Everything Robert Goddard was completely theoretically possible. And had been understood, theoretically, for a couple of hundred years.
Yeah, these kind of comments, like the well-known claim that Bumble Bee flight “violates the laws of aerodynamics” drive me nuts. The Chinese had rockets as far back as the 7th century, and rockets have been used in warfare for nearly a thousand years. Nobody thought Goddard’s ideas were kooky in the least.