The big issue with SSTO's is that their lower payload hurts economic viability if they aren't reusable, and reuse is an all-or-nothing endeavour. This runs counter to the incremental development strategy that most of the new space actors are aiming for.
Are there any studies of SSTO's made of more modern, lighter materials like carbon fiber?
The lack of projects is mostly due to the withdrawal of government support for such projects. The Sabre Engine is being slowly funded and that is pretty much it. SSTOs are expensive bleeding edge machines that are nowhere near as technologically mature as TSTO-RLV's. They would take possibly decades and billions to just get to a demonstration system. I can't see private companies jumping at this by themselves.
Quote from: scienceguy on 01/19/2017 08:43 PMAre there any studies of SSTO's made of more modern, lighter materials like carbon fiber?VentureStar and Skylon. VentureStar was cancelled, Skylon is still in development. Both are liable to start "lively" discussions around here.