[simne]

Problem is another. To got additional speed from other body, you need to move very close to it and in perigee use some powerful acceleration to increase rotation speed fast, so could not use slow acceleration engines, like ion engines.

Idea of gravity-assist acceleration, mechanically is just rotation of pair tightly tied bodies (and cut tie in right moment, so one body got acceleration and other got deceleration), but as it is impractical to tie for example to Moon with rope, used gravity force.

What also interesting, gravity-assist could use not only orbital speed of large body, but also got some acceleration from rotation of large body, as for gravitation, large planet is not just one material point, but system of few smaller (sub)bodies, and closer (sub)bodies give more acceleration than others.