An aircraft that can hover before it lands offers considerable advantages in regard to operating site flexibility. Also, during an approach to a hover, there is much more flexibility with regard to track, speed and approach angles compared to the conventional fast jet approach case. The Harrier family has offered these operational advantages for over 30 years. It is what STOVL is all about.
However, the Harrier has never broken free from the constraints of a limited thrust weight ratio. As the thrust has increased over those 30 years so has the aircraft weight. As a result, its ability to land back from the hover with a full range of expensive weapons has been limited effectively to cold temperatures and sea level altitudes.
So for me, any Harrier replacement must break out from this bind. If it does not offer a step change in hover payload capability please don’t waste my taxpayer’s money on it.
Of the two JSF B model prototypes one is “relatively” simple and uses only direct jet lift from its 119 while the other is more mechanically complex and couples a shaft driven fan to essentially the same engine when it joins the circuit.
Somebody will doubtless correct me if I am wrong, but it seems to me that giving up say 6000lb of direct lift to drive a fan that is going to return you some 18000lb of lift is a very good piloting deal in regard to the all important hover capability that the Harrier lacks.
But what about the mechanical complexity involved? Where would the Harrier have been if it had not been simple and thus reliable? Not in service with anybody is probably the answer to that.
But we are 35 years on from the days when the Harrier was designed. My motorcar in those days was powered by a very simple Ford 100E (ah!) 64 BHP side valve engine and the wipers were driven by vacuum because nobody trusted electrics. The car was far from reliable despite its 1000 mile servicing schedule. Today my car is so mechanically and electrically complex that my mind boggles. But it is also totally reliable (no need to even touch wood) and needs negligible servicing despite being turbocharged and producing 220 BHP. It also does more mpg than my 100E and reaches the top speed of that Ford in second gear out of six.
So that is why I would pick the JSF that has the much better hover performance, even if it is more complex than the Harrier.