  Two recent test failures of Hypersonic Technology Vehicle 2 impose a strike
to the increasingly growing enthusiasm, not only on the United States side. It
is important to find out the exact failure reason, otherwise a solution is
impossible. In this Note, we propose a potential failure reason from the
perspective of lateral stability analysis. We argue that the time variant
pressure fluctuations, which are normally omitted in classical aircraft
dynamics analysis, could not be neglected in dynamic analysis of hypersonic
vehicles. To demonstrate the idea, a hypersonic model is imagined in this work
and its aerodynamic parameters are estimated using fundamental fluid
principles. Pressure fluctuations are thereafter estimated by an empirical
formula. A lateral dynamic equation is set up, taking those time variant
fluctuations into account. The resulted equation is a Mathieu differential
equation. Numerical solutions of this equation show that the inclusion of
fluctuation terms generates more complicated dynamics and should be considered
in flight controller design.
