A high-velocity impact-ignition testing system was used to study the dynamic response of brittle thermite projectiles impacting an inert steel target at velocities of 850 and 1200 m/s. The projectiles included consolidated aluminum and bismuth trioxide that were launched by a propellant driven gun into a catch chamber equipped with high-speed imaging diagnostics. The projectiles passed through a break-screen at the entrance to the chamber and either fragmented upon penetrating the break-screen or remained intact prior to impacting the steel target. In all cases, the projectiles pulverized upon impact, and a reacting debris cloud spreads through the catch chamber. At lower impact velocities, the fragmented and intact projectiles produced similar flame spreading rates of 217-255 m/s. At higher impact velocities, the intact projectile produced the slowest average flame spreading rate of 179 m/s because debris rebounding was limited by the length of the projectile and the resulting debris field was highly consolidated in the radial direction. In contrast, the fragmented projectile rebounded into a well dispersed debris cloud with the highest, 353 m/s, flame spreading rate. A kinetic energy flux threshold was proposed as a means for describing the shift in observed debris dispersion and flame spreading rates. A reactivity model was developed based on particle burn times using a computational fluid dynamics code that incorporated heat transfer and particle combustion in a multiphase environment to understand how the particle size influenced flame spreading. Results from the model show a trade-off between faster reactivity and increased drag inhibiting movement for smaller particle debris.