SpaceX has finally had success with pressure testing its prototype Starship, the giant rocket which is intended to carry astronauts to Mars. This is a significant step forward as all of the previous full-scale Starship prototypes have failed during pressure testing, with dramatic and explosive results.

SpaceX CEO Elon Musk posted about the successful test on Twitter, including a video of the test in action, showing the stainless steel tube of the SN4 prototype withstanding the pressure of its tanks being pumped full of liquid nitrogen.

SN4 passed cryo proof! ???? pic.twitter.com/EJakThZRGF — Elon Musk (@elonmusk) April 27, 2020

Musk also confirmed that the test reached a pressure of 4.9 bars, which he described as “kind of a softball” target, but “enough to fly.”

The next phase, Musk announced, will be a static fire test with the Raptor engine. This test will involve just one engine, as opposed to the three which will be used in the next prototype and in the final rocket. A static fire test is where the rocket is brought to a launch pad and is tethered to the ground. Then the engines (or engine, in this case) are fired at full thrust for a short time, but the tether prevents the rocket from actually lifting off the ground.

The static fire test allows the engineers to check that propellant is getting to the engines as it should, and that the engines are generating the correct force at an appropriate temperature. Because the rocket stays on the ground, these factors can be measured safely without worrying about the integrity of the rocket.

The static fire test of the SN4 is planned for later this week.

This will then be followed by a hop test, in which the engines are fired and the rocket lifts a short way off the ground. Musk has confirmed that the plan is to test a 150-meter hop with just one Raptor engine in the SN4.

The prototypes of the Starship so far began with the Mk1 and Mk2, which were both retired after the Mk1 was partially destroyed in a pressure test. The next prototype was the Mk3, then renamed as SN1, which was also destroyed in a pressure test. The next pared-down and partially assembled SN2 prototype passed its pressure testing. Then the team moved on to the full-scale SN3 prototype for static fire testing and short hop testing, but this also failed its pressure test.

This leaves the SN4, the latest version, as the only full-scale prototype to have survived pressure testing.

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