Funds & goals
X-33 one BILLION dollars. Delta Clipper around 60 million dollars. One never flew, the other flew. Obviously VERY different goals but the X-33's entire approach violated most aerospace rules of thumb. Too many expensive, experimental "advances" in one craft. We still couldn't build a STSO today. TSTO and other schemes are more achieveable.
Besides the Clipper org was not PC (BMDO) and had powerfull beaurecratic enemies.
