What NASA wants to see from SpaceX’s second Starship test flight

SpaceX's Starship rocket sits on a launch pad in Texas, waiting to take off on a test flight.
Enlarge / SpaceX’s Starship rocket stands on a launch pad in Texas and is waiting to take off for a test flight.


NASA managers are in South Texas this weekend as SpaceX prepares to launch its second full-size Starship rocket. If the rocket flies perfectly, it will enable further testing as early as next year to prove SpaceX can transport hundreds of tons of super-cooled methane and liquid oxygen between two spacecraft in orbit.

This is a fundamental part of the architecture SpaceX has developed to fly Starship missions beyond low-Earth orbit. With two fixed-price contracts from NASA worth more than $4 billion, SpaceX is poised to develop and operate two crewed lunar landers based on the Starship design (and likely many more missions, assuming the first two work as advertised).

Depending on who you ask, SpaceX may need to launch a dozen or more tankers to replenish the methane and liquid oxygen tanks of the Starship lunar lander, whose tanks have already been emptied in order to reach low-Earth orbit following the launch from above of a Super Heavy Booster . The Starship lander will use this fuel to move out of low Earth orbit toward the Moon, descend with astronauts to the lunar surface, and climb back into space to take the crew to their Earth return vehicle – an Orion spacecraft.

But first, SpaceX needs to get Starship into space. That’s the goal for Saturday with the second test flight of the full-size Super Heavy rocket and Starship upper stage. If all goes according to plan, Starship will accelerate to nearly 17,000 miles per hour, just below the speed needed to reach a stable orbit around Earth. This keeps the vehicle on a trajectory to naturally re-enter the atmosphere after a trip around most of the world and make a targeted splashdown near Hawaii.

The spacecraft’s upper stage will ultimately evolve into multiple designs, creating a fleet of landers, space transporters, tankers and fuel depots. It’s a bold bet on the future of space exploration.

Unlocking the future

“Tomorrow is a test and we’re going to learn a lot either way,” Lisa Watson-Morgan, who leads NASA’s Human Landing System program, said in an interview with Ars this week. “We’d like to see it go perfectly “But honestly, if not, it will still be a great learning event and it will still give us progress on the schedule for the various flight tests, and then we will also know the areas that we need to go deeper into.”

Despite different cultures, NASA – the often bureaucratic and conservative government agency – and SpaceX – the fast-moving company that likes to design, break and fix things – are closely linked. Without SpaceX, US astronauts would still be entirely dependent on Russian help to fly to the International Space Station. Without NASA’s support, SpaceX probably wouldn’t be here today.

NASA officials said their collaboration with SpaceX to develop the Crew Dragon spacecraft for ferry flights to the space station laid the foundation for their more recent partnership on the Starship lander, NASA’s first human landing system, a key part of the agency’s Artemis program Returning astronauts to the moon.

“The spacecraft being tested (Saturday) is clearly not as sophisticated as the human landing system spacecraft we will eventually have,” Watson-Morgan said. “It’s a precursor or an early demonstration.”

There is a roadmap for SpaceX to quickly bring Starship to maturity and introduce new technologies and capabilities through a series of test flights scheduled to run through 2024. The big priorities for Starship are to continue to iterate the design to meet the needs of NASA’s lunar lander and to launch new generation Starlink internet satellites that will allow SpaceX to build out the broadband network significantly faster than it could with its existing Falcon 9 rockets would be possible.

How quickly SpaceX moves forward with Starship depends on many things. At the top of that list is the outcome of the Starship test flight, scheduled to launch Saturday from Cameron County, Texas. SpaceX has a 20-minute launch window that opens at 7 a.m. CST (1:00 p.m. UTC).

Ars has covered all the upgrades and changes SpaceX has made since the nearly 400-foot-tall rocket’s first test flight in April. Watson-Morgan said NASA engineers will watch how everything performs during the next test launch. These include the modified launch pad, the 33 Raptor engines on the Super Heavy booster and six more on the Starship upper stage, the flight software, guidance, navigation and controls, and the new “hot staging” technology that SpaceX will test on the second test flight.

Starship has six Raptor engines that propel the vehicle into orbit, where it can be refueled for further Raptor firings to reach more distant destinations such as the Moon.Enlarge / Starship has six Raptor engines that propel the vehicle into orbit, where it can be refueled for further Raptor firings to reach more distant destinations such as the moon.

“All the engineering disciplines, all the functional systems that you would expect, the NASA team gets visibility into all of that and then from that we can see how their system, their design and development, how it matures,” Watson-Morgan said. “We give feedback and our experience base when we see things.”

At least five of the booster’s 33 Raptor engines failed during Starship’s first test flight earlier this year. After analyzing the data from this flight, engineers determined that there were fuel leaks that led to fires in the booster’s engine compartment. The Raptor engines, which each burn methane as fuel and produce about half a million pounds of thrust, will need to be much more reliable for SpaceX to move to on-orbit refueling tests and Starlink launches with Starship, not to mention meet the strict safety requirements of the NASA standards for astronauts.

