Ramjet
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.
Scramjet
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.
Dual-mode scramjet
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.
Inward-turning inlet
Wedge-shaped inlet structure with reduced surface area, and lower cooling requirement.
Source: Flight International
