# How to calculate gravity with multiple bodies?

I'm looking at building something space-related, and trying to figure out how realistic gravity calculations would work.

The specific question I want to answer is, if I have a sun and an arbitrary number of planets in the system, and I know what the position and mass of each of them is, and I have a spaceship at position P trying to accelerate in direction D, how much thrust would that require if the same acceleration in purely gravitically "flat" space would require 1 unit of thrust?

For the sake of simplicity, assume that the entire solar system exists in a 2-dimensional plane.