Larry is correct
People in my branch participated in thermal protection development for X-33. As the X-33 design proceeded, the projected payload margin for VentureStar grew less and less as the predicted test flight range for X-33 decreased. When the liquid hydrogen tank blew up there was a significant "negative" payload. The failure of the liquid hydrogen tank was really the straw that broke the camel's back. X-33 effectively discredited the concept of Single Stage To Orbit (SSTO). Nobody is going back there.
Gary