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
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.