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Why do airliners use the engines the way they currently do and not one behind another?? Would it not lead to better thrust and in case any one of the 2 engines placed together has to fail, the 2nd one will help keep it moving and with the fuel, we wouldnt completely lose all the thrust the engine used to provide.

What is wrong with my idea?

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    $\begingroup$ I think this is essentially an afterburner. After the air goes through the first engine the oxygen level in the air is low so the efficiency of the second engine is lower $\endgroup$
    – Wyatt
    Commented May 26 at 14:36
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    $\begingroup$ Thanks @Wyatt I did not think of that point :) $\endgroup$ Commented May 26 at 14:39
  • $\begingroup$ @Wyatt, except If your engine fails you can't just turn on the afterburner. It's part of the engine, not a separate backup engine. It relies on all the sections in front of it operating properly. $\endgroup$ Commented May 26 at 14:46
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    $\begingroup$ @MichaelHall Yeah that’s true. The second engine would still receive hotter and less oxygenated air, making it less efficient. (Given the first one is on) $\endgroup$
    – Wyatt
    Commented May 26 at 14:53
  • $\begingroup$ Duplicate of What would happen if put a jet engine behind another jet engine? $\endgroup$
    – mins
    Commented May 29 at 14:34

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It wouldn't increase thrust.

Thrust means the gases coming out the back of the engine are moving faster than the gases coming in the front. The bigger the speed difference, the more the thrust.

So if you speed up the air coming in (by feeding it the fast exhaust from another engine), then you're reducing its thrust.

A more precise explanation is offered by our beloved Peter, of course.

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If one looks at the interior of a jet engine we do see many compressor and turbine blades one behind another.

With jets, it is most efficient to have one place where incoming air is heated to drive the turbine blades, which drive the compressor (and fans).

Indeed, the trend has been from many smaller jet engines to 2 larger jet engines.

Theoretically, the 2 could be replaced by one even larger single engine (imagine a 747 with a giant fan jet mounted on the back just like a Heinkel 162, but 2 engines provide safety in redundancy).

The English Electric Lightning did have 2 jet engines mounted one on top of another, rather than side by side, but no advantage is gained by having one jet behind another.

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This question is a bit like asking why car designers don't replace V8 engines with a pair of daisy-chained 4 cylinder engines. Whatever two separate engines connected inline can do, will almost certainly be done better when they are integrated into a single engine. This is why there are zero racing sports cars with dual engines in them. The closest thing I can think of is the Bugatti Veyron W16, which is like two V8s attached together. However, this is still a single engine, rather than two separate V8s connected together.

When you consider that a jet engine has to accelerate the air coming into it to actually provide thrust, it becomes clear that the outgoing air from one engine does not make for good incoming air for the other. The exception is if the front engine could accelerate the exhaust to high supersonic speeds, enabling the rear engine to operate as a [sc]ramjet. Unfortunately, a turbojet cannot accelerate air from subsonic to multiple mach numbers, but even if it could, the mass of the exhaust stream would have to be very small because the incoming air volume is not large enough to support such a large multiple of output velocity (the average flow rate between the input and output of the engine must be the same + fuel mass).

But even if you could design a turbojet with this multiplicative capability, there is still the question of why you would then use two separate engines rather than combining them into the same engine. In this case, all the hard work goes into the front engine, so there is no obvious benefit to adding the back engine at all.

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This is an easy physics. We don't need to think about engineering.

Jet engines work by compressing air, adding heat through combustion, and then using the hot, expanding gases to generate thrust. Placing one engine directly behind another would expose the rear engine to the already hot exhaust of the front engine. This would reduce the efficiency of the rear engine because it would compress air less efficiently (reduced air density due to preheating).

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Most airliners already have 2 engines, and can operate on one just fine.

Your suggestion would be very costly, adding massive weight and complexity, and burning way more fuel for additional engines that would be operating at reduced efficiency due to breathing in the oxygen depleted exhaust of the engine in front of it.

There are four engined airplanes out there, but they are largely going away due to the performance and reliability of modern twins. And if for some reason you did want to design a new four-engined airplane, you would probably want to configure them side by side and not in line as you suggest.

In short, it would create way more problems than it would solve.

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    $\begingroup$ I’m also imagining that if the first engine fails, it’s likely to lose parts, which will be caught by the second. $\endgroup$ Commented May 26 at 15:54
  • $\begingroup$ Depends on the cause... failed high pressure fuel pump, no. If birds FOD out the forward engine, then they & broken fan/turbine blades all go directly into the rear engine, and immediate dual-engine failure. $\endgroup$
    – Ralph J
    Commented May 26 at 19:25

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