Scramjet

A scramjet (supersonic combustion ramjet) is a special type of ramjet jet engine where fuel combustion takes place while the air in the engine is moving faster than the speed of sound. In a ramjet, air is slowed to subsonic (slower than the speed of sound) speeds before fuel is burned. Unlike ramjets, scramjets do not use shock cones to compress incoming air; instead, shock waves from inside the engine compress the air by slowing it down very quickly. This means that scramjets can operate very efficiently at extremely high speeds. However, a scramjet relies on the aircraft moving much faster than the speed of sound in order to work; scramjets do not work at subsonic, or even low supersonic, speeds.
As of November 2022, scramjets are still experimental. This means that they have not been used on commercial airliners or in active military scenarios. Successful scramjet tests have been conducted, however.