I've decided to make a list of propulsion systems that were either shown in trailers, gameplay footage or were talked about by the devs along with corresponding sources.
Those are the new propulsion systems we can expect to see in the game:
Nuclear pulse drive (Project Orion) shown in the trailer
Inertial confinement fusion drive (Project Daedalus) shown in the trailer
Metallic hydrogen rockets mentioned by Nate Simpson in an interview with IGN
Magnetic nozzles s
I think you're referring to this, from 2017:
https://www.sciencealert.com/the-world-s-only-metallic-hydrogen-sample-has-disappeared
Whereas I am referring to this, from June 2019:
https://www.sciencealert.com/french-scientists-believe-they-have-created-metallic-hydrogen
https://arxiv.org/ftp/arxiv/papers/1906/1906.05634.pdf
In which it states:
Note how all their observations that they used to convince people that the hydrogen had in fact become metallic reversed w