Surviving reentry is the key goal for SpaceX’s fourth Starship test flight

After three test flights, SpaceX has shown that the world's most powerful rocket can reach space. Now, engineers must demonstrate the company's next-generation Starship vehicle can get back home.

This will be the central objective for the fourth Starship test flight, which could happen as soon as early June, according to Elon Musk, SpaceX's founder and CEO.

"Starship Flight 4 in about 2 weeks," Musk posted on X, his social media platform, following a Starship countdown rehearsal Monday at the Starship launch site in South Texas. "Primary goal is getting through max reentry heating."

Nearly ready to fly

With Monday's practice countdown, SpaceX completed one of the final major tests on the next Starship rocket before it is ready to fly. SpaceX's launch team loaded more than 10 million pounds of super-cold methane and liquid oxygen propellants into the Super Heavy booster and Starship upper stage.

The nearly 400-foot-tall (121-meter) rocket was fully stacked on its launch pad for the countdown rehearsal, which ended, as planned, before the ignition of the booster's 33 Raptor engines. SpaceX then drained the cryogenic propellants from the rocket, and ground teams removed the Starship upper stage from the booster Tuesday for more work on the ship's heat shield. A few days before launch, SpaceX will install the rocket's self-destruct mechanism, which would be used to destroy the vehicle if it flies off course and threatens populated areas.

These are the final major tasks for the team at SpaceX's Starbase facility before Starship is ready to take off on its fourth test flight. SpaceX is also waiting for a commercial launch license from the Federal Aviation Administration, which is overseeing SpaceX's internal review of the previous Starship test flight in March.

The FAA classified the outcome of that flight as a mishap because the Starship lost control and disintegrated during reentry before it could make a targeted intact splashdown in the Indian Ocean. For SpaceX, the March flight was a resounding success. It was the first time a Starship test flight reached near orbital velocity, with full-duration burns by all 39 Raptor engines on the rocket's first and second stages.

After the first and second Starship test flights last year, the FAA withheld a launch license for the next flight until regulators could review all the results of SpaceX's investigation into what went wrong on the prior mission. The FAA is responsible for ensuring commercial space launches do not endanger the public.

Thirty-three engines fired to power the Super Heavy booster and Starship rocket into the sky on its second test flight in November 2023.

Credit: Stephen Clark / Ars Technica

Last week, an FAA spokesperson said SpaceX requested that regulators approve the next Starship launch before the formal conclusion of the mishap investigation into the March test flight. Instead, SpaceX asked the FAA to approve the next launch after determining the events of the previous flight did not put the public at risk. This could speed up the licensing process.

"If the FAA agrees no public safety issues were involved in the mishap, the operator may return to flight while the mishap investigation remains open, provided all other license requirements are met," the FAA spokesperson said.

SpaceX is already testing hardware for several rockets slated to fly later this year, positioning the company to ramp up the Starship launch. The company is also building a second launch pad in Texas and plans two Starship launch sites in Florida to support a faster launch cadence.

But there's a lot more SpaceX needs to iron out with the Starship design before it becomes operational.

Reentry or bust

SpaceX had several goals for the last Starship test flight. These tests included the opening and closing of Starship's payload bay door in space. This "Pez door" would be used to release satellites into orbit on future Starship missions. SpaceX aborted an attempt to reignite one of Starship's six Raptor engines in space because the vehicle was rolling too much as it coasted through space.

Another experiment SpaceX completed on the March test flight involved the transfer of liquid oxygen between two tanks inside Starship. This tank-to-tank transfer of liquid oxygen was part of a demonstration supported by NASA funding. Agency officials said this demonstration would allow engineers to learn more about how the fluid behaves in a low-gravity environment, gathering useful data before SpaceX attempts an ambitious refueling mission with two Starships.

The ship-to-ship refueling demonstration probably won't happen until 2025 at the earliest. But it's a critical milestone for SpaceX and NASA, which selected Starship as the first vehicle to ferry astronauts to and from the surface of the Moon for the agency's Artemis lunar program. Getting Starship to the Moon will likely require 10 or more refueling missions to gas it up in low-Earth orbit. The timing for NASA to return astronauts to the lunar surface largely hinges on the readiness of Starship.

