Aliens plan to invade Earth but hero, scientist, and girl friend stop them

I read this before 1992 or so. It was more of a 'B' novel.

Humans make contact with aliens who begin invading. A human hero, I think he's a military officer (US Navy?), gets to a human scientist with a very fast space ship. They head towards the alien home planet which circles Bearnard's Star with aliens chasing them (I think).

Alien and human ships use some sort of field drive (not a reaction drive). Ships of both species travel FTL but the trip takes weeks or months.

The alien world is wild / jungle like. Some human crew members get chomped by the native fauna / flora(?). The discover the aliens have a weakness (which I don't recall). Humans travel home and free the world.