This is a hidden ending but one that you would not be expecting to be as easy. Whether you are siding with Phineas or working with The Board, you will come to a point where you will go to the Hope during the Kept Secret But Not Forgotten mission.
In this mission, you will have a choice to skip the ship. This will then bring you to a console where you can skip the ship. For this ending, you will need the Dumb stat. Ada will advise you not to do it, but you can convince her easily. This will then allow your ship to sail in the Sun and destroy everyone on it along with itself.