SLS vs Starship: the main differences between NASA and SpaceX heavy launchers

Today, NASA and SpaceX are each developing a heavy launcher capable of sending humans onto the Moon and on Mars. With the Space Launch System (SLS), built by Boeing, NASA is acquiring a launcher for its future manned exploration missions to the Moon, Mars and a asteroid. It will also be used to send probes around the most distant worlds of the Solar system. Finally, it can be used to send tens of tons of infrastructure into low orbit. As part of the Artemis return to the Moon program, it will be used to launch the Orion capsule. This future launcher will use two thrusters propellant booster solid directly derived from Solid Rocket Booster (SRB) of the Space Shuttle. Only difference, if the boosters for the spaceship had four booster segments, those of the SLS will have five. With a height of 54 meters, they will be the tallest boosters ever built.

As for SpaceX, it is developing the giant launcher Starship whose main objective is to send hundreds of humans to Mars to colonize the red planet. As a reminder, “Starship” refers to the space transport vehicle and the upper stage of the launcher. The main floor, i.e. the booster needed to launch the Starship is called “Super Heavy”. This future “all-purpose” and reusable transport system must, as soon as it is put into service, replace in a very short time, the entire current range of launchers and cargo and manned transportation systems from SpaceX. That is to say the Falcon Heavy and the Falcon 9used for launching satellites, refueling the international space station and crew rotation. This launcher will also be used for commercial missions to the Moon and NASA plans to use it to land its astronauts on the moon.

And yes, unlike the SLS, the Starship is a fully reusable transport system capable of landing on the Moon or Mars. The Falcon 9 and the Falcon Heavy only partially because only the main floor is recovered. The upper stage of each launcher is lost with each launch.

Installed on its Kennedy Space Center launch pad, the SLS is preparing for a series of tests in view of its maiden flight which could take place in June or July, although NASA is not ruling out a launch as early as May. This mission, Artemis I, will consist of an unmanned test flight around the Moon, following a trajectory similar to that of the mission Apollo 8, using the gravity moon to earn speed and to propel itself nearly 70,000 kilometers beyond the Moon, nearly half a million kilometers from Earth – farther than any human has ever travelled. On his return trip, Orion will perform a overview from the Moon before returning to Earth. The mission will last approximately 20 days and will end with a dip in the Pacific Ocean.

A rollercoaster around the Moon and a point-to-point flight

As to SpaceX Starship, the date of the maiden flight is also very uncertain. In the last news, Elon Musk predicted it during the month of May. Unlike the SLS Moon Roller Coaster, the Starship’s maiden flight will be less ambitious. It is a so-called point-to-point flight, a flight in space which aims to connect Texas to thearchipelago of Hawaii, without achieving a full orbit around the Earth.

On the performance side, the SLS will be available in three versions with different transport capacities depending on the version. The block 1 version, the only one under development, is the one that will be used to launch the first three Artemis missions. This version will have a capacity of 95 tons in low orbit and 27 tons for lunar missions. With the block 1B version, the capacity increases to 105 tons in low orbit and 42 tons sent to the Moon. The block 2 version is expected to launch 130 tons into low orbit and 46 tons for lunar missions. As for the Starship, it will be capable of sending more than 100 tonnes into low orbit and 21 tonnes into a geostationary transfer orbit. But, and this is the particularity of the Starship, if it is refueled in orbit, its performance increases significantly and goes from 100 to 150 tons bound for the Moon or Mars for example.

Unsurprisingly, the SLS won’t be cheap, but then not at all. In 2019, Ars-Technica estimated that the cost could be more than $2 billion to launch the rocket once a year. In March 2022, a study concluded that it could cost twice as much, up to $4.1 billion. The Starship, on the other hand, will obviously be cheaper, but certainly not as cheap as Elon Musk says, who estimates its cost at only $2 million at “cruising speed”, thanks to the efficiency savings that come from the reuse.