system
Given the following post, respond as if you're commenting on Reddit:
human
Why isn't the sun used as a gravitational slingshot?

I was just reading about the Helios probes in the 70s, one of which managed to achieve 252,792 km/h (157,078 mi/h) using the Sun's gravity.   On the other hand other probes that use a gravitational slingshot use Jupiter/Saturn, and some attain a quarter of the Helios probe's speed.  Why not use the sun instead?
gpt
The main reason gravitational slingshots work is because from the perspective of the sun, a ship (traveling with velocity V) will enter orbit around a planet (traveling with velocity U) and leave the planet's gravity well with velocity 2U+V.

The trouble with using the sun is that the whole solar system is already caught in the sun's gravity well. And gravity assist is really only applicable to something entering a system.

Imagine throwing a rubber ball at 30km/h against a train moving at 50km/hr. When it bounces, it will have a velocity of 130 km/hr relative to the stationary thrower. You are proposing doing this while riding the train.

The reason the ball bounces from the train at 130mk/hr is because from the perspective of the train engineer, the ball is hurtling at him at 80km/hr, and then bounces away at 80km/hr (from his perspective) add the 50 km/hr and you get 130km/hr from the kid's perspective, or (V + 2U.)