In the paper the optimization of jet engines for combat aircrafts is discussed. This optimization is referred to the selection of the valúes of the fan pressure ratio or by—pass ratio; máximum turbine inlet temperature; overall pressure ratio and máximum reheat temperature which optimize the specific fuel consumption and the ratio of the thrust to the nozzle throat área, assuming this last parameter to be an indicative of the thrust/weight ratio. The selection is carried out taking into account the aircraft missions for which the engine design is optimized

Mera Diaz, E.

Sánchez Tarifa, Carlos

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

A STUDY ON THE OPTIMIZATION OF JET ENGINES 
FOR COMBAT AIRCRAFTS 
C. SSnchez Tarifa* 
E. Mera Diaz** 
Abstract 
In the paper the optimization of jet 
engines for combat aircrafts is discussed. 
This optimization is referred to the se-
lection of the valúes of the fan pressure 
ratio or by—pass ratio; máximum turbine 
inlet temperature; overall pressure ratio 
and máximum reheat temperature which opti-
mize the specific fuel consumption and the 
ratio of the thrust to the nozzle throat 
área, assuming this last parameter to be 
an indicative of the thrust/weight ratio. 
The selection is carried out taking into 
account the aircraft missions for which 
the engine design is optimized. 
Nomenc lature 
Selection of the valúes of these va-
riables is carried out in order to opti-
mize the thrust/weight ratio and the spe-
cific fuel consumption according to the 
missions for which the engine is optimiz-
ed. The thrust/weight ratio depends not 
only on the cycle characteristic but also 
on the specific details of the engine 
design and on the engine size. From the 
cycle point of view it may be taken as re-
presentative of that variable the ratio of 
the thrust to the nozzle throat área. 
Other important engine characteristics, 
such as life cycle cost, maintainibility 
and reliability have to be taken into ac-
count. However, they are outside of the 
scope of the present work. 
CC 
RH 
FN 
ov 
altitude 
Nach number 
combustión chamber outlet temperature 
reheat outlet temperature 
by-pass ratio 
fan pressure ratio 
over-all pressure ratio 
II- Missions 
k Introductions Scope of the Work 
The present work is devoted to the 
study of the optimization of the principal 
functional variables of a typical modern 
military engine for combat aircrafts. This 
optimization is referred to the fundamen-
tal engine characteristics: thrust/weight 
ratio and specific fuel consumption. 
Unlike comercial aircrafts, which al-
ways fly at practically the same condi-
tions; combat aircrafts are designed to 
opérate at very different flight condi-
tions, and the optimization of their engi-
nes greatly depend on the type of missions 
for which the aircraft is optimized. 
Jet engine for modern combat aircraft 
are of the turbofan type, with reheat and 
a convergent or convergent-divergent ex-
haust nozzle of variable área. 
During development optimization is an 
important task and it consists in the se-
lection of the principal variables of the 
thermodynamic cycle: by-pass ratio or fan 
pressure ratio, combustión chamber outlet 
temperature, overall pressure ratio and 
máximum reheat temperature. 
"*SENER7-Professor. Escuela Técnica Supe-
rior de Ingenieros Aeronáuticos. Poly-
technic University of Madrid. 
• «Aeronautical Engineer. 
Among the great number of missions as-
signed to combat aircrafts, there are two 
specially significant with respect to en-
gine optimization: they are the intercep-
tion missions and the air strike or ground 
attack mission. In the first of these mis-
sions, typical of a combat or air superioi— 
ity fighter, the aircraft takes off, climbs 
at optimum climb speed and flies at máxi-
mum or near máximum horizontal speed to 
intercept the target at full reheat condi-
tions. In this mission up to 75-85% of the 
fuel may be consumed with the reheat burn-
ing and, therefore, fuel consumption in the 
reheat is of fundamental importance. On the 
other hand, in the strike missions the re-
heat is utilized only for take off and for 
attacking the target, and less than 10% of 
the fuel may be consumed when the reheat 
is operating. 
In this last mission, and in missions 
of long range or long loiter; dry condi-
tions of the engine (reheat off) at high 
altitude and in subsonic or supersonic 
cruise, become specially important in the 
process of the engine optimization. 
III Assumptions 
a) The engine is assumed to be a two 
shafts turbofan, with reheat and a conver-
gent noz zle . * 
b) Typical adiabatic efficiencies of 
compressors and turbines are taken and as-
sumed to be constant. Typical pressure-
losses coefficients will also be taken for 
The case of a convergent-divergent 
nozzle will be treated separately. 
the combustión chamber, reheat, by-pass 
duct and nozzles, as well as for the mixing 
process of primary and secondary flows at 
