Chapter 1 – 3 Spool vs 2 Spool engine
Advantages of a 3 spool engine:
• More flexible due to aerodynamic matching at part load
• Lower inertia of rotating components
• Easier to start as only one spool needs to be turned by the starter
• Allows for higher ratios of fan air flow to engine flow. This allows for increased thrust without a corresponding increase in jet velocity and reduction in propulsive efficiency leading to high SFC.
Note: The older turbo jet engine accelerates a small mass of air to a very high velocity to achieve a given amount of thrust. To obtain the same amount of thrust from a turbofan, it takes a much larger mass of air and does not need to accelerate it to as high of a velocity, thus giving a lower SFC.