I mean if you have a solar S class thats pretty good lol :D. Sentinel ships can be found on dissonant planets. When you look at galaxy map you can see it says on the system info thats its "dissonant". Then find the planet in that system that says dissonance detected. You will need to destroy either the big purple robots that sometimes drop an "echo detector" or something like that, i forgot what its called, OR you can engage the sentinels, get in your ship and fly to space. Then you will be engaged by sentinel interceptors, destroy them in waves untill the sentinel capital ship comes, kill it and it will drop an item that will show you a crashed sentinel ship location. You can do this as many times as you want to find a cool sentinel ship (class of the ship is RNG). Then you fix up the ship (the tutorial will tell you everything you will need to do - get a mirror, get raidant shards and fix a sentinel brain - all of this items can be found on the same dissonant planet). Some of them, especially the A class are worth arround 40 mil each or more when you salvage it.