SN4 becomes first full-scale Starship prototype to pass cryogenic proof test

by Michael Baylor

Starship SN4 became SpaceX’s first full-scale Starship prototype to pass a cryogenic proof test on Sunday evening at SpaceX’s launch site in Boca Chica, Texas. The cryogenic proof is one of the first tests necessary to prove the vehicle’s flight-worthiness. With the successful test, SpaceX teams will now work towards a static fire, and then potentially a hop to 150 meters.

SN4 follows the Mk1, SN1, and SN3 full-scale Starship prototypes. The previous full-scale builds all failed cryogenic proof testing. The cryogenic proof is when the Starship is filled with liquid nitrogen to verify that its tanks can withstand flight pressures.

Until Sunday, the only Starship prototype to pass cryogenic proof testing was SN2. However, SN2 was only a partially assembled tank section rather than a full-scale vehicle.

With the prior setbacks, the pressure was on SN4 both literally and figuratively to make it past the cryogenic proof test.

There was optimism ahead of SN4’s proof, as the most recent failure of SN3 was not a result of a welding problem. Welding issues had plagued the previous prototypes. Instead, a simple test configuration mistake caused a substantial pressure difference between the liquid oxygen and methane tanks.

Consequently, the liquid oxygen tank collapsed under the weight of a fully fueled methane tank above it.

This mistake was not one that was likely to be repeated with SN4.

In the end, SN4 passed the cryogenic proof test – hitting 4.9 bar. SpaceX CEO Elon Musk admitted in a tweet that this was “kind of a soft ball…” However, “that’s enough to fly,” he added. It is now expected to move on to engine testing within the coming days.

Currently, SpaceX has three flight-ready Raptor engines waiting for the opportunity to participate in the testing. One of these engines will be installed on SN4.

A Raptor engine is not installed until after the cryogenic proof test, as that test uses hydraulic pistons to simulate the forces created by Raptors during flight.

A Raptor engine in Boca Chica, Texas. Credit: BocaChicaGal for NSF

After SpaceX performs the Raptor installation on SN4, teams will need to conduct checkouts of the engine on the vehicle. These will include gimbal, ignitor, and fuel pre-burner tests, among others.

Only then will SpaceX be ready to attempt a static fire.

April 29 was originally the target for a static fire test, but a one day delay with the cryogenic proof test means that the static fire is now likely targeting no earlier than April 30.

Like SpaceX’s previous static fire attempts with the smaller Starhopper vehicle, it may take a few tries before the company can successfully execute the static fire test. After all, it will be the first time that SpaceX fully fuels a Starship with cryogenic methane and liquid oxygen.

As a result, there is a good chance that small kinks with the ground support equipment will have to be worked out ahead of the static fire.

If all goes well with the static fire, SpaceX may then elect to proceed with a small hop of the Starship SN4 prototype.

Until the testing incident with SN3, SpaceX had planned to hop that vehicle to around 150 meters. SpaceX is currently hoping to now perform that flight with SN4. However, test plans are constantly evolving.

What is certain is that SN4 will be limited to only small hops that do not require a relight of the Raptor engines, according to a tweet from Musk.

Musk explained that this is because the vehicle will not be outfitted with the control surfaces necessary for the vehicle to perform flights where the engines are shut down and then relit.

Either the SN5 or SN6 vehicles will get flaps installed, according to Musk. The flap design has been modified slightly from what SpaceX most recently unveiled with the Starship Mk1 vehicle.

Both of the upcoming SN5 and SN6 vehicles are also expected to get nosecones, something that a Starship prototype has yet to have with it at the pad.

The nose cone will not only house the payloads for the operational Starships, but it will also include the liquid oxygen header tank. The placement of the header tank in the tip of the nose cone helps keep the center of mass of the vehicle forward. This is important for when a Starship is descending through an atmosphere without a payload.

The construction of the SN5 vehicle is already well underway. Numerous sections have already been spotted inside of the large production tents at SpaceX’s Boca Chica facility. Furthermore, a piece of the SN5 tank section was moved into the high bay just hours after SN4 was rolled to the pad on Thursday afternoon.

While SN4 will not be receiving a nose cone, the vehicle has had several heat tiles installed on its surface. A cluster of tiles has been placed along both the engine section and towards the top of the tank section.

The production version of Starship will eventually have an entire side of the vehicle covered in heat tiles to allow the vehicle to safely reenter Earth’s atmosphere and eventually enter the atmospheres of other planets.

Including heat tiles on SN4 will allow SpaceX to gain experience with the mounting process and to verify that the tiles stay properly secured to the Starship during testing.

For daily updates on the progress in Boca Chica, subscribe to the NASASpaceflight YouTube channel.

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