Battle of the Giants — The Big Turbofans

Airliner history has been evolving towards these engines for decades. Twin engine aircraft have lower maintenance costs but need disproportionately large engines (because putting it bluntly, if one fails, they have only one left). This has traditionally made the twin configuration unsuitable for very long range aircraft, which cannot afford to carry all-around avoidable powerplant weight, and unsuitable for very large aircraft, which would demand development of a new, very big engine.

Despite these obstacles, twins have been getting bigger and flying further. The A330 was the first to break through the barrier of the 747’s thrust requirements and demand engines of a power that it alone could use. The 777, incredibly, not only demands more thrust again but in some versions combines it with the extreme range once thought the domain of three or four engined aircraft. The longer range not only comes with punitive powerplant weight but demands even greater takeoff weights and therefore yet more thrust — an engine size now far beyond what any other aircraft can use.

Now think about the economic problems involved here: more thrust means more development cost, but selling the engine on only one or two aircraft models means fewer sales to spread the cost around. Worse, the aircraft are twins, so each airframe sale means just two engine sales instead of three or four. And then all three manufacturers insist on competing in the market, dividing the revenue by three.

But it’s too late to go back now. Before they all took the plunge, Pratt & Whitney and Rolls-Royce might have merged to take on industrial giant General Electric, especially since Rolls is by far the smaller company and GE the largest. But all three have now already spent the development money on their new engines, so little can now be saved. Further, Rolls has begun to outsell GE, and must now feel little need to surrender its independence.

The new engines divide into three thrust ranges, with a fourth in prospect:

70,000lb or so (roughly 300kN) from the PW4168 and Trent 700 for the A330.

Around 84,000lb (875kN) from the engines for the lighter 777 A version, the PW4084, Trent 885 and GE90-85B. The P&W and RR engines in this category are quite different to their cousins on the A330. All three engines in this category can be derated to under 78,000lb (850kN) if the customer prefers.

Upwards of 90,000lb (400kN) is generated for the 777’s heavyweight B version by the already certified Trent 890 and from the forthcoming Trent 895, PW4090 and GE90-92B. These engines are physically little different to the previous category.

The 777’s amazingly rapid weight growth is raising the prospect of a C version, which would require a fourth thrust range from the engines, probably over 100,000lb (450kN).

To understand these engines one must almost stop thinking of them as jets at all. Early turbofans were turbojets that bled off part of their compressor air to create a bypass flow, because it is more efficient to create thrust by accelerating a larger mass of air more slowly. But in these latest engines the front compressor fan has become the whole point of the exercise. The turbine engine core is now little more than a colossal turboshaft; its main output is not its own thrust but shaft power to drive the big fan at the front.

So to develop these big engines the makers had to do two things: first, create a more powerful core by enlarging and refining their old ones or, in the case of GE, by building an entirely new one; second, they had to fit a bigger fan on the front of that core, both to handle the extra power and to increase the bypass ratio for better fuel economy. GE and Pratt’s also had to develop the more efficient wide chord fan blades that Rolls had been using since the early 1980s. The resulting engines compete against each other in four main technical areas: thrust, fuel consumption, weight and reliability.

Thrust

As this is written, the most powerful jet turbine ever built is the Rolls-Royce Trent 890, the first to be certified at 90,000lb (400kN). All three engine makers first targeted a Boeing-specified 84,000lb (875kN) for the 777. But Rolls decided during testing that its engine was somewhat understressed at that level, so it turned up the juice and lifted the baseline thrust to 90,000lb (400kN), giving itself a significant sales advantage.

Available thrust is one of the few technical factors that can alone make or break a sale. An airline might live with other shortcomings but there are times when it simply must have a certain amount of thrust. So everyone is working on larger engines.

The planned super Jumbos — the A3XX and 747-500X and 747-600X — would have provided an important new market for these engines except for GE — the lowest rated GE90 is likely to be too powerful. However, GE and Pratt’s — erstwhile cutthroat competitors — have teamed for a brand new 70,000lb (300kN) and above class fan for the big Boeings (and perhaps later the A3XX) leaving Rolls on its own to offer a Trent 800 development. (Boeing has ruled out using existing A330 engines or uprated 747-400 powerplants.)

Fuel Economy

The GE and P&W engines use less fuel than the Trent, because they have bigger, more efficient fans. GE, bent on pushing technology to the practical limit, has used the largest fan, a vast assembly whose 3.12m diameter rivals that of the 737 fuselage. This is a heavy design choice, but is justified if it can achieve its theoretical efficiency. Indeed, the GE90 was supposed to consume 5% less fuel than its rivals for a given thrust. Unfortunately, this technical tour de force is not yet producing the goods, and the GE90’s fuel consumption is not significantly different to the PW4084’s. Rolls concedes that the Trent is behind both.