“We expect the Raptor engines to perform better in this test,” Watson-Morgan told Ars. The engines on the next Starship test flight will be a mix of first- and second-generation Raptors during the first launch In April mainly the older engine model was used.

“By the time of the Human Landing System Starship, we will have a much later generation Raptor and thus have the engine predictability required for the human rating of the spacecraft itself,” Watson-Morgan said.

Everyone wants to know, but no one knows

There are many unanswered questions about what it will take to send a spacecraft to the moon, but the basic architecture is there. SpaceX will launch a Starship depot into orbit a few hundred miles above Earth, where it will await deliveries of fresh methane and liquid oxygen from a wave of Starship tankers.

The depot will have used its initial supply of 1,200 tons (or more) of fuel just to get into orbit. The Starship tankers replace the ability to carry 150 tons or more of cargo into low Earth orbit with additional fuel capacity, allowing each tanker to dock at the depot and gradually fill its tanks.

At some point during this sequence, which will span several weeks to several months, SpaceX will launch the human-rated landing version of Starship without anyone on board. Once the depot is at capacity, Starship will dock with it, receive a fresh supply of methane and liquid oxygen, and then fire up its Raptor engines for the trip to the moon.

All of this will launch on the Super Heavy launch vehicle, probably alternating between launch pads in Texas and Florida.

But there are many unknowns. Can two spacecraft safely connect and connect their fuel transfer lines in orbit? The answer to that is probably yes. SpaceX has a lot of experience with this technology when docking its Dragon spacecraft with the International Space Station.

How fast will cryogenic methane and liquid oxygen vaporize in orbit? This won’t be so easy to know until SpaceX collects data on the thermal behavior of these fluids, each stored hundreds of degrees below zero after hanging around in space for weeks to months. During this time, the Starship depot will experience extreme temperature fluctuations as it travels between the day and night sides of Earth. Small temperature fluctuations in the tanks can cause methane and liquid oxygen to evaporate or change from liquid to gaseous state and be lost.

How efficiently can SpaceX actually transport fuel in zero gravity? Is fuel lost during the transfer? Engineers need flight tests and demonstrations to find the final answer to these questions, and when those tests are conducted depends on the outcome of this weekend’s Starship launch.

Artistic illustration of two spaceships docked belly-to-belly in orbit. Enlarge / Artistic illustration of two spaceships docked belly-to-belly in orbit.


NASA and SpaceX engineers are working together to “analyze things like void space, tank collapse, sloshing, how the fluids should get from one ship to the other, navigation and guidance control, how the two ships remain stable “So we work hand in hand with them and our experts do independent analysis that we provide to SpaceX.” Watson-Morgan said.

One of the next steps is a flight demonstration using the “Tipping Point” technology development award that SpaceX received from NASA in 2020. At the time, NASA described this $53 million award as a “large-scale flight demonstration of transferring 10 tons of cryogenic fuel, specifically liquid oxygen, between the tanks of a spacecraft vehicle.

“We are interested in the resulting science and expect a demonstration of fuel transfer later next year.” [between two Starships]“This is really when we start to mature the systems, and that’s when it gets really exciting for HLS because these are the building blocks that we need, and frankly, that’s never been done successfully in orbit before.”

“Complex and challenging”

Only after a fuel transfer demonstration can SpaceX and NASA learn answers to some of the long-simmering questions about the Starship lander. One of those questions is the number of tanker launches SpaceX will need to make a single Artemis landing mission possible.

“We have a general idea, but I hesitate to say exactly what that is because SpaceX is still designing the spacecraft, the launch vehicle and the fleet — the tankers and the depot,” Watson-Morgan said. “That’s why we really need, next year, the fuel transfer demonstration test, because that will then help us say, ‘Okay, we see the boil-off, we see the sizing, we see how long it takes to transfer this liquid.’

Watson-Morgan suggested that the range of the number of Starship refueling flights for a single Artemis mission could be in the “high single digits to low double digits.” SpaceX founder and CEO Elon Musk has indicated that the company is considering expanding Starship’s capacity with larger fuel tanks. This could help reduce the number of tanker flights.

Musk said in June that switching to a “hot staging” technique for Starship would increase the rocket’s payload capacity, but Watson-Morgan said it would have no impact on the HLS program. “Given the analysis we are seeing and future projections, hot staging for us should have no impact on the number of tankers or our operating concept,” she said.

Lakiesha Hawkins, deputy associate administrator of NASA’s Moon-Mars Program Office, reported a different number of tanker flights Friday in a meeting of the NASA Advisory Council’s Human Exploration and Operations Committee. Hawkins suggested that SpaceX will need launches somewhere in the “high teens” for an Artemis landing mission, with those flights departing from Texas and Florida about every six days.

“In order to meet the required schedule and handle fuel boil-offs, etc., there will be a rapid succession of fuel launches,” Hawkins said, adding that this approach is “complex and challenging.”

An artist's illustration of Blue Origin's Blue Moon Mark 1 lander, a precursor to the company's manned lunar lander.Enlarge / An artist’s illustration of Blue Origin’s Blue Moon Mark 1 lander, a precursor to the company’s manned lunar lander.

