NASA's Space Launch System (SLS) is a Space Shuttle - derived heavy launch vehicle developed for future human space exploration. Launchspace engineers think of it as the Phoenix of cancelled Constellation Program launch concepts. As you will recall, retirement of the Space Shuttle coincided with a replacement family of two launch vehicles, the Ares I and the Ares V.

The Ares I development got as far as a single demonstration launch of the SRB-based first stage with a dummy second stage. This was known as the Ares I-X launched on October 28, 2009. However, the Ares I design was plagued with a number of serious flaws that had been aired by NASA, Launchspace and others.

Design challenges included excessive longitudinal vibrational amplitudes, extreme structural sensitivities to transverse loads during ascent, stability and control complications and significant redesign of the Shuttle's Solid Rocket Boosters (SRBs) to fit the Ares I First Stage mission requirements.

All this and the additional complications of huge cost overruns and the required in-orbit rendezvous with a cargo ship (Ares V) for missions beyond low earth orbit (LEO). In summary, it proved to be an unmitigated engineering and economic disaster.

Mercifully, President Obama cancelled the Constellation Program and signed the NASA Authorization Act of 2010 which transformed the Ares I and Ares V vehicle designs into a single launch vehicle that would be usable for both crew and cargo launches. The embodiment of the single launcher is the SLS.

The early version (Block I) will not carry an upper stage, but is projected to lift up to 70 metric tons to LEO. Block II is intended to include an integrated upper Earth Departure Stage (EDS) with a lifting capacity of at least 130 metric tons to LEO. This is 12 metric tons more than the Saturn V could carry. Thus, if SLS Block II is built it will represent the most capable launch vehicle ever built.

The SLS with the EDS would be capable of lifting astronauts and hardware beyond LEO to other near-Earth destinations such as asteroids, the Moon, Mars and Earth's Lagrangian points. SLS could also support trips to the International Space Station (ISS), but such missions are expected to be fulfilled by commercial space operators, such as Orbital Sciences and SpaceX.

The SLS program does not include the crew quarters. However, NASA is separately developing the Orion Crew and Service Module which will be integrated with the SLS at the launch site. Astronauts will return to earth in a capsule-shaped, four-person crew module. SLS will operate out of NASA's Kennedy Space Center, Florida. The first flight-test of the Block I SLS is scheduled for 2017.

There has been a great of push back from several groups, claiming the SLS and human spaceflight beyond ISS are not needed and cannot be justified. However, as one space industry publication puts it: As long as Sen. Richard Shelby is alive, NASA will build the SLS, because the space agency needs the Alabama Republican, (ranking member of his party on the Senate Appropriations Committee) and Shelby needs the SLS to keep his constituents at the Marshall Space Flight Center (MSFC) happy.

Thus, SLS funding will continue at an annual level of about $1.3 billion, with some added advanced-technology money.

In the meantime, MSFC is pursuing ways to broaden its constituent base, but so far has not announced any good news. To further weaken the SLS case a recent analysis by the Air Force and National Reconnaissance Office (NRO) offered no mission requirements for a Saturn V-class launch vehicle. Given the expected costs of using the SLS and the limited justification for such a capability, it is difficult to be optimistic about its future.