Weight

Fan size drives engine weight, particularly because a bigger fan forces up the weight of the casing, which must absorb the enormous energy of a broken blade flying off the hub. With the smallest fan (2.79m) the Trent is the lightest engine, reversing the tradition of RB211s being heavy. But the PW4084 (fan diameter 2.85m) is not much heavier and therefore has the magic combination of moderate weight and good fuel consumption, which greatly benefits payload and range.

The GE90, even with its fan built of carbon fibre, cannot help being the heaviest. But note that the GE90’s big fan must be able to handle bigger thrusts more comfortably — GE must be hoping the 777’s weight quickly reaches levels that force its opponents to fit larger fans. It might be disappointed, though: Rolls says it can get more than 95,000lb (425kN) out of the Trent without replacing its fan while Pratt & Whitney is planning a 98,000lb (436kN) version with the same fan.

Reliability

This includes failure rate, maintenance and holding good fuel consumption between overhauls. A low failure rate is particularly important to attain the crucial approval to fly over water without quick access to a runway — Extended Twin OPerationS (ETOPS).

In theory, the Trent has a reliability advantage in its rather complicated three shaft layout, courtesy of its RB211 origins. The PW4000, GE90 and most other engines have two shafts (one runs inside the other) which create two different parts of the compressor, each working at a more optimal speed than could be achieved with just one shaft. A three shaft layout takes this a step further and separates the fan from the first section of the compressor proper. This gives more compression from each stage of compressor blades, which allows fewer stages and therefore less length. Shortness in turn makes the engine stiffer and therefore more reliable. But even if the Trent has better reliability, as claimed, a lower failure rate would be a fairly marginal advantage: engines fail very rarely, whether they have two shafts or three.

The GE90 has had initial service problems, but this is no surprise: it is a completely new engine. Pratt’s and Rolls are taking no comfort in GE’s difficulties, because everyone knows that bugs get ironed out.

But ETOPS affects airlines crucially, and General Electric must have expected more trouble getting approval for a new engine. The GE90 did not get immediate ETOPS approval when it entered service last year and is now scheduled to gain it around now, a little behind the Trent, which entered service only in April. The PW4084 has had ETOPS since it began service with United in mid 1995. Combine that with its lightness and fuel efficiency, and you can see how Pratt & Whitney moves them out the door.

The Market

All of these technical factors have had to jostle with price and politics as the engine makers battle for orders. Three big deals, two of them supposedly marked by massive discounting, have shaped the market.

United launched the 777 and chose the PW4000, giving Pratt & Whitney a significant lead over the other two. All subsequent buyers could then choose to play safe and stick with the engine that was either the only one actually ordered or, later, the one that had racked up the most orders, which it remains, with more than 40% of the 777 market (and most A330 sales, too).

The PW4000 did not remain the only engine on the 777 for long, because in 1991 GE achieved one of the most amazing feats in recent engine marketing history: it kicked Rolls off British Airways. The deal meant the Trent was now fighting an uphill battle to become a third engine to get into this very limited market while BA, a committed RB211 user, had endorsed the dangerous competition of GE’s all-new engine. But there have been accusations that GE made the deal so attractive that it lost money, effectively buying Rolls’s customer. BA had made the purchase of an engine repair facility part of the deal and GE reportedly paid far more than anyone else thought it was worth.

For all this, GE has only about 25% of the big fan market, behind Rolls-Royce, which has 30% to 35%. Rolls was also accused of buying business when it clinched its breathtaking order last year from Singapore Airlines, a Pratt & Whitney user. Including an important order from SIA’s leasing subsidiary — which helps second hand values — Rolls got its engines onto 34 firm 777s and 33 options. It was a watershed deal, cementing the Trent’s future. It also meant that the Trent had the entire 777 market in Southeast Asia.

Now one of the most interesting prospective customers will be Qantas, which may buy a handful of big twins (five or so?). The Australian airline is 25% owned by BA, so commonality would suggest a purchase of 777s with GE90s. But like BA, Qantas is a traditional RB211 user. And it is surrounded by airlines that have all chosen Rolls. Maybe an even more important issue is whether the same engine is suitable for a later super jumbo purchase — an issue that must weigh heavily on BA’s mind, too. This will indeed be an engine order to watch.