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During WW2, the first jet engines appeared en mass in combat aircraft. Britain and Germany made the first ones.

But why were turbojets first? Why not turboprops, which are easier since the turbo-machinery is smaller for a given thrust. Turboprops have propellers so it would've been a much easier evolution too. If I'm not mistaken, turboprops can fly higher and faster much more easily than piston-props, so it would've conferred an advantage.

So why were turbojets first? Did the concept of a propeller attached to a turboshaft just not cross the minds of engineers during the war? Was it tunnel vision onto the much more powerful turbojets?

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  • $\begingroup$ Or maybe the gearbox was the component creating the problem, this was a huge power to transfer. $\endgroup$ – mins Oct 23 '17 at 11:48
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    $\begingroup$ The Merlin had a reduction gearbox, for instance the Merlin XII Merlin fitted with 0.477:1 reduction gear installed in some Spitfire II's with three-bladed Rotol constant-speed propeller. Rated at 1,150-horsepower (857 kW) at 3,000 rpm at 14,000 feet.[19]) $\endgroup$ – Koyovis Oct 24 '17 at 10:16
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The development of turboprops and jets were going on more or less concurrently before and during the initial stages of WWII -their first test runs were within years of each other. You have to understand that there was a war going on- any (significantly) better aircraft engine would be the difference between life and death; and that would be the one that would get the resources. The turboprop was not to be that engine. During development, the turbojets showed improvements and promise that the turboprops never did.

Turboprops have propeller but the engine was completely different- instead of developing another type of engine (which will certainly be less powerful compared to the present ones) and making it to run the same propeller, it is easy to see why the concept of an engine without propeller is more appealing- for similar configuration, you have to show a significant improvement downright, while the new idea can survive (atleast initially) on promise. Add to this the fact that the piston engines reached their peak during WWII in performance and power, it would have been a tall order for the turboprops to better them.

Generally speaking, while turboprops of today may be faster than the piston engined aircraft, it was not necessarily true till the Soviets developed the Tu-95. In case of jet engines however, the difference was substantial. One of the initial problems in Me-262 was that it was too fast for prevailing tactics. During their introduction, the jets didn't offer any meaningful increase in flight ceiling.

For a turbprop, you need three parts- a turbojet, a gearbox and a propeller. A turbojet eliminates two of these components and makes the the engine simpler to construct (even compared to some piston engines), an important consideration in wartime.

At the time of the war, the primary thrust of engine development of improving the performance of combat aircraft- and the jet engines brought a paradigm shift in this regard by increasing speed, which the turboprop was never able to do. Even today, the turboprops are not popular for their power, but for their endurance, and their application in combat role is rather limited; in a situation like WWII, where the combat aircraft were paramount, their ascent would have been inconceivable. It was not tunnel vision, but facing the reality of appropriating the limited resources for surviving today and winning tomorrow.

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Propeller planes already existed, and turboprops were an answer to a question that was not asked in times of war. The main driving factor behind development of the turbojet was to provide great climbing power and high airspeed for intercepting enemy bombers, which the turboprop would not have been able to do. Fuel economy and engine life were of secondary importance, the Junker-Jumo 109 of the Me-262 lasted only 10 flight hours!

The piston propeller engines were very sophisticated towards the end of WWII, the super-charged Merlin piston engine powering the Spitfire developed over 2000 hp.

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But the Me-262 and the He-178 were way faster than the Mustangs & Spitfires that escorted the bombers, and could only be shot down when lined up for landing. From the wiki:

Too fast to catch for the escorting Allied fighters, the Me 262s were almost impossible to head off. [Note 7] As a result, Me 262 pilots were relatively safe from the Allied fighters, as long as they did not allow themselves to get drawn into low-speed turning contests and saved their maneuvering for higher speeds.

Turboprops would not have resulted in such an advantage, since the propeller is the limiting factor for airspeed. The lighter weight of a turbine would have been beneficial: the Merlin weighed 600 kg, and for instance the PW123 turboprop delivers 2,380 hp and weighs 450 kg - but only beneficial in the lower speed turning performance which the jet fighters learned to avoid.

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