RJAMZ - I may have been a little hasty saying you can't get 40% lower trip costs in an NMA with more floor Area than an 788. I don't really know.
Is it possible? What exactly is included in the definition of trip costs?
When I say trip costs I consider only fuel and airport fees. They are easy to calculate and isolate. The shorter the flight distance the greater the airport fees become percentage wise. On a short 500nm flight between two busy airports the fees can nearly equal the cost of fuel.
For an aircraft that will be doing short flights there is then a big incentive for the designer to reduce empty at the expense of fuel burn. The A321LR is a good example of this, it is underwinged which is great to keep empty weight and airport fees down. But the stepped climb significantly increases fuel burn on long 3000+nm flights. People do not realise just how bad a stepped climb is for fuel burn. Lucky for Airbus they have only had to compete with 30 year old 757 and 767 designs.
For an aircraft that will be doing long flights there is a big incentive to improve lift to drag ratio with a big wing. Empty weight is not a big issue, you might add 10T to the empty weight with a big wing but reduce trip fuel burn on a long flight by 5T. Total takeoff weight might be heavier with more airport fees but on a long flight the fuel saved exceeds the extra fees.
The 787-8 for example is slightly overwinged the 787-10 is slightly underwinged. The 777X is slightly overwinged but the 777-300ER is slightly underwinged.
I expect the 797 to be slightly underwinged like the 787-10 but not as underwinged like the A321LR. On a 4000nm flight that would see an A321 start its climb at 29,000ft the 797 at that flight length would go straight to 40,000ft. Only when loaded for a 5000nm flight would the 797 need to step climb. This would give the 797 a big and often understimated advantage on a 4000nm flight.
CASM often includes purchase price which requires big assumptions. I personally find it hard to accurately calculate.
I was thinking along the lines of what RR did with the Trent 700 to upgrade it to the 7000. From you're saying you don't believe that is possible on the Trent 500. If RR decides to scale down Trent 1000TEN wouldn't there be a possibility that engine would heavier than its competitors?
The Trent 1000TEN is state of the art and has excellent fuel burn and power to weight ratio. An engine perfectly scaled down to 70% of the thrust would be a perfect choice for the 797. It would be a big investment for RR as every part would have to be brand new and scaled down. Any part retained from the larger engine would start to make it overweight and non optimised.