In the 1980s and 1990s, Boeing studies conducted under a NASA contract evaluated 21 HSCT configurations designed for Mach numbers between 2.4 and 10.0. A screening process was used to evaluate the concepts on the basis of risk versus benefit. These analyses showed that speeds above Mach 3.2 would be impractical. Of the 21 configurations, 6 were further developed and analayzed.