Blue origin

In a 2021 report, the Government Accountability Office wrote that SpaceX needed 16 launches to support a single crewed landing mission. Musk wrote on social media around the same time that such a high number was unlikely. He suggested a maximum of eight tanker flights to refuel a moon-bound spacecraft in low-Earth orbit – possibly as few as four.

That doesn’t seem to be a number NASA is counting on for Artemis III. “It really depends on tank expansion, tank size and how long they have to hang around,” Watson-Morgan said.

The Starship depot will remain in low Earth orbit while the tankers gradually arrive. Then, once the Starship lander reaches its orbit around the moon, it must wait for the arrival of astronauts flying from Earth on NASA’s Space Launch System rocket and Orion capsule.

“So there’s this orchestration,” Watson-Morgan said. “That’s why I say it depends on a lot of things, and right now we’re really just focused on getting to orbit and fully understanding the fluid dynamics associated with propeller transfer, which will be an amazing engineering feat that’s never been done before has given.” now finished.

“Depending on how this whole thing goes, we’ll determine the proper steps for the unscripted demo and go from there,” she said. SpaceX and NASA plan to launch a spacecraft to the moon for an uncrewed end-to-end demonstration before committing to using the spacecraft for the crew lunar landing on the Artemis III mission.

Not Apollo 2.0

Artemis II, scheduled to launch in late 2024 or, more likely, 2025, will be the first flight of astronauts on the SLS rocket and the Orion spacecraft, following a highly successful uncrewed test flight a year ago. Artemis III is officially scheduled for late 2025 in NASA’s plan, but this will undoubtedly be pushed back several years.

The readiness of the spacecraft is one of the main factors determining when Artemis III will fly, but there are other challenges. Axiom Space is developing new spacesuits that astronauts can wear on the lunar surface, and the Orion spacecraft will fly for the first time with a docking port on Artemis III. Although the SLS rocket has flown before, the manufacturer of its core stage, Boeing, has not proven it can deliver on time.

“Yes, the lander is absolutely important. Without them we can’t go anywhere. But without the suits, we can’t go anywhere,” Jim Free, NASA’s associate administrator for exploration systems development, said at Friday’s advisory board meeting.

In May, NASA commissioned Blue Origin to develop a second manned lander. SpaceX has a contract for the Artemis III and Artemis IV landers, and Blue Origin will take care of Artemis V. The companies will compete for contracts for downstream Artemis missions.

Blue Origin’s lander concept also requires multiple launches to get the lander to the moon and fill it with fuel, although not as many as SpaceX’s larger Starship vehicle.

It may seem like all of this is unnecessary considering NASA launched the Apollo lunar lander on the same Saturn V rocket as the manned Apollo command module. The next version of the SLS rocket, the “Block 1B” variant, scheduled to debut in the late 2020s, will be capable of sending almost as much payload to the moon as the Saturn V.

But we know that the Space Launch System is expendable and expensive. Producing a single SLS Block 1B rocket will cost at least $2.5 billion, the NASA inspector recently reported. Relying on this rocket for everything would cripple NASA’s space exploration program.

And the Apollo lunar module never remained on the lunar surface with a crew for longer than three days. On its first flight, the Starship lander will remain on the Moon for six and a half days, and its endurance will increase on subsequent missions. To do more than repeat Apollo, NASA and its contractors must take on new challenges.

Apollo 17 commander Gene Cernan drives a lunar rover, with the lunar lander visible in the background.  Apollo 17 spent 75 hours on the lunar surface, the longest lunar stay of any manned mission.  Eugene Cernan, commander of the Apollo 17 mission, controls the lunar vehicle at the Taurus-Littrow landing site at the start of the mission's first moonwalk.  The lunar module can be seen in the background.Enlarge / Apollo 17 commander Gene Cernan drives a lunar rover, the lunar lander can be seen in the background. Apollo 17 spent 75 hours on the lunar surface, the longest lunar stay of any manned mission. Eugene Cernan, commander of the Apollo 17 mission, controls the lunar vehicle at the Taurus-Littrow landing site at the start of the mission’s first moonwalk. The lunar module can be seen in the background.

NASA/Harrison Schmitt

“This is not Apollo 2.0,” Hawkins said. “We want to move as quickly as possible, but we are also establishing an ecosystem there on the lunar surface and therefore not just the more complex approach to the landing system, but all the elements that support Artemis, including the Gateway Station, as well as the Surface facilities (like rovers and habitats) that we place on the Moon.

NASA’s overall strategy remains focused on sending humans to Mars. The lunar missions are a small foretaste and offer an opportunity for practice and technology development in the Earth’s neighborhood. NASA recently completed an architectural review for future human exploration of Mars, but those plans are even more unclear than the details of how the spacecraft will get to the moon.

Still, NASA views the Artemis lunar missions, while historic and challenging in their own right, as just a stepping stone. A human mission to Mars will likely require things like in-space refueling, fuel depots, and advanced propulsion and life support. It’s all part of Artemis.

“We definitely try not to be short-sighted in this regard,” Hawkins said.