Lisa Watson-Morgan, manager of NASA's Human Landing System, said she was pleased with the results of the propellant transfer experiment on the March test flight. "They were looking for a quantity of oxygen to go from one tank to the other. We were looking at settling ... and vapor pull-through. Those are typically the indications of a successful test."

The goal for the experiment was to transfer at least 10 metric tons of propellant from a header tank to the main liquid oxygen tank. "From the data that I have from people, it seems that it was successful," she said.

While SpaceX tried several different experiments on the March test flight, the focus for the next mission will be on controlling the reentry of the rocket's Super Heavy booster and Starship upper stage. Both vehicles broke apart during their descent on the last flight.

"I think SpaceX put a lot in this last test to see where they were," Watson-Morgan said in an interview with Ars. "So, now they know, in general, areas they need to work on, and part of their commercial approach is reuse of hardware. It's not a NASA requirement, (but) it's great that we could potentially see lots and lots of hardware and rapid turnaround rates. They will have to produce more for us if they don't get their reusability right. So it's important to them."

SpaceX officials would like to see the Super Heavy booster for the next test flight, named Booster 11, make a controlled pinpoint splashdown in the Gulf of Mexico just offshore from Starbase. Halfway around the world, the Starship upper stage, known as Ship 29, will try to survive the blistering reentry back into Earth's atmosphere.

Starship is dressed in about 18,000 hexagonal heat-absorbing ceramic tiles to protect its stainless-steel structure during reentry, when temperatures peak at about 2,600° Fahrenheit (1,430° Celsius). In the final moments of the March test flight, onboard cameras captured spectacular video of the pinkish-orange glow of plasma flowing over the vehicle high over the Indian Ocean. This video was downlinked back to Earth through SpaceX's Starlink broadband network.

This rear-facing camera on Starship shows plasma building up around the vehicle during reentry over the Indian Ocean.

Credit: SpaceX

On the next flight, Musk wants to see Starship survive the highest temperatures of reentry with all systems functioning. Eventually, SpaceX wants to return Super Heavy and Starship back to the launch site with propulsive landings. Assuming an on-target splashdown in the Gulf of Mexico on the next mission, Musk has said SpaceX could try landing a Super Heavy booster back at the launch pad in Texas as soon as the fifth test flight later this year. Due to the harsher conditions of reentry from orbital velocity, it will take longer for Starship to reach this level of maturity, but Musk said SpaceX could start landing ships in Texas next year.

"For Flight 4, it's mostly going to be a repeat of Flight 3 without the propellant transfer, without the Pez door open, without the other items that SpaceX was working to demonstrate," she said.

NASA's main interest in the fourth Starship launch will be seeing that SpaceX replicates the good engine performance it demonstrated on the last two flights.

"For us, primarily, it's the successful light of those Raptor engines and achieving main stage with all of them on Booster 11," Watson-Morgan said. "Getting Raptor to where we can consistently see the performance in each test flight, that's going to give us a good indication that SpaceX and NASA are well on our way to making all the updates necessary for the HLS (lunar landing) mission. So, in line with that, we definitely want to see successful hot staging and then lighting of the Raptors on Ship 29."

Watson-Morgan said SpaceX is not planning to attempt a Raptor engine restart on the next Starship test flight. Eventually, SpaceX must demonstrate this capability for future Starships to drop out of orbit and return to Earth, or to head to the Moon and take off from the lunar surface.

Achieving an in-orbit engine restart—necessary to guide Starship toward a controlled reentry—is a prerequisite for future launches into a stable higher orbit, where the ship could loiter for hours, days, or weeks to deploy satellites and attempt refueling.

The Starship launch trajectories for the rocket's initial test flights do not reach a stable orbit, meaning the vehicle will naturally reenter the atmosphere before completing a full lap around the Earth without requiring a deorbit burn.

Reigniting Raptor engines in space is "important" for future missions, Watson-Morgan said, "but we’ve got to get the other fundamentals right first. If we can't light all 33 engines on the booster, and if we can't light all six engines on the ship, then we're going to have trouble getting to where we need to go," she said. "So it's basically a building-block approach."