NASA still wants to build the heavy-lift Space Launch System, and as long as Sen. Richard Shelby is alive, it will. The U.S. space agency needs the Alabama Republican, who is the ranking member of his party on the Senate Appropriations Committee, and he needs the SLS to keep his constituents at the Marshall Space Flight Center happy. So the fairly level funding of $1.3 billion for the big rocket, plus some extra advanced-technology money, in the agency's fiscal 2015 budget request is no surprise. SLS received $1.6 billion in fiscal 2014, with no serious challenges on Capitol Hill despite continued grumbling from other space constituencies—“New Space” and science for starters—that could use that kind of money for their own purposes.

The project is bending serious metal, such as this “confidence dome” friction stir-welded at the Michoud Assembly Facility in New Orleans. In an effort to broaden their own constituent base, managers at Marshall have been seeking other payloads for the SLS, so far with no apparent good news. A U.S. Air Force/National Reconnaissance Office analysis is said to have turned up no requirement for a Saturn V-class launch vehicle except—being the military—the need to sustain the U.S. industrial base for big rockets.

The Science Mission Directorate certainly would be able to use a heavy-lifter for a robotic outer-planet mission, both to avoid time- and money-consuming gravity-assist routes to distant targets and to carry more radioisotope thermoelectric generators for more power to run its instruments. But those would be multibillion-dollar flagship-class missions, and the money is not there. The recent discovery of water over the South Pole of Europa, apparently spewing from the subsurface ocean on the Jovian moon, may add a compelling flyby science objective that could be more affordable and probably would not need a launch vehicle the size of the SLS (AW&ST Feb. 17, p. 37).

Commercial satellites have been growing in both directions—larger and smaller. Crewmembers on the International Space Station have just finished deploying the first “flock” of 28 cubesat-size imaging satellites for San Francisco-based Planet Labs (AW&ST Jan. 20, p. 20), while commercial communications satellites weighing six or seven tons are fairly common. Neither would need a launcher the size of the SLS, since even the largest can be accommodated on existing launch vehicles. If there is a need for heavier satcom birds, they are unlikely to outgrow the 23 tons Elon Musk says he will be able to place in geostationary transfer orbit with his planned Falcon 9 Heavy.

That leaves human spaceflight, which has been the stated reason for developing the SLS since it was conceived as a compromise alternative to turning U.S. access to space over to the private sector entirely. The compromise has evolved into a policy of leaving low Earth orbit (LEO) to commercial carriers like the Falcon 9 flown by Musk's SpaceX, the Orbital Sciences Antares, and the older Atlas V and Delta IV fielded by United Launch Alliance, while using the government rocket to launch the high-risk missions beyond.

A private attempt to send a crew of two on a 2018 Mars flyby came to naught, in part because SpaceX did not want to participate. And while Musk hopes to use the planned Falcon 9 Heavy to take humans to Mars eventually, that vehicle remains a paper rocket for now. SpaceX is focused on fulfilling its commercial cargo contract with NASA, and on clearing the safety hurdles that will allow it to launch NASA's astronauts on the basic Falcon 9 under the agency's commercial crew program. It doesn't seem likely that NASA and its congressional backers will trust human lives anytime soon to a 27-engine vehicle that bears an unfortunate resemblance to the ill-fated Soviet N-1 Moon rocket, which had 30.

By comparison, the engineers working on the “Inspiration Mars” human flyby project for space-tourism pioneer Dennis Tito concluded that they would need an SLS for the mission, even with a slip to a 2021 opportunity that can get to Mars with a gravity assist from Venus (AW&ST March 3, p. 18). Taber MacCallum of Paragon Space Development Corp., which conducted a life-support bench test for Tito, says a deeper dive into the requirements for either Mars launch opportunity revealed a need for the SLS variant with the 105-ton-to-LEO capability, the second variant NASA plans to develop. That intermediate version would need a restartable cryogenic “dual-use” upper stage before the final 130-ton version with the J2X engine.

All three SLS variants are designed to be human-rated, using engines like the RD-25 developed for the space shuttle and the Saturn V vintage J2X. Of course, if the commercial economy begins to grow in LEO, as NASA managers hope it will, there probably will be a need for a big launcher like the SLS to orbit the space factories, hotels and who knows what else the dreamers foresee in the post-ISS era. But for missions beyond LEO, right now it appears the future of SLS will depend on whether we want to send humans out there.