DIRECT to Triumph
Pieces of shuttle hardware live again; Space Launch System reuses the barrel sections and domes from the External Tank (larger propellant feed lines will be necessary for the heavy-lift variant.) The main propulsion system comes from the shuttle orbiters, including the Space Shuttle Main Engines. Three will be mounted on a diagonal for the crew-launch variant, and five will be mounted in an "X" for the heavy-lifter. Spare engines from the shuttle will be used on early missions, but Pratt & Whitney Rocketdyne will redesign the nozzle and other components to make the engines cheaper to produce and throw away on future flights. And for initial flights, the rocket will use 5-segment SRB's developed for the original Ares program.
A significant part of the Space Launch System announcement is that it's a declaration of victory for the DIRECT team. Since the end of 2006, DIRECT has been agitating for a return to a shuttle-based system as the Ares rockets underwent a death-spiral of requirements creep and weight growth. It was the first proposal I'm aware of that used a flexible system to mount engines to the core stage, using an appropriate number of engines for the mission. They popularized the concept of using the heavy-lifter, sans upper stage, to launch a manned capsule. By the time of DIRECT 3.0 in May 2009, the team had embraced the use of expendable SSME's to get maximum performance out of the propellants stored in the core stage. Aside from one aspect, the rocket proposed for SLS is DIRECT.
The biggest difference is in the solid rocket boosters. Back in 2006, the DIRECT team wanted to use the stock 4-segment SRB's from the shuttle. The idea made sense back then, when NASA had barely begun their development. But the 5-segment stretched SRB got a significant portion of NASA spending during Project Constellation, to the point where it wouldn't be much more expensive to make the switch now that most of the development costs have been sunk. So SLS is using the 5-segment solids on initial missions, with rocket vendors free to develop alternative boosters for future variants of the rocket. The shuttle tankage at the core of SLS has been stretched to accommodate the attach points for the longer boosters.
Will SLS be built? I'd give that prospect a 50-50 shot. The program will receive a development budget of $3 billion per year for the next several years, fitting into the budget wedge once occupied by the Space Shuttle program. In earlier times that would not have been a problem for the federal government, but these are now times of austerity. And if SpaceX will have their 53-ton launcher flying by 2014-5, it will be hard for NASA to justify their 74-ton launcher by 2017. If Congress does keep SLS alive, the ulterior motive will be providing a stimulus to the tech industry to keep good jobs in Florida, Alabama and other states that figured heavily into the Space Shuttle program.
The tragedy of Project Constellation is that SLS is virtually the same as the original Ares V rocket, specified by the ESAS report in summer and fall 2005. Great sums of money have been spent and little real progress was made in developing the rocket's core or its ground facilities. The death spiral started in early 2006 when NASA adopted RS-68 engines for the core of Ares V. RS-68 was cheaper to produce and throw away than SSME, but it burned fuel less efficiently. Unwilling to sacrifice the monster rocket's performance, NASA widened the tank to 10 meters diameter. This meant a major redesign of the Mobile Launch Pad, the need for new crawler-transporters, and possibly even changes to the crawlerway and hardstand at the launch site to shore it up. The entire rocket was so heavy that NASA needed a sixth RS-68 under the core. And nestled between the plumes of two SRB's, the RS-68's were being exposed to far greater temperatures than their ablative-cooled nozzles could handle. Ares could either switch back to SSME, or pay for a new regeneratively-cooled nozzle on the RS-68. Such was the state of the program before it was euthanized in February 2010.
Did the DIRECT guys do any breakthrough, original engineering analysis to show why their idea was better? No. They simply kept in touch with the engineers within NASA who knew that Ares V was doomed to fail as long as the program continued marching towards unachievable payload masses and without regard for development budgets and schedules. The final SLS design probably owes much more to NASA engineers saying "You know what? Our original Ares V design wasn't so bad, and it could have worked if management didn't start messing with it." But in the spirit of maintaining an independent sanity check, the DIRECT team should be proud of keeping a viable idea alive and within the view of policymakers, while NASA kept chasing an ever-growing Ares V down a rabbit hole.