The Delta Clipper, or DC-X, was developed by McDonnell Douglas for the Ballistic Missile Defense Organization (BMDO) Single Stage Rocket Technology Program in the early 1990s to prove the practicality, reliability, operability and cost efficiency of a reusable, single-stage-to-orbit flight vehicle. The DC-XA evolved from the DC-X. The DC-X's airframe was extensively modified by replacing existing systems with a composite hydrogen tank; a Russian-built aluminum-lithium alloy liquid oxygen tank; a composite intertank to connect the hydrogen and oxygen tanks; and an auxiliary propulsion system which included a composite liquid hydrogen feedline, a composite liquid hydrogen valve, a liquid-to-gas conversion system reaction control system, and a Russian auxiliary power unit providing redundant hydraulic power for flight control.