Overcoming Flight Instability in the Boundary Layer Transition Experiments

The Boundary Layer Transition Experiment (BOLT)-1A, conducted in June 2021, featured a highly instrumented asymmetric payload designed to measure hypersonic boundary-layer transition during flight. Shortly after first-stage separation, the vehicle entered a large-amplitude roll-pitch lock-in condition that persisted through ascent and significantly degraded performance. Subsequent BOLT flights prioritized eliminating this instability. This work demonstrates that an angle-of-attack divergence, triggered when an asymmetric vehicle crosses resonance, serves as the precursor to the roll-pitch lock-in observed during the BOLT-1A flight. Metrics are introduced to quantify the severity of unstable resonance crossing, and a distinction is made between small-amplitude lock-in and large-amplitude lock-in. The latter provides the mechanism for sustained roll-pitch lock-in and explains the behavior seen in BOLT-1A. In contrast to BOLT-1A, the BOLT-1B flight in September 2024 successfully avoided roll-pitch lock-in. The strategy used to ensure a stable resonance crossing on BOLT-1B is described, and its implications for future asymmetric hypersonic flight experiments are discussed.