The Gold Eater (The Gold Chronicles) ~ by: D. Girard Watson

After their victory against the Spanish, David, Harriet and their crew are sent on a peace mission to Sumatra, a planet that is the home of mysterious humans, who may hold the secret to gold-powered space travel. David and Harriet attempt to survive imprisonment, sickness, and war while unearthing a secret that could impact the future of Earth in unimaginable ways.