Here's an interesting article on syntactic metal foams, with a brief mention of RLV applications. As materials continue to improve SSTO becomes more and more achievable. It's still not trivial, but it certainly seems within reach for vehicles that aren't loaded up with pet hobbyhorses like lifting reentry or linear aerospikes.

I notice that this is copywrite 1999. Anyone know anything more recent?

Metal foams made this way aren't really that new. I first heard about them 10-15 years ago.

There are a lot of nifty materials technologies out there that could be applied to the SSTO problem, like gamma-TiAl/SiC metal matrix composites. Indeed, much of the work on MMC's was done during the X-30 NASP program, an HTHL SSTO.

Really though, at some point economics comes into the picture. I think this will preclude the more exotic materials from the first generation of RLV's. This may very well mean going to a TSTO architecture.

But if both stages are fully reusable, and more importantly, have airplane-like operability, it will be more than worth the performance hit of using old-fashioned 2000-series aluminum alloys.

Dunno. Aluminum isn't such a great material in some ways- it melts at a low temperature and it has no minimum stress for metal fatigue. For a reusable, metal fatigue becomes a strong possibility and a potentially big problem.

True, but Al has a very high thermal conductivity so you have more leeway in dealing with thermal issues.

As for fatigue, the fracture mechanics of Al alloys are pretty well understood. After all, aluminum airframes take thousands of pressurization cycles without major crack growth. (Note I didn't say *zero* crack growth :-).