Don't think that only the Rolls large fan engines are triple-shaft .....

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Doubtless because the Rolls was designed later. Both current Pratt and GE engines are nothing more than refined and extended versions of the engines first designed in the early 1960s for the C-5 Galaxy. Rolls were a few years after this putting the RB211 design together, in which time technology had advanced notably.

There can't be a fundamental difference because Pratts and GE never redesigned for triples, and Rolls never redesigned to go back to twins.

In the days of steamships 100 years ago there were compound steam engines which had either two, or three, or four stages of expansion of the steam. None triumphed over the other, but then the steam turbine, and then the diesel, came along and swept it all away.

Like they are trying to do with the geared fan of today :)

Going back a bit further in history -

P&W's first production axial-flow engine, the J57, had two spools to permit matching the pumping of the front compressor stages to the rear. A single-spool fixed-geometry compressor likes to run over a narrow speed range, and two spools (w/ interstage bleed valves) was P&W's solution to the problem. The civil version was the JT3C; later a smallish front fan was added in the JT3D (a.k.a. TF33).

A bit later GE introduced the variable stator (variable geometry) system on the single-spool J79. And it didn't require bleed valves. Being a quite a bit lighter and capable of Mach 2 inlet temperatures, it competed readily with the J57 in the military market; but its civil CJ805 model saw few customers and its Convair jetliners didn't fare so well in the market.

As both companies developed their early high-bypass fans, their compressor technologies began to grow more alike. P&W added some variable stators on the JT9D, and GE found variable bypass (or bleed) valves necessary on the CF6-50.

When our company flew 744's out of SYD we often where just behind the Qantas 744 with the RR engines. (we are PW-4056 at 56,750 lbs T/O thrust). We would always outclimb them, and yes we knew their pax load, which was always around the same as ours. I always felt guilty holding them down and wonder what the advantages of a better SFC is if you are disadvantaged in the race for levels. I must admit the Rollers are very nicely cowled, look good, very British, built like a concrete outhouse.

RB211-524 as fitted to B747.
 

From RR Maintenance Data handbook: The QEC weight for the -524 is 13,636 lbs (6187 Kgs). The basic engine in Testable Standard is 11,000 lbs (5000 Kgs). I know that CX contended that the triple spool engine held onto it's SFC longer in service than comparable thrust two spool engines. They also pioneered some ultra longhaul sectors with -524D4 200 series aircraft and never had the need to invest in SP's. The fact is all modern engines are a far cry from anything which previously was available, well over 20,000 hours on wing being not a rare feat.

From a Ground Engineers point of view three spool every time.

Just ask anybody has had to grapple with the TCCS (Turbine Case Cooling System) defects, a very primative system!!

tristar 500

What is the reason for the three spool affecting the TCCS:confused:

Hi everyone!

Wow i posted my question a couple of days ago and it has generated quite a few responses. Really appreciate all your answers guys! :)
Thanks to those that answered I have a bit more knowledge and understanding now!

regards,
Andrew