In the flat, sandy landscape on the southern outskirts of Berlin, preparations are under way to start by mid-year full-scale testing of the gearbox that is to become a central part of Rolls-Royce's future commercial engine family.
The manufacturer has at its Dahlewitz site – near the German capital's still-to-open Brandenburg airport – built a €65 million ($69 million) test facility for the gearbox, which is being developed under Rolls-Royce's UltraFan programme in partnership with engineering group Liebherr.
UltraFan represents a second stage in Rolls-Royce future technology programme, with the first – dubbed Advance – concentrating on the development of a new, more thermally efficient engine core. Advance is intended to deliver efficiency gains of 20% – versus today's Trent 700 – in a conventional turbofan architecture from 2020, while for UltraFan the target is a further 5% for engines after 2025, to be achieved by decoupling the fan from the core, similar to Pratt & Whitney's PW1000G geared turbofan. Advance and UltraFan are both demonstrator projects aimed at maturing technology for potential future engine programmes.
Rather than following Pratt & Whitney's approach of introducing a gearbox in a medium-thrust range – the PW1000G-family spans engines of 13,000-35,000lb thrust – Rolls-Royce opted to develop its gearbox for large powerplants of more than 70,000lb. This is no surprise as the UK manufacturer's current commercial engine models are all in the high-thrust segment. But chief engineer of civil aerospace strategy and future programmes Phil Curnock also says it will be easier to scale down the gearbox to smaller engines than going in opposite direction at a later stage.
Testing of the gearbox began in September 2016 on an attitude test rig that is installed on a gimbal to simulate an aircraft's pitch and roll movements. This is aimed, in particular, at assessing oil flow – crucial for both lubrication and heat absorption – in different phases of flight. While the rig can simulate conditions of the engine operating or windmilling up to 30,000ft, the set-up cannot represent torque inside the engine. This will be assessed on a power rig in a dedicated part of the test facility.
While the planetary gearbox has an approximately 80cm (31.5in) diameter and weighs a few hundred kilogrammes, it has been designed to transfer power up to 100,000hp. Chief engineer and programme head for UltraFan technologies Mike Whitehead likens the energy that is being transferred between two gear-teeth to the power of an entire race group of Formula 1 cars.
To absorb vibrations during the testing, the power rig has been installed on top of a 2,400t concrete block that rests on more than 700 adjustable steel springs. The rig will be driven by two electric motors that transfer their power through a system of gearboxes and linkages which is, in principle, similar to a car's rear wheels being externally spun in order to turn, via the differential, the car's gearbox and engine. That arrangement, Whitehead explains, allows the manufacturer to modulate shaft speeds and torque in order to simulate gearbox conditions in different flight scenarios. The power rig has been additionally equipped with actuators to simulate stresses on the aero engine's shaft and gearbox when the aircraft pitches up at full power during take-off.
Rolls-Royce declined to specify oil volume and temperature in the UltraFan gearbox. But the manufacturer indicates that the oil flow circulating through the power rig at full operation – which utilises shipping container-sized cooling systems outside the test facility – is "in the order of swimming pools".
Developing an oil system for the gearbox is one of the central challenges in the UltraFan programme, says Curnock. Much of the test programme is directed at "really understanding heat management" inside the gearbox across a range of thrust settings and atmospheric conditions, he says. Oil needs to be sprayed around the gears for lubrication and heat absorption but must also be reliably extracted in order to be cooled. "We need to get oil in and, crucially, out again," notes Curnock. Whether the gearbox will be included in the engine core's oil circulation or have its own separate subsystem has not been decided yet, says Whitehead.
Rolls-Royce opted to design its gearbox with a stationary ring-gear – unlike Pratt & Whitney's – and five planet gears revolving around the central, turbine-powered sun gear. Thus, the planet gear's movement drives the fan, while in Pratt & Whitney's star-gear design the five planets are stationary and the ring gear drives the fan. Curnock says Rolls-Royce's configuration achieves a higher reduction ratio – if the gears were to have the same dimensions to a Pratt & Whitney-style system – and it ensures that the fan and turbine turn in the same direction.
Liebherr has been chosen as partner because of its experience in manufacturing gearboxes for a range of applications including cranes and helicopters, says Whitehead. Rolls-Royce and Liebherr established a jointly owned subsidiary, Aerospace Transmission Technologies, to develop the gearbox. Whitehead adds that a key reason for the Swiss-headquartered engineering group's selection was its capability to manufacture its own gear-grinding machinery. Liebherr has a tradition of producing core equipment in-house rather than relying on external suppliers.
Rolls-Royce intends to test-fly the gearbox in a demonstrator engine by 2021. Once the technology becomes available, Curnock says it will enable the manufacturer to build larger, more efficient fans, while the core can be optimised for greater thermal efficiency. The decoupling of the turbine and fan allows the core to operate at high speed and high temperature, while the fan can turn at slower speed for greater propulsive efficiency.
Already for the next turbofan generation – based on the Advance technology programme – Rolls-Royce intends to use a composite fan and fan case. In 2008, the manufacturer established with GKN Aerospace a joint venture at that UK company's facility on the Isle of Wight in order to develop production processes for the fabrication of composite fan blades. Rolls-Royce has since become the sole shareholder of that entity, and has decided to move the operations to its Bristol site by the end of 2017. However, Curnock says no decision has yet been taken where a composite fan blade production facility will be located.
Meanwhile, ground tests with an Advance core demonstrator are set to begin in Derby "around" mid-year, says Curnock. That "Advance 3" demonstrator comprises a four-stage intermediate-pressure compressor, 10-stage high-pressure compressor, two-stage HP turbine and one-stage IP turbine. This configuration represents a change from Trent-series engines, which tended to have more IPC stages and a smaller HPC. Curnock says the demonstrator is "really about understanding that core change and architecture change before we go anywhere near [developing] a new product".
The demonstrator will be operated with a Trent XWB fan and Trent 1000 low-pressure turbine. It will also include a lean-burn combustor that features two separate fuel-flow systems to provide maximum thrust when high power is required, and a more efficient combustion process during cruise. While the principle of having two fuel-flow systems is established, Curnock says integration of such an arrangement in to an engine is a challenge. The combustor will be separately assessed in a Trent 1000 engine for a series of ground tests – including cold temperature trials in Canada – before flight tests begin in 2018.
In the hot section, Rolls-Royce intends to start employing ceramic matrix composites for seal segments. Curnock says their use will be gradually expanded, first to stationary turbine vanes and eventually also to blades. A CMC research and development facility was formally opened in California in 2016.
While the Advance programme still envisions an engine with a typical Trent-style three-shaft architecture, Rolls-Royce will with UltraFan remove the low-pressure turbine altogether and power the fan with a multiple-stage IPT. But the IPC, HPC and combustor will be identical to those in the Advance core, and the HPT will require "minimal" change, Rolls-Royce says.
Asked whether the manufacturer was adopting a two-shaft architecture, Curnock insisted that the future engine series would still have three shafts. "The front one is quite short," he acknowledges. But he argues that the core technology is rooted in the manufacturer's Trent architecture, with the future IPT operating operating at higher speed and temperature as well as being more compact than a conventional LPT. "A lot of the knowledge we gain from our three-shaft capability," he says, adding: "That IP turbine is much more like one of my IP turbines from my Trent history than it is like the LP [turbine]."
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