Enlarge / United Launch Alliance's final Delta IV Heavy rocket, seen here in December when ground crews rolled it to the launch pad at Cape Canaveral Space Force Station, Florida. (credit: United Launch Alliance) This is the rocket that literally lights itself on fire before it heads to space. It's the world's largest rocket entirely fueled by liquid hydrogen, a propellant that is vexing to handle but rewarding in its efficiency. The Delta IV Heavy was America's most powerful launch vehicle for nearly a decade and has been a cornerstone for the US military's space program for more than 20 years.