SLS vs Starship: Differences between NASA and SpaceX heavy launchers

While SpaceX’s Starship and NASA’s Boeing-built SLS could both make their first flight this spring, let’s find out the similarities and what differentiates these two heavy launchers that promise to go to the Moon and Mars.

Rocket Comparison

Today, NASA and SpaceX are developing a heavy launcher capable of sending humans to the Moon and 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 an 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 program back to the Moon, it will be employed to launch the Orion capsule . This future launcher will use two thrusters solid propellant booster  directly derived  from the Space Shuttle ‘s Solid Rocket Booster (SRB). 

The only difference, if the boosters for the space shuttle 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 Starship launcher, the main objective of which 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 stage or booster, which is needed to launch the Starship is called “Super Heavy”. This future “all-in-one” and reusable transport system must, as soon as it is put into service, replace in a very short time, the entire current range of SpaceX launchers and freight and manned transport systems. i.e. the Falcon Heavy and the Falcon 9, used for launching satellites, refueling the International Space Station and bringing crews. 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 are only partially so because only the main stage is recovered. The upper stage of each launcher is lost with each launch.

Installed on its launch pad at the Kennedy Space Center, the SLS is preparing for a series of tests in preparation for its maiden flight which could take place in June or July, although NASA is not ruling out a launch from the month of may. 

This mission Artemis I, will consist of an unmanned test flight around the Moon, following a trajectory similar to that of the Apollo  8 mission, using lunar gravity  to gain speed  and to propel itself nearly 70,000 kilometers beyond the Moon, nearly half a million miles from Earth – farther than any human has ever travelled. On its return trip, Orion will perform a flyby from the Moon before returning to Earth. The mission will last approximately 20 days and will end with a dip in the Pacific Ocean.

NASA SLS Specs

• Height: 98m
• Weight: 2.6 million kg
• Payload weight to low-Earth orbit: 95 tonnes
• Payload weight to the Moon: 27 tonnes
• Thrust: 39 million newtons
• Solid fuel: Polybutadiene acrylonitrile
• Liquid fuel: Oxygen and hydrogen
• Top speed: 39,500km/h

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

As for SpaceX’s Starship , the date of the test flight is also very uncertain. The last I heard, Elon Musk was planning it sometime in May. Unlike the SLS Moon Roller Coaster, the Starship’s test flight will be less ambitious. It is a so-called point-to-point flight, a flight in space which aims to connect Texas to the Hawaiian archipelago , without achieving a complete 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 peculiarity of the Starship

Unsurprisingly, the SLS won’t be cheap, but then not at all. In 2019, Ars Technica  estimated the cost could be over $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.

SpaceX Starship Full Specs

SLS vs Starship: Which is the ultimate heavy lift rocket?