Summary

  • McDonnell Douglas' MD-94X was meant to combine turbofan power with turboprop fuel savings but ultimately faced low demand.
  • The type was designed to rival Boeing's 7J7, but the high development costs of propfan engines led to the project's cancelation.
  • Even though the propfan engine design offered 60% fuel savings, airlines showed little interest in propfan-powered planes.

The McDonnell Douglas MD-94X was a planned propfan-powered airliner. Also called an open rotor engine or an unducted fan, the engine is related to the turboprop and turbofan, yet distinctly different. The design was meant to offer the speed and performance of a turbofan with the fuel savings of a turboprop.

McDonnell Douglas announced the intention to build the MD-94X in 1986, with production scheduled to start in 1994. The aircraft would seat between 160 and 180 passengers, possibly using a twin-aisle configuration.

Efficiency was everything

In order to understand why the idea of building a propfan-powered engine became popular, it is important to look at the landscape of the commercial aviation industry in the 1980s and 90s. The 1970s, which preceded the era of propfan-powered developmental projects, was defined by multiple oil shocks, times in which the price of jet fuel shot skywards.

For airlines, oil shocks were extremely challenging situations to deal with and had significant negative impacts on airline bottom lines. As Simple Flying has discussed multiple times, airlines typically operate on wafer-thin profit margins, making it much more critical to reduce costs as much as possible.

For most carriers, fuel is undeniably the single most significant expense, one which becomes an even larger portion of a passenger's ticket price the longer a flight is. An even bigger issue emerges when one recalls that the cost of crude oil fluctuates daily, and airlines have extremely limited control over the price of jet fuel.

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As a result, airlines will seek out any possible improvement in fuel efficiency when it comes to new aircraft, and manufacturers are always looking to create more fuel-efficient planes to capture demand from their competitors, according to Airways Magazine. In the 1980s, a new concept for a jet engine began to emerge, one which promised to deliver unprecedented fuel efficiency with an impressive power output.

The propfan as a concept

When the idea of propfans first came to light, two major kinds of engines were being used on commercial passenger aircraft. First, we have turboprops, offering the best fuel economy on the market, with propellers generating the majority of thrust. However, they were simply not as powerful as other kinds of engines.

The second primary engine type was the turbofan, which ignited fuel in a combustion chamber and generated thrust with a fan blade, combining to allow for a significantly higher power output than the typical turboprop. However, as you may have guessed, the biggest downside with turbofan engines was their higher fuel consumption, as efficient advancements like high-bypass designs had yet to enter production.

The propfan engine offered a unique level of efficiency and, as a result, had manufacturers confident they could draw airlines' interest in the concept. With the same fuel economy as a turboprop but the power output of a conventional turbofan, the propfan was set to be the ideal engine for airlines of the time.

According to New Atlas, the design combined both turboprop and turbofan components, with a small intake chamber and a rear-attached fan with many heavily twisted blades. This fan would have far more blades than a typical turboprop but significantly fewer than the ducted fan on a traditional turbofan.

The plane was to compete with a similar offering from Boeing

McDonnell Douglas developed the MD-94X to compete with Boeing's proposed replacement for the Boeing 727, a 150-seat aircraft called the "Boeing 7J7." The 7J7 was supposed to be a highly fuel-efficient aircraft, but, like the MD-94X, it never got built due to the price of oil coming down.

At the time, McDonnell Douglas said that for them to make the plane, the fuel price needed to be at least $1.40 a gallon or higher. While similar in design to the MD-80, the MD-94X incorporated new technologies such as canard nose planes, laminar and turbulent boundary layer control, side-stick flight control, and aluminum-lithium alloy construction.

Airlines were not interested in propfan-powered planes

Despite touting that the MD-94X's propfan engines could yield up to a 60% reduction in fuel savings, the airlines would ultimately find little interest, and both the MD-94X and Boeing 7J7 programs were canceled. By the time the aircraft concept was ready to market, oil prices had dropped to the point where the risk of flying a novel airliner was simply not worth the potential fuel consumption gains.

At the same time as the MD-94X was being proposed, McDonnell Douglas was working on propfan-powered commercial variants of the MD-80. Designated the "MD-91X and the "MD-92X," the MD-91X would have seated between 100 and 110 passengers while the larger MD-92X could accommodate 150 passengers.

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According to McDonnell Douglas, both aircraft were planned to enter service in 1991 and 1992. On the downside, at the cost of $1.6 million per engine, the propfans were much more expensive than the Pratt & Whitney JT8D-200 series engines already being used on the MD-80.

A propfan-powered military variant of the MD-91X, called the "P-9D", was also on the table to be built for anti-submarine warfare. The plan was to use the new plane to replace the United States Navy's fleet of 125 Lockheed P-3 Orion aircraft. Again, this too got shelved when the Navy selected a derivative of the P-3 Orion, later named the "Lockheed P-7A. Ultimately, there was no demand and little market benefit to continue with the planned MD-94X, and the project was scrapped.

About McDonnell Douglas

Headquartered at St. Louis Lambert International Airport (STL) near St. Louis, Missouri, McDonnell Douglas was a prominent American aerospace manufacturing corporation and defense contractor. Initially, two separate companies were formed by Massachusetts Institute of Technology (MIT) graduates, James Smith McDonnell and Donald Wills Douglas. Both men started their careers working for the Glenn L. Martin Company before leaving to create their own businesses.

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The company was founded in 1967 when McDonnell Aircraft and the Douglas Aircraft Company merged. Before being taken over by Boeing in 1997, the company produced well-known passenger aircraft, such as the DC-10 and MD-80. For the military, McDonnell Douglas manufactures the F-15 Eagle and the F/A-18 Hornet multi-role fighter.