We use FORTRAN to develop missile models because the language is fast, has low overhead and provides the percision that is needed for flight calculation. It is legacy code that incorporates differential equations to do calculations and so having low overhead is beneficial to speed. If it runs too slow or doesn't provide enough precision we can't use it. Even recurrsive functions cannot be used if it doesn't meet real time timing constraints. FORTRAN is fast and easier than assembler.

For new clients I always get half up front. It does two things, 1. insures the clients interest in the project 2. insures you get something for your work if the client bails. I usually require NDA from the client so they don't drop me for someone cheap once we're making progress.