The National Launch System was a study authorized in 1991 by President George H. W. Bush to outline alternatives to the Space Shuttle for access to Earth orbit. Shortly thereafter, NASA asked Lockheed
A National Launch System engine being test-fired at NASA's Stennis Space Center
During the lifetime of the Space Shuttle, Rockwell International and many other organizations studied various Space Shuttle designs. These involved different ways of increasing cargo and crew capacity
HLLV
The Ares I launch vehicle would have used a Five-Segment SRB
The Super Guppy's "humpback" design
The Space Shuttle Enterprise unpowered test vehicle would have been the basis for the unpowered orbiter