The classical distinction between spacecraft and payload doesn’t fit LISA well, because the LISA spacecraft are not just providing the infrastructure for the instruments. Each LISA spacecraft is part of the instrument itself because it protects the free-falling test masses from disturbances.
The LISA spacecraft must thus be designed and built with the gravitational requirements of the free-falling test masses in mind.
The usual structural and thermal analysis of the spacecraft has therefore been extended to include gravitational effects as well to ensure that the requirements on gravity gradient at the position of the test masses is fully met. In addition, the payload controls the position of the spacecraft during science operations, rendering the spacecraft effectively a part of the instrument.
The importance of the co-design and the co-operation of spacecraft and payload is captured in the term “sciencecraft”.
The core features of the payload have been stable for more than a decade: the interferometric measurement system, the telescope, the gravitational reference sensor. Their design has evolved and reached maturity. Many of the design features and concepts of LISA, e.g., the optical bench, have been demonstrated on LISA Pathfinder (LPF).