engine reliability

[curmudgeon]

While looking for info on the RR Derwent engine via Google I came across this letter from the UK CAA: http://www.caa.co.uk/docs/536/LTO2420.pdf
It looks like time between inspections for Derwents ran to many 100s of hours and some people have probably been pushing well beyond that.

Have looked inside one of these things now. Very cunning design to keep most metal parts bathed in new cold air while the flame ran down the middle of an airstream. No wonder in the early days centrifugal engines were preferred to axial flow.

[Wuzak]

Having just been looking at the Whittle engines, I realised for the first time that they were of the centrifugal compressor/axial turbine layout.

The early Whittle engines made by Rover and then RR (Wellend) used reverse flow combustors - the air from the compressor moved forwards before turning and operating on the turbine.

The Halford H1 was also a centrifugal compressor/axial flow turbine design, IIRC. It had straight through combustors. As did the Derwent.

I should think that the axial compressor/axial turbine types would have had a similar arrangement, where the flame is central in teh chamber, and it is surrounded by cooler air.

Tony Buttler mentions in his book Secret British Projects 1935-1950 that the British had developed much better heat resistant metals for use in the turbine blades than had the Germans. It is most likely there where the gains in reliability were made.

The other disadvantage of axial flow turbojets is that they require several stages of compressor to gain the necessary compression ratio.

This meant that there were a lot of compressor blades whirring around at great speed. Any imperfections in their manufacture could have disaterous effects.

[ChrisMcD]

I have a feeling that the German's problem was the lack of heat resistant metals.

What the Brits did was to adapt techniques used for dentistry!

Apparently the corrosion resistant alloys (ie Nimonics) that could be cast with very precise accuracy into dentures, were also sufficiently heat resistant to be made into turbine blades.

http://www.foundrytradejournal.com/features_ftj/news_item.asp?NewsID=121

amazing but true!

[Wuzak]

Thanks Chris. An interesting article.

[Wuzak]

http://content.answers.com/main/content/wp/en/3/32/Jumo004.jpg

A small cut-away of a Jumo 004. It shows the combustors having bypass air around them.


More insight into German turbojet problems on the Stormbirds site:

http://www.stormbirds.com/project/technical/technical_3.htm