The De Havilland Comet was the world’s first jet-powered airliner, but the Jet Age really began with the debut of the Boeing 707 and Douglas DC-8 in the late 1950s. These aircraft were larger, more capable, and more technologically advanced than the Comet, thereby popularizing the concept of the jetliner. Over 500 DC-8s and over 1,000 707s (including the shorter 720) were sold, establishing jet engines as the preferred method of large aircraft propulsion to this day.

Even today, these aircraft still look fairly modern. They popularized many design elements that are used today, including swept wings and podded engines. The engines themselves, however, are the part of the aircraft that have seen the most changes. The most striking difference between early jet engines and modern powerplants is that newer engines are several times larger. Here is how much engines have grown since the start of the Jet Age, and how engine technology has evolved.

The Very First Jet Engines

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The initial De Havilland Comet 1 was equipped with four Halford H.2 Ghosts, while later variants were equipped with the Rolls-Royce Avon. The Rolls-Royce Avon also powered early versions of the Sud Aviation Caravelle, while initial variants of the Boeing 707 and Douglas DC-8 were powered by the Pratt & Whitney JT3C. What these engines all have in common is that, unlike modern turbofan engines currently in production, these engines are turbojets.

This makes them inherently smaller and less efficient than modern engine models. A turbofan and a turbojet are similar to one another, except that a turbofan engine features a fan at the front of the engine. The fan drives a significant amount of air around the engine core through the bypass ducts, which increases thrust without dramatically increasing fuel burn. A turbojet engine, on the other hand, moves all air through the engine core to generate thrust.

This results in extremely high fuel burn, which is why turbojet engines have largely become obsolete for subsonic aircraft. The Pratt & Whitney JT3C has a diameter of roughly 39 inches (99.06 cm), which is significantly smaller than even early turbofan engines like the Pratt & Whitney JT8D.

It’s roughly 10 inches (25.4 centimeters) smaller than the General Electric CF34, an engine model used on several regional and business jets. However, the JT3C was applied to long-range airliners. In addition, the JT3C was rated to produce a maximum of roughly 12,000 lbs of thrust, meaning that an early Boeing 707 quadjet was powered by less than 50,000 lbs of thrust.

Comparing The First Turbofans

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The Rolls-Royce Conway was the first turbofan engine to enter commercial service. It was used to power the Handley Page Victor strategic bomber and the Vickers VC-10, but was most notable for powering variants of the Boeing 707 (707-420) and Douglas DC-8 (DC-8-40 series). These variants were based on variants of the 707 (707-320) and DC-8 (DC-8-20/30) that featured the Pratt & Whitney JT4A. As a turbofan engine, the Conway burned significantly less fuel than the JT4A.

This meant that the 707-420 had a roughly 2% longer range than the 707-320. However, the Conway found few customers because Pratt & Whitney released the JT3D so soon after it. The JT3D was offered on new-build examples of the 707 and DC-8, while the JT3D was also available as a retrofit for already-delivered units. Not only were the JT3D and Conway more efficient than the turbojet engines, but they were more powerful as well, greatly improving performance.

Pratt & Whitney also developed the JT8D, which was used on the Boeing 727, 737, McDonnell Douglas DC-9, MD-80, and the rare Dassault Mercure. Despite its relative unpopularity, the Conway was a revolutionary engine, and the JT3D, as well as the JT8D, became the dominant engine options used on airliners in the 1960s.

The Conway has a bypass ratio of 0.3:1, and was fairly compact with an engine diameter of 42 inches (107 cm). Meanwhile, the JT3D had a significantly higher bypass ratio of 1.42:1 and a fan diameter of 53 inches (130 cm). Early variants of the JT8D, meant for smaller short-haul airliners, had a bypass ratio of 0.96:1 and a diameter of 49 inches (125 cm).

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The Debut Of The High Bypass Turbofan

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The Lockheed C-5 Galaxy featured the world’s first high-bypass turbofan engine. It was equipped with four General Electric TF39s (which were adapted to develop the popular CF6 line), and these engines had a diameter of 97 inches (246 cm), along with a bypass ratio of 8:1. Even by today’s standards, this bypass ratio is considered excellent. Commercial CF6 variants varied in size from 86.4 inches (219 cm) to 96 inches (244 cm).

