Page 183 - The Jet Engine

temperatures up to 700 deg. C., combustion will not cooling corrugations, to form a single skin. The rear
take place below 800 deg. C. If however, the of the heatshield is a series of overlapping 'tiles'
conditions were such that spontaneous ignition could riveted to the surrounding skin (fig. 16-4). The shield
be effected at sea level, it is unlikely that it could be also prevents combustion instability from creating
effected at altitude where the atmospheric pressure excessive noise and vibration, which in turn would
is low. The spark or flame that initiates combustion cause rapid physical deterioration of the afterburner
must be of such intensity that a light-up can be equipment.
obtained at considerable altitudes.
10. For smooth functioning of the system, a stable Propelling nozzle
flame that will burn steadily over a wide range of 14. The propelling nozzle is of similar material and
mixture strengths and gas flows is required. The construction as the jet pipe, to which it is secured as
mixture must also be easy to ignite under all a separate assembly. A two-position propelling
conditions of flight and combustion must be nozzle has two movable eyelids that are operated by
maintained with the minimum loss of pressure. actuators, or pneumatic rams, to give an open or
closed position (para. 4.). A variable-area propelling
nozzle has a ring of interlocking flaps that are hinged
to the outer casing and may be enclosed by an outer
CONSTRUCTION
shroud. The flaps are actuated by powered rams to
Burners the closed position, and by gas loads to the interme-
11. The burner system consists of several circular diate or the open positions; control of the flap
concentric fuel manifolds supported by struts inside position is by a control unit and a pump provides the
the jet pipe. Fuel is supplied to the manifolds by feed power to the rams (para. 18).
pipes in the support struts and sprayed into the flame
area, between the flame stabilizers, from holes in the
downstream edge of the manifolds. The flame CONTROL SYSTEM
stabilizers are blunt nosed V-section annular rings
located downstream of the fuel burners. An 15. It is apparent that two functions, fuel flow and
propelling nozzle area, must be co-ordinated for sat-
alternative system includes an additional segmented isfactory operation of the afterburner system, These
fuel manifold mounted within the flame stabilizers. functions are related by making the nozzle area
The typical burner and flame stabilizer shown in fig. dependent upon the fuel flow at the burners or vice-
16-4 is based on the latter system. versa. The pilot controls the afterburner fuel flow or
the nozzle area in conjunction with a compressor
delivery/jet pipe pressure sensing device (a pressure
Jet pipe ratio control unit). When the afterburner fuel flow is
12. The afterburning jet pipe is made from a heat- increased, the nozzle area increases; when the
resistant nickel alloy and requires more insulation afterburner fuel flow decreases, the nozzle area is
than the normal jet pipe to prevent the heat of reduced. The pressure ratio control unit ensures the
pressure ratio across the turbine remains unchanged
combustion being transferred to the aircraft structure. and that the engine is unaffected by the operation of
The jet pipe may be of a double skin construction afterburning, regardless of the nozzle area and fuel
with the outer skin carrying the flight loads and the flow.
inner skin the thermal stresses; a flow of cooling air
is often induced between the inner and outer skins. 16. Since large fuel flows are required for afterburn-
Provision is also made to accommodate expansion ing, an additional fuel pump is used. This pump is
and contraction, and to prevent gas leaks at the jet usually of the centrifugal flow or gear type and is
pipe joints. energized automatically when afterburning is
selected. The system is fully automatic and incorpo-
13. A circular heatshield of similar material to the jet rates 'fail safe' features in the event of an afterburner
pipe is often fitted to the inner wall of the jet pipe to malfunction. The interconnection between the control
improve cooling at the rear of the burner section. The system and afterburner jet pipe is shown diagram-
heatshield comprises a number of bands, linked by matically in fig. 16-5.

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