The announcement of the new joint venture between Rolls-Royce and Pratt & Whitney on mid-sized engines was hardly a statement of marriage, but the vows made by the two aero-engine giants on 12 October nevertheless secure their long-term future in the huge market for mid-sized aero-engines up to 2030.

The deal provides for future joint development of engines to power the forthcoming replacements for the Airbus A320 and Boeing 737 ranges and aircraft up to the size of the Boeing 757. It also includes the sale of Rolls-Royce’s stake in International Aero Engines, at once providing a useful $1.5 billion payment from P&W for R-R’s equity and program share in the venture. Long-term support of the V2500 engine for existing operators is solidified, and R-R receives an agreed payment for each hour flown by the current installed fleet of V2500-powered aircraft for fifteen years from completion of the transaction.

All very “elegant”, in the words of Mark King, R-R’s civil aerospace president. But what, exactly, does the joint venture really mean in terms of how the two companies structure what, to-date, have been very different approaches to a market estimated at 45,000 engines over the next two decades?

A quick look through AIN’s articles on the subject over the past few years reveals, for example, that R-R has always rejected geared turbofans for the mid-size engine market. “Gearing the turbofan produces no improvement on an advanced turbofan”, R-R’s head of strategic marketing, Robert Nuttal said at the 2010 Farnborough air show. The future, he added, lies not in re-engining but in an all-new engine which is “fully integrated” with the airframe. As part of that, he points to the company’s considerable R&D commitment to providing a “step change” in technology for future mid-sized aircraft, which means open rotors.

At the same air show, P&W’s geared turbofan guru, Bob Saia, vice-president, next generation product family, rejected the open rotor solution being pursued by R-R. “We believe we can get better fuel efficiency than open rotors with our engine by leveraging the technology we already have,”  he told AIN.

The wording of last month’s announcement demonstrates how things have changed. “This new joint venture will focus on high bypass ratio geared turbofan technology.  In addition, the venture will collaborate on future studies for next generation propulsion systems, including advanced geared engines, open rotor technology and other advanced configurations”. In other words, R-R now embraces a future geared fan engine, while P&W has opened the door to open rotors.

Nuttal says that what he really meant was that gears “do work at certain bypass ratios. The rejection of geared turbofans for the current generation of aircraft was, he told AIN, because there was “no business case” for an engine of the GTF’s 12:1 bypass ratio. At the higher 20:1 bypass ratio of a future engine, however, gears begin to make sense to drive the bigger fan. “The physics of geared turbofans changes at higher bypass ratios because the weight reduction in the low-pressure turbine offsets the increased weight of the gears,” he said Nuttal.

Gears will also be essential to any open rotor engine, to drive its complex contrarotating, variable pitch rotors. P&W’s geared turbofan work has won the company a huge amount of gearbox knowhow, but R-R also has considerable experience through its large turboprop programs. The joint venture also brings P&W’s United Technologies co-subsidiary, propeller blade specialist Hamilton Sundstrand, to the table.

It seems to be a win-win solution for all parties. By joining with P&W, Rolls has pulled off a brilliant coup, ensuring it has access to all three potential solutions to power the forthcoming all-new generation of midsized aircraft, whatever their configuration: conventional two and three-shaft turbofans, through its Advance 2 and 3 R&D programs, as well as geared turbofans and open rotors. For its part, P&W has gained control over IAE, enabling it to offer a “unified approach” to the pursuit of all future A320 and A320neo orders, and the US company gains access to the major effort R-R is putting in to open rotors.

On the surface, the deal, which caught the industry by surprise, appears to have wrong-footed arch-competitor CFM. The hugely successful GE/Snecma partnership has captured a significant slice of both A320 and A320neo orders, capitalising on its ability to offer airlines engine “packages” for both aircraft. Having digested the news, however, CFM’s sales teams will shed few tears, the company’s Leap engines having sold well on the A320 and A320neo and continued its monopoly on the Boeing 737MAX. CFM is also heavily involved in a number of open rotor R&D programs.

R-R will have no involvement in the current PW1000 series of geared turbofans, apart from minor investment in ongoing development. The early success of the GTF concept in finding a home on the A320 (which some observers believe took R-R by surprise) points to possible development of a second generation of geared turbofan engines that would include the UK company.

Asked about the potential for further development of the V2500, Nuttal said “The future of the engine is up to Pratt & Whitney. The main thing is we’ve provided a seamless transition for existing customers. We are very keen to help ensure the V2500 continues its long and successful career.”

For the aero-engine industry, as well as for Boeing and Airbus, the joint venture announcement was momentous, setting the agenda for Rolls-Royce and Pratt & Whitney’s approach to the mid-sized engine market for the next 40 years or more (CFM celebrates its 40^th in 2014). But the story does not end here. The risks of developing an all-new engine, and in this case one which might have a totally different configuration to that of existing powerplants, are huge. Such programs require a lot of trust, understanding and flexibility on both sides - elements that both parties will have to preserve if the venture is to succeed.