The Woracle

Anyone think open rotors are for real this time round? By open rotor I mean propfans, unducted fans - engines with the fans on the outside. Engine manufacturers are again talking about open rotors, but as part of a long-term technology plan that begins with today's turbofans, advances through ultra-high bypass engines (those with the fans on inside, like Pratt & Whitney's Geared Turbofan) and finally gets to open rotors around the end of next decade.

But airlines like easyJet, with its "eco-friendly" aircraft design, are pushing for open-rotor engines by 2015. And NASA, which has begun developing technology for a 2015-timeframe next-generation narrowbody, says it has had requests to revisit open rotors.

Propfans came close to a launch in the late 1980s, but fuel prices were too low. This time round the driver is emissions and the environment.

Reuters is reporting that US Congressional negotiators have agreed to fund contined development of the General Electric/Rolls-Royce F136 alternate engine for the Lockheed Martin F-35 Joint Strike Fighter.

The Pentagon had sought to cancel the F136 to save money, and the $480 million in FY2008 funding will have to come from somewhere, but the preservation of a competitive engine choice is good news for the JSF programme, and for F-35 operators.

Although it looked likely, the F136's survival can't have been harmed by Pratt & Whitney's recent issues with its F135 primary engine. The failure of an engine on a test stand in late August has proved a problem for Lockheed as it works to get the first F-35 back in the air after a six-month hiatus.

Not exactly a surprise, but Bombardier has announced it is working exclusively with Pratt & Whitney to power its CSeries airliner with the Geared Turbofan. The GTF has already been selected to power the Mitsubishi Regional Jet. Neither aircraft has been launched yet, but if they are Pratt will find itself developing two sizes of GTF simultaneously: 17,000-19,000lb thrust for the MRJ and 23,000lb for the CSeries.

And the larger of those two engines is going to look a lot like the engine Pratt would offer for the next generation of Airbus and Boeing singe-aisle airliner. As Pratt continues to say its preferred route to market for the next-gen narrowbody is via International Aero Engines, a GTF launch on the CSeries is going to make for some interesting conversations with its V2500 partners.

And while we have all given Bombardier a hard time for its indecision on the CSeries, the market might just be on its side this time. With more airlines calling for a new generation of narrowbody, an environment-friendly 110- to 130-seater available in 2013 might just attract some buyers unwilling to wait another five years for a new Airbus or Boeing.

To celebrate 100 years of Flight International, we want to discover the "100 Greatest" in aviation; by determining the top twenty civil aircraft, military aircraft, engine, people & moments. Here the best engine is put forward!

To kick things off on the "100 greatest engines", I have nominated the Rolls-Royce Nene. Something of an off-the-wall choice, I know. But as well as powering some key British and American aircraft, the Nene gave several countries their first taste of jet power - notoriously the Soviet Union, where Nenes provided by the British government were reverse-engineered to produce the MiG-15's engine.

R-R's Nene - worthy of the '100 greatest'? (from Flight's cutaway archive)

So far, the popular vote is going to General Electric's J79, the afterburning turbojet produced by the thousands to power Lockheed F-104s and McDonnell Douglas F-4s, as well as the B-58 Hustler and A-5 Vigilante. Other countries have had equally pivotal fighter powerplants, notably Britain's Rolls-Royce Avon, France's Snecma Atar and Russia's Tumanksy R-11. Think they belong on the "100 greatest"? Then get busy nominating!

Do you agree with this choice?Why not nominate your own favourite of the following categores in our "100 Greatest" area: Greatest Civil Aircraft? Greatest Military Aircraft? The Greatest Engine? The Greatest Person? The Greatest Moment?

How's this for timing? The day after the Pentagon says it will try again to cancel the General Electric/Rolls-Royce F136 alternative engine for the F-35, the Lexington Institute defence think-tank publishes a study arguing "one engine is enough" to power the Joint Strike Fighter. Either the Institute's Loren Thompson worked through the night or he knew what was coming. Knowing Loren, it was the latter.

The Pentagon tried to cancel the F136 last year, but Congress put the money back into the budget. So eliminating funding for the GE/R-R engine (right, in afterburner) from the defence department's FY2009 budget request sets the stage for another showdown with the lawmakers. It will be interesting to see what influence the Lexington Institute's report has on this year's debate.

No one argues that annual competitions to power the JSF would bring down the purchase price of the engines, or that having two powerplants would avoid the potential for an engine-related grounding of the entire F-35 fleet. But do the advantages outweigh the extra cost of developing, producing and supporting two engines over their service lives? Most DoD studies - and the Lexington report - say no.

So it comes down to the emotive issue of industrial base and keeping GE in the fighter engine business. Citing GE's domination of the commercial engine market, Loren Thompson argues that keeping the alternative JSF engine would probably cement the company's dominance over domestic rival Pratt & Whitney "until mid-century, of not forever".

