I was wondering if this might make sense as commercial aircraft design has not changed significantly (from the outside) in the last 50 years?
You are absolutely right - refitting new engines to older airplanes makes a lot of economical sense. After all, two thirds of fuel economy gains in the last 50 years were due to better engines.
However, some conditions conspire to make this harder than it might look:
- The better efficiency was gained by increasing the bypass ratio of engines, which means that the new ones had more diameter for the same thrust. This makes them hard to fit to airframes designed for the smaller engines, and adding a longer landing gear would be too expensive - then it is indeed better to buy a new airframe.
- Designs with tail-mounted engines would have center-of-gravity problems with newer, heavier engines. Tail-mounting engines was very popular after the Sud Aviation Caravelle pioneered this, but rules out those designs (BAC One-Eleven, Vickers VC-10, Boeing 727, Illyushin 62, DC-9, Tupolev 134 and 154) for refitting with more modern engines.
Add to this that airframes have a limited life time, and fitting new engines makes sense for only a limited number of aircraft.
Yes, this can and does happen.
Two examples that I think are excellent illustrations of the concept are turbine conversions of the Beech 18 and the Douglas DC-3. In these cases the old radial piston engines were replaced with turbo-prop engines. Other examples also exist, including less drastic modifications.
An example of a Beech 18 modification is the Volpar Turbo Beech conversion which used Garrett TPE-331 engines. Information on that conversion here, and an example of the operational use of such modified aircraft here.
It's certainly possible for aircraft to be retrofitted with newer engines. One example is the DC-8 Series 70, which had the older JT3D engines replaced with more efficient CFM56 engines. However, there are many reasons for this process not being more common.
There are many costs associated with changing engines. The engine may need a new nacelle design to fit the specific aircraft. This will probably require a new or at least revised strut design to mount to the wing. The wing may also need to be reinforced, as newer engines tend to be heavier. All of the systems attached to the engine, including electrical, hydraulics, and bleed air may also need changes. Avionics will also need to be updated to work with the new engine type.
Certification also should not be overlooked. Assuming the aircraft can use an existing engine that is already certified, the aircraft will still need to be certified to fly with that engine. This means a lot of paperwork with the regulators, and probably additional testing. However, engines tend to be designed for a specific aircraft type. For example, the CFM LEAP engine will power the A320, 737, and C919. All of these aircraft are similar in size and compete for the same market, yet CFM is doing testing and certification of a different version of the LEAP-1 for each of these aircraft. Each manufacturer chose to design their aircraft for a different engine version, which makes it seem unlikely that a new engine would just happen match the needs of an existing aircraft without extensive modification of the engine.
Aircraft fuselages are designed with a limited life span. As they age, maintenance costs increase as more work is required to repair damage from fatigue and regular wear and tear. Even with new engines, the airframe would still have a limited life span.
As other answers have pointed out, a new airplane design will be more efficient than just putting new engines on an old aircraft. Owners of the older aircraft would have to see a benefit in paying to upgrade their old aircraft, and having those aircraft out of service for an extended period, versus just buying a new plane that will be more efficient and will have many more flight hours left ahead of it. And some company will need to find enough of these customers to justify the large upfront costs listed above in making such an upgrade possible.
While the design of aircraft may not look different to previous generations of aircraft, the technology behind them is vastly different, from lightweight carbon fibres, to far more aerodynamic wings. New design aircraft provide vastly better operating economics than aircraft based off an old design but with newer engines.
Take for example the A350, which Airbus claim delivers a 25% fuel reduction per seat when compared to existing aluminium long-haul aircraft. The A330neo on the other hand, which is a re-engined version of the A330 (but also containing revised wingtips and some other minor aerodynamic enhancements), is only expected to deliver a 14% fuel burn advantage per seat over the current A330.
And this is with a newly built A330neo rather than taking an old A330 and putting new engines on an existing frame.
There are many reasons why putting new engines on older airframes are not a good idea. First off, simply swapping out the engines is not feasible - the chances are the new engines will have different mounting points, and different fuel flow requirements, requiring a complete redesign of the fuel flow system. You would also need to revise the avionics, to take into account revised engine operating parameters. Operating procedures for pilots would also be different between models, leading to either the need for separate pools of pilots per engine type, or if pilots are crossing between models, an increased risk of pilot error leading to an accident.
But the biggest reason why this is not done, is that aircraft fuselages have a limited lifespan. With every flight, you are pressurising the aircraft, then de-pressurising it for landing. This causes metal fatigue to the frame. This alone is the single biggest cause of aircraft being retired. It is required that aircraft undergo a "D-Check" roughly every 6 years. This involves stripping the entire aircraft, and removing all paint from the outside, so the state of the fuselage can be checked for signs of metal fatigue. This is a hugely expensive and time consuming process, and as a result most aircraft only every go through a few of these before they are retired.
So re-fitting an aircraft with newer engines would not be feasible as the air frame would have exceeded its lifespan anyway and would need replacing, so far better to just buy a new build aircraft.