In July 1985, Rolls-Royce's technical director Gordon Lewis stated that the firm sought the involvement of the Royal Aircraft Establishment's (RAE) propulsion group, and that Rolls-Royce was not prepared to invest its own funds into engine development for HOTOL. By the second half of 1985, work had commenced on the two-year concept-of-proof study. Early on, there was considerable pressure to demonstrate the project's feasibility and credibility in advance of final decisions being taken by the ESA on the Hermes and what would become the Ariane 5 launch system, thus the work concentrated on the validation of critical technologies involved.