Hypersonic speed

Hypersonic speed occurs when an aircraft or another airborne object travels at a rate much faster than the speed of sound. Precise definitions of the term usually identify Mach 5 as the cutoff point for hypersonic speed. Such velocities trigger changes in the molecules, air pressure, and air density interacting with the aircraft or object, creating challenges related to aerodynamics, vehicle design, and engineering.

According to the National Aeronautics and Space Administration (NASA), the only crewed aircrafts ever documented to fly at hypersonic speed are the North American X-15 rocket-powered airplanes and NASA’s Space Shuttles, which achieved hypersonic velocities while re-entering Earth’s atmosphere. Other operational aircrafts capable of reaching hypersonic speeds are crewless glide vehicles with military applications. For many years, high research, development, and design costs inhibited the evolution and advancement of hypersonic aeronautics before breakthrough scramjet technologies led to a wave of progress during the 2000s and 2010s.

Background

The speed of sound can change depending on the surrounding air temperature. At sea level and with a temperature of 59 degrees Fahrenheit (15 degrees Celsius), sound travels at 761.2 miles per hour (1,225 kilometers per hour). Sound moves through warmer air at a faster rate and cooler air at a slower rate due to changes in the movements of gas molecules that occur in warmer or cooler conditions.

Vehicles traveling at velocities exceeding the speed of sound are said to “break the sound barrier.” In aviation, the velocity that a supersonic aircraft must reach to cross this threshold is lower, as modern aircraft typically fly at relatively high altitudes where the air temperature is much colder than it is on Earth’s surface.

Given this variability, scientists use the specially devised Mach number system to measure object velocities relative to the speed of sound. Named in honor of groundbreaking Austrian physicist Ernst Mach (1838–1916), the dimensionless system assigns a value of Mach 1 to the speed of sound, whatever that speed may be given considering the temperature and air conditions. Aviators also use a hierarchical regime to classify aircraft speeds relative to the speed of sound: subsonic speeds fall below Mach 0.8, and speeds between Mach 0.8 and Mach 1.2 are known as transonic as velocities in this range can break the sound barrier. Supersonicspeeds exceed Mach 1.2, and the hypersonic range covers speeds between Mach 5 and Mach 10. Scientists use the term high hypersonicto describe speeds in the Mach 10 to Mach 25 range.

Most commercial and passenger airplanes travel at speeds in the high subsonic or transonic ranges, usually at rates between about Mach 0.8 and Mach 0.9. This means that they travel at approximately 80 to 90 percent of the speed of sound at their cruising altitudes. Very few civilian or commercial supersonic aircraft have ever operated, and speeds in the hypersonic range currently remain the exclusive domain of military and space travel programs.

Overview

From a practical standpoint, the only known objects that interact with Earth capable of traveling at hypersonic speeds are advanced aircraft, military aviation systems, and objects such as meteorites that enter the atmosphere from space. The North American X-15, a series of research planes jointly developed by NASA and the US Air Force, was the first winged aircraft in history to reach hypersonic speeds. According to the Smithsonian National Air and Space Museum, the X-15 achieved a velocity of Mach 6 during a crewed flight undertaken after the aircraft completed a series of test flights in 1959. Later generations of the X-15 reached a peak velocity of 4,534 miles per hour (7,297 kilometers per hour), which equated to Mach 6.72 at the atmospheric altitude at which the speed was reached. NASA’s Space Shuttle fleet, which was operational between 1981 and 2011, represents the only other crewed aircraft ever to attain hypersonic speeds.

At hypersonic speeds, aircraft require designs that account for the unique aerodynamics that occur at such rapid velocities. Molecular changes occur at both lower and higher hypersonic speeds. At the lower end, the molecular bonds in the air particles in contact with the aircraft vibrate, increasing the amount of force they exert on the aircraft as it speeds through them. At higher hypersonic speeds, these molecules actually disintegrate, enveloping the aircraft in a layer of plasma that has an electrical charge. Hypersonic aircraft also trigger major swings in localized air pressure and air density through the dual effects of air expansion and shockwaves, which in turn generate very high levels of heat.

Though the practical and commercial capabilities of hypersonic flight have significant appeal, practical considerations have long imposed major limitations on its development. Engineering functional hypersonic aircraft is very costly, both in terms of financial and human resources. It also has a mixed safety profile, limiting its uses in civil aviation. However, promising scramjet advancements achieved during the 2000s and 2010s led to noteworthy innovations in hypersonic aviation, though thus far they have mainly been applied to military aircraft and cruise missiles. Scramjets are supersonic combustion ramjets, in which a combination of airborne fuel combustion and supersonic airflow propel specially designed aircraft to theoretical speeds of at least Mach 15.

China, Russia, and the United States lead the list of countries known to be pursuing hypersonic missile programs. According to a 2020 report published in Sciencemagazine, the Chinese and Russian militaries are both developing advanced hypersonic missile systems, prompting the US Department of Defense (DOD) to commit an annual budget of more than $1 billion to the development of hypersonic technologies. Popular Mechanicshas reported that the US military has developed hypersonic missiles capable of reaching speeds of more than 6,000 miles per hour (10,000 kilometers per hour). Military experts note the strategic value of weapons with such capabilities, as they are functionally unstoppable and therefore represent a next-generation deterrent. Competition among major military powers to advance their hypersonic missile programs has instigated what some analysts describe as a new arms race akin to that of the Cold War (1945–1989).

Numerous private companies are working to apply advancements in scramjet technology to commercial aviation. One such enterprise is the United Kingdom-based Reaction Engines, which has set a goal of creating passenger airplanes capable of flying at Mach 5 by the 2030s. Such aircraft would be capable of flying from Los Angeles to Tokyo in two hours or London to Sydney in four hours.

Following the 2022 Russian invasion of Ukraine, media reports suggested Russia was employing hypersonic weaponry. In February 2024, Ukraine claimed to have recovered debris of a Russian Zircon hypersonic cruise missile following an attack on Kyiv. Russia allegedly fired the weapon from aboard a naval vessel. Such weapons have been deemed indefensible.

Partially in response to Russian employment of hypersonic weapons, both the U.S. Air Force and Navy have accelerated their developmental programs. In March 2024, the U.S. Air Force announced the successful testing of a hypersonic missile carried aboard a B-52 bomber. In April 2024, the U.S. Navy displayed a model of its new Mako hypersonic missile that is designed to be launched from smaller-sized fighter aircraft.

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