The fastest air-breathing vehicle ever built was the NASA X-43. Only three of these aircraft were built, with one being destroyed and two operating successfully. The X-43 testbed aircraft measured 12 feet in length with a width of five feet. While they looked similar in appearance, they were engineered differently internally to operate more effectively at a different Mach number.

This is regarded as the fastest NASA airplane ever built, although it might not remain the fastest, as Australia’s Hypersonix is working with NASA and has intentions to build a vehicle capable of Mach 12 powered by hydrogen fuel. The X-43 only successfully flew twice, with the second flight coming tantalizingly close to Mach 10. The fastest NASA manned aircraft (a rocket plane) remains the North American X-15 that flew on the edge of space, more on that below.

NASA’s X-43 Hypersonic Air Vehicle

Hypersonic speeds are classified as greater than Mach 5 or five times the speed of sound. The program lasted eight years and cost around $230 million. It was considered a high-risk, but high-reward program designed to tackle the challenges of hypersonic flight that had never been attempted. When the program was initiated, no aircraft had been propelled to hypersonic speeds by an air-breathing engine.

The NASA X-43 is the fastest aircraft ever built by NASA. The experimental unmanned hypersonic parasitic aircraft was launched from a B-52 Superfortress and reached an impressive Mach 9.6. The aircraft was designed to be single-use, so each aircraft only flew once. Only three were built, and only three flights were carried out, with two of those flights being successful.

To achieve their enormous speed, they were carried by the B-52, then a winged booster aircraft would fire before being discarded. Finally, the X-43’s scramjet engine would kick in and take it to its maximum speed. The aircraft was built to test hypersonic flight, with the program first starting in the late 1990s. The first flight took place in 2001, but it failed, and the aircraft was destroyed.

Using A Scramjet Engine

In contrast to rockets, scramjet engines are air-breathing. NASA says, « Unlike rocket-powered vehicles like the space shuttle, scramjet (supersonic combustion ramjet) powered vehicles offered more airplane-like operations, promising increased affordability, flexibility, and safety for ultra-high-speed flights within the atmosphere and into Earth’s orbit. » This comes with a range of advantages over their rocket-powered counterparts.

As scramjets don’t require their own oxidizer (something rockets require), they are able to be smaller and lighter. This means that they can carry greater payloads. Scramjet technology is seen as being the propulsion of future hypersonic missiles, hypersonic airplanes, and the first stage of two-stage-to-orbit reusable launch vehicles. They could also be used for single-stage-to-orbit reusable launch vehicles.

A ramjet uses subsonic combustion of fuel in a stream of air compressed by the forward speed of the aircraft. This is a major limiting factor of scramjets; they have to already have that forward speed to work. Normal jet engines use fan blades to compress the air. Ramjets operate between Mach 3 to Mach 6. A scramjet is a ramjet in which the airflow through the engine remains supersonic, and they are believed to be able to operate at up to at least Mach 15.

The Three Flights

The first X-43A flight took place in 2001. However, it failed as the boost failed and had to be destroyed early in the flight. This meant the research vehicle itself was not tested as it was unable to reach the needed test conditions. NASA then worked to fix the issues and was able to launch two successful test flights three years later. Both of those flights set speed records with the scramjets operating for around 10 seconds, and the vehicles then glided for ten minutes before crashing in the ocean as intended.

The first and second vehicles were designed to fly at Mach 7, although, as stated, the first vehicle failed. The second vehicle successfully flew to Mach 6.8 on 27th March 2004. The third vehicle was designed for Mach 10, and it reached Mach 9.6 on 16th November 2004. The previous record was held by an SR-71 Blackbird reconnaissance plane at Mach 3.2. At these speeds, the vehicle’s head served the same purpose as pistons in a car by compressing the air as fuel for combustion.

Somewhat confusingly, the Smithsonian National Air and Space Museum says, « NASA X-43 recorded speeds over Mach 10. The X-43A used a rocket booster to get to speed after being dropped out of a B-52. » It is unclear why the Smithsonian says the aircraft achieved a speed of over Mach 10, as NASA gives the speed at Mach 9.6.

