Jet engines all do the same job—turn fuel into thrust—but how they do it depends heavily on speed. Turbojets, ramjets, and scramjets each occupy a different slice of the velocity spectrum, and the physics inside them changes dramatically as airflow transitions from subsonic to hypersonic. A turbojet uses rotating machinery to do the hard work. Incoming air is compressed mechanically by axial or centrifugal compressors, mixed with fuel, ignited, and expanded through a turbine and exhaust nozzle. The turbine steals some energy to keep the compressor spinning, and the remaining momentum produces thrust. Turbojets are efficient at low to high subsonic speeds and were common in early jet aircraft, but their moving parts limit efficiency and survivability at extreme Mach numbers. A ramjet removes the compressor and turbine entirely. Instead, it relies on the aircraft’s forward speed to compress incoming air through shock waves in the inlet. Fuel is injected into this compressed air and bur...