# How does a boeing 707 have 4 engines instead of 2? [duplicate]

I was reading a article Link It stated that lower bypass ratio jet engines have higher thrust than high bypass ratio engines, but the boeing 707 has 4 of these low bypass ratio engines and only travels about 600mph. Most modern high bypass ratio engines travel at about 514mph(push boeing 737-800 aircraft at an average of this speed) and only have 2 of these high bypass ratio engines Boeing 737-800 engine.I am having trouble understanding how this works.

• There was about a 35-year gap between the 707 and 787; that's a lot of time for engine technology and efficiency to improve in Sep 2, 2015 at 23:16
• Another "why A has X engine instead of Y" question
– Him
Sep 2, 2015 at 23:46
• This is too incomplete to be an answer, but using rough numbers from Wikipedia: The 707-320B has a max takeoff weight of 333,600 pounds and has four engines producing 19,000 pounds of thrust each. The 787-10 has a maximum takeoff weight of 553,000 pounds and has two engines producing 76,000 pounds of thrust each. Each engine on the 787 produces as much power as all four engines combined on the 707. So even though the 787 weighs 200,000 pounds more than the 707 and has half as many engines, it actually has more power per pound of aircraft weight. Sep 2, 2015 at 23:55
• @Ethan, this is literally an exact clone of your two previous "Why does [airliner] have 4 engines instead of 2?" questions. If you need clarification about this, why not ask for it in an existing question?
– egid
Sep 3, 2015 at 2:23
• Can you try rephrasing it that way, then? The answers are currently reading between the lines to get that question. Currently it very much reads as 'why does this airplane not have fewer engines?'.
– egid
Sep 3, 2015 at 2:28

Boeing 707 was initially designed to use the P&W JT3C turbojet engine. Early versions of the '707 (like the one in the first picture of your question) and C-135 (including KC-135A) used those engines. Wikipedia reports a maximum thrust of 12,030 lbf.

The JT-3D is one of the firsts early low-bypass ratio turbofans (1958/59). It's based on the JT3c and, according to Wikipedia, has a bypass ratio of 1.42:1 and a maximum thrust of about 17,000 lbf.

The CFM-56 first run was in 1974, more than 15 years later. The CFM-56 is a high-bypass ratio turbofan, with a bypass ratio of 5.5:1 and a maximum thrust of 19,500 lbf for the CFM56-7B18 model that powers the Boeing 737-600 as reported on Wikipedia.

So the answer to your question is that at the time the Boeing 707 was designed, the only available engines were turbojets with a limited (for today standards) thrust.

The article you're asking about refers to modern jet engines. Today low-bypass ratio turbofans are used mainly in military supersonic aircrafts, were fuel efficiency is less important than thrust and supersonic efficiency. The opposite applies on modern civil aircrafts.

Engine technology has improved tremendously since the days of the Boeing 707. Today's engines are larger, more powerful, more efficient and quieter than their predecessors.

The engines used in first production Boeing 707s (from 1958), the Pratt & Whitney JT3C turbojet, developed a thrust of 50KN (11200 lbf). This engine was the first one (in US) to develop more that 10000 lbf. The same engine (Pratt & Whitney J57) was used in the B-52B stratofortress.

This engine was selected as it was the best available (in fact, the J57 designers won the Collier trophy for their efforts). In fact, the engine was so successful that aircrafts were designed around it- half of the century series USAF fighters used them, as did 707's competitor, the Douglas DC-8.

As for Boeing 737, which had its first flight around 10 years later, the Pratt & Whitney JT8D used was a low bypass turbofan engine, having a bypass ratio of 0.96:1. Again, it was developed from a military engine, the Pratt & Whitney J52 (which powers Northrop Grumman EA-6B Prowler, among others).

One pattern we can see clearly is that most of the early civil aircraft engines were developed from military turbojets. The reason was that only these powerplants could generate the required thrust at that time and today's high bypass turbofans were not even in the picture.

However, as the airliners became fuel conscious and noise became a major issue, high bypass turbofans were developed exclusively for civil use and the development of civil and military engines diverged. With time, the bypass ratios have increases with decreases in fuel consumption.

Source: compositesworld.com

As a comparison, the Rolls Royce Trent-1000 engines used in Boeing 787 can generate 350KN (78000 lbf) of thrust, more than 6 times that of JT3C. So, for meeting the thrust requirement of the four engined Boeing 707, you don't even need a single 787 engine.

In the context, it is more useful if you compare the engines used in the same aircraft over the years. The most powerful engine used in the 707, the Pratt & Whitney JT3D-7 increased thrust over JT3C by over 50%. In case of 737, the 737 MAX uses CFM International LEAP, which doubles the thrust on the PW JT8D.

As for the article, what I get is this: For a given power available to the bypass fan, it can be used in either the two ways- take less air and increase the pressure of it more or take more air and have a lesser increase in pressure.

In general, the military turbofans have a lower bypass and higher thrust to weight ratio. For example, the Pratt & Whitney F119 used in F-22 Raptor has a bypass ratio of 0.3:1 and a thrust to weight ratio in excess of 9, while the General Electric GenEx used in Boeing 787 has a bypass ratio of 9.6:1 and a thrust to weight ratio of 5.