a solid rocket booster to achieve baseline velocity. The X-51 first achieved simulated Mach 5 velocities during ground testing at NASA’s Langley Research Center, with further tests conducted to observe acceleration between Mach 4 and Mach 6 to confirm hypersonic thrust.

WaveRider’s first powered flight testing occurred in 2010. It flew for 200 seconds and achieved Mach 5 velocity, and though this was short of a planned 300-second total flight duration, it still set the record for the longest hypersonic flight time of 140 seconds, beating the X-43. 

In 2013 the X-51 flew for a 4th time, reached Mach 5.1, and it lasted 210 seconds — good enough to beat its own previous record.

After this test, out of fuel, WaveRider splashed down into the Pacific Ocean. It was the completion of the X-51 program, considered a successful demonstration of the potential for hypersonic flight.

It was also exactly 100 years after René Lorin filed the first Ramjet patent.