Why the SLS mega-rocket is not reusable like SpaceX

The huge SLS rocket has proven that it works. She successfully lifted off on November 16, sending the Orion spacecraft to the Moon. But unlike a SpaceX Falcon 9, the SLS is not reusable. It is not a coincidence.

She finally took off. “She” is the Space Launch System (SLS) rocket, which has finally made its maiden flight on November 16, 2022 after months of delay. It is with this launch vehicle that humanity will return to the Moon during the decade of 2020. But it will not be with the rocket that has just left Earth: it has actually become useless.

This is one of the distinguishing features of SLS: it was not manufactured on the same model as SpaceX launch vehicles, with a reusable first stage (a feature that can also be found elsewhere, such as the New Glenn launcher from Blue Origin). This means that each SLS is disposable, just like Ariane. Most rockets in the world work this way.

This lack of recovery is noted in this chart from the European Space Agency, published on November 15. It can be seen that the two lateral thrusters are fired two minutes after takeoff, followed by the main stage after eight minutes. At no point is the possibility of recovering them mentioned – the same for the second floor, by the way.

The US space agency says the same thing: when they have burned their propellant […]the boosters are discarded, easing the load for the rest of the space flight “. According to NASA, the boosters fall back into the Atlantic Ocean, 225 km off the coast of Florida, while the main stage ends up in the Pacific Ocean, between Hawaii and California.

Knowing the success SpaceX has experienced with its recovery policy, it sounds inevitable.

After all, when the American company operates the Falcon 9, the main stage automatically returns to Earth and is replenished several times. Ditto with the Falcon Heavy and its two side boosters (which are actually parts of Falcon 9 launchers). SpaceX can thus chain the shots together and lower costs because it does not have to rebuild a rocket from scratch.

All the fuel must be used to send to the Moon

To this question, NASA has provided an explanation on its official website: including such a capacity would logically require that part of the fuel be reserved (or something added) to ensure that the main stage is controlled back to Earth. But SLS uses its power to maximize the cargo the rocket can send to the Moon. »

Adding fuel would affect the total mass of the SLS. Ditto if it was necessary to provide a propulsion system cut for this kind of mission. It would also be necessary to possibly imagine a foot of landing in case you want the stage to be positioned correctly with the vertical. Again, this adds bulk and complexity (and risk of failure).

The main floor (in orange) is not small. It would take a significant amount of fuel for a controlled return to Earth. // Source: NASA/Joel Kowsky (cropped photo)

Another element deserves consideration. It’s one thing to judge NASA’s choice for SLS today with SpaceX’s successes in mind. It is something else to place oneself in the time when the American mega-rocket project was formalized. And since SpaceX had not yet proven the relevance of this model.

The SLS project was formalized in September 2011 in the wake of the Saturn V rocket from the time of the Apollo missions (it was also not reusable). Certainly at that time SpaceX already existed. Several of its rockets had successfully lifted off and put payloads into orbit. It was not until April 2014 that the central stage of a Falcon 9 could be recovered for the first time.

During these two and a half years, the design of the SLS has progressed in parallel, increasingly determining the final design. In addition, the project had already absorbed significant funds. As for SpaceX, it still took a few years to reach the stage where this recovery became a routine operation. SLS was then nearing completion.

At the turn of the year 2010, who could have imagined the extent of the upheaval that the era of reusable launchers would bring to the space sector? “New Space” players, like SpaceX and Blue Origin, no doubt. But among the already established organizations, this topic was obviously not yet such an obvious and critical topic.

SpaceX Falcon 9
SpaceX’s first success in recovery dates back to 2014. But at that time, NASA had already launched the development of SLS for a few years. // Source: SpaceX

Would it have been possible to redirect the development of the SLS along the way, assuming that the integration of such a capability would be useful to the Artemis program? Since it was not intended for this in the first place, it was to take the risk of starting from scratch, or almost. With a rocket that has already cost nearly $27.5 billion, multiplied by budget overruns, that would have been a tough pill to swallow.

In addition to funding issues, these design changes would likely have further delayed the start of the Artemis mission—if only to conduct a test campaign to validate the recovery of the SLS first stage. It exposes itself to delaying the astronauts’ return to the Moon for some years. And, by extension, the start of a mission to Mars. Is it reasonable when China is also on the way and has great ambitions in this area?

Further

The Tiawen-1 landing pad.  // Source: CNSA (cropped photo)

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