When playing around with the NASA trajectory browser, I noticed that practically every mission to the Jupiter system followed the mold (order of magnitude of $\Delta V$):
- Start in Earth Orbit (of course), into Orbit around the Sun (5 km/s)
- Deep-Space Maneuver (DSM) to change course for an Earth flyby (500 m/s)
- Earth flyby with further change in velocity (100 m/s)
Now, I understand that acceleration is more efficient when done in a gravity well, and that changes in direction require less fuel when the craft is moving fast - as is the case during the flyby.
But I can't figure out why the $\Delta V$ applied in the flyby can't be applied the same way with the first acceleration, as the position in the gravity well and the speed relative to Earth will be almost the same.
So, why Earth flybys?