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The Boeing 747 That Never Was, the Fabled -500, was intended to be a stretch model version of the 747-400 by integrating new technologies from the 777. At the time, the ‘Triple Seven’ introduced a number of innovative building technologies to commercial aviation.
The breakthroughs would be leveraged to stretch the 747-500 18 feet longer than the -400 and accommodate a total of 462 passengers. The extra-long double-decker would have a 250-foot fuselage complemented by a 251-foot wingspan, also intended to be made using advances derived from the 777 program.
Unfortunately, Boeing’s Board of Directors ultimately declined to approve the project in 1997. Despite initial interest from carriers like Singapore Airlines and Qantas, overall demand was low. The success of the 747-400 meant that Boeing was unable to secure a minimum of 30 firm orders. While the 747-500X was cancelled, many of its concepts eventually influenced the development of the 747-8, the final production variant of the jumbo jet.
Going Back to Props: The Radical 747-500X
One of the most novel design features considered for the extra-long version of the 747 was to change out the power plants from turbofan engines to propfan engines. Boeing’s concept was to dramatically increase fuel efficiency by using the turboprops in a rear-facing pusher configuration. This was not intended to be the only option, as the -500 was also pitched with turbofan engines, but the complications of its noise and maintenance challenges made customers very skeptical.
The proposal to equip the 747-500 with propfan engines introduced severe engineering obstacles that threatened the aircraft’s structural integrity and passenger comfort. While propfans offered up to 30% better fuel efficiency, according to the Smithsonian, than contemporary turbofans, their design created issues that standard jet engines do not face.
Unlike a turbofan, which keeps its high-speed airflow contained within a nacelle, a propfan’s slipstream interacts directly with the wing. The powerful shockwaves and vortices created by the exposed, counter-rotating blades caused massive vibrations. The wing design alone was estimated to cost over $5 billion. Boeing found it difficult to justify such an expense for a derivative aircraft that required entirely new tooling and a full flight test program.
If a blade were to break off a propfan, there is no engine shroud to stop it. It essentially becomes a high-speed projectile that could pierce the wing’s fuel tanks or the passenger cabin. To make the -500 safe for commercial use, Boeing would have had to armor the wing and fuselage with heavy shielding, which would have cancelled out much of the fuel savings the propfans were meant to provide.
Simpler But Not Cheaper Than A Clean Sheet Jet
The premise of the 747-500 program was to make it a technological bridge between the classic 747 jumbo jet and the revolutionary Boeing 777. The legendary plane maker meant to reduce development costs while offering a jet in a whole new class by integrating cutting-edge components and manufacturing processes with the template of the 747.
The immediate and insurmountable obstacle was also the core design feature, the wing. The 747-500X would have moved away from the standard 747 wing in favor of a 251-foot (76.5 m) wingspan based on 777 aerodynamics. The new wing profile would significantly reduce drag while also improving lift to increase the range to 8,700 nautical miles. These wings were designed with advanced composite materials and structures that could carry significantly more fuel than the -400 model.
Aside from the fact the wing alone was deemed to be too expensive, there was also the issue of integrating a fully digital fly-by-wire, glass cockpit, as Key Aero touches on. And also re-engining the plane with the immensely powerful General Electric GE90 turbofans. These engines were the most powerful in the world at the time, after being custom-designed for the Triple Seven. In the end, the cost just wasn’t justifiable despite the fact that these improvements would have made the aircraft more efficient, safer, and superior performing across the board.
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Two major carriers stood out as the most vocal proponents of the new variants due to their specific route networks. Singapore Airlines was the most interested party when Boeing introduced the -500X. British Airways also had an interest in the -500X, as well as the -600X, and it was the world’s largest 747 operator at the time. United Airlines had options to upgrade their 747-400 orders to either the -500 or -600 but in the end the carrier opted for twin jets instead. Qantas showed interest in the -500X for ultra-long-haul missions, but few others came forward, according to Found And Explained.
