(Repeats Monday's story with no changes to text)
* MS-21 first jetliner using composite wings without autoclaves
* Move foreshadows changes in the way many jets may be built
* Boeing pondering same technology for possible mid-market plane
* Upcoming technology choices to shape jet industry for decades
By Tim Hepher
PARIS, May 29 (Reuters) - Russia's new jetliner, which conducted its maiden flight on Sunday, may have a hard time challenging the sales duopoly of Boeing and Airbus, but it does point the way to radical changes in how they could be building jets in the future.
The MS-21, a new single aisle airliner produced by Russia's United Aircraft Corporation, is the first passenger plane borne aloft by lightweight carbon-composite wings built without a costly pressurised oven called an autoclave.
The manufacturing process provides a test for a technology already being assessed by Western rivals, who are looking for cheaper and faster ways to build some of their aircaft with composites, according to aerospace executives and suppliers.
Even as it sets up the world's largest autoclaves to make wings for its giant 777X, Boeing is exploring alternatives for its "New Midsize Airplane" (NMA), in the middle of the market between its big wide-body jets and best-selling 737.
"There's a good chance part of the NMA will be built without autoclaves," a person familiar with the project said.
A Boeing spokesman said it was studying mid-market opportunities and declined further comment.
Sources say Boeing's choice of technology for its two-aircraft NMA family will lay the foundation for the next generation of its money-spinning 737, expected to appear from 2030 and last well into the second half of the century.
Boeing has not yet discussed this part of its strategy publicly, but industry sources said it may include a trio of jets seating 160 to 210 people and built using broadly the same production system as the one developed for the NMA.
Both families of planes are likely to be built for 30 years and stay in service for another 20-30. So today's technology choices represent a colossal 75-year bet.
Airbus too is monitoring the technology as it considers how to respond to Boeing's mid-market jet, CNN reported last month. Airbus has declined to comment on the report.
Composites have been used in aviation since the 1970s but achieved a breakthrough in the past decade as the Boeing 787 Dreamliner and Airbus A350 entered service, promising to save money on fuel by replacing most metal parts with lighter carbon.
Those are long-haul jets, which means that the savings on fuel are worthwhile even though the planes are expensive to build. For short-haul planes that burn less fuel, like the NMA or future 737, it is more important to find cheaper ways to build them, and avoiding the need for autoclaves could help.
Betting on technology that does not require an autoclave is a gamble also for composite suppliers like U.S.-based Hexcel , Solvay of Belgium and Toray of Japan, whose share of aerospace manufacturing is growing.
At a recent JEC composites fair in Paris, Hexcel and Solvay showcased out-of-autoclave prototype parts as they gear up to supply manufacturers on a bigger scale.
"It's one of the big questions now in aerospace: how to produce out-of-autoclave on a large scale and at high speeds," said Henri Girardy, business development manager at Hexcel Composites, adding jetmakers would accept no cut in performance.
Boeing's Dreamliner and the Airbus A350 are built from carbon fibre already impregnated with resin, called "prepreg", which is supplied to jetmakers, tailored by machines into plane parts and cured inside giant pressure-cooking autoclaves.
Analysts say these parts cost around 30-40 percent more to produce than aluminium.
Under the new technology, instead of using fibre that is pre-impregnated with resin, parts are made from a dry-fibre engineered textile which is placed in a mould and then infused with resin under a vacuum.
The parts can then be cured in an oven without pressure, a process estimated to cost 25 percent more than metal. Ultimately, that gap needs to narrow significantly or disappear.
Boatbuilders and windfarm makers have used this method for years. Secondary airplane parts have also been made that way.
But although Canada's Bombardier partly used the technique for its CSeries, it was rare for flight-critical parts before the designers of the new Russian plane chose it for the wing.
The MS-21 has yet to score large sales but has been able to catch onto the latest manufacturing wave at a time when Western giants are starting to think beyond their recently upgraded models to future designs.
"This is an excellent technology demonstrator because it is on a real programme and a primary-structure part," said Frank Nickisch, global director of strategic projects at Solvay Composite Materials, which provided materials for the MS-21.
He noted that the wings on the MS-21 are of comparable size to a Boeing 737 or Airbus A320.
Still, more needs to be done to bring down costs. And composite firms are not yet ready to support output at speeds planemakers have in mind.
Industry sources say Boeing is expected to build 20-30 of its proposed mid-market planes a month, about twice the rate for the 787. Future 737 and A320 replacements are likely to be closer to the 60-80 mark.
Nickisch said the proposed new Boeing middle-market aircraft would be a good test case for whether the industry can switch to a higher tempo using the widely expected composites.
Yet tough planemaker targets on cost, performance and output require progress on three fronts: cheaper materials, wider automation and computer simulation to ensure new technology works first time, following widespread delays with new jet programmes.
"Prepreg technology has been developing for 40 years, based on day-to-day knowledge. But to bring in a new process we need to be able to predict it first, with databanks," Girardy said.
Writing by Tim Hepher; Editing by Peter Graff