The Gravity Research Group indicated those companies had constructed "rigs" to improve the performance of Thomas Townsend Brown 's gravitators through attempts to develop materials with high dielectric constants (k). Gravity Rand Limited provided a set of guidelines to help management conduct research and nurture creativity. Articles about the gravity propulsion research by the aerospace firms ceased after 1974. None of the companies featured in those publications had filed retractions. The following aerospace firms have been cited in the works published from 1955 through 1974: