"'La force motrice' of Reusable Launcher Development: The Rise and Fall of the SDIO's SSTO Program, From the X-Rocket to the Delta Clipper" Introduction. NASA commissioned me to document the development of the X-33 in March of 1997. The X-33 is an advanced technology demonstrator vehicle intended to flight test technologies deemed critical for eventually building a reusable single-stage-to-orbit rocket transport. Those technologies include a metallic thermal protection system, an aerospike engine, and composite cryogenic hydrogen tanks. As part of the history project, I chose to write about the SDIO's SSTO Program as a predecessor to the X-33, even though...