Ok Ill start things out.
I think the most important aspect is rocket science (when they are in space) is conserving fuel. Knowing when and where to aim the rocket when it blasts off of Earth. Look at Voyager 2. It didnt visit all 4 gas giants just to study them, but when it gets close to the planets, the small arch in the orbits give them a gravity boost for speed. Otherwise they would have never gotten that far out there in space that quick. An incredible billiard shot indeed.
Same technique was used in Apollo 13 or else they may have never gotten back to Earth. In the 70s when Mariner 10 was launched for a Mercury mission, they aimed it straight for Venus at the right time.