  Assessing the desirability of hypersonic glide vehicles (HGVs) requires
understanding what capabilities they may provide compared to alternative
systems that could carry out the same missions, particularly given the
technical difficulties and additional costs of developing hypersonic gliders
compared to more established technologies. Part 1 of this paper considers the
primary motivations given for developing HGVs and summarizes the current
development programs of the United States, Russia, and China. It then analyzes
the vulnerability of hypersonic boost-glide vehicles (BGVs) to missile defenses
and finds that evading the most capable current endo-atmospheric defenses
requires that BGVs maintain speeds significantly higher than Mach 5 throughout
their glide phase. Achieving these high speeds has important implications for
the mass and range of these weapons, which in turn affect what delivery
vehicles can carry them and how their performance compares to other means of
delivery, as is discussed in Part 2.
