From what I understand, they are trying to keep their unique type shape count very low relative to the shuttle, which should make tile replacement a lot cheaper. Nonetheless, the tiles seem like the weakest aspect of the whole design. Recall that the early proposals didn't have tiles at all, so the tiles existing now is already and acknowledgement that not everything is going according to the ideal path.
I wonder whether they considered using a metallic heat shield with heat resistant alloys. The cancelled VentureStar space plane was planned to use this approach:
Yeah, but it isn't clear whether they ever considered a metallic heat shield. At some point in the past it was considered an improvement over ceramic tiles, since the VentureStar was planned as a Space Shuttle successor that fixed its shortcomings.
