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3-shaft engine family
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Three-shaft engine design

Large aero engines built around the three-shaft design concept are unique to Rolls-Royce and were introduced with the entry into service of the first of the RB211 series in the 1970s.

This heritage has been continued into today's Trent family of high-thrust, high bypass engines powering the new generation of widebodied jets from Airbus and Boeing. Today, Rolls-Royce has accumulated more than 100 million hours of service with three-shaft engines.

The engineering principle involves low, intermediate and high pressure systems, each consisting of a number of compressor and turbine stages, being mounted on independent shafts which run at their optimum aerodynamic speeds. As a guide, in the Trent 800 the low pressure system will operate at 3,000 revolutions per minute (rpm), the intermediate pressure system at 7,500 rpm and the high pressure system at 10,000 rpm. The fan needs to rotate relatively slowly, being limited by the stress and aerodynamic tip speed of the blade.

Optimum engine efficiency, governed by the maximum pressure and temperature achieved in its core, is enhanced by the three-shaft design. The pressure of the air as it enters the combustion and turbine components in a Trent engine, is around 40 atmospheres, with turbine entry temperatures above 1600 C - an environment significantly in excess of the components' melting point. This necessitates advanced materials including ceramic coatings, and cooling technologies.

A further benefit of the three-shaft design is the ease in which it can be optimised for specific aircraft applications. The Trent family provides a thrust range from 53,000lb to potentially over 100,000lb by scaling its modularised components; the fan, compressor, combustor and turbine systems. The result is low-risk, cost-effective development, with lessons learned in-service being fed back into new versions of the engine family.

Three-shaft engines are typically of short length, and rigid, compared to longer, two-shaft engines of competitors, which do not feature an intermediate system of compressors and turbines. The benefit to operators of Rolls-Royce engines in this context is that three-shaft powerplants tend to flex less in flight, leading to less wear, and therefore better performance retention.

The simplicity and inherent efficiency of their design also means that modern three-shaft engines produce significant weight benefits. A Trent 800-powered Boeing 777 is around 7,500lb lighter than the same aircraft powered by the heavier of the competing engines.


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