Rolls-Royce has dusted off a mothballed test rig that it last used for ‘propfan’ or ultra-high bypass work in the 1980s, and fitted it with 21st century-shaped fan blades for its first open rotor test in two decades. The one-sixth scale ‘rig 145’, which measures almost 3 ft in diameter, is on its way to a wind tunnel in the Netherlands where it will undergo all-important noise testing in July.
Noise was the Achilles heel of the open rotor concept when it first emerged after the oil crisis of the 1970s. While the noise levels were fairly acceptable in the 1980s and early 1990s, there was no way the first generation open rotor engines could have met modern, let alone future noise requirements. In particular, it was the strange tones created by the two sets of counter-rotating blades in both the General Electric and Pratt & Whitney/Allison engines that really put the kibosh on them.
A Rolls-Royce artists impression of its open rotor concept is deliberately 'vague' when it comes to blade design.
No pictures of the ‘rig 145’ have yet been released, largely because Rolls has a patent pending on key aspects of the design. It also wants to keep its competitors guessing for as long as possible, though that’s where things could get complicated. While Pratt & Whitney is pretty much going it alone with the geared turbofan, General Electric and Snecma are busy forging even stronger links over a joint open rotor project. GE is also reviving work it finally shelved on the GE36 in the early 1990s, while NASA is rapidly restoring its old propfan test rig to help the effort.
The issue for Rolls could be that work on rig 145, which is due for completion early in 2009, comes out of DREAM (valiDation of Radical Engine Architecture systems), a European Union research project with major Snecma involvement. Under this, Rolls is focusing on a geared open rotor and Snecma on a direct-drive concept. The two are also embarking on the next stage, and will work together on the follow-on Clean Sky joint technology initiative (JTI). With a budget of around $2.4 billion (US), Clean Sky is Europe’s first large-scale aeronautical research program. The JTI includes at least two open rotor demo engines.
No-one seems to be able to explain yet how the relationship will work out, yet there can be no doubt that should open rotors finally succeed this time, Snecma is on to a winner! The French engine maker has plenty of low pressure spool and advanced composite fan manufacturing experience to bring to the party and was actually responsible for around 30% of the GE36.
The GE36, seen here on a 727 testbed, involved Snecma.
What about Rolls? Aside from its own 1980s project, the company has access to much of the work done by Allison on the 578-DX propfan. It also has a lot of valuable counter-rotating propeller experience, even though this was all with puller designs and not a pusher like Rolls’ rig 145 demo. Most of this comes from work with one of the last, and largest production propulsion systems of its kind in the west, the counter-rotating propeller on the Rolls-Royce Griffon. The ultimate Griffon-powered machine was the Avro Shackleton – a sound that once heard could never be forgotten, and not one likely to be repeated with rig 145!
To see (and hear) a Shack flypast, click below: