text: 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 
  the minimum obtainable. This pref- 
  than 
  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: 