The simplest of jet engines, the ramjet compresses air to subsonic speed by the ram-effect of its forward motion. The compressed air is mixed with fuel within the engine, and ejected through a supersonic nozzle. The ramjet has no significant moving parts.
A supersonic-combustion ramjet (scramjet) is a ramjet in which combustion of the fuel/air mixture occurs at supersonic speeds. The scramjet can therefore achieve greater speeds than a ramjet that slows the incoming air to subsonic speeds before entering the combustion chamber. The scramjet, however, requires acceleration to supersonic speed before it will operate.
This refers to an engine that can operate in both subsonic- combustion and supersonic combustion modes – usually as part of a turbine-based combined cycle or a rocket-based combined-cycle concept.
Turbine-based combined cycle
A propulsion system in which a gas turbine engine and a ramjet/scramjet are integrated in a dual-flowpath configuration. The turbine-based combined cycle is aimed at Mach 5 to 8.
Rocket-based combined cycle
A propulsion system in which rocket thrusters are integrated in the ramjet/scramjet flowpath. Aimed at speeds of Mach 5 and beyond.
Two-dimensional engine inlet
Rectangular inlet with straight sidewalls and upper and lower inlet walls.
Three-dimensional engine inlet
An asymmetric, or round inlet that produces lower skin friction, but higher-pressure rise.
Wedge-shaped inlet structure with reduced surface area, and lower cooling requirement.
Source: Flight International