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Recently NASA shut down its Space Shuttle program. Between the first launch on April 12, 1981, and the final landing on July 21, 2011, NASA's space shuttle fleet --Columbia, Challenger, Discovery, Atlantis and Endeavour -- flew 135 missions, helped construct the International Space Station and inspired generations. What made NASA do this ?
Does NASA have plans for an even bigger reusable spacecraft?
This is an old question, but nobody has properly answered it.
The ultimate answer is the Columbia disaster. This disaster demonstrated that the growing expense of, and inherent risks in, the Shuttle program precluded long term use of the Shuttle. From chapter 9, page 210 of the Report of Columbia Accident Investigation Board (emphasis theirs):
Even so, based on its in-depth examination of the Space Shuttle Program, the Board has reached an inescapable conclusion: Because of the risks inherent in the original design of the Space Shuttle, because that design was based in many aspects on now-obsolete technologies, and because the Shuttle is now an aging system but still developmental in character, it is in the nationʼs interest to replace the Shuttle as soon as possible as the primary means for transporting humans to and from Earth orbit.
The decision to retire the Shuttle came shortly after the CAIB made their report. In early 2004 when President George W. Bush announced of his Vision for Space Exploration where he said (emphasis mine):
To meet this goal, we will return the Space Shuttle to flight as soon as possible, consistent with safety concerns and the recommendations of the Columbia Accident Investigation Board. The Shuttle's chief purpose over the next several years will be to help finish assembly of the International Space Station. In 2010, the Space Shuttle -- after nearly 30 years of duty -- will be retired from service.
NASA immediately began following this mandate. Starting in 2004, NASA began the long process of continuing Shuttle program operations (after returning to flight) to complete the construction of the International Space Station, and then retire the Shuttle in 2010. (Ultimately that retirement would occur 2011 rather than 2010. President Obama added two additional flights to the original manifest.)
Part of the process begun in 2004 was a decision to make various lifetime buys of parts that needed to be replaced on every Shuttle flight. They knew exactly how many more flights there would be needed. Add parts for a couple of contingency flights, and they knew exactly how much to buy. Many of those parts were one of a kind items. There were specialty bolts and connectors of non-standard dimensions and made of exotic alloys. There were vintage 1970s era pieces of electronics. Many of these were made by mom and pop fabricators. They stayed in business primarily because they were doing something good for the country. When they fulfilled those lifetime buy purchases, many of those mom and pop fabricators simply went out of business. They retired with the Shuttle.
This process was largely complete in 2008. By 2009, the decision to terminate the Shuttle program was irrevocable. The logistics chain was gone. For more on this, see Wayne Hale's NASA Blog: Shutting Down the Shuttle.
The cost of running the space shuttle never met the predictions, and was more expensive (Largely due to the standing army of folk it took to prepare and manage it).
Then after the Columbia broke up on re-entry the lack of options for aborts, bailouts, etc, and the inherent danger in the foam on the main tank hitting the wings the notion slowly morphed into the idea of replacing it.
Initially the Constellation program was suggested as Shuttle SRB as the first stage for Ares-1, and then a heavy lift Ares-V with two SRB's and a stretched shuttle main tank, but with the payload on top, instead of on the side.
The thinking was, SRB's were at this point well charecterized and mostly thought to be understood. The equipment to make the main tank and stretch it was not a big deal.
Retiring the complex shuttle component was thought to make it cheaper to run, since a capsule would be much simpler than the shuttle orbiter.
The DIRECT guys, ex-NASA folk were pushing a similar plan, but focussing on commonality and minimal changes to keep costs down.
However the shuttle program had turned out to be less about space access and more about a jobs program. Tens of thousands are employed in the shuttle program in many states and thus there is a strong desire to keep the jobs going. Alas, that was the truly expensive part of the program.
Thus we got Constellation a compromise that started looking like the DIRECT plan, and slowly changed to be less and less of a good deal.
Then it was cancelled and replaced with the Space Launch System, which sadly is probably a worse approach. (I wish it were not so). The focus was changed as well to invest in and then hire commercial carriers (Like SpaceX (crewed Dragon), Boeing (CST-100), SNC (Dream Chaser)) for operations just to the space station. NASA would then focus on the 'hard' missions, since it was becoming clear that commercial options might actually succeed.
Thus the Orion capsule (remained mostly intact from Constellation) is designed for the radiation environment outside LEO, and for longer term missions than any of the other commercial providers. Alas, to do so it is too heavy to launch on pretty much anything other than Delta-4 Heavy (and maybe one day a Falcon Heavy? Who knows!). So the single SRB approach was tossed out, and the SLS booster, which needed a stretched SRB for additional thrust was required (Constellation needed them as well, but DIRECT tried very hard to avoid needing modifications).
Alas, the SLS appears to be as much of a jobs program, and is looking less and less likely to fly seriously. Current plans are a launch of the Orion capsule on a Delta-4 Heavy in 2015 (This did happen, Dec 2014), and then the next launch on an actual SLS is 2018 (Which has since been delayed till at least 2022). The next flight is possibly 4 years after that, which is of course, completely untenable.
Time will tell, and the success of SpaceX with a Falcon Heavy will have a huge effect upon the program. If Falcon Heavy launches, and is available at the cost currently advertised it will be very hard to maintain the need for SLS at it Shuttle sized cost.
The simple answer is that between operating shuttle and station, there were no funds left in the human spaceflight budget to develop anything new to go somewhere else or do something different, assuming a flat budget. By all indications, a flat budget is an extremely good assumption.
They had to pick one to kill, and dropped the one that had been operational for a few decades, as opposed to the one that just became operational.
Now they have money to do new and different things. The saga of what exactly those new and different things should be is a whole 'nother story.
