AEROSPACE PROPULSION AAT
TOPIC : COMBINED CYCLE ENGINE
NAME : P L N S SUMANTH
ROLL NO. : 18951A2149
CLASS : AERO -2A
INTRODUCTION TO COMBINED CYCLE
ENGINES

Propulsion technology which can meet the flight mission requirement of
supersonic/hypersonic flight is one of the major challenges during
supersonic/hypersonic civic airliners technology development.

It is certain that no individual engine can operate efficiently and economically along
the entire operating range. For this reason different combined engine cycles are put
forward to exploit the performance benefits of multiple engines at different Mach
numbers.

Among these combined engine cycles are turbine-based combined cycle engine
(TBCC) and rocket-based combined cycle engine (RBCC).
TURBINE BASED COMBINES CYCLE
ENGINE

TBCC engine is an engine which integrates the components and operating
modes of gas turbine engine and ramjet engine into a common flow path, in
order to provide superior performance to any individual engine across a wider
flight range.

It is recommended as the propulsion system for the hypersonic civic airliner
due to the advantages such as being reusable, horizontal taking off, lower
fuel burn, less emission and not having the need to carry oxidizers.

One of the best examples of the TBCC engines is the turbo-ramjet engine.

It is a combination of turbojet and ramjet engine cycles
COMBINED CYCLE ENGINE

For aircraft designed to fly mixed missions (i.e., at subsonic, transonic,
and supersonic flight speeds) with low levels of fuel consumption, it is
desirable to have an engine with the characteristics of both a high-bypass
engine (for subsonic flight speed) and a low-bypass engine (for supersonic
flight speed).
COMBINED CYCLE ENGINE

If the components of an engine are designed to accommodate the extreme
limits of flow, pressure ratio, and other conditions involved in both highbypass and low-bypass operation, the engine may be operated at either
extreme of bypass ratio or at any bypass ratio between those extremes by
means of a valve (or valves) in the bypass stream (in conjunction with a
variable exhaust nozzle). When the valves are closed, they restrict the flow in
the bypass stream to achieve low bypass for supersonic flight. When the
valves are open, the bypass is increased to its maximum value for efficient
subsonic flight.
TURBO-RAMJETS

As noted above, the ramjet provides a simple and efficient means of
propulsion for aircraft at relatively high supersonic flight speeds.

It is, however, quite inefficient at transonic flight speeds and is completely
ineffective at subsonic velocities.

The turboramjet has been developed to overcome this inadequacy. In this
system a turbofan engine is built into the inlet of a ramjet engine to charge
the latter with a pressurized stream of air at subsonic flight speed where ram
pressure is insufficient for effective ramjet operation.

During supersonic flight the fan blades, if they are of variable pitch, may be
feathered so that they do not interfere with the flow of ram air to the
ramjet. A separate inlet to the core engine that drives the fan may be closed
off so as not to expose the turbomachinery to the hostile environment of the
high-temperature ram air.
TURBO RAMJETS

During supersonic flight the fan blades, if they are of variable pitch, may be
feathered so that they do not interfere with the flow of ram air to the ramjet. A
separate inlet to the core engine that drives the fan may be closed off so as not to
expose the turbomachinery to the hostile environment of the high-temperature ram
air.

Another variation of the turboramjet does without the core inlet and the
core compressor altogether.

Instead, the aircraft carries a tank of an oxidizer, such as liquid oxygen. The
oxidizer is fed into the core combustion chamber along with the fuel to support the
combustion process, which generates the hot gas stream to power the turbine that
drives the fan.

During supersonic flight, the fan may be feathered, and a surplus of fuel may be
introduced into the core combustor. The unburned fuel passes through the fan
turbine and undergoes combustion in the ramjet burner when it mixes with the
fresh air entering via the bypass stream from the fan.