Ramjets

Ramjets can be classified according to the type of fuel, liquid or solid; and the booster. In a liquid fuel ramjet (LFRJ), hydrocarbon fuel (typically) is injected into the combustor ahead of a flameholder which stabilises the flame resulting from the combustion of the fuel with the compressed air from the intake(s). A means of pressurizing and supplying the fuel to the ramcombustor is required, which can be complicated and expensive. Aérospatiale-Celerg designed an LFRJ where the fuel is forced into the injectors by an elastomer bladder which inflates progressively along the length of the fuel tank. Initially, the bladder forms a close-fitting sheath around the compressed air bottle from which it is inflated, which is mounted lengthwise in the tank.

This offers a lower-cost approach than a regulated LFRJ requiring a turbopump and associated hardware to supply the fuel. A ramjet generates no static thrust and needs a booster to achieve a forward velocity high enough for efficient operation of the intake system. The first ramjet-powered missiles used external boosters, usually solid-propellant rockets, either in tandem, where the booster is mounted immediately aft of the ramjet, e.g. Sea Dart, or wraparound where multiple boosters are attached alongside the outside of the ramjet, e.g. SA-4 Ganef. The choice of booster arrangement is usually driven by the size of the launch platform. 

A tandem booster increases the overall length of the system, whereas wraparound boosters increase the overall diameter. Wraparound boosters will usually generate higher drag than a tandem arrangement. Integrated boosters provide a more efficient packaging option, since the booster propellant is cast inside the otherwise empty combustor. This approach has been used on solid, for example SA-6 Gainful, liquid, for example ASMP, and ducted rocket, for example Meteor, designs. Integrated designs are complicated by the different nozzle requirements of the boost and ramjet phases of flight. Due to the higher thrust levels of the booster, a differently shaped nozzle is required for optimum thrust compared to that required for the lower thrust ramjet sustainer.

This is usually achieved via a separate nozzle, which is ejected after booster burnout. However, designs such as Meteor feature nozzleless boosters. This offers the advantages of elimination of the hazard to launch aircraft from the ejected boost nozzle debris, simplicity, reliability, and reduced mass and cost, although this must be traded against the reduction in performance compared with that provided by a dedicated booster nozzle.