DIRECT's approach had a resemblance to that of the 1991 National Launch System effort. Proposed jointly by NASA and the Department of Defense as an alternative to the Titan IV, the design was based on the same solid rocket boosters and modified external tank, but instead of the reusable Space Shuttle main engine, specified four of the proposed disposable, less expensive Space Transportation Main Engines. The United States Congress did not appropriate funding for the development. A great deal of reference material exists in the public domain regarding NLS.