However, I am still surprised that it is cheaper to modify what they've got rarther than build new from scratch.
I was told, many years ago, that the reason for keeping the same fuselage and wingplan (although the wings are made of new materials) is to avoid airworthiness certification of a new design within contemporary legislation. ie. in terms of aerodynamics, it is no more than a modification of an existing aircraft. Yes, the politicians and BAe execs will say its a new aircraft with new systems, but its still the same old design/shape and it will be a Nimrod, but this time a later mark.
If it had a new fuselage and new wings it would, by law (so I was told), be a new design and, as such, the longitudinal stability problem caused by the modification would not exist, because an aircraft designer would not have drawn up such a piece of sh....
We reap what we sow...
Thats how I understand it.
Ed Sett