National Launch System

[1] Shortly thereafter, NASA asked Lockheed Missiles and Space, McDonnell Douglas, and TRW to perform a ten-month study.

A NASA history from 1998 says that reusable single-stage-to-orbit (SSTO) rockets and space planes such as the McDonnell Douglas DC-X and the Lockheed Martin X-33 seemed attainable and represented smaller, simpler alternatives to the sprawling Shuttle program.

[citation needed] In 1994, the United States Air Force Evolved Expendable Launch Vehicle (EELV) program led to the development of the Delta IV.

NLS research on the STME, a simpler SSME, served as a starting point for the greatly simplified RS-68 that powered the Delta IV EELV rocket.

The similarities include a lengthened Shuttle external tank-like core stage, four engines meant as expendable versions of the SSME, and large solid rocket boosters (with five segments instead of four).

The NLS launch family would have shared a common liquid-fuel engine.
Proposed NLS family of launch vehicles.
A National Launch System engine being test-fired at NASA's Stennis Space Center