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SpaceX's SN4 Starship prototype passes key pressure test

The newest prototype of SpaceX's Starship Mars-colonizing spacecraft just passed a crucial pressure test, likely setting the stage for a test flight in the near future.

Starship SN4 survived a "cryo pressure test" late Sunday night (April 26) at SpaceX's South Texas site, near the village of Boca Chica, company founder and CEO Elon Musk announced.

"SN4 passed cryo proof!" Musk said via Twitter late Sunday Texas time (early morning on Monday, April 27, EDT), in a post that included a "relieved" emoji. "Great work by SpaceX engineering!" he added in another tweet.

Video: See SpaceX's Starship SN4 pass a critical pressure test in time-lapse video
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SpaceX's Starship and Super Heavy rocket in pictures

Musk's excitement is understandable, for success was far from guaranteed. This trial, in which the vehicle is filled with frigid liquid nitrogen to simulate the conditions experienced during operational missions, which will use ultracold propellant, felled three previous prototypes over the past five months. 

The SN4 can now progress to the next step of the development campaign: engine tests, which will culminate with a static fire of its single Raptor engine on the ground. In yet another late-night tweet, Musk said SpaceX aims to conduct the static fire later this week.

Provided that goes well, the SN4 ("Serial No. 4") will be cleared to fly, on an uncrewed test with a target altitude of 500 feet (150 meters) or so. SN4 would be the first full-size Starship prototype to fly, and the second of any sort; a stubby test vehicle called Starhopper got off the ground briefly last year but was soon retired. 

Future Starship prototypes will go higher, powered by more Raptors. For example, the SN5 will sport three of the powerful, next-generation engines, Musk said in another tweet

The operational Starship, whose design SpaceX is homing in on via rapid prototype iteration, will feature six Raptors. The 165-foot-tall (50 m) spaceship will launch off Earth atop a gigantic rocket known as Super Heavy, which will have room for 37 of the engines, Musk has said.

Both vehicles will be reusable. Super Heavy will come down to Earth for vertical touchdowns shortly after liftoff, just as the first stages of SpaceX's Falcon 9 and Falcon Heavy rockets do now. Each Starship, meanwhile, will be able to fly many missions between Earth and Mars, or Earth and the moon, or anywhere else people (or payloads) need to go. (Starship will be powerful enough to launch itself off Mars and the moon, which have relatively weak gravitational pulls. Super Heavy is needed to get the craft off Earth, however.)

And SpaceX will make many Starships, if all goes according to plan. Musk's long-term vision involves sending a huge fleet of the 100-passenger vehicles off to Mars every 26 months, when the Red Planet and Earth are properly aligned for interplanetary trips. He sees Starship and Super Heavy as potentially slashing the cost of spaceflight enough to make Mars colonization — a driving ambition for the billionaire entrepreneur, and the main reason he founded SpaceX back in 2002 — economically feasible.

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

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