Lockheed did research on behalf of NASA on practical configurations for SST. This was pre-competitive and Lockheed did internal competitive research as well, but the cross-breeding was natural. Configurations studied were based on SCAT-4, -15, -16 and -17 for SST with a cruise speed of Mach 3 built with titanium, and a SCAT-17 derivative for a cruise speed of Mach 2.2 built with aluminium. Boeing did the same (or so). Will post later. BTW, SCAT-17 and SCAT-17 Aluminium were posted here by Scott some time ago. I repost them for completeness of view.