F-35 keeps flying on Pratt power as argument continues

Timing is everything. Bad timing is breaking your engine on the very day your customer tries for a third time to cancel your rival and a new report rubbishes the benefits of competing engines. But that's what happened to Pratt & Whitney on February 4, when a turbine blade on an F135 Joint Strike Fighter engine broke during "proof testing". That was the day the Pentagon told Congress it still intends to cancel the General Electric/Rolls-Royce F136 alternative engine.

The blade problem is not news. That's why the engine was being proof tested, to see if it was susceptible to a unique blade vibration that causes high-cycle fatigue. This follows a blade failure on a test stand in August last year. What is news is that this latest incident will delay the first flight of Lockheed Martin's F-35 STOVL JSF, which was scheduled for mid-year.

The two blades that have failed so far were in STOVL F135s - Pratt thinks the problem is tied to the heavier load on the third turbine stage in the STOVL engine, because the low-pressure turbine has to work harder to power the shaft-driven lift fan. The latest engine to fail was to power the first F-35B - aircraft BF-1 - now Pratt will have to proof-test a replacement engine, which will delay the start of STOVL ground testing by a month - maybe more, they don't know yet.

It's not every day a new form of propulsion makes its first flight: the turbojet in August 1939 (Heinkel He178), the ramjet in April 1949 (Leduc 010), the scramjet in July 2002 (University of Queensland HyShot). Now it's the turn of the pulsed detonation engine (PDE) - a simple, lightweight powerplant that promises efficient operation over a wide range of speeds from 0 to Mach 4.

In a PDE, combustion is supersonic (detonation) rather than subsonic (deflagration), resulting in the more efficient conversion of fuel into thrust. PDEs have few moving parts. A fuel/air mixture is injected into a tube and ignited, creating a supersonic detonation wave that travels down the tube and is expelled, producing a pulse of thrust. Grouping several tubes together and firing each many times a second produces constant thrust.

It's taken a few years longer than planned, but the US Air Force Research Laboratory and partners ISSI and Scaled Composites finally accomplished the first PDE-powered flight in late January. The modified Long-EZ was powered by a four-tube PDE, each tube firing 20 times a second, producing 200lb peak thrust. The flight was short, just a few tens of seconds, taking place within the length of the Mojave runway, but it was a first.

(Pictures by Alan Radecki)

On the subject of PDEs - pulse detonation engines - General Electric has just been assigned a US patent for a turbofan that uses a rotating pulse-detonation system, rather than low- and high-pressure turbines, to drive the fan and compressor. Basically, each turbine stage is replaced by a radial array of raked and angled pulse-detonation tubes.

According to the patent, the first ring of PDE tubes (58) is fired first to start the booster compressor (28) turning. Once the booster is up to speed it provides compressed air to the remaining PDE rings (59), which then fire up to drive the fan (20).

As a PDE ring turns, each tube sequentially passes first a port (50) allowing air into the tube, then a fuel injector (52), then the igniter (54), which initiates a supersonic detonation wave (66) to power the ring's rotation.

Having just spent two days at Pratt & Whitney's Connecticut facilities, some of the company's fervour for the Geared Turbofan appears to have rubbed off on me. With oil at $110 at barrel and jet fuel at $2.70 a gallon, an engine that promises to reduce aircraft fuel burn by 12%, and noise and emissions by 50%, seems to make a lot of sense.

And those figures are for the GTF at entry into service in 2013. Pratt is promising to continue reducing fuel burn by 1% a year, for a 20% reduction by 2020 - with consequent reductions in carbon dioxide emissions. Conventional turbofans will keep getting better too, but Pratt believes the physics of its geared-fan architecture give the GTF a 6% advantage the others can't catch up with.

Where's the catch? Well there is the gearbox. But, having seen it, I'm inclined to believe Pratt's claims that it has mastered and matured the technology. For a 30,000shp gearbox, it's small - about 18in diameter and weighing around 200lb - and simple. Then there's the torture chamber Pratt has constructed to test the gearbox to its limits, and beyond - like 2.5 times the maximum misalignment caused by gyroscopic forces on the fan during take-off rotation.

Airbus and Boeing are interested, Pratt says, but they are waiting until the company flight tests its GTF demonstrator. Flights on the company's Boeing 747SP engine testbed are planned to begin in late June/early July. By then the GTF could have been formally launched on the Mitsubishi Regional Jet, and perhaps Bombardier's CSeries.

Up till now, airline interest in new engines has been about dramatically reducing their carbon emissions. That has encouraged Airbus and Boeing to consider holding off and waiting for new technology to mature, like open rotors. But with oil prices soaring, fuel burn may take the lead, and airlines may start demanding action sooner rather than later. That would appear to put Pratt and its GTF in pole position.