IN PURSUIT OF PERFECT POWER
SABRE is a hybrid rocket and jet, billed as ‘the biggest breakthrough in aerospace propulsion technology since the invention of the jet engine’
MODERN JET ENGINES ARE arguably among the most complex machines ever made. Years of toil and billions of dollars go into designing and developing these devices that are worth their weight in silver. And since commercial engines must keep expensive airliners with hundreds of passengers flawlessly flying, they have no room for error. Indeed, jets are built not just at Six Sigma quality standards (3.4 defective features per million instances), but when possible, even better. Engine technologists strive to machine thousands of components to precise specifications using ever lighter and tougher materials, and packing them into the smallest possible space. Their aim is to deliver ultimate levels of power and efficiency.
However, power comes at a price. Aviation fuel is now much cheaper than in the mid-2014, when it constituted almost half an airline’s operating cost. But the airlines know that another sharp rise is certain, sooner or later. That is why there is no slackening of demand for new and even more fuel-efficient engines like Pratt & Whitney’s PurePower PW1000G, a geared turbofan engine that entered commercial service in January this year and CFM International’s Leading Edge Aviation Propulsion (LEAP) turbofan that should follow in July.
Power also needs to be ‘purified’ because of concerns over climate change. According to the influential Air Transport Action Group (ATAG), aviation produced 770 million tonnes of CO in 2 2015 out of over 36 billion tonnes generated by human activity. Although commercial aviation is responsible for just two per cent of worldwide CO emissions, these may have a substantial impact 2 on the atmosphere because they are released at high altitude. Therefore, pressure is mounting on the industry to achieve perceptible progress in improving fuel efficiency and reducing emissions and noise. And it has set an ambitious target: to slash net carbon emissions to half of their 2005 level by 2050. This requires huge investments and a massive leap in technology.
FANNING FRENZY. For the first few decades of aviation, it was the piston engine coupled with the whirring propeller that provided power for flight. The Jet Age dawned in the early 1950s with fuel-guzzling turbojets. Then turbojets morphed into turbofans in the 1960s and it took only a short period for these fuelefficient engines to power practically every commercial jet.
In a turbofan, most of the cold air is sent around the engine while a small proportion flows through the core. This is because it is more efficient to accelerate a large amount of air a little than a small amount a lot. The proportion of air flowing around the core against that passing through it is called the ‘ bypass ratio’. Over the years, air intakes have become ever larger to accommodate larger and better fans. And modern turbofans have bypass ratios of up to 9:1 — one measure of their ultra-efficiency. But the longer fan blades mean their tips are now moving at close to the speed of sound. And that is not heartening because the resulting shock waves could trigger potentially dangerous vibrations.
Pratt & Whitney believes that conventional engines have practically reached their limit of improvement and new thinking is necessary. It spent about 30 years and over $10 billion to develop its PurePower brand. PurePower engines use large fans (81 inches in diameter on the Airbus A320neo), but a special gearbox makes the fan turn slower than the turbine. This is possibly the biggest advance in propulsion philosophy since the arrival of the turbofan. The gearbox keeps the efficiency of each component at its peak and increases the bypass ratio to 12:1. Overall, the A320neo’s PurePower engine promises 15 per cent lower fuel consumption than the standard A320 as well as a dramatic reduction in engine noise — up to 75 per cent lower on the ground. Naturally there is a price to pay — increased weight and aerodynamic drag. And the gearbox adds complexity to the engine, heightening the risk of something going wrong some day.
Apart from being offered as one of two options on the A320neo, variants of Pratt’s PurePower engine will also be installed in Bom- bardier’s CSeries regional jets, the Mitsubishi Regional Jet (MRJ) and Embraer’s E-Jets E2 regional jets. It will also be offered as an option on Russia’s Irkut MC-21, a 150- to 212-seat narrow-body airliner.
CFM International, owned by Safran and General Electric, prefers the tried and tested route. It feels it can make a better engine than PurePower using conventional turbofan architecture, without the added weight and drag of a gearbox, not to mention its complexity. The CFM LEAP is a high-bypass turbofan, offered as an option for the A320neo and as the exclusive engine for the Boeing B737 MAX as well as the Commercial Aircraft Corporation of China’s new COMAC C-919 airliner. LEAP has more than 4,000 different parts and some are subjected to temperatures of approximately 2,700 degrees Celsius while turning at 2,400 RPM. CFM copes with these harsh conditions by using advanced lightweight composite materials such as carbon fibre fan blades, achieving what it calls ‘the ultimate refinement of the traditional turbofan engine.’ The turbine shroud, another highly stressed portion, is made from ceramic matrix composites (CMC). CMCs have been around for decades but only recently has the technology matured enough to make it practical to use in jet engines.
