the design service life with trouble-free opera- 
tion. The following items describe the critical 
areas encountered during the operational use 
of the turbojet engine: 
(1) The limiting exhaust gag tcmpcra;wcs pro- 
vide the most important restrictions to the op- 
eration of the turbojet engine. The turbine 
components are subject to centrifugal loads of 
rotation, impulse and reaction loads on the 
blades, and various vibratory loads which may 
be inherent with the design. When the turbine 
components are subject to this variety of stress 
in the presence of high temperature, two types 
of structural phenomena must be considered. 
when a part is subject to a certain stress at some 
high temperature, weep failure will take place 
after a period of time. Of course, an increase 
in .tcmperature or stress will increase the rate 
at which creep damage is accumulated and 
reduce the time required to cause failure. An- 
other problem results when a part is subjected 
to a repeated or cyclic stress. F&&e failure 
will occur after a number of cycles of a varying 
stress. An increase in temperature or magni- 
tude of cyclic stress will increase the rate of 
fatigue damage and reduce the number of cycles 
necessary to produce failure. It is important 
to note that both fatigue and creep damage are 
cumulative. 
A gross overstress or overtemperature of the 
turbine section will produce damage that is 
immediately apparent. However, the creep 
and fatigue damage accumulated through pe- 
riods of less extreme’ overstress or overtem- 
perature is more subtle. If the turbine is 
sibject to repeated excessive temperatures, the 
greatly increased rate of creep and fatigue 
damage wiIl produce failure early within the 
anticipated service life. 
Generally, the operations which produce 
the highest exhaust gas temperatures are 
starting, acceleration, and maximum thrust 
at high altitude. The time spent at these 
temperatures must be limited arbitrarily to 
prevent excessive accumulation of creep and 
fatigue. Any time spent at temperatures in 
NAVWEPS OO-SOT-RO 
AIR.PLANE PERFORMANCE 
excess of the operational limits for these con- 
ditions will increase the possibility of early 
failure of the turbine components. 
While the turbine components are the most 
critically stressed high temperature elements 
they are not the only items. The combustion 
chamber components may be critical at low 
altitude where high combustion chamber pres- 
sures exist. Also, the airframe structure and 
equipment adjacent to the engine may be sub- 
ject to quite high temperatures and require 
provision to prevent damage by excess time at 
high temperature. 
(2) The c~mprcs~or Jtall or surge has the pos- 
sibility of producing damaging temperatures 
in the turbine and combustion chamber or un- 
usual transient loads in the compressor. While 
the stall-surge phenomenon is possible with 
the centrifugal compressor, the more common 
.occurrence is with the axial flow compressor. 
Figure 2.13 depicts the pressure distribution 
that may exist for steady state operation of 
the engine. In order to accelerate the engine 
to a greater speed, more fuel must be added to 
increase the turbine power above that required 
to operate the compressor. 
Suppose that the fuel flow is increased be- 
yond the steady state requirement without a 
change in rotative speed. The increased com- 
bustion chamber pressure due to the greater 
fuel flow requires that the compressor dis- 
charge pressure be higher. For the instant 
before an engine speed change occurs, an in- 
crease in compressor discharge pressure will be 
accompanied by a decrease in compressor flow 
velocity. The equivalent effect is illustrated 
by the flow components onto the rotating com- 
pressor blade of figure 2.13. One component 
of velocity is due to rotation and this compo- 
nent remains unchanged for a given rotative 
velocity of the single blade. The axial flow 
velocity for steady state operation combines