Singapore Airlines was focused on ultra-long-haul flying between Singapore, New York and London, which made the range of the -500X appealing. This was the same reason that Qantas expressed interest. The ability to skip a stop for fuel and instead overflying Dubai or other midway points had a strong appeal to both of these airlines.
British Airways and United were more interested in the -500X because of its very large capacity for passengers. BA was struggling with severe congestion at the London Heathrow hub despite the fact that it already flew the world’s largest fleet of double-decker jetliners. United similarly wanted high-capacity and long-range planes for trans-Pacific routes.
Ultimately, none of these airlines placed firm orders. Many were simultaneously window shopping for the competing Airbus A3XX, which became the A380. The $5 billion development cost of the new 747 variants meant Boeing could not offer them at a price point that beat the upcoming twin-engine Boeing 777.
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How The -500X Lives On In The 747-8
Although a very different aircraft in many regards, the 747-8 is considered to be the spiritual successor to the -500X program. Paralleling the way that the 747-500 was planned to incorporate features from the latest and greatest twin jet in the Boeing line, the 747-8 incorporated a great deal of technology developed for the 787 Dreamliner. Much of that same technology first began development in the mid-1990s when the -500X and -600X concepts were originally presented.
Right off the bat, the 747-8 has a nearly identical fuselage length because the Boeing engineers aimed for essentially the same cabin stretch as the -500X was proposed with. The -8 was produced with an overall length of 250 feet and two inches. Another one of the very close similarities can be found in the power plants. And while the 747-8 did not go “full” fly-by-wire for all surfaces, it did adopt fly-by-wire spoilers and outboard ailerons. This mirrors the major goal of the -500X to modernize the heavy mechanical controls using the Boeing 777’s FBW system.
Just as the -500X was planned to be built with GE90 engines designed for the 777, the final iteration of the ‘Queen of the Skies,’ as the 747 is called, incorporated power plants also derived for use on the 787. The 747-8 ultimately used the General Electric GEnx-2B, which is the direct descendant of the GE90 architecture Boeing wanted for the -500X. These engines include the chevron-edged nacelles and composite fan blades first envisioned for the next-gen 747 series.
The 747-500X program was the first time Boeing seriously proposed abandoning the original 1960s wing for an entirely new design. The 747-8 features a completely redesigned wing that is thicker and deeper, similar to the concepts studied for the -500X. The 747-400 was famous for its vertical winglets, while the -500X proposed replacing these with more efficient, tapered extensions. Launching off of that legacy, the 747-8 was made with raked wingtips, a feature that allows for better lift-to-drag ratios than traditional winglets.
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Remixing The 747: The -500 Meets The 787
The 747-500X was proposed at a time when aluminum was still the optimal material for constructing a commercial aircraft. That changed dramatically when the 747-8 was developed, and a new carbon fiber composite material developed under the 787 Dreamliner program helped realize much of the modernization that had been too costly or unrealistic before. Both programs were attempts to “lighten” the massive 747 airframe using the materials breakthrough of the decade, but the 747-8 was able to produce an aircraft that both fit the needs and the budget of operators around the world.
The 747-500X failed because the 777 technology of the time was too expensive to retrofit into the 747’s old architecture, as it required a $5 billion total wing redesign. By the time the 747-8 arrived, computing power and manufacturing had advanced so much that Boeing could integrate 787 technology more cheaply and efficiently than they ever could have with the 777.
Also, because the 747-8 was able to move off the drawing board and onto the production line, the Dreamliner DNA worked its way into the cabin of the iconic double-decker. This mashup of the venerable Queen of the Skies with ultra-modern interior design created the most luxurious, comfortable, and modern cabin ever found inside a 747.
The most immediate change was the move from flat, low ceilings to cove lighting and arched panels. On the Dreamliner, Boeing used recessed LED mood lighting to simulate different times of day. Boeing brought this exact lighting system to the 747-8. Boeing also wanted the “moment of arrival” on the 747-8 to feel like stepping into a luxury hotel rather than a metal tube, which inspired the creation of an iconic entryway with the atmosphere of a grand gateway instead of a simple hatch.
