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, as well as investigating further reusability. A large focus of these designs were related to developing new shuttle boosters and improvements to the central tank, but also looked to expand NASA's ability to launch deep space missions and build modular space stations. Many of these concepts and studies would shape the concepts and programs of the 2000s such as the Constellation, Orbital Space Plane Program, and Artemis program.
Shuttle-derived vehicles
Shuttle-C
The Heavy Lift Launch Vehicle was a study by NASA to turn the Space Shuttle launch stack into a dedicated uncrewed cargo launcher. The external tank and Space Shuttle Solid Rocket Boosters (SRBs) would be combined with a cargo module that took the place of the shuttle orbiter and included the Space Shuttle Main Engines. A ballistic return pod would be used as the main engine structure and carry 2-4 SSMEs as well as mount the payload/booster stage. It would be recovered via parachutes in the Australian outback or northern Mexico. Small lifting wings were added to allow more accurate landings. Various Heavy Lift Launch Vehicle concepts were investigated between 1984 and 1995 and it would eventually become known as the Shuttle-C, which lacked reusable engines and ballistic return pods.
The Shuttle-C concept would theoretically cut development costs for a heavy launch vehicle by re-using technology developed for the Shuttle program. End-of-life and Space Shuttle hardware would also have been used. One proposal involved converting the Columbia or Enterprise into a single-use cargo launcher.
With the Shuttle-C, it was thought that the lower maintenance and safety requirements for the uncrewed vehicle would allow a higher flight rate. Before the loss of Space Shuttle Challenger, NASA had expected about 14 shuttle flights a year. In the aftermath of the Challenger incident, it became clear that this launch rate was not feasible for a variety of reasons.
The Shuttle-C would also launch an uncrewed lunar lander and propulsion module while a second vehicle would launch the Crew Exploration Vehicle to perform lunar missions. The Shuttle-C would additionally act as the crewed component of the International Lunar Resources Exploration Concept proposed in 1993.
In the early 1990s, NASA engineers planning a crewed mission to Mars included a Shuttle-C design to launch six non reusable 80-ton segments to create two Mars ships in Earth orbit. After President George W. Bush called for the end of the Space Shuttle by 2010, these proposed configurations were put aside.
Magnum
The Magnum was a large Super heavy-lift launch vehicle designed by NASA's Marshall Space Flight Center during the mid-1990s. The Magnum would have been a booster around tall, on the scale of the Saturn V, and was originally designed to carry a human mission to Mars. It was to have used two strap-on side boosters, similar to the Space Shuttle Solid Rocket Boosters (SRBs), but using liquid fuel instead. Some designs had strap-on boosters using wings and jet engines, which would enable them to fly back to the launch area after they were jettisoned in flight. The Magnum was designed to carry around 80 tons of payload into low Earth orbit (LEO).
National Launch System
The National Launch System (or New 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 Missiles and Space, McDonnell Douglas, and TRW to perform a ten-month study.
A series of launch vehicles was proposed, based on the proposed Space Transportation Main Engine (STME) liquid-fuel rocket engine. The STME was to be a simplified, expendable version of the RS-25 engine. The NLS-1 was the largest of the three proposed vehicles and would have used a modified Space Shuttle external tank for its core stage. The tank would have fed liquid oxygen and liquid hydrogen to four STMEs attached to the bottom of the tank. A payload or second stage would have fit atop the core stage, and two detachable Space Shuttle Solid Rocket Boosters would have been mounted on the sides of the core stage as on the Shuttle. Period illustrations suggest that much larger rockets than NLS-1 were contemplated, using multiples of the NLS-1 core stage.
Upgraded boosters
Early studies looked at alternate booster and external tank configurations such as: