NASA is, the Agency urges in a news release, "ready to move forward with the development of the Space Launch System—an advanced heavy-lift launch vehicle that will provide an entirely new national capability for human exploration beyond Earth's orbit. The messy politics behind the story aren't innovative (the Apollo program was canceled to make way for the Shuttle, and the Shuttle has now been ditched to make way for the SLS, with bitter discussions and budget controversies along for the ride, as ever) but the rocket itself is going to be. Because NASA's next "big stick" binds together 50 years of research and lessons into one 21st century rocket.
NASA's news release notes the rocket will "incorporate technological investments from the Space Shuttle program and the Constellation program in order to take advantage of proven hardware and cutting-edge tooling and manufacturing technology that will significantly reduce development and operations costs." This is an important decision because one of the things that's hardest to recover from a canceled engineering enterprise of this size—the Bush-era Constellation program, the first attempt at trying this mission—is the expertise and physical tools and production line capabilities. These devices and systems take even more innovation than the rockets they put together, and even now the capability is quietly evaporating as staff are laid off at the end of the Shuttle program.
To this end, the SLS will use a liquid hydrogen-oxygen engine system, including the "RS-25D/E from the Space Shuttle program" (its main engine) and the "J-2X engine for the upper stage." It'll also use "solid rocket boosters for the initial development flights" based on the Shuttle's own SRBs, including a 5-segment unit that was developed for the canceled Ares-1 crew rocket and only tested this last week. Later boosters will be re-engineered "based on performance requirements and affordability," but mean that the system has a modular launch capability—saving money and allowing it to expand from its initial launch mass of 70 metric tons to a massive 130 metric tons later in its program.
Those reusable RS-25 engines are directly from the Shuttle, and the design of the SLS involves building a slightly more conventional-looking rocket stack on top of the familiar orange Shuttle fuel tank. But the rest of the innovations have an older heritage. The J2 engine that'll power the upper stage into orbit is a direct evolution of the engine that powered the second stage of the Apollo Moon rocket—the Saturn V. And much about the techniques of flying such a massive rocket into space and recovering its crew-return capsule (in this new SLS design, it's the Orion capsule rescued from the Constellation program) will be re-learned from the Apollo program expertise.
With just six years to pull SLS into shape, NASA and its engineering and science teams have a huge job ahead of them. But it's worth considering one unusual fact: The Apollo program can be considered as a pinnacle of space technology because using 1960s-era systems it successfully landed and returned many astronauts from the surface of the Moon. It was canceled to make way for the Shuttle, arguably the most complex single machine mankind's ever made—and a pinnacle of engineering from computing tech through rocketry to materials science. But NASA was already innovating the Saturn V Apollo rocket before it was canceled. One design from the 1970s, using a relatively simple evolution of the rocket's design, would've been able to haul pretty much the entire core ISS bulk—hundreds of tons of it—into orbit in a single shot...in the early 1980s.