| BlueSky Business Aviation News | |||||||||||||||||
| . | |||||||||||||||||
Some say that the Comet was killed by the fatigue issue, which may have some superficial validity to it, but the real reason the Comet was eclipsed by the 707 and the slightly later DC-8 wasn't related to fatigue. It was economic productivity that truly killed the program. Both the 707 and the DC-8 could carry twice as many passengers twice as far as the Comet for comparable costs. The profit margins were simply too high to ignore. (It has ever been thus).
Ticket prices dropped to something middle-class people could afford. Engine development Those first couple of decades of jet flight were all about learning how to design and build an efficient, safe, and reliable airplane, both structurally and from a systems perspective as well. The Comet, for example, was a much smaller airplane than the 707 with shorter legs because the engines available in 1951 were less powerful and less fuel efficient than those that would appear before the end of the decade, and even less fuel efficient than the brand new turbofan engines that would be introduced in the early 1960s. Fuel efficiency is crucial for engine designs; this is the source of aircraft range, payload, too, but mostly range. The turbofan The Rolls Royce Conway was the first turbofan engine to enter service, and the first to sport a two-spool arrangement. As is often the case, the first application for the Conway was a military bomber, the Handley Page Victor. Next, it was successfully installed on both the 707 and the DC-8, and then the Vickers VC10. (Ever seen a VC10? It's a rear-engine configuration with two engines per side in a dual installation that only the IL-62 shares. The RAF still has a few modified as tankers. They are rare.) But, the Conway was rapidly overshadowed by the Pratt & Whitney JT3D turbofan, a derivative of the original JT3C straight-pipe turbojet used on the 707. The JT3C itself a derivative of the J57 which was started off in life as a turboprop design for the then brand spanking new XB-52 strategic bomber program. That's right. Turboprop. I kid you not. However, the chief engineer for the B-52 made the comment that "Life is too short for propellers" and the commitment was made to go all jet, a bold move at the time given the B-52's ambitious range requirements and the performance of the engines then available. But, I digress. One of the reasons the JT3D eventually outsold the Conway was because the Conway was designed to be installed inside the strucural envelope of the wing, and so was limited in bypass (and probably growth potential as well). The JT3C and D engines were designed to be installed in a pod below and forward of the wing and so did not face the same structural constraints as the Conway. Burying the engine Installing engines inside flight structure is, on balance, a terribly limiting design solution. The Comet used it because the design couldn't meet the program requirements if it didn't. Military programs, of course, have their own logic, but both the Victor and the Vulcan bombers used "buried" engines. Notice that the Comet's military derivative, the Nimrod, is a very difficult design for a turbofan, making it all but impossible to install the very fuel efficient high-bypass ratio turbofans currently available. In contrast, when the 707's military version, the KC-135, was re-engined with high-bypass engines, the strucural redesign was very limited. Other aircraft with buried engines, like the #2 engines of the 727, the DC-10, and the Lockheed L1011 are all very expensive and difficult to up-engine beyond a certain point (the problem is the structurally limited air mass flow rate, if you are curious). Podded engines Early jet engines were somewhat less reliable than those we have available today. There was also a certain amount of understandable caution from military folks about burying the engine. The previous generation of piston engines contained a fair amount of rotational energy, but the turbojet took that to a whole new level. A disintegrating jet engine could and would destroy whatever structure was around it, and there really is no amount of armor that can be economically wrapped around it, though we do still make an effort. For a bomber that is expecting to be shot at, a buried engine might give one pause. Even in commercial service, an exploding engine, while incredibly rare, is a very serious event with catestrophic potential. You may recall the A380 which lost an engine in climb, causing extensive damage to the aircraft. Boeing did quite a lot of research on the question of engine location during the B-47 project, and out of that research came the by now very familiar location of the engines below and forward of the wings. This was the location with the lowest drag penalty. The first generation of engines, of course, didn't have quite enough power to overcome the additional drag of an external mount and still meet the other program requirements. But those early aircraft with buried engines have an elegance and cleanness of line that no aircraft with externally mounted engines can match. The high-bypass engines The next big stop, of course, were the engines for the wide-bodies. General Electric won the competition to power the astonishing Lockheed designed C-5 Galaxy transport with the TF-39, the very first high bypass ratio turbofan with a bypass ratio of 8:1. The Conway, the first turbofan, in contrast, had a bypass ratio of 0.3:1, about the same as one of today's advanced fighter jet engines. GE went on to develop the CF6 family of engines from the TF-39, and the CF6 was the original engine for the DC-10. Interestingly, Pratt & Whitney's JT9D was the loser in the C-5 competition. The JT9D for the 747 was second of the high-bypass ratio engines with a bypass ratio of 5 to 1. Bypass is just the amount of air that enters the engine inlet but doesn't go through the core where the air is highly compressed and then burned to generate power; this air "bypasses" the engine core--it adds thrust, reduces noise, and improves the propulsive efficiency of the engine. The "bypass ratio" tells us the proportion of the air that goes through the bypass ducts instead of the core. An engine with a bypass ratio of 5 has five pounds of air pushed through the bypass ducts for every pound of air that is run through the core. It's hard to see the ducts from the front, since they are covered by the fans, or the first stage of the engine. That fan and the implied bypass duct is the reason we call them "turbofans." High-bypass history At the same time as the C-5 competition, Rolls Royce had bet the company (a disturbingly common occurrence in aviation) on their new engine, the RB211. The RB211 was the engine for the Lockheed L1011 TriStar, the third entrant in the civilian wide-body airliner market. As you might expect, Rolls had done an enormous amount of work in the '60s on turbofans. Once again, they set a new technical standard with the three-spool engine; the JT9D and CF6 engines are two-spool designs. The RB211 was designed to be an enormous advance in jet engine technology, complete with a composite fan blade, something which had not been attempted previously. In order to sell the engines, Rolls had promised a miracle engine on a very tight schedule for a ridiculously low price. Things didn't quite work out as planned, and Rolls went bankrupt. It was VERY exciting there for a bit since Lockheed couldn't deliver any airplanes without the engines (redesign to accept a competitor's engine was too expensive to contemplate as the program was already over cost and late, a rather traditional state of affairs). The US government under Nixon rescued Lockheed, and the UK government under Heath rescued Rolls, salvaging both programs and both companies. However, in spite of the technical, financial, and political heroics, the L1011 was not successful and was cancelled after 250 aircraft were built. Industrial change Along in here somewhere, industry practice changed. Previously, an airframe was designed with only one engine in mind. Engine development is hideously expensive, and engine manufacturers needed exclusivity to (hopefully) recoup their cost of development, fund new research and still leave a profit for the shareholders. Very ambitious, really. But, with the advent of three high-bypass engines and three wide-body aircraft, something happened. Airlines wanted to be able to choose their engine provider, and the smart airframer would design things so that could happen as painlessly as possible. The L1011 had only the one engine option and it faired poorly (though not because of the engine issue). The DC-10 was originally designed with the GE CF6 engine, but later could be had with a P&W JT9D. The 747 would eventually offer all three engines as options. Interestingly, a decade later, the Boeing 767 would be designed to use the same engines as the 747, potentially simplifying fleet logistics. Today, it is a rare design that doesn't allow an airline to choose an offering from any of the big three engine houses. The future Engine development, like airframe development, can't stand still. It's develop or die, I think. Often we focus on the product and lose sight of the process, but it's the process that truly counts, and the ongoing development process for aircraft engines is still going strong. The newest products, like P&W's PW1000G geared turbofan offer truly amazing performance with features never before seen in a civilian engine like variable geometry nozzles; GE's LEAP-X is moving into new territory with a 10:1 bypass ratio; and Rolls is again headed towards the technical high ground with something called an "open rotor" engine, which promises a gain of 10% (which is ENORMOUS) in fuel efficiency. One of the things I learned early on in my studies of aircraft design is that a great deal of airframe development depended directly on engine development. Airlines don't operate gliders, you know. For airplane designers like me, the current crop of existing engines and those that are near term are very exciting. I can't wait to see what the next twenty years brings. Terry Drinkard is currently consulting on an aviation start-up. His interests and desire are being involved in cool developments around airplanes and in the aviation industry. Usually working as a contract heavy structures engineer, he has held positions with Boeing and Gulfstream Aerospace and has years of experience in the MRO world. Terry’s areas of specialty are aircraft design, development, manufacturing, maintenance, and modification; lean manufacturing; Six-sigma; worker-directed teams; project management; organization development and start-ups. Terry welcomes your comments, questions or feedback. You may contact him via terry.drinkard@blueskynews.aero
|
|||||||||||||||||
