Grumman H-33 Shuttle Orbiter concept

Some drawings of the H-33, a Shuttle design from when the Shuttle was meant to be a two-stage vehicle with a manned flyback first stage, and the orbiter itself would have considerable propellant loading itself. This would have led to a much larger orbiter. While this would have had the downside of a much larger underside requiring thermal protection tiles, it also would have meant a lower density orbiter, which would have a lower thermal loading per unit area. Either lighter tiles or perhaps metallic “shingles” would have been used.