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.
