For about a decade, NASA and SpaceX have each been building a rocket aiming to send humans to the Moon for the first time since the 1970s. Years of development and many billions of dollars later, both rockets are now standing on their launch pads ready to blast off on their inaugural flights, possibly within the next few months.
The Starship rocket has been on a launch pad at SpaceX’s test site in Texas since early February. SpaceX CEO Elon Musk said on March 21 its first orbital flight is expected to launch in May, provided that the rocket’s engines are produced and installed on time. NASA’s Space Launch System (SLS), which was rolled out to a launch pad at the Kennedy Space Center in Florida last week, is targeting a similar launch window.
Largest rockets ever built
Both Starship and SLS are unprecedented in size, thrust power, and payload capacity. Whichever flies first will be the most powerful spacecraft launched in history. Both rockets consist of an upper stage, designed to carry cargo and crew, and a chunkier lower stage to boost the upper-stage capsule to their planned altitudes.
SLS, which stands at 322 feet tall with its upper stage, Orion, is slightly shorter than the Saturn V rocket (363 feet) that sent American astronauts to the Moon in 1969, but has 15 percent more thrust force, meaning it can lift a larger mass. SpaceX’s complete Starship is 394 feet tall, making it the tallest rocket ever built. The initial version designed for Earth orbital flight will have about 500,000 pounds, or 230 tons, of thrust power at sea level, Musk said in a tweet this week. Later versions of Starship will likely increase thrust as it aims for more distant destinations.
Two Moon rockets for different purposes
While NASA's SLS was built with the Moon in mind all along, Starship was originally designed as a rocket to conquer Mars. If a rocket is powerful enough to carry the payload necessary for a crewed mission to the Red Planet, it's certainly capable of flying astronauts to closer destinations, including the Moon. In fact, lunar and Earth orbital missions will be the main functions of Starship, at least in the beginning.
In September 2018, SpaceX signed its first lunar passenger: Yusaku Maezawa, a Japanese fashion tycoon. He will fly in a future Starship for a multi-day trip around the Moon. The same rocket is expected to replace SpaceX's workhorse Falcon 9 to deliver future Starlink satellite missions to low Earth orbit.
Neither SpaceX nor NASA has demonstrated that their rockets can reach Earth's orbit, a prerequisite for deeper space exploration. Starship's test record is more encouraging. To date, SpaceX has conducted five high-altitude test flights to 10 kilometers with different prototypes of Starship's upper stage. Its upcoming flight with the booster attached will aim for Earth orbit, which starts at 160 kilometers. The test won't tell us if Starship is ready for the Moon or Mars. But, if successful, it will mark a major milestone in Musk's quest for interplanetary travel and allow SpaceX to soon use the rocket for regular Starlink launches.
The SLS, although never flown for orbital tests, will aim for the Moon on its first go. The upcoming mission, dubbed Artemis-1, will send an un-crewed Orion capsule to the Moon's orbit for a month-long journey. Future Artemis missions will attempt more complex tasks: orbital intercepts, cargo landings, and eventually humans landing on the Moon's surface.
NASA officials said in February Artemis-1 will have three launch windows between April and June. The SLS is currently being prepared for a "wet dress rehearsal," or fueling test, which will run a countdown until 10 seconds before engine ignition. A wet dress rehearsal is the final test before a launch.
The Starship test will require a flight license issued by the Federal Aviation Administration (FAA). The agency expects to complete its review process for SpaceX before the end of March. NASA missions don't require a FAA license.
Vast cost difference in the two rockets
Despite the two rockets' many similarities, the SLS is a significantly more expensive project than Starship.
Since its inception in 2011, the SLS program has cost NASA at least $20 billion, according to a 2019 report by the U.S. Government Accountability Office. A more recent assessment from the Office of Inspector General, the federal auditor of NASA programs, estimated the first four SLS missions would each cost more than $4 billion, eight times the initial projection set in 2012. The operational cost was described as "unsustainable" by NASA Inspector General Paul Martin during a House Science Committee hearing on March 1.
Boeing, the lead NASA contractor building the SLS, argued that, when adjusted for inflation, the cost of developing SLS is only a quarter of that of the Apollo-era Saturn V rocket.
Elon Musk has estimated that the development cost of Starship is less than 5 percent of that of Saturn V, which translates into $5 billion when adjusted for inflation, per CNBC's calculation. Once in use, its operational cost would be less than $10 million per launch, Musk said during a SpaceX media event in Texas last month. That's significantly lower than what SpaceX currently charges for a launch with its smaller Falcon 9 rocket: $67 million.