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Fig. 16-4 Typical afterburning jet pipe equipment.
hydraulic medium, but some systems use fuel.
Nozzle movement is achieved by the hydraulic
operating rams which are pressurized by an oil
pump, pump output being controlled by a linkage
from the pressure ratio control unit. When an
increase in afterburning is selected, the afterburner
fuel control unit schedules an increase in fuel pump
output. The jet pipe pressure (P6) increases, altering
the pressure ratio across the turbine (P3/P6). The
pressure ratio control unit alters oil pump output,
causing an out-of-balance condition between the
hydraulic ram load and the gas load on the nozzle
flaps. The gas load opens the nozzle to increase its
exit area and, as the nozzle opens, the increase in
nozzle area restores the P3/P6 ratio and the
pressure ratio control unit alters oil pump output until
balance is restored between the hydraulic rams and
the gas loading on the nozzle flaps.
THRUST INCREASE
19. The increase in thrust due to afterburning
depends solely upon the ratio of the absolute jet pipe
temperatures before and after the extra fuel is burnt.
For example, neglecting small losses due to the
afterburner equipment and gas flow momentum
changes, the thrust increase may be calculated as
follows.
175
Fig. 16-5 Simplified control system.
176
Fig. 16-6 A simplified typical afterburner fuel control system.
20. Assuming a gas temperature before afterburning
of 640 deg. C. (913 deg. K.) and with afterburning
of 1,269 deg. C. (1,542 deg. K.). then the
temperature ratio = 1,542 = 1.69. 913
The velocity of the jet stream increases as the
square root of the temperature ratio. Therefore, the
jet velocity = ^/T.69 = 1.3. Thus, the jet stream
velocity is increased by 30 per cent, and the increase
in static thrust, in this instance, is also 30 per cent
(fig. 16-8).
21. Static thrust increases of up to 70 per cent are
obtainable from low by-pass engines fitted with afterburning
equipment and at high forward speeds
several times this amount of thrust boost can be
obtained. High thrust boosts can be achieved on low
by-pass engines because of the large amount of
oxygen in the exhaust gas stream and the low initial
temperature of the exhaust gases.
177
Fig. 16-8 Thrust increase and temperature
ratio.
Fig. 16-7 A simplified typical afterburner nozzle control system.
22. It is not possible to go on increasing the amount
of fuel that is burnt in the jet pipe so that all the
available oxygen is used, because the jet pipe would
not withstand the high temperatures that would be
incurred and complete combustion cannot be
assured.
FUEL CONSUMPTION
23. Afterburning always incurs an increase in
specific fuel consumption and is, therefore, generally
limited to periods of short duration. Additional fuel
must be added to the gas stream to obtain the
required temperature ratio (para. 19). Since the
temperature rise does not occur at the peak of
compression, the fuel is not burnt as efficiently as in
the engine combustion chamber and a higher
specific fuel consumption must result. For example,
assuming a specific fuel consumption without afterburning
of 1,15 lb./hr./lb. thrust at sea level and a
speed of Mach 0,9 as shown in fig. 16-9. then with
70 per cent afterburning under the same conditions
of flight, the consumption will be increased to
178
Fig. 16-9 Specific fuel consumption
comparison.
Fig. 16-10 Afterburning and its effect on the rate of climb.
approximately 2.53 lb./hr./lb. thrust. With an increase
in height to 35,000 feet this latter figure of 2.53
lb./hr./lb. thrust will fall slightly to about 2.34 lb./hr./lb.
thrust due to the reduced intake temperature. When
this additional fuel consumption is combined with the
improved rate of take-off and climb (fig. 16-10), it is
found that the amount of fuel required to reduce the
time taken to reach operation height is not excessive.
179
Rolls-Royce Dart
Armstrong Siddeley Viper
The Viper was designed as a result of
experience gained with the larger Sapphire
turbojet. Originally built as a 1,640 lb thrust
short-life engine for target drones, it later
emerged as a long life engine for the Jet
Provost. Subsequently the engine was
developed by Bristol Siddeley as the power
plant for civil executive jets, and Rolls-Royce
for present generation trainers and light strike
aircraft with a maximum thrust of 4,400 lb
(5,000 lb with reheat).
17: Water injection
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