NASA X-15 Rocket Airplane

While the X-43 may have been the fastest aircraft NASA has ever flown, it may not be what people have in mind when they think of aircraft. Back in the 1960s, NASA teamed with North American to build the X-15 hypersonic rocket-powered aircraft to explore traveling on the edge of outer space. The X-15 was manned, it was also launched from B-52 motherships, and it looks much more like an aircraft.

Remarkably, the X-15 still retains the world’s manned flight speed record of 4,520 mph or Mach 6.7. It also retained the world’s altitude record (354,200 feet) for many years. The program was a forerunner to the Apollo mission that took man to the moon. Among the pilots who flew the X-15 was none other than Neil Armstrong. The aircraft’s record speed was set in October 1967 when William J. Knight reset the record, a record that remains unbroken.

A total of 12 pilots flew some 199 flights in the X-15. Highlighting how the X-15 blurs the line between an aircraft and a spacecraft, eight of these pilots and three of these flights exceeded an altitude of 50 miles. This meets the Air Force spaceflight criterion and qualifies the pilots as astronauts. Two of these flights flew over 62 miles, which meets the definition of going out of space by the Fédération Aéronautique Internationale.

The Race To Develop Hypersonic Vehicles

Countries today are racing to build hypersonic missiles, with Russia claiming to already have them. However, Russia is using a cheat definition for its missiles. For example, the Kh-47M2 Kinzhal (NATO reporting name AS-24 Killjoy) air-launched ballistic missiles do achieve hypersonic speeds in certain phases of flight, but so do all ballistic missiles. Even Germany’s original V-2 Rocket achieved around Mach 4.7. According to the US Army, the Kinzhal slows to Mach 3 in its terminal phase, like other non-hypersonic ballistic missiles.

To be considered a hypersonic missile, the vehicle needs to both maneuver at hypersonic speeds and to sustain speeds of Mach 5+ during the terminal phase as the vehicle encounters more air resistance. Many companies are working to develop these hypersonic vehicles, not only as missiles, but also for other roles, like intelligence. Companies are also working to find ways to make these vehicles powered by scramjet engines and not rockets.

One notable startup in Australia called Hypersonix has attracted the investment of the British Ministry of Defence, Australia, and the United States (including NASA). Hypersonix is using its 3D printed hydrogen-powered scramjet engines. One experimental vehicle in development could even have a maximum speed of Mach 12. Hypersonix is one of the few companies working in the field that explicitly says it wants to use this technology for hypersonic commercial passenger travel eventually.

The Hypersonic Missile Race

After the NASA X-43 program, the United States Air Force took over the successor Boeing X-51 Waverider program. The Waverider was also a scramjet experimental aircraft designed for hypersonic flight, although it was designed for slower flight at Mach 5. After unsuccessful tests, it successfully set a new record in 2013 for the longest duration of a powered hypersonic flight. Today, the Air Force is pursuing several hypersonic programs. Among these is Air-Launched Rapid Response Weapon (ARRW), which is being developed by Lockheed Martin, and it is also launched from B-52s.

Another USAF program is the Hypersonic Attack Cruise Missile (HACM) led by Raytheon. Many countries are working to develop hypersonic programs. Australia has emerged as a notable player and is cooperating with the United States as part of the SCIFiRE Hypersonics program. The Royal Australian Air Force says the new hypersonic weapon being developed with a Mach 5-class precision strike missile to be powered by an air-breathing scramjet engine. It will be carried by the country’s F/A-18F Super Hornet, EA-18G Growler, F-35A Lightning II, and P-8A Poseidon aircraft.

Meanwhile, Australia’s Hypersonix startup says its Spartan scramjet is the world’s first entirely 3D-printed scramjet engine. Hypersonix wants to develop hypersonic technology for civil and commercial purposes. However, as with the development of jet and rocket engines in the 1940s, this is a technology that is likely to be first developed for military purposes before maturing and finding its way into the civil sector.