As mentioned earlier, there are two configurations of the  axial compressor currently in use: the single rotor/spool and  the dual rotor/spool, sometimes referred to as solid spool and  split spool (two spool, dual spool).  One version of the solid-spool (one spool) compressor uses  variable inlet guide vanes. Also, the first few rows of stator  vanes are variable. The main difference between variable inlet  guide vane (VIGV) and a variable stator vane (VSV) is their  position with regard to the rotor blades. VIGV are in front of the  rotor blades, and VSV are behind the rotor blades. The angles  of the inlet guide vanes and the first several stages of the stator  vanes are can be variable. During operation, air enters the front  of the engine and is directed into the compressor at the proper  angle by the variable inlet guide and directed by the VSV. The  air is compressed and forced into the combustion section. A  fuel nozzle that extends into each combustion liner atomizes  the fuel for combustion. These variables are controlled in  direct relation to the amount of power the engine is required  to produce by the power lever position. Most turbofan engines are of the split-spool compressor type.  Most large turbofan engines use a large fan with a few stages  of compression called the low-pressure spool. These turbofans  incorporate two compressors with their respective turbines and  interconnecting shafts, which form two physically independent  rotor systems. Many dual rotor systems have rotors turning  in opposite directions and with no mechanical connection to  each other. The second spool, referred to as the high-pressure  spool and is the compressor for the gas generator and core of  the engine, supplies air to the combustion section of the engine. The advantages and disadvantages of both types of  compressors are included in the following list. Even though  each type has advantages and disadvantages, each has its use  by type and size of engine. bearing support surfaces and splines for joining the  turbine  shaft. The drum-type and disk-type rotors are illustrated in  Figures 1-50  and  1-51 , respectively.  The combination of the compressor stages and turbine stages  on a common shaft is an engine referred to as an e ngine  spool. The common shaft is provided by joining the  turbine  and compressor shafts by a suitable method. The engine’s  spool is supported by bearings, which are seated in suitable  bearing housings. The centrifugal-flow compressor’s disadvantages are: The centrifugal-flow compressor’s advantages are: Figure 1-51. Disk-type compressor rotor. Stator blade Rotor blade •  High pressure rise per stage, •  Efficiency over wide rotational speed range, •  Simplicity of manufacture and low cost, •  Low weight, and •  Low starting power requirements. •  Its large frontal area for a given airflow and •  Losses in turns between stages.  1-43 Figure 1-50. Drum-type compressor rotor.  Air flow Air flow