These engines powered the Airbus A300, A310, A330, Boeing 747, 767, McDonnell Douglas DC-10, and the MD-11. In commercial service, the Pratt & Whitney JT9D beat the CF6 by over a year with the Boeing 747-100. It was sized similarly to the General Electric CF6, with a typical fan diameter of 93.4 inches (237 cm). It was used on early versions of the Boeing 747, 767, as well as the Airbus A300, A310, and the McDonnell Douglas DC-10-40.

Early CF6 variants were replaced by the improved CF6-80 series, while the JT9D was replaced by the Pratt & Whitney PW4000 series. Rolls-Royce initially developed the similarly sized RB211 for the Lockheed L1011 TriStar, and the engine was also placed on the Boeing 747. A smaller derivative was designed for the Boeing 757, while future widebodies were offered with the Rolls-Royce Trent series.

The Boeing 777 was offered with the Trent 800, featuring a fan diameter of 110 inches (280 cm). The Trent 800 competed with the PW4000-112, which had a diameter of 112 inches (284 centimeters), as well as with the General Electric GE90, which had a fan diameter of 123 inches (312 cm).

21st Century Widebody Engines

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The Rolls-Royce Trent 900 was a derivative of the Trent family used on the Airbus A380. These engines measured roughly 116 inches (295 cm), making them nearly three times the size of the original Rolls-Royce Conway turbofans designed in the 1950s. The Rolls-Royce Trent 1000 used on the Boeing 787 is actually slightly smaller, at 112 inches (285 cm), while the comparable Rolls-Royce Trent 7000 and General Electric GEnx are roughly the same size.

The Airbus A350 uses a new variant of the Rolls-Royce Trent family named the Trent XWB, which is the largest and most powerful engine ever built by Rolls-Royce. Its fan diameter is 118 inches (299.7 cm), and it’s capable of producing roughly 97,000 lbs of thrust in its Trent XWB-97 variant, which powers the Airbus A350-1000. Meanwhile, the less powerful Trent XWB-84 is designed for the Airbus A350-900. Apart from the power rating, the two engines are largely identical.

For years, the General Electric GE90-115B equipped on the Boeing 777-300ER was the world’s largest and most powerful engine. It had a fan diameter of 128 inches (330 cm) and was rated for over 115,000 lbs of thrust. However, the new Boeing 777X is equipped with the General Electric GE9X, which is a more advanced and even larger engine.

The GE9X actually has a lower power rating of roughly 110,000 lbs of thrust (though it is capable of producing more power). It also has a fan diameter of 134 inches (304 centimeters), making it the world’s largest turbofan engine.

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Examining Narrowbody Engines

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The CFM56 is the world’s most successful jet engine of all time, and it was primarily used on narrowbody aircraft. The CFM56-5B was prominently featured on the Airbus A320 family, and it has a fan diameter of roughly 68.3 inches (173 cm). Meanwhile, the CFM56-7B engine offered on the Boeing 737NG has a fan diameter of roughly 61 inches (155 cm). This is larger than even the Pratt & Whitney JT8D-200 offered on the McDonnell Douglas MD-80.

The Boeing 757’s Rolls-Royce RB211-535Es had a fan diameter of roughly 74 inches (188 cm), and the Pratt & Whitney PW2000 had similar specifications. The newer Pratt & Whitney PW1000G and CFM LEAP families are equipped on a variety of airliners, but they’re most prominent on the Airbus A320neo family. The PW1000G variant on the A320neo has a fan diameter of 87.6 inches (223 cm), while the CFM LEAP-1A has a fan diameter of 78 inches (198 cm).

As a whole, engine sizes have grown substantially since the start of the Jet Age. This is partly due to the design of a turbofan engine, which leads to greater fuel efficiency for larger engines with higher bypass ratios, as well as increased power requirements today compared to the 1950s. A General Electric GEnx, for instance, is nearly 60 inches (152 cm) wider than a Pratt & Whitney JT3D, while producing more than three times the thrust, and being significantly quieter too.