What aspect of the project are you referring to when you say "someone who knows why". I would imagine everyone on the team should know the "why".

How about, a Sr. Software Engineer is someone who can work on, whatever technical problem your team is trying to solve, with little to no supervision. They can also be expected to occasionally provide technical direction/assistance to others on their team.

Within that title you can always set grades to allow for progression in salary and experience. Sr. Software Engineer II or III and so on.