Why aren't there any single turbofan airliner?

Question

A configuration which looks like a DC-10, without engines mounted under the wings. Let's say 737 sized.

Let's forget about redundancy security advantages of twin or quad engines, since they're obvious and not my concern about this particular question.

One single engine is more efficient than two similar engines. Efficiency multiplies, two engines are less efficient, four even worse.

Center of gravity problem doesn't exist since wings can be moved backward. (of course yaw stability will be corrected inherently)

Modern turbofans family can provide a single stock powerful enough version to fly a 737 class size.

In the event that debris is released from the engine, one canard configuration may be designed, in order to prevent hydraulic failure and loss of control.

To summarize, are there (except redundancy safety concerns) any reason why there are no single turbofan airliners?

Answer 1

Airliner designs must be certified in order to fly commercial services. In the US, this means compliance with the Code of Federal Regulations, Title 14, Subchapter C, Part 25. In §25.901(c) it says:

For each powerplant and auxiliary power unit installation, it must be established that no single failure or malfunction or probable combination of failures will jeopardize the safe operation of the airplane except that the failure of structural elements need not be considered if the probability of such failure is extremely remote.

Engines do fail from time to time, so there must be a second unit which can take over if one fails. It's the same for fuel pumps, instruments or even wheels: Every one has a second unit which can take over if the first fails. For wheels, this is very easy to check: You will not find a single airliner landing gear leg which has only one wheel. All have two wheels or more.

Or pilots: The minimum number of pilots for safe operation is two. One can fly the plane (so she/he can land it if the other pilot is incapacitated), but two are needed to start any revenue flight.

I could continue that list, but I guess the concept has become clear: For every element on an airliner there needs to be a second, redundant one, except for structural elements with a very remote probability of failure.

This has been learned the hard way: Some of the first aircraft designed for passenger service (like the Junkers F-13 or the Fokker F.II) had a single engine. Some of the multi-engined ones even became dangerous death traps when one engine failed. Over time, the desire to increase safety made redundancy the supreme design principle for airliners.

Military operators have fewer qualms about single engine designs, because using fewer engines improves performance and lowers acquisition and operation cost.

Answer 2

There is an aircraft with a single engine in the middle of fuselage- the Northrop Grumman RQ-4 Global Hawk- and it has the wingspan of a 737.

Global Hawk and B737; image from reddit.

For the engine to power an airliner the size of 737, it would have to be more powerful and huge. This leads to a few issues:

• It is going to eat up cabin space. In an age where airlines are finding creative ways to squeeze more people into the available space, this is a no-go.

• The large engine means that there should be sufficient clearance between the engine and the fuselage- else, it would result in more drag and reduced thrust, thereby reducing efficiency. Increasing clearance is quite difficult for fuselage mounted high bypass engine due to weight implications.

• Passengers in rear seats may not have a pleasant journey due to engine noise.

• Mounting the engine in rear incurs weight penalty through various ways- you have to strengthen the rear fuselage and at the same time, you lose wing bending relief, requiring strengthening and you have cg issues.

• Mounting engines above the fuselage is a not maintenance friendly either.

• Just to note, redundancy and safety are significant issues, especially in a commercial airliner.

Answer 3

This question explains with pros and cons of dual vs single engine.

I think that the main reason is additional safety that comes from redundancy. Apart from that an airliner has much more people on board, an airliner with its the only engine failed would be much more dangerous for the people on the ground than some tiny single engine aircraft.

Answer 4

There are a number of problems I can think of if an airliner the size of a Boeing 737 has a single turbofan engine:

• Insufficient thrust. The engine must now produce twice the power.
• Unbalanced Center of Gravity. The tail-heavy configuration would make tail strike (both during ground operations and takeoff / landing) must more likely. Also, the effects of the elevator would be greatly reduced.
• Lack of redundancy. Even before ETOPS, airliners need at least 3 engines to fly oceanic routes.
• Loss of critical controls in the event of an un-contained engine failure. Many critical components, such as hydraulics, elevator and rudder, are located near the tail. In the event that debris is released from the engine, there is a high chance that these components will be damaged, rendering the plane uncontrollable.