(Note: The background section of this page uses most of the same words as Terra Futura's page on ramjets to save time.)
Early airplanes used piston engines similar to those used in cars, buses, and trains. As planes became common in the 1930s, radial engines started being used. In the early years of air travel, the radial engine was the dominant aircraft engine. The radial engine was used to turn a propeller which helped the airplane fly. After World War II, gas turbine engines started being used for propulsion. The earliest were turboprop engines which continued to use a propeller. In more large scale aircraft, the turboprop was initially replaced with the turbojet. But turbojets were inefficient and noisy. So they were replaced with turbofans.
Turbofans became commonplace after they were first used on the Boeing 707. The jet age was born. This was followed by the 727. Early turbofans were low bypass. The fan had multiple stages. It could go faster. In the 1960s, turbofans became high bypass. There was only one fan. They more efficient than ever before. By the early 21st century, the fastest turbofan used for passenger travel was the Boeing 787. It reached Mach .89 once it finally got in the air. In the 21st century, turbofans were mainly used for subsonic travel. Other engines went supersonic.
Ramjets were faster than turbofans because they had no fan. The movement of air was not impeded. The most notable use of ramjets was the SR-71 Blackbird which traveled at Mach 3. That was where ramjets usually worked best. The maximum speed ramjets could do without trouble was Mach 6. The LAPCAT A2 approached that speed, but it topped off at Mach 5. There was one problem. Ramjets did not work at subsonic speeds. So a turbofan often accompanied it. In the mid-21st century, ramjets were going faster and becoming scramjets.
Tech Level: 11-12
A supersonic combustion ramjet or scramjet was a ramjet in which compression was reduced. Therefore, combustion was not slowed down. This allowed the scramjet to operate at hypersonic speeds. The earliest application was in scramjet missiles. These missiles came in two stages. A rocket stage accelerated the missiles to Mach 12 and then came off. The missile stage could potentially go faster. These missiles were used during World War III. Scramjets were also used in fighter planes during that time, too. The aircraft was reconfigurable to deal with subsonic, supersonic, and hypersonic speeds. After World War III, scramjets were used in commercial aviation. Most planes could travel at Mach 12. The sonic boom had already been eliminated by this time. There was no pollution and little-to-no maintenance. The fastest scramjet planes were capable of orbital spaceflight at Mach 25. This resulted in increased globalization on a scale unheard of before.