the reheat inlet. 
c) The complex cooling system of a 
high temperature military engine is treat-
ed in a simplified way. The cooling flow 
rates of turbines, reheat and exhaust 
nozzle are related to the combustión cham-
ber outlet temperature and to the reheat 
temperature respectively; and it is as-
sumed a partial recovery of the energy of 
the cooling flows in both turbines. 
d) Average valúes will be taken of the 
exhaust área of the nozzle, for both on 
and off reheat conditions. 
e) Average valúes of the specific heats 
are taken in each phase and for each in-
dividual case. Combustión processes are 
treated numerically utilizing hydrocarbon 
combustión tables. 
With all these assumptions, calcula-
tions are simple and suitable to be treat-
ed with a computer programme. 
IV. ReSHA-*5_5!lA_2°S£AH5A2£5 
IV. 1 By Pass Ratio and Fan Pressure Ratio 
By equalizing the pressures of the 
primary and secondary flows at the reheat 
inlet, a relationship is obtained among 
the four variables: by-pass ratio, A fan 
pressure ratio * F „ . combustión chamber 
outlet temperature T„_ and over-all pres-
sure ratio *ov* External conditions: alti-
tude a and fligh Nach number M influence 
that relationship. 
Fig. 1 showB results for K=0 and a=0 
for different temperatures T„_. 
Large fan pressure ratios are essen-
tial in order to obtain high valúes of the 
thrust/weight ratio, and low valúes of the 
specific fuel consumption, when the reheat 
is working (Figs. 2, 3 and 4) , which im-
plies low valúes of the by-pass ratio ac-
cording to Fig. 1. 
These low valúes of A give good super-
sonic flight conditions, but it deterio-
rates subsonic cruise conditions (Fig. 5) ; 
due to the fact that the optimum valué of 
A that minimizes specific fuel consumption 
occurs at very high valúes of this parame-
ter (Fig. 6) , even higher that those nor-
mal ly utilized in turbofan engines for 
commercial aircrafts. 
As a consequence, engines optimized 
for interception missions or combat air-
crafts, must have small valúes of the by-
pass ratio, of the order of 0.20-0.50 in 
practice. The minimum possible valué of A 
is determined by a minimum air flow requi-
red to have combustión in the secondary 
flow and by the cooling requirements of 
reheat and nozzle. This minimum valué of 
A is of the order of 0 .10 - 0 . 20 . ' •' 
Engines optimized for strike fighters 
or for aircrafts that require long range 
missions at subsonic speeds, higher valúes 
of the by—pass ratio have to be selected, 
of the order of 0.5-1. 
IV. 2 Combustión Chamber Outlet Temperature 
Combustión chamber outlet temperature 
does not influence directly the thrust/ 
weight ratio when the reheat is operating, 
but it allows the selection of larger va-
lúes of the fan pressure ratios (Figs. 1 
and 2 ) . On the other hand, it does influen-
ce directly the thrust/weight ratio at dry 
conditions. 
High T temperatures improve the spe-
cific fuel consumption when the reheat is 
operating (Fig. 4) at supersonic condi-
tions, but it deteriorates subsonic cruise 
conditions, specially at low by-pass ra-
tios (Fig. 5) . * 
High temperatures T are always desi-
rable in engines for combat aircrafts. At 
todays level of materials technology, 
those high temperatures are made feasible 
by means of large turbines cooling flows, 
which may offset further gains above a cer-
tain limit. Actually, máximum inlet turbi-
ne temperatures are of the order of 1800-
2000 K; but there are research programmes 
in progress to increase these temperatures 
up to near the stoichiometric level by de-
veloping new types of materials. 
IV.3 Overall Pressure Ratio 
For a given by-pass ratio, there is a 
valué of the overall pressure ratio * Q V 
for which the fan pressure ratio x F N is 
máximum (Fig. 7) , and then the specific 
fuel consumption is minimum at reheat ope-
rating conditions. However the specific 
fuel consumption is not a very sensitive 
function of *QV above that minimum valué 
(Fig. 8) . 
On the other hand, at dry conditions 
the specific fuel consumption has a mini-
mum valué as function of *ov> which occurs 
at very high valúes (40 or even higher) of 
this variable, at subsonic flight condi-
tions (M=0.85), as occurs in engines for 
commercial aircrafts. The valué of »ov 
that minimizes the specific fuel consump-
tion is considerably smaller at supersonic 
flight conditions. 
It has also to be considered that high 
valúes of » Q V would créate severe tempera-
ture problems in the high pressure com-
pressor at supersonic flight conditions. 
At very large by-pass ratios results 
would be inverse. 
In practice, modérate valúes of the 
overall pressure ratio (of the order of 
25) are a good compromise for combat air-
crafts that do not require the capability 
of cruising at supersonic speeds at dry 
conditions. 
If supersonic cruise is required with-
out reheat lower valúes («15-20) of »ov 
appear to be more suitable. 
IV.4 Reheat Máximum Temperature 
