An endlessly repeated media trope is that NASA’s new launch vehicle program, the Space Launch System (SLS), is some kind of entitlement boondoggle, a gigantic “make-work” project for the agency – a derision summed up through the use of the derogatory phrase, “jobs program.” Mocked as a “rocket to nowhere,” the current SLS program takes a substantial fraction of the agency’s exploration budget, about $2 billion out of the $8.5 billion per year designated for human spaceflight. It is designed to launch massive payloads (initially, about 70 metric tons to low Earth orbit, later up to 120 metric tons) into space, allowing the use of pre-fueled departure stages to send payloads beyond LEO into deep space. In contrast to some misleading promotional slight of hand, the SLS will not “take astronauts to Mars” but it could launch ready-to-assemble pieces for a human Mars mission into space (it would take between 8 and 12 launches of an augmented SLS to get a fully fueled manned Mars vehicle into space and prepared for departure to Mars).
Why does the SLS draw such invective? It is claimed by space advocates that commercial launch can do the same job as SLS at a fraction of the cost. Less often spoken directly, but clearly part of this meme, is that the part of the money NASA currently spends on SLS should instead be given to “New Space” private companies to develop and produce heavy lift launch services at lower cost (predominantly, but not exclusively, to SpaceX for their “Falcon Heavy” launch vehicle), thereby freeing up money that NASA could spend on hardware such as habitats, landers and other peripheral equipment needed for future human deep space missions.
How much of this story is true? Why are we building a rocket whose mission and destination are uncertain? Would it be better to use “privately developed” heavy lift launch vehicles (HLV) for human deep space missions? And why is a heavy lift vehicle desirable anyway? Could we not do the same thing using smaller launch vehicles and space-based propellant depots?
Human missions to deep space destinations require large amounts of mass in LEO because we need to protect and preserve the lives of the crew, take all consumables and fuel for the journey, and carry the equipment and supporting machines needed to carry out the mission objectives. The main benefit of using an HLV is that fewer individual launches are needed to get the same amount of mass into space – with fewer launches of larger payloads, a lower launch cost per unit mass is realized. A few larger pieces are more easily assembled in space than are a multitude of smaller ones. The cost buy down is mitigated somewhat by the assumption of more risk, as the loss of a single HLV will more greatly impact the mission campaign than the loss of a single smaller vehicle. But the benefits of fewer launches overall and less complex on-orbit operations are usually judged to outweigh these drawbacks.
There are also historical reasons for the use of an HLV. The agency’s experience with the Apollo program created the mindset among many mission designers that very heavy lift launch vehicles enable deep space missions. Even the Space Shuttle was sized to Saturn V proportions; although its gross liftoff weight (2000 metric tons) was comparable to Apollo-Saturn (2950 metric tons), it had much lower payload capacity because of the requirement for the 100-ton orbiter to return and be reused. With modifications, the Shuttle stack was able to use launch infrastructure created for the HLV Saturn V. Likewise, the existing Shuttle support systems and infrastructure are now being modified for the forthcoming SLS program.
Complaints against the SLS come from several streams of thought. It is argued that building the SLS is merely a program to make a rocket, but one having no place to go. In fact, this arrangement was exactly the course recommended by the 2009 Augustine committee, who contended that the agency should become “destination agnostic” and focus on a “Flexible Path,” in which systems would be developed first and destinations chosen afterwards. President Obama seemingly embraced that recommendation in 2010 by terminating the Moon as a destination for the American human space program. We were going to visit an asteroid instead (sometime), but finding no suitable targets for human missions, the “Asteroid Redirect Mission” concept was embraced. SLS can certainly support that mission, for what it is worth (which is not much).
Another criticism of SLS is that NASA should not be spending its budget making rockets – especially a new launch vehicle system – and instead should purchase launch services from the “private” sector. Certainly NASA does this already for robotic missions and satellites, so why not let commercially available launch vehicles serve the human program as well?
No existing commercial launch vehicle (nor any anticipated in the near future) has the launch capacity of the SLS. The largest extant commercial LV is the Delta-IV Heavy, which can put a bit less than 30 metric tons into LEO, less than half the capacity of the core SLS. Critics of SLS claim that the advent of SpaceX’s “Falcon Heavy” vehicle will render SLS unnecessary, but that launch vehicle was announced in 2011 and we have yet to see even a structural test article of it. It is stated that this vehicle will be able to put about 53 metric tons into LEO, significantly less than the 70 ton payload of the SLS core. The acceptance of this lower performance by its advocates is predicated on a proclaimed vastly lower cost, but as no Falcon Heavy has yet to fly, we have no idea of what its cost would be. Moreover, there are good reasons to question the technical viability of the Falcon Heavy. Released design details show that it consists of 3 Falcon 9 rockets, strapped together and burning simultaneously. Such a configuration would consist of 27 engines, all of which must burn for the same duration and thrust level. The Soviet Union once had a launch vehicle (the N-1) that had 30 rocket engines; it flew four times, each flight ending in a catastrophic fireball, largely as a result destabilizations following an engine-out condition.
The problem lies not with the rocket that we are building – it lies with the mission that we do not have. SLS did not cause the destinations for human spaceflight to go away – that was a considered and deliberate decision by the current administration. In response, the Congress (who had twice voted their overwhelming support for the goals of lunar return, in two different authorization bills) mandated the construction of SLS, largely because NASA was dragging its feet on doing anything about it. Congress was concerned that an important national resource – the industrial and technical infrastructure (including its human resources) to build and fly HLV rockets – was being lost through neglect and attrition. They asked the agency to come up with a specific design for an HLV system but received no cooperation. So, they consulted external technical experts to derive the specifications of a general purpose HLV and mandated this design in the authorization. Its purpose was to make sure that the vehicle would be built and to assure that our national capability in this area would not be lost.
Congress reasoned that the SLS could be used for any deep space mission a future administration might consider. The reinstatement of the lunar surface as a prime candidate for future missions is clear from the language of the 2010 NASA Authorization, which specified the technical details for SLS, and at the same time, listed a variety of destinations for America’s human spaceflight program. In the preamble to that bill, cislunar space is listed as a destination in 4 of the 7 provisions of that section, while the presence of humans on the lunar surface is mentioned in 3 of those sections. These declarations were ignored by the agency.
A final ridiculous criticism of the SLS program is that it is simply a “jobs program” for NASA. Well, of course it is! Federal programs create jobs for the people who bring them to fruition. The unstated implication of the “jobs program” epithet is that it is useless “make-work,” like a work crew digging a ditch and then re-filling it, all simply to collect a paycheck. But “jobs programs” are now a fundamental part of our national existence (Try Googling the phrase “jobs programs” and see how many hits you get). However, some federal “jobs” aren’t make-work, but instead create or maintain national capabilities that are determined to be important or vital to the security and prosperity of the nation. We supported an elaborate industrial and technical defense establishment for the 50 years following World War II not to “make jobs” but because we believed that such capabilities were critical to the national interest. The SLS program was created because it was feared that once lost, the ability to make and operate a heavy lift launch system might vanish forever. There are always repercussions when you create a power vacuum.
No one but NASA (Saturn, Shuttle) and the Soviets (Energia) have successfully operated an HLV System. Such a system is needed for deep space human missions – at least until we can transition to a space-based transportation system provisioned by lunar resources, rather than one which requires that everything be brought up from the deep gravity well of Earth. The SLS is not perfect – it certainly isn’t the system I would have chosen to build (Shuttle side-mount could have been flying now for a fraction of what we have spent to date on SLS). But it exists and it will provide a capability that we can use to go back to the Moon and to the planets beyond.