Engine Degradation and 'mature fuel burn'

A bit of a technical one here, but have heard much about how the triple core Trents and RB-211 suffer much less from degradation in performance and fuel burn than the twin core competitors such as the GE-90.

My question is partly how does this happen, and also what kind of percentage degradation is there, and how do the two sets compare over time fuel burn wise ( I understand the GE-90 has a lowere fuel burn initially), obviously this is pretty vague with various models (and for different aircraft) available, but is there some general rule?
Cheers

I will be the first to admit that there is a lot I do not know about the topic, but perhaps I can contribute some information.

Most airlines have some way to capture actual fuel burn on each flight. This data gets fed back into the computer and a performance factor for each individual airplane is continually updated (the computer calculates planned versus actual). As the burn gets higher, due to engine wear, weight of the airplane changing, dirt accumulating, etc. the flight planning system is fed this data and gives a little more fuel to successive flights. Of course, things like an engine change, or an aircraft overhaul/interior modification cause the calculation to be reset. The initial setting is normally from manufacturer supplied data.

The wear of an engine can be monitored within fractions of percentage points. As I say, I am not an expert, but I believe it would be rare for an engine burn to increase more than 5% due to wear alone, but that can be significant on a long haul flight. I must say that I am unaware of an degradation differences due solely to the number of spools.

Keep in mind also that engines on newer airplanes (330/340/777/etc) have extensive on-board computer monitoring capability. The data captured is very detailed, all the critical parameters and then some. This data can be read in real time at the airline headquarters or wherever their engine monitoring is done. It is possible to see an individual engine deteriorating to the point where it would fail. The trick is to run it as long as possible with on-wing maintenance and then change it before it fails. This capability is an important factor in the success of ETOPS.

Multiple stage axial flow compressors and turbines would, in an ideal world, allow every stage to rotate at its own ideal speed however this is not practicable as it would mean that every stage would have to be on its own shaft. The real world answer is to accept either 2 or 3 shafts and that there is a compromise in the efficiency of the stages sharing the same shaft. These inefficiencies can be reduced by having three shafts rather than two (all stages on the same shaft being closer to optimum) at the expense of the extra weight and complexity of having a further shaft. It took RR some time to sort out the triple spool engine as initially they wore out very quickly but those issues have been resolved now. I have flown with the RB-211, Trent and the CFM56 and I’m embarassed to say that out of all of them I actually slightly prefer the CFM56. I’m not dissing the RR’s it’s just that I find the CFM the easiest to live with. In fact I shall spend today in close proximity with 4 of them for the first time since July and frankly I’m quite looking forward to it.