due to thc~ low combustion pressure and values 
of c, from 2.0 to 4.0 are typical with aftet- 
burner operation. 
The turbojet engine usually has a strong 
preference fot high RPM to produce low specif- 
ic fuel consumption. Since the normal rated 
thrust condition is a particular design point 
for the engine, the minimum value of c, will 
occur at or near this range of RPM. The 
illustration of figure 2.10 shows a typical vati- 
ation of c, with percent maximum RPM where 
values of RPM less than 80 to 85 percent pro- 
duce a specific fuel consumption much greater 
than the minimum obtainable. This pref- 
erence for high.RPM to obtain low values of 
C, is very pronounced in the fixed geometry 
engine. Turbojet engines with multispool 
compressors tend to be less sensitive in this 
respect and are more flexible in their operating 
characteristics. Whenever low values of cI ate 
necessary to obtain range or endurance, the 
preference of the turboiet engine for the design 
operating RPM can be a factor of great 
influence. 
Altitude is one factor which strongly affects 
the performance of the turbojet engine. An 
increase in altitude produces a decrease in 
density and pressure and, if below the tropo- 
pause, a decrease in temperature. If a typical 
nonaftcrbutning turbojet engine is operated at 
a constant RPM and true airspeed, the vatia- 
tion of thtust and specific fuel consumption 
with altitude can be approximated from figure 
221. The variation of density in the standard 
atmosphere is shown by the values of density 
ratio at vatious altitudes. Typical values of 
the density ratio at specific altitudes are as 
follows: 
Altitude, ft.: Dews@ ra#ie 
scaleeel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Loo0 
5,ooo.. :. . .a617 
lO,coo.............................. .7385 
.?2#XQ. .4976 
35,cao . . . . . . . . . . . . . . . . . . . . . .3099 
40,oal.. . . . .2462 
~,OUO. . . . .lS32 
NAVWEPS 00-8OT-80 
AtRPlANE PERFORMANCE 
If the fixed geometry engine is operated at a 
constant V (TAS) in subsonic flight and con- 
stant N (RPM) the inlet velocity, inlet ram, 
and compressor pressure ratio are essentially 
constant with altitude. An increase in alti- 
tude then causes the engine air mass flow to 
decrease in a manner very nearly identical to 
the altitude density ratio. Of coutsc, this de- 
crease in mass flow will produce a significant 
e&ct on the output thrust of the engine. 
Actually, the variation of thrust with altitude 
is not quite as severe as the density variation 
because favorable decreases in temperature 
occut. The decrease in inlet air temperature 
will provide a relatively greater combustion 
gas &ergy and allow a greater jet velocity. 
The increase in jet velocity somewhat offsets 
the decrease in mass flow. Of course, an in- 
crease in altitude provides lower temperatures 
below the tropopause. Above the tropopause, 
no further favorable decrease in temperature 
takes place so a more rapid variation of thrust 
will take place. The approximate variation 
of thrust with altitude is represented by figure 
2.11 and some typical values at specific alti- 
tudes ate as follows : 
RIrio of Tbrvrt at dri14 Altitude, ft. : ( ) Thi ti I,‘ bwl 
Scalevel............................. 1.m 
5,ooo................................ ,888 
lO,ooo............................... .785 
2o,ooo............................... ,604 
35,Mx)............................... .392 
40,Ko. .315 
=Jo,ocQ ._._..,...._....._,.,.__.,..... .180 
Since the change in density with altitude is 
quite rapid at low altitude turbojet takeoff pet- 
formance wil1 Abe greatly affected at high alti- 
tude. Also note that the thrust at 35,000 ft. 
is approximately 39 percent of the sea level 
value. 
The thrust added by the afterburner of a 
turbojet engine is not affected so greatly by 
altitude as the basic engine thrust. The use of 
afterburner may provide a thrust increase of 50 
percent at low altitude or as much as 100 per-