SpaceX's Elon Musk reveals next Starship's Raptor engines, explains latest failure

SpaceX CEO Elon Musk took to Twitter to better explain what happened when the latest full-scale Starship prototype failed during one of its first tests, while later revealing the rocket engines set to power a future prototype’s first flight.

On April 3rd, SpaceX initiated the third full-scale Starship prototype’s first cryogenic (ultra-cold) pressure test by loading the ~30m (100 ft) tall rocket’s upper propellant tank with what was likely 400+ metric tons (~900,000 lb) of liquid nitrogen. For several hours, liquid nitrogen – a chemically-neutral propellant simulant – was loaded and offloaded several times. Finally, around 2:07 am local (07:07 UTC), the liquid oxygen tank below the methane tank abruptly crumpled, reminiscent of a plastic bottle with the air partially sucked out of it. After several long seconds of gradual crumpling, gravity did what gravity does and pulled the heavy upper tank to the ground, shredding the rest of the rocket’s thin steel skin.

Both unfortunate and a positive development, Musk has recently confirmed Teslarati’s earlier speculation that based on videos of the anomaly, a bad test design and operator error(s) – rather than a technical fault of the rocket itself – could have been the cause of Starship SN3’s failure. In other words, barring future operator error-related failures, Starship SN3’s second cryogenic test went quite well and should mean no delays to Starship SN4’s ongoing assembly.

Particularly in light of Elon Musk’s statement that operator error and a bad test design caused Starship SN3’s failure, the ship’s April 3rd performance was quite impressive. That SN3 remained vertical for several seconds after its aft tank crumpled and likely lost pressure – despite carrying a load equivalent to a fully-loaded Boeing 747 passenger jet – suggests that the vehicle’s structure is extremely robust.

In his explanation, Musk revealed that the rocket failed because the lower (liquid oxygen) tank had not been pressurized enough to withstand the stress of a methane tank fully loaded with liquid nitrogen. Musk’s description almost makes it sound like one or several people failed to account for the fact that liquid nitrogen is nearly 25% heavier than the cryogenic methane it was simulating.

Thankfully, while a prototype likely worth several million dollars and at least a month of work was lost, that means that SpaceX should be able to move on to SN4 with confidence – at least as long as it can rectify whatever allowed those operator mistakes to occur.

In the same Twitter session, Musk – presumably burning the late-night oil at SpaceX’s Boca Chica, Texas Starship factory – also posted a photo showing three operational Raptors in the same frame, a definite first for the cutting-edge rocket engine. Had Starship SN3 survived its cryogenic proof tests last week, SpaceX’s plan was to install and static fire either one or three Raptor engines. A successful static fire campaign would have then been followed soon after by a full-scale Starship’s inaugural flight test, potentially seeing the ship fly as high as Starhopper’s final August 2019 hop.

Now, while Musk says SpaceX may still “reuse much of [SN3’s] thrust section,” the company’s Starship test plan will now rely on SN4 – the next full-scale prototype. It’s far more likely that SN4 will reuse almost no structural aspects of SN3, but even that might cause just a few weeks of delays. Based on a particular assembly step completed on April 4th, Starship SN4 is only four weeks away from the launch pad under the assumption of zero improvements to the speed of production, assembly, and outfitting. Knowing SpaceX, SN4 could be fully stacked and outfitted even sooner.

For now, it looks like we’ll thus have to wait at least a few more weeks to see Starship attempt another cryogenic proof test and – potentially – breathe its first fire with one or several Raptor engines.

Eric Ralph: Eric Ralph is Teslarati's senior spaceflight reporter and has been covering the industry in some capacity for almost half a decade, largely spurred in 2016 by a trip to Mexico to watch Elon Musk reveal SpaceX's plans for Mars in person. Aside from spreading interest and excitement about