The Skylark of Space (ePub)

The Skylark of Space by E.E. Smith was written between 1915 and 1921 while Smith was working on his doctorate. 

At the beginning of the story, Seaton accidentally discovers a workable space drive in combining pure copper with a newly discovered element "X" (suggested to be a stable transactinide element in the platinum group) in solution. Having failed to re-create the effect, Seaton realizes that the missing component is a field generated by DuQuesne's particle accelerator, and thereafter sets up a business with his millionaire friend, Martin Crane, to build a spaceship.

DuQuesne conspires to sabotage Seaton's spaceship and build his own from Seaton's plans, which he uses to kidnap Seaton's fiancée, Dorothy Vaneman, to exchange for the "X". In the resulting fight, DuQuesne's ship is accidentally set to full acceleration on an uncontrolled trajectory, until the copper 'power bar' is exhausted at a vast distance from Earth's solar system. Using an "Object Compass" that once locked on an object, always points toward that object, Seaton and Crane follow DuQuesne in their own spaceship (the eponymous Skylark) to rescue Dorothy and her fellow-hostage, Margaret "Peg" Spencer, until the Skylark discovers DuQuesne's ship derelict in orbit around a massive dead star (resembling a cold neutron star).

Having obtained the hostages, Seaton extracts a promise from DuQuesne to "act as one of the party until they get back to Earth", in which relationship they leave orbit and travel further in search of additional fuel. On an Earthlike exoplanet, they obtain "X" from an outcrop almost purely of that mineral; then leave that planet in search of copper.