Increasing the reheat máximum tempera-
ture TEJJ directly increases the thrust/ 
weight ratio, at the cost of a less impor-
tant increase in the specific fuel consump-
tion (Figs. 2 and 4) . 
Máximum valúes of TRH are limited, on 
one hand, by the cooling problems of the 
reheat and the exhaust nozzle, and on the 
other hand, because when TRH approaches 
the adiabatic combustión temperature, com-
bustión instabilities problems (buzzing 
and screeching) become more severe. In 
practice, limiting valúes of 2000-2200 K 
are not surpassed. 
Nozzles 
The optimization studies have been 
carried out, for simplicity, on engines 
with convergent nozzles, and considering 
that results would have been very similar 
with convergent-divergent nozzles. 
However, most modern engines for combat 
aircrafts utilize convergent divergent 
nozzles. With the high nozzle pressure ra-
tios, typical of these engines at super-
sonic flight and at high altitude condi-
tions; ideal thrust gains of the order of 
25-30% may be obtained, with similar reduc-
tions in the specific fuel consumption. 
In practice, thrust gains are lower, 
due to losses, additional cooling air and 
especially due to geometrical constraints 
typical of axi-symmetrical nozzles made of 
a certain number of sliding petáis. 
On the other hand, if optimum valúes 
of the exhaust and throat áreas would be 
adapted for each flight condition, each 
área would change independently and it 
would require two independent actuation 
systems, with an important penalty in 
weight and cost. 
Convergent-divergent nozzles with only 
one actuation system are Bimpler and widely 
utilized in practice. Some sort of mecha-
nical link between the convergent and the 
divergent petáis is utilized, giving a re-
lationship between the exhaust and the 
throat área. The selection of this rela-
tionship is the basis of the optimization 
process of the nozzle according to the 
missions. 
Fo example, a law of variation such as 
A-A (Fig. 9) would give priority to high 
altitude combat conditions in detriment of 
range, and it would be suitable for an air-
craft optimized for interdiction missions. 
On the other hand, a law of variation such 
as B-B, would be more suitable for a long 
range fighter or for a strike aircraft. 
Bi-dimensional nozzles, which are being 
developed, overeóme some of the aforemen-
tioned problems, at the cost of being more 
voluminous, probably heavier and with dif-
ficult sealing problems. However, their 
main asset is the possibility of thrust 
vectoring, which increases the maneuvera-
bility of the aircraft. 
VIA_5££S£«S£*£ 
1. Braybrock, R. : A New Engine for the 
next European Fighter. Jane's Defense 
Weekly. February 1984. 
2. Moxon, J.: Front-line Power. Flight In-
ternational. April 1986. 
3. Sánchez Tarifa, C : Problemaa de Desa-
rrollo de Turborreactores Civiles y Mi-
litares. Cosmo 86, Gerona. Nay 1986. 
Proceedings. 
These convergent-divergent nozzles re-
quire also an important optimization proc-
ess according to the missions for which -the 
engine design is optimized. 
For each flight conditions (Mach num-
ber and altitude), there are optimum valúes 
of the exhaust and throat áreas of the 
nozzle giving máximum valúes of the thrust 
gain. However, this requires considerable 
variations of the ratio of the exhaust área 
to the throat área, when the nozzle- changes 
from conditions adapted, for example, at 
full reheat (combat) at high .altitude and 
high Mach number, down to typical cruise 
conditions (Fig. 9) . Petáis constraints may 
limit such large variations of the nozzle 
área ratio. 
¡X 
3 
BV -PASS RAT'O A 
F i g . 1 . - Fan p r e a s u r e r a t i o as f u n c t i o n o f 
t he b j - p a s s r a t i o . 
9v - =ASS 3ATI0 A 
F i g o . 4 and 5. - S p e c i f i c f u e l c o n a u í p t i o n 
as f u n c t i o n o f t h e b y - p a s s r a -
t i o . Reheat on and o f f • 
3Y-PASS RATIO A 
BV-PASS 1AT10 A 
F i g o . 2 and 3 . - T h r u s t / n o z z l e á r e a r a t i o 
as f u n c t i o n o f t h e by -pano r a -
t i o . Reheat on and o f f . 
F i g . 6 . - S p e c i f i c f u e l c o n s u m p t i o n aa func-
t i o n o f t h e b y - p a s o r a t i o a t sub-
s o n i c s p e e d . 
OVERALL PRESSURE RMIO TTo. OVERALL PRESSURE RATIO 
F i g - 1 . - Fan p r e s s u r e r a t i o a s f u n c t i o n 
o f t h e o v e r a l l p r e s s u r e r a t i o , 
s h o v i n g t h e m á x i m u m v a l u é o f * 
FU" 
F i f i , fl.- S p e c i f í c f u e l c o n n u m p t I o n a s 
f u n c t i o n o f t h e o v e r a l l p r e s s u r e 
r a t i o . 
'¿foñwéffi*\*%^ 
•f<\ C H U I S E " ^ X ^ 
¿ 
MACH NUMBE" 
Fig. 9.- Convergent-divergent nozzles. 
Laws of variation of the ra-
tio of the exhaust área to 
the throat área. 


A study on the optimization of jet engines for combat aircraft

Abstract

Similar works

Full text

Available Versions

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

Archivo Digital UPM