St. Peter's between Bramante and Michelangelo
Raphael Sanzio
The first architect put in charge of the project of St. Peter's was Raphael Sanzio, a good friend and colleague of Bramante. His interpretation of the work was to return to the original plan (with the apsidal terminations on the arms) and to lengthen the east arm so that it became a nave, fronted by a narthex.
This drawing explains to the wide facade that would front the narthex and nave, asenvisioned by Raphael.
Baldassare Peruzzi
Upon the death of Raphael in 1520, Baldassare Peruzzi became architect of St. Peter's.
Peruzzi returned to the centralized plan originally put forth by Bramante, but revised the exterior into three pavilions with alternating pediments, the central one being triangular.
Antonio da Sangallo the Younger
The most extensive revision of Bramante's plan was attempted by Antonio da Sangallo the Younger who proposed a return to Raphael's concept of a Latin cross plan.
The Sangallo plan is in actuality a combination of the Greek cross with an extension that is a separate space forming a transition to the narthex. It is complicated and its functionality is questionable.
The exterior expression of the Sangallo plan is a compilation of many small elements. The size of the individual members of the vocabulary and the stacking of the layers in all the volumes--main body, towers, dome, etc.--is clearly the work of someone unable to handle a building of this scale successfully.
The model reinforces the problems seen in the drawing.
The interior space would probably have been much more successful than the exterior expression.
This elaborate model allows a person to actually "enter" the space--somewhat like Gulliver among the Lilliputians.