Invented by Rene Lorin in 1913, ramjets are jet engines which require no moving parts, and operate at subsonic speeds (as opposed to supersonic ramjets, or scramjets). Ramjets also differ slightly from pulsejets, in that they burn fuel at a constant rate. The basic concept of the ramjet is to force air into a chamber, the shape of which is designed to achieve the appropriate compression required for operation. Because ramjets rely on this concept, they must be accellerated to roughly 400 mph before they will generate any thrust.

Airflow enters the ramjet where it is compressed, mixed with fuel, ignited and released. Because of the amount of air flowing through the jet, a special, slow-burning fuel is required to avoid the immediate extinguishing of the burn, and due to the low compression ratio at which the ramjet operates, it is very inefficient.

Variations on the ramjet concept include both liquid and solid fuel ramjets, variable geometry ramjets (which provide for a broader range of thrust, but are much, much more complicated), pulsejets, ramrockets, scramjets and a nuclear ramjet. There have even been experiments with adding ramjets to the tips of the rotors on a hellicopter, but with limited success.