Jet Propulsion A Simple Guide to the Aerodynamics and Thermodynamic Design and Performance of Jet Engines

The emphasis of Part 1 of the book has been overwhelmingly towards the aerodynamic and thermodynamic aspects of a jet engine. These are important, but must not be allowed to obscure the obvious importance of a wide range of mechanical and materials related issues. In terms of time, cost and number of people, mechanical aspects of design consume more than those which are aerodynamic or thermodynamic. Nevertheless this book is concerned with the aerodynamic and thermodynamic aspects and it is these which play a large part in determining what are the desired features and layout of the engine. Clearly, an aerodynamic specification which called for rotational speed beyond what was possible, or temperatures beyond those that materials could cope with, would be of no practical use.

An aircraft engine simultaneously calls for high speeds and temperatures, light weight and phenomenal reliability; each of these factors is pulling in a different direction and compromises have to be made. Ultimately an operator of jet engines, or a passenger, cares less about the efficiency of an engine than that it should not fall apart. Engines are now operating for times in excess of 20,000 hours between major overhauls (at which point they must be removed from the wing), and this may entail upward of 10,000 take-off and landing cycles. In-flight engine shut-downs are now rare and the rate for the fleet of modern civil aircraft is one shut-down in about 250,000 flying hours. As a result most pilots will never experience a compulsory engine shut-down during their whole careers.

The speeds of some of the components of an engine are comparable with the shells from some guns with the potential to do terrible damage to the pressurized aircraft fuselage and the passengers. The fan blades are the largest rotating items and the fan is surrounded by a containment ring so that in the event of a fan blade detaching the parts will not travel sideways to hit the fuselage. Much engineering effort is expended in design and testing of the containment ring around the fan and this ring adds considerable extra weight to the engine. Other rotating items are not designed to be contained but, instead, enormous care is taken in the manufacture of the discs to ensure failure is very rare.