The promising Russian PD-35 engine and the Rolls-Royce Trent 1000 for the Boeing 787 belong to the class of large turbofan engines. However, their internal structure differs. The PD-35 uses 2 shafts, while the British engine is built around 3 coaxial rotors.
At first glance, the difference is only one shaft. But it is precisely this that determines the operation of the compressors and turbines. "Pervy Tekhnichesky" went to study the material.
Rolls-Royce's Third Shaft
The Trent 1000 received 3 independent rotors: low, intermediate, and high pressure. Each of them connects its own compressor to a turbine.
The 2.85 m diameter fan works together with the low-pressure system. Behind it is an 8-stage intermediate-pressure compressor, followed by a 6-stage high-pressure compressor.
Each cascade has its own shaft and turbine. This design allows the fan and compressors to rotate at different speeds. Rolls-Royce uses precisely this three-shaft architecture in the Trent family of engines.
How the two shafts of the PD-35 are arranged
The PD-35 does without an intermediate rotor. Instead of 3 independent shafts, the Russian engine uses 2.
The first shaft connects the fan and booster stages of the low-pressure compressor with the low-pressure turbine. The second connects the high-pressure compressor with the high-pressure turbine.
The PD-35 gas generator operates on a separate high-pressure shaft. It includes a high-pressure compressor, a combustion chamber, and a high-pressure turbine.
Russian designers retained the two-shaft architecture but applied it to an engine with a thrust class of up to 35 tf.
Two against three
The three-shaft design gives the Trent 1000 more freedom in choosing speeds. Each rotor can operate in its own range, which helps optimize fan and compressor modes.
However, this comes at the cost of increased structural complexity. The shafts are arranged coaxially one inside the other and require a complex system of supports and bearings.
During operation, the Trent 1000 faced serious problems with the durability of several parts. These led to unscheduled inspections, operational restrictions, and downtime for Boeing 787 aircraft. The causes of the problems were related to specific engine elements and their operating loads, so they cannot be attributed solely to the three-shaft design.
The PD-35 does not have an intermediate rotor from the outset. The Russian engine chose a simpler kinematic scheme: 2 shafts instead of 3.
One extra shaft does not in itself make the Trent 1000 better, and the absence of a third does not make the PD-35 a less modern engine. Rolls-Royce divided the engine's work between 3 independent rotors, while Russian designers solved the same problem using 2.