After building a ground-up simulation of the vehicle's behavior under thrust, I re-tuned the stability algorithm to the inertia I calculated from the results of a bifilar pendulum test. The calculation was incorrect, due to an extra set of parentheses used, and the algorithm was then tuned to the incorrect vehicle inertia. The calculated inertia was much smaller than it should have been, so the simulation predicted a vehicle requiring less control authority, hence the loose oscillation during the test. The error was found soon after. For more info: https://twitter.com/joebarnard http://www.bps.space