The next change in DIRECT's development was in response to NASA dropping the Space Shuttle Main Engine on the Ares V design due to the high manufacturing cost of the SSME engines and the difficulty in producing the required number of units per year with existing manufacturing facilities. NASA specified five RS-68 engines as the core engines for Ares V. The DIRECT proposal specified that its core should include two RS-68 engines. Additional performance for carrying payloads to Low Earth Orbit would be provided by upgrading the main engines with Regenerative Cooling Nozzles to improve their efficiency.