text: area is too large, incomplete expansion will 
  take place; if the exit area is too small, an over 
  expansion tendency results. The exit area can 
  affect the upstream conditions and must be 
  properly proportioned for overall performance. 
  When the ratio of exhaust gas pressure to 
  ambient pressure is greater than some critical 
  due, sonic flow can exist and the nozzle will 
  be choked or limited to some maximum flow. 
  When supersonic exhaust gas velocities are re- 
  quired to produce the necessary momentum 
  change, the expansion process will require the 
  convergent-divergent nozzle illustrated in fig- 
  ure 2.9. With sui?icient pressure available the 
  initial expansion in the converging portion is 
  subsonic increasing to sonic velocity at the 
  throat. Subsequent expansion in the divergent 
  portion of the nozzle is supersonic and the re- 
  sult is the highest exit velocity for a given 
  pressure ratio and mass flow. When the pres- 
  sure ratio is very high the final exit diameter 
  required to expand to ambient pressure may be 
  to the 
  very large but is practically. limited 
  fuselage or nacelle afterbody diameter. 
  If the 
  exhaust gases exceed sonic velocity, as is porsi- 
  ble in a ramjet combustion chamber or after- 
  burner section, only the divergent portion of 
  the nozzle may be necessary. 