WASHINGTON — A prototype of SpaceX’s Starship next-generation launch vehicle passed a pressurization test April 27, one that had destroyed three of its predecessors.

The Starship SN4 vehicle, on a pad at SpaceX’s facility in Boca Chica, Texas, was loaded with liquid nitrogen, a test designed to confirm its ability to hold cryogenic propellants at pressure. That test came a day after a pressurization test where the tanks were filled with gaseous nitrogen at ambient temperatures.

SpaceX Chief Executive Elon Musk confirmed the cryogenic pressurization test was a success. “SN4 passed cryo proof!” he tweeted.

https://twitter.com/elonmusk/status/1254632509863866368

Three previous Starship prototypes all failed that test in spectacular fashion. In November 2019, a bulkhead in the Starship Mark 1 vehicle, which the company showed off to the media two months earlier, ruptured during a cryogenic pressurization test, sending debris flying. The company said at the time that the outcome “was not completely unexpected.”

A second Starship vehicle, SN1, was also destroyed in a cryogenic tanking test Feb. 28, this time with the vehicle bursting near its base. Musk said that the failure in that test appeared to be with a “thrust puck” at the base of the tank that takes the load from the vehicle’s Raptor engines.

The company then stripped down the next Starship prototype in development, SN2, to its tanks, which alone passed a pressurization test in early March. However, the Starship SN3 vehicle collapsed in another cryogenic pressurization test April 3. Musk said at the time that a test configuration error, rather than a flaw with the vehicle itself, may have caused the vehicle to crumple.

With the cryogenic pressurization test completed, SpaceX will install a Raptor engine in the vehicle and perform a static-fire test as soon as later this week, Musk said. That will be followed by a short “hop” test of the vehicle where it flies to an altitude of 150 meters, similar to a flight by a smaller “Starhopper” vehicle in August 2019.

Musk said in an April 26 tweet that Starship SN4 will be “physically ready” for that flight in a few weeks. “Approvals may take longer.” That flight would require an experimental permit from the Federal Aviation Administration’s Office of Commercial Space Transportation, as was the case for Starhopper’s test flight last August.

SpaceX is working on the next Starship prototype, SN5, which Musk said will have three Raptor engines and the vehicle’s distinctive nose cone section. Musk said in tweets earlier in the month that the vehicle’s flaps and other aerodynamic surfaces “are undergoing redesign for mass reduction & simplicity.” The revised flaps would be installed on either the SN5 or SN6 vehicles.

For all the challenges SpaceX has had testing the Starship prototypes, Musk said the biggest challenge is developing the production line for the vehicle. “The thing that’s really hard is the production system,” he said in an April 24 online discussion hosted by Hack Club, a student group. Musk said in the conversation that he was at Boca Chica overseeing work on Starship.

“Designing a rocket is 1x hard, then building one of it is 10x,” he said. “Building the production system is at least 100x, possibly a 1000x.”