A
ramjet, sometimes referred to as a
flying stovepipe or an
athodyd (an abbreviation of
aero
therm
odynamic
duct), is a form of
airbreathing jet engine that uses the engine's forward motion to compress incoming air without an
axial compressor. Ramjets cannot produce thrust at zero airspeed; they cannot move an aircraft from a standstill. A ramjet-powered vehicle, therefore, requires an
assisted take-off like a
rocket assist to accelerate it to a speed where it begins to produce thrust. Ramjets work most efficiently at
supersonic speeds around . This type of engine can operate up to speeds of .