Molykote From United States of America, joined Aug 2005, 1343 posts, RR: 15
Reply 1, posted (8 years 8 months 2 weeks 6 days 1 hour ago) and read 2242 times:
Quoting Poitin (Thread starter): Since I keep on hearing that the new generation of narrow bodies is waiting on the development of new engines, are these them?
My opinion: "I don't know."
RR themselves held the stance (even recently) that the 2 shaft configuration was optimal for engines up to 25,000-35,000 lb thrust while noting the following characteristics of a 3 shaft vs 2 shaft arrangement.
- Blade/vane (relative) velocities are more closely optimized for flow conditions at each point. (Editorial: this is pretty obvious and has been touted since the advent of the 3 spool)
- Three shafts add mechanical complexity but reduce reliance on variable geometry features.
- "The main benefit is that higher thrust can be developed from a shorter, lighter engine than an equivalently rated 2 shaft layout."
("The Jet Engine" - 2005)
Obviously RR is now open to the possibility that a 3 shaft design may now be appropriate.
Given that a new engine (regardless of manufacturer) will be required for 737/narrowbus replacement, I wouldn't expect a clean sheet GE design to give up much (if anything) to a clean sheet RR design. With the high cycle accumulation likely for this airframe/engine type I'd be reluctant to assume that a (presumably) more expensive, complicated engine is a better option.
All else equal, higher cycles will mean more wear and tear "per hour" and thus shorter overhaul intervals for an engine under this service schedule. A more complicated engine requiring more frequent overhauls would of course require a meaningful fuel burn savings to justify its business case (also assuming a higher purchase cost due to the higher complexity). Much of this cost depends on how well RR would strike a balance between robust build and light weight (i.e. the 757-200 with RB211s will see engines on wing for 30,000hrs+ but is underworked compared to other installations - The flipside here is the boner-inducing performance that we all know and love!).
The geared turbofan is a cool concept but I can't guess at when one may see service. The science/physics behind the differences between a GTF and a normal turbofan are thousands of years old but the engineering required to make the GTF a slam dunk (reliability and clean integration) are unknown to me.
I'm a propulsion engineer but certainly am not claiming knowledge at a higher level than the published works of any OEM. The next generation engines in this class (given the time frame involved) are going to be a large advance over the current engines that guessing is quite a tall order for those not in the know at an OEM.
All RR really says in the article is that if fuel burn criteria X is greater than unit cost criteria Y then criteria X will be a larger factor in driving design decisions. I'm personally not so sure we will see a 3 shaft engine in this class in the next generation of narrowbody (737/A320) powerplants but I'll be excited to eat my words if it happens.