early jet engines

[63tb]

I told you it was a slow day

I've seen pictures of WWII German and British jet engines. The German engines appear tube shaped. British engines have barrel shaped chambers around them.  Is one design better than the other? Which type is used today?

63tb

[MiloMorai]

http://tanks45.tripod.com/Jets45/Jets45-Engines.htm

[HoHun]

Hi 63tb,

>I've seen pictures of WWII German and British jet engines. The German engines appear tube shaped. British engines have barrel shaped chambers around them.  Is one design better than the other? Which type is used today?

The British jets were mostly of the radial flow (centrifugal) compressor type while the German jets were of the axial flow compressor type.

Axial flow jets have been dominant since the 1950s, I think.

However, in the early days, centrifugal compressors had the benefit of being well-known technology, so it was easier to optimize their design. The axial-flow compressors of the Jumo 004B and the BMW003A were not very efficient, but that couldn't be attributed to being axial-flow in principle because the later BMW003C and D compressors were much better.

Apparently, the lack of high-temperature resistant materials on part of the Germans didn't affect the engine performance as the engineers had found ways of working around the limitations (at a cost in development time and engine lifespan, of course).

The best feature of the Rolls-Royce Welland engine compared to the Jumo 004B was its superior turbine efficiency (87% vs. 79%), which seems to have been due to a more advanced design.

The German engines on the other hand had been put into series production at a very early stage in their development, so their lack of efficiency was the inevitable price for getting jet aircraft into action in 1944.

Compared to the Welland (as only WW2 jet engine the Allies produced in numbers), the Jumo 004B had a ca. 30% higher specific fuel consumption and a 55% higher specific weight. Due to its smaller cross-section, it had 2.6 times the thrust-per-frontal-area, though, which illustrates why the German aircraft industry with its focus on high-speed flight had chosen the axial layout.

So in summary, one could say that the German jet engines were relatively crude, thirsty and heavy, but this was the result of a deliberate decision to get them into combat-worthy jet aircraft as early as possible. In the comparison between axial and radial flow designs, neither could secure a decisive advantage in WW2.

(Summarized from von Gersdorff et al., based on an 1950 analysis by Robert Schlaifer.)

Regards,

Henning (HoHun)

[Kurfürst]

Excellent summary, HoHun !

[63tb]

Wow, you guys are amazing!

Thanks

63tb

[MiloMorai]

Originally posted by HoHun
Compared to the Welland (as only WW2 jet engine the Allies produced in numbers), the Jumo 004B had a ca. 30% higher specific fuel consumption and a 55% higher specific weight. Due to its smaller cross-section, it had 2.6 times the thrust-per-frontal-area, though, which illustrates why the German aircraft industry with its focus on high-speed flight had chosen the axial layout.
 

The Welland, W2B, only powered 20 Meteor F1s and the first 15 Meteor F-3s. After that the Meteor F-3 was powered by the  Derwent B.37.

From a Bill Gunston book on aero engines

"The best TBO for the Jumo 004B was 30hrs, whereas the Welland and Derwent passed type test at 500hrs and had a service TBO of 150hrs."

[Kurfürst]

Keep in mind the Jumo 004B was 'spared' of expensive alloys, as it was a combat engine. 30 hours of TBO - in reality, it was 50 hours as per the 004B manual - was perfectly enough for combat aircraft of limited lifespan and did not strain strategic resources. The complete engine could be replaced in about 15 mins anyway, provided the plane wasn`t yet shot down after flying some 30-40 combat missions - unlikely.

The Jumo004A was the development version, and as such used high alloys freely - as a result, TBO time was 200 hours.

The LW could choose between producing a few expansive engines and basically no combat service with a few aircraft, or introduce a cheaper version of the same engine in large numbers - over 8000 produced - which could have some effect on the air war and see combat early and in numbers.

The Meteor had it`s share of problems as well, and in any case, it`s 'participation' in WW2 was limited to a few eventless patrols by a handful of aircraft.

[Kev367th]

Axial vs centrifugal compressor -

Even today it depends on what the eventual use is.

Some even use both an LP (low pressure) axial compressor and an HP (high pressure) centrifugal compressor.

Eg. Gem engine used in a Lynx heli has both, 4 axial stages and a single centrifugal stage.

Each stage in an axial consists of a set rotating blades and a set of fixed blades. The fixed blades guide the air so it strikes the rotating blades at the optimum angle on the next stage.

Centrifugal compressors are generally more rugged, easier to manufacture and more useful when size is an issue. One centrifugal stage can compress the air more efficiently than a number of axial stages. Basically you end up with a short, fat engine.

Axial engines use a longer thinner design (aerodynamically and space wise more preferable in a combat aircraft ) with multiple stages to compress the air.

Pure centrifugal based engines are rare now, they tend to be either a hybrid or pure axial.

[MiloMorai]

Originally posted by Kurfürst
Keep in mind the Jumo 004B was 'spared' of expensive alloys, as it was a combat engine. 30 hours of TBO - in reality, it was 50 hours as per the 004B manual - was perfectly enough for combat aircraft of limited lifespan and did not strain strategic resources. The complete engine could be replaced in about 15 mins anyway, provided the plane wasn`t yet shot down after flying some 30-40 combat missions - unlikely.

Here we go with Barbi believing a manual and not reality. Yup, unlikely 30-40 missions could be completed.

The Jumo004A was the development version, and as such used high alloys freely - as a result, TBO time was 200 hours.

Still less than half of the British engine.

The LW could choose between producing a few expansive engines and basically no combat service with a few aircraft, or introduce a cheaper version of the same engine in large numbers - over 8000 produced - which could have some effect on the air war and see combat early and in numbers.

There was a reason that 8000 had to be produced. They did not last long enough.

The Meteor had it`s share of problems as well, and in any case, it`s 'participation' in WW2 was limited to a few eventless patrols by a handful of aircraft.

There was no need for them to take on the LW as the Allied prop jobs were doing a good job of taking care of the few LW jets.