SpaceX just rolled a completed Starship prototype to the launch pad for the third time in two months and began stacking the next rocket just hours after its assembly facilities were vacated.

SpaceX began building the latest Starship prototype – known as serial number 4 (SN4) – around March 23rd. Exactly 31 days later, SpaceX lifted the vast steel rocket onto a Roll Lift transporter and carried it roughly a mile down the road to the company’s Boca Chica, Texas test and launch facilities. In just a few hours, technicians lifted the rocket off its transporter and onto a fixed launch mount made out of thick steel beams, expediency made possible partly by the addition of new mounting points and hold-down clamps.

Sitting atop the late Starship SN3 prototype’s salvaged skirt, landing leg, and service section, the fate of Starship SN4 remains to be seen and the path it has taken to the pad is paved with the remains of several former prototypes. For the most part, that should be a positive aspect. Given how apparent it is that SpaceX is very quickly learning from past mistakes, SN4 has the best chance yet of successfully passing its proof tests and graduating into Raptor static fire and (perhaps) flight testing. However, if things don’t go as planned, SpaceX is perhaps just a week or two away from completing the next prototype – Starship SN5.

SN4 is now on the launch mount at SpaceX Boca Chica launch site. There will be more pics and video later. 🤩🚀@NASASpaceflight https://t.co/nCG7E9XtKM pic.twitter.com/IMRRB1GZJw — Mary (@BocaChicaGal) April 23, 2020

Starship SN4 rolled to the launch pad on Thursday, April 23rd, exactly one month after work on the rocket began. (Elon Musk)

A few hours after SpaceX lifted Starship SN4 onto its steel launch mount, CEO Elon Musk revealed an aerial photo of the rocket and its pad facilities taken with a drone. Recently painted gray and refurbished to undo damage done by Starship SN3’s April 3rd, that mount is currently configured with a strong metal frame and three powerful hydraulic rams. A nearly identical jig was damaged during SN3’s last test when a minor tsunamic of liquid nitrogen – used to safely simulate ultra-cold and explosive liquid oxygen and methane propellant – washed over the mount after the rocket burst.

Much like an ice cube can violently crack and pop when it rapidly changes temperature, untreated steel (almost always cheaper than the alternative) can also be catastrophically damaged by rapid temperature changes (thermal shock). This appears to be exactly what happened to the first hydraulic ram mount, which had visible cracks in photos taken after Starship SN3’s April 3rd demise.

Starship SN4 was installed on top of a launch mount and hydraulic ram stand on April 23rd. (NASASpaceflight – bocachicagal)

SpaceX appears to have had no issue at all acquiring a replacement in a matter of weeks and it arrived and was installed several days ago. The purpose of the hardware is relatively simple: simulate the stresses one or three Raptor engines will create when ignited and ensure Starship’s ‘thrust puck’ and engine section can survive those stresses while filled with cryogenic liquid methane.

Each ram attaches to the thrust puck with the same hardware an actual Raptor uses, including the rods each engine needs for thrust vector control (TVC; i.e. active steering). In the event that Starship SN4 passes its cryogenic proof test(s) and engine stress simulation(s) with flying colors, SpaceX has already built, acceptance-tested, and shipped three Raptor engines to Boca Chica, where they are waiting inside an assembly tent for their call to action.

Once a Starship prototype passes acceptance testing and three Raptor engines can be installed, it will be a first for SpaceX’s next-generation rocket engine. For example, if SN4 makes it through testing and is ready to proceed into static fire operations, it will be the first time Raptor has operated in a multi-engine setup – always a significant milestone for any launch vehicle, including SpaceX’s own Falcon 9 and Merlin engines.

In case SN4 does make it to the other side, SpaceX is already prepared with both road closures and NOTAMs (Notices To Airmen) for static fire and hop tests spread out over the next week or so.

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