Beyond supersonic? Defining the 4 speeds of flight

Flying at high speeds is a dream that millions of people around the world share.

Not only is it a thrill to imagine traveling at supersonic speeds or even hypersonic speeds, it’s also tempting to imagine arriving at your destination faster. How fast can you fly — exactly? Here’s a look at how the aerospace industry categorizes the speeds of flight and the type of aircraft capable of achieving those speeds.

Typically, we measure the speed of an aircraft by its Mach number, which is a velocity relative to the speed of sound (approximately 770 mph or 1,239 kmh at sea level). Mach 1 is the speed of sound.

Four general categories define the speed of flight: subsonic, transonic, supersonic and hypersonic. Each is relative to a Mach number.

Subsonic

At this speed, an aircraft is traveling slower than the speed of sound — less than about Mach 0.8. Subsonic aircraft include everything that flies slowly, including all general aviation aircraft, such as the Cessna 172, ultralights, and even paragliders.

Commercial aircraft, such as the Boeing 777 and Airbus 330, and smaller regional jets that have less than 100 seats, are subsonic as well. Most older military jets also fall into the subsonic category. Examples include the F-100 Super Sabre, which was developed in the 1950s and flown by the U.S. Air Force for 25 years.

Transonic

At this speed, an aircraft is approaching the speed of sound but hasn’t yet reached and surpassed Mach 1. At some places on the aircraft the speed will exceed Mach 1, while at others it will be less than Mach 1. There are a handful of aircraft that fly deep in the transonic regime, including the Cessna Citation X and the Gulfstream G650.

The line between subsonic and transonic is blurry. There are even transonic flows on both of the subsonic commercial airliner examples mentioned above. In some cases, you can even see the shadow of the shocks on the upper wing. Click here to watch the shock wave formation on a Boeing 737 in transonic flight.

Supersonic

At this speed, the entire aircraft is experiencing supersonic airflow and traveling at speeds faster than Mach 1. Generally, supersonic speeds range from Mach 1.2 to Mach 5. Boom’s Overture will fly comfortably in the supersonic regime at Mach 1.7.

Rockets, such as the Space Shuttle, fly at supersonic speeds immediately after liftoff and for about 45 seconds until about two minutes after launch. During this time, the shuttle accelerates from Mach 1 to Mach 5.

Many types of military aircraft are also capable of supersonic flight. For example, the F-4 Phantom II first took to the skies in 1960 and exceeded Mach 2. Nicknamed the “Flying Footlocker,” it was retired in 2013.

Hypersonic

At this speed, an aircraft is traveling faster than Mach 5.

The hypersonic X-15, a joint venture that NASA conducted with the Air Force, the Navy, and North American Aviation, Inc., flew at Mach 6.7. The X-15 flew from 1958 to 1969 and provided insights that later contributed to the Mercury, Gemini, and Apollo piloted spaceflight programs. It also helped inform the Space Shuttle, which flies at hypersonic speeds while in the earth’s upper atmosphere. (It slows down to supersonic speeds as it re-enters the lower part of the earth’s atmosphere.) Most recently, China launched its experimental “waverider” Starry Sky-2 hypersonic aircraft, which soared at about Mach-5.5 for 400 seconds (after being carried by a rocket to an altitude of 18 miles or 30km).

The reason for the distinction between supersonic and hypersonic is due to temperature changes. At speeds above Mach 5, most metals will melt or become so soft that they can’t be used for any type of structure. As a result, hypersonic aircraft must go to extreme measures for heat protection (such as the tiles and blankets protecting the space shuttle). While it’s challenging to imagine what happens to an aircraft at such temperatures, here are two examples: aluminum will melt at approximately 1,200° F or 648° C and steel will melt at approximately 2,500° F or 1,371° C.

To learn more about speed versus temperature, click here for a great blog about why air is hot when you fly fast and why there’s no such thing as “cooling air” once you’ve achieved Mach 1.