All in all, the PurePower and LEAP are technological marvels that seem set to power the bulk of the global commercial fleet for a decade or more. As to which will fare better in the market, airline head honchos are notoriously conservative and usually prefer icecream in “any variant of vanilla”. That is perhaps why the LEAP has more than 10,500 firm orders and commitments against over 7,000 orders and options for PurePower. However, these are early days and the rivals are yet to prove themselves in commercial conditions. GE9X: BIGGEST AND BEST? Since narrow-body planes dominate aviation, technologists mostly concentrate on developing new and better engines to power these. For the big jets, the underdevelopment Rolls-Royce Trent 7000, designated the exclusive
engine for the Airbus A330neo, has a 112-inch fan diametre and 10:1 bypass ratio. General Electric is also going all out to incorporate advanced technologies in the underdevelopment GE9X power plant. It claims the GE9X will be its most fuel-efficient engine ever, with a bypass ratio of 10:1. It will have a significant fuel burn savings: 10 per cent better specific fuel consumption (SFC) than the GE90-115B engine that powers the Boeing B777-300ER and five per cent better than any comparable engine in service in 2020. It generates thrust of 1,05,000 pounds (467 kN). Its fan is 133.5 inches in diametre — a world record. This helps draw in more air using less energy and operating quieter. The engine uses CMC materials in its core for the fan case and fan blades. CMCs have twice the strength and better thermal capabilities than their metal counterparts yet weigh just a third. The GE9X, that will power the Boeing B777-8 and B777-9 airliners, has already attracted over 700 orders and is scheduled to enter service around 2020. DOUBLE BUBBLE. Jet engine technologists like to do their thing — making complex machines deliver the most power with the greatest efficiency — working independently. At the same time, airframe designers separately strive to build aerodynamically efficient structures and worry about the power plant only at the appropriate time. In fact, they usually offer two or three different engines as options to customers. For instance, the standard A320 may be powered either by CFM International’s CFM56 or by the V2500 from International Aero Engines. Inevitably, when airframe and engine are married up, there is a drop in predicted performance which is inconsequential. However, now that fuelefficiency is a burning issue, aviation technologists have realised that today’s tube-and-wing design is nearing its limit of exploitation and integrated engine-airframe design is indispensable.
Integration is what the Massachusetts Institute of Technology’s D8 future aircraft design concept is all about. It is similar in size to the Airbus A320/Boeing B-737 and claims to offer potentially huge benefits like 71 per cent reduction in fuel burn, 60 effective perceived noise level in decibels (EPNdB) less noise and 87 per cent reduction in landing and take-off (LTO) cycle emissions of nitrogen oxides (NOx) compared with a B-737-800 aircraft. Every part of the airframe and engine will be meticulously reconfigured to maximise efficiency and minimise operating costs. The fan will be large and feature an impressive bypass ratio of 20:1. The wide ‘ double-bubble’ fuselage will generate increased lift, so smaller wings can support the aircraft’s weight and a lighter landing gear and smaller tail will suffice. In turn, smaller engines and less fuel capacity will be needed. The twin engines will be integrated with the fuselage for a clean high-aspect-ratio wing with low drag. Mounted at the rear end, they will re-energise the slow-moving boundary layer flow over the fuselage and promote boundary layer ingestion (BLI) which increases efficiency. This too means smaller engines, which reduces weight and fuel carriage even more. Indeed, it is a classic virtuous circle and may endow the D8 with hard-to-beat efficiency. However, the D8 will not happen in a hurry because numerous technological challenges remain to be overcome. It is expected only around 2035, a timeframe that NASA calls ‘ N+3 generation’. Beyond that, what? SABRE RATTLING. Modern jets satisfy the human need for speed over short distances, but for longer journeys, time in the air can be rather irksome. For instance, despite Emirates’ efforts to make its passengers comfortable, how many would fancy spending 17 hours 15 minutes to cover 14,203 km on its Auckland to Dubai Boeing B-777-200LR flight? In a couple of decades, the same trip may be accomplished in a fraction of the time, thanks to SABRE.
UK’s Reaction Engines is developing a reusable space plane, Skylon, powered by a Synergetic Air-Breathing Rocket Engine (SABRE). SABRE is a hybrid rocket and jet, billed as “the biggest breakthrough in aerospace propulsion technology since the invention of the jet engine.” Although SABRE will provide an economical way to place satellites in orbit, it can also function as a commercial airliner. The first full ground-based engine test is scheduled for 2020 and the first unmanned test flight should happen about ten years from now. The UK Government has invested £60 million towards this next-generation engine that is likely to make low-cost high-speed flight feasible.
Unlike rocket engines that must carry their own oxygen thus significantly increasing weight and drag, SABRE will ‘ breathe’ air from the atmosphere. It will feature two operating modes: initially the air-breathing mode cruising at Mach 5 through the atmosphere; then if required, using stored oxygen and hitting Mach 25 as a conventional rocket in space. A SABRE-powered airliner will take off from a runway, reach any point on the earth in less than four hours and land on a runway. Now that is near-perfect power!
THE PUREPOWER AND LEAP ARE TECHNOLOGICAL MARVELS THAT SEEM SET TO POWER THE BULK OF THE GLOBAL COMMERCIAL FLEET FOR A DECADE OR MORE
CFM LEAP uses advanced lightweight composite materials such as carbon fibre fan blades, achieving what it calls ‘the ultimate refinement of the traditional turbofan engine’
A cutaway view of PurePower PW1000G engine
Cutaway of the SABRE engine and (inset) Skylon in flight showing the SABRE engine