Airliner technology continues to evolve year-after-year, in the pursuit of better fuel economy which leads to better financial yields for operators. There’s been a number of initiatives over the years from all aircraft manufacturers, but in the 1980s unducted fan (UDF) technology was showing promise for reducing fuel consumption substantially on new model aircraft for the 1990s. McDonnell Douglas was one of the manufacturers that pursued this new technology for nearly a decade before abandoning it as a future airline powerplant.
An Attempt To Push Beyond The Turbofan
Modern jetliners operate on turbofan engines, which quite simply have a ducted fan attached in front of a turbojet engine. Going into detail on turbofan construction and operation would require a separate article; but the turbine creates the bulk of the thrust, the ducted fan creates additional thrust and lowers the overall noise footprint of the engine, and this is what modern jetliners use to operate their aircraft.
In 1973 when OPEC engaged in an oil embargo with nations that were perceived to support Israel during the Yom Kippur War, oil prices shot up overnight resulting in an increase from .36 cents a gallon to .53 cents a gallon. By 1974, a barrel of oil had quadrupled versus the previous years. The embargo ended six months later, but the price of oil and subsequently gas prices continued to climb. A gallon of gas reached an average of $1.19 a gallon just six years later, a substantial climb in price, nearly four times higher than 1973. But it didn’t stop in 1980, gas just continued to climb in price leading to a serious effort to find and update powerplants to be more fuel efficient.
NASA actually led the way to look for new technology and concepts to reduce fuel burn, by working with manufacturers and providing NASA funded research grants. Manufacturers looked at ways to make their aircraft more fuel efficient in light of the overnight fuel crisis which led to increased fuel prices. Suddenly aircraft that burned massive amounts of fuel were a pain point for airlines. One of the key powerplants that showed promise was Unducted Fan (UDF) technology. Two key powerplant producers, Pratt & Whitney and General Electric invested in this technology with prototype engines, and Boeing and McDonnell Douglas provided aircraft test beds and perceived new aircraft models to present to airline customers. Additionally, other manufacturers such as Fokker, British Aerospace, and Tupolev all invested in testing out alternative prop-based powerplants to varying degrees over the years.
UDF: The Unducted Fan Emerges As An Option
While little information is available, McDonnell Douglas was looking at UDF technology for the newly launched MD-80 series as an option. Interestingly enough, the early rendition in the form of a 1/100 scale model differs considerably from the powerplants that did eventually take flight, featuring forward facing prop fans. It was a close rendition of the eight bladed Allison 501-M78 and a Hamilton Standard propeller, used in a NASA propfan test. These early units were modified turbofan engines with the fan placed outside the engine nacelle on the same axis as the compressor blades, and assumed this was an early design study from 1981.
NASA had released their internal studies on propfan technology to engine makers who were interested in pursuing the technology. By 1984 NASA had awarded a contract for $20m to further study and pursue the concept of an open rotor powerplant to General Electric (GE). GE was working closely with Boeing on the UDF as a possible new short haul aircraft powerplant, dubbed the 7J7 for a market introduction in 1991. The 7J7 was a widebody short haul aircraft with a range of up to 2700 nautical miles initially. But Boeing ended up pitching a hybrid-narrowbody concept at 155 inches to airlines as the final configuration, with a 2-2-2 seating configuration ensuring there was no middle seat. That was depending on the airline customer you talked to, as it was said Boeing kept changing their minds as to what the aircraft was and would be.
GE unveiled their engine concept at the 1984 Farnborough Air Show, promising up to 30% reduction in fuel burn without decreasing inflight cruise speeds. Two key points for GE’s proposal was twin contra-rotating fans which kept fan blade length to a reasonable size (12 feet) vs. a single prop at nearly 20 feet in diameter. The second selling point was the lack of a gearbox, which transfers power from the turbine to the propeller. Beginning in 1986, the GE36 flew a considerable number of test flights, achieving a cruise speed of Mach .84 at an altitude of 39,000 feet on Boeing’s test 727 aircraft.
There was significant interest from British Airways as a replacement for its 737-200s, SAS, and American Airlines. However, British Airways selected the Boeing 737-300 instead. American Airlines had concern if the 7J7 could be stretched further using the same powerplant. Internally at Boeing, the 7J7 was viewed as a political football: a possible new line of fuel-efficient aircraft, or the grim reaper for the 737 line. In the end American Airlines went with the Fokker 100 aircraft based on availability. The engine technology while progressing, wasn’t going fast enough to guarantee a launch of the 7J7 by 1991. With competition brewing from McDonnell Douglas not to mention the newly introduced Airbus A320, Boeing ended up postponing and subsequently canceling an UDF powered aircraft. But McDonnell Douglas pressed on.
