Prototype never uses the same engines for production.
But F-35 prototypes use the same engine for production... Oh wait, F-35 is no longer in prototype stage; it's already in serial production. Silly me.
But what's with all the problems it has with unfinished software, de-lamination of its composite skin, lack of fire suppression systems, neutered performance envelope, problematic helmet, etc.? If F-35 is in production, shouldn't its unit price be stabilized by now and total program cost be coming down?
Maybe ice bomb has some answers.