Allison Engine Company which already had a head start on a similar concept, partnered with Pratt & Whitney in 1987 to produce a prototype UDF for a test run on an McDonnell Douglas airliner. While Allison had produced a smaller prototype and tested in conjunction with NASA back in 1981, it was playing catch up to the GE36 in 1985, which flew first. Pratt & Whitney would not be able to develop and certify an original engine design to meet the aircraft launch dates, and thus partnered with Allison to speed up development. The engine was proposed in two sizes, with the smaller variant a 10k shaft horsepower and 23:1 compression ratio, with a three stage boost compressor and power turbine for 100 passenger aircraft; and a larger 15k shaft horsepower aircraft for up to 160 passengers. The engine was dubbed the 578-DX, but unlike the GE36, the 578-DX would in fact have a reduction gearbox, and claim an additional 7% fuel savings as a result. But having a gearbox meant more maintenance and reliability headaches and airlines were wary. Allison claimed that the gearbox would need an overhaul after 30,000 hours, and was in fact reliable (although this would never be proven).
McDonnell Douglas Goes All In On UDFs
McDonnell Douglas was already planning its next range of airliners based on the successful MD-80 series. The first of these next generation airliners was the MD-90, an updated version of the MD-80 with all new engines and a glass cockpit, better range and less fuel consumption. It would also address noise by having a lower noise profile than the current JT8D engines. But this was expected to be a stop-gap measure until UDF technology matured and eventually replaced jet powered MD-80 and 90 aircraft. Subsequently, McDonnell Douglas began marketing the MD-91 and MD-92 aircraft as early as 1985, all powered by UDF engines by GE or P&W.
The MD-91 would have been the smallest capacity of the offerings, at 114 passengers (2 class) or 130 passengers (all economy) and just under 129 feet in length. It would utilize the MD-87 fuselage, but with the wing shifted 3 feet further back. The MD-90 (traditionally powered) would carry 153 passengers (2-class), or up to 172 passengers in all economy configuration, with a fuselage length of 152 feet. Finally, the MD-92 was slightly larger at 157 feet in length, utilizing the MD-88 fuselage (lengthened 133 inches forward of the wing), and 165 passengers (2 class) or 173 passengers in all economy. All aircraft would feature a composite horizontal stabilizer, with split powered elevator and rudders and a full glass digital cockpit. There were subsequent other aircraft marketed like the MD-94, but this was proposed to be a clean sheet design to compete with Boeing’s 7J7.
There was the perception an open rotor aircraft suggests interior noise to passengers. McDonnell Douglas claimed that the interior cabin noise levels of the MD-91/92 would be 80dB, which would make it up to 6dB quieter than the jet powered aircraft. McDonnell Douglas claimed that the demonstrator registered 82dB during testing, and that airline executives who flew on marketing flights made statements like “very quiet, better than expected” or “airplane very quiet, particularly in the last row.” This conflicted with reports of other members of the travel industry who said it was very noisy inside, a different type of prop sound, but certainly noticeable and generating some cabin vibration (that would be addressed later on). The marketing test flights were carried out of Long Beach, and flew airline personnel, media, military, and other industries (181 people carried total). The aircraft would fly out of LGB and out over the Pacific Ocean, directly over Catalina Island, then it would turn left and south where it would make a U-turn over San Clemente island and descend back to LGB.
Ultra-Loud and Ultra Efficient
The GE36 was also tested on the McDonnell Douglas MD-80 testbed for a future MD-9x series of aircraft a year later in 1987. It was modified from an 8×8 blade set up (8 blades on each counter-rotating prop) to an 10×8 with the rear prop containing 8 blades versus the forward props 10 blades. This configuration resulted in quieter operation, and that a commercial production version would have different blade counts on each prop to achieve the desired noise profile and best performance. Most of the test flights with the McDonnell Douglas MD-80 test bed were performed with the GE36, as the P&W option was way behind schedule.
Most testing on the MD-80 testbed was with the GE36, which achieved a maximum speed of Mach .865 at 37,000 feet. This powerplant would accomplish 165 hours over 93 flight tests, substantially more than the 578-DX. McDonnell Douglas began to refer to the UDF with another acronym: UHB – Ultra High Bypass, and came up with further variations and aircraft types utilizing the UHB acronym. They were marketed with up to 40% less fuel burn, but ignoring the substantial cost increase to acquire these new powerplants. McDonnell Douglas was even discussing offering powerplant retrofits on existing MD-80 series aircraft. Once Boeing cancelled the 7J7, McDonnell Douglas in marketing materials to airlines began saying you can’t get UDF technology from anyone but McDonnell Douglas. This was all based on the GE36, because up to this point the 578-DX had not flown on an McDonnell Douglas airframe having missed numerous milestones in the schedule due to a variety of issues.
McDonnell Douglas was so confident in what the UDF offered that it entered a military variant when the Navy began a search for a new patrol aircraft, specifically the Long-Range Anti-Submarine Warfare Capable Aircraft (LRAACA) competition launched in January 1987. This competition was seeking replacements for the ageing P-3 Orion aircraft. No other manufacturer other than Lockheed had interest in building a derivative to the P-3. But the Navy wasn’t going to just hand Lockheed a contract without bidding it out, and thus expanded the scope to include commercial aircraft derivatives in March of 1987.
McDonnell Douglas submitted an MD-91 derivative, ironic given the MD-91 hadn’t officially flown yet. The designation for the derivative was P-9D, and it was the UDF powered MD-91 but with additional equipment. McDonnell Douglas was offering either engine (P&W or GE) as a choice to the Navy, even though the 578-DX hadn’t flown on the MD-80 test bed. The P-9D was presented as having a 45%+ lower fuel burn than the current P-3 Orion. The P-9D would have carried the latest electronics and sensors, along with AGM-84 Harpoon Missiles and operating with a crew of 11. Nonetheless, in October of 1988, the Navy selected the Lockheed proposal (P-7) which was significantly lower in cost than the McDonnell Douglas proposal, while also being judged technically superior with less technical risk. McDonnell Douglas soldiered on with the commercial MD-91 and MD-92, flying the UDF prototype to the Farnborough Airshow in 1988 equipped with the GE36 engine in hopes of drumming up sales.
The 578-DX would undergo ground testing from late 1987, with scheduled in-flight testing (installed on an MD-80 test bed) planned for early 1988. Due to repeated mechanical and engineering issues encountered on the ground, the 578-DX didn’t take to the air until April of 1989, nearly two years late. But when it did, it reportedly flew without issue at a speed of up to Mach .77 at an altitude of 30,000 feet. McDonnell Douglas was even experimenting with the idea of replacing the turbofans on the DC-10 with UDF powerplants, with one example being given the conceptual designation “UHB-270”, but never appeared outside of model form.
After years of delay, McDonnell Douglas killed off the UDF variants of its new MD-9x airliners in May of 1989, one month after successful testing of the 578-DX. It was clear that the engine delays would further push the MD-9x series of aircraft further into the mid-late 1990s before airline customers could take delivery. And GE wasn’t interested in moving forward with their UDF unless McDonnell Douglas had firm orders for at least 150 aircraft. Caught in chicken-and-egg scenario, airlines were generally reluctant to order a brand new type without the manufacturer fully committing to the type. But McDonnell Douglas wasn’t going to commit to anything, from guaranteeing performance data to manufacturing the aircraft without orders.
The End of the Road For The UDF
What killed the UDF concept? Price, price and price. The price of fuel when the UDF concept kicked off was at the top of the market, and climbing by the day. But when the UDF concepts finally flew, the price of gas had begun to drop. From a high of $1.31 a gallon in 1981, by 1986 gas was averaging .86 a gallon. Gas was no longer perceived as a financial gating issue compared to the early 1980s. The second problem was the price of the powerplant. Pratt & Whitney’s 578-DX was forecast to be 40% higher in cost to purchase than a turbofan that was currently available for Boeing and McDonnell Douglas aircraft. The economics just didn’t quite work the way they expected. Finally, no airline customer put in a firm order for the prop-fan powered airliners. All three of these issues killed the UDF concept.
But all was not lost, and a lot was learned from the UDF exercise. The technology to build the UDF’s fan blades was used to develop the fan blades for the GE90, one of the engines that powers the Boeing 777 aircraft. The technology using composite materials is now used in many aircraft parts. And there are still companies, GE included, who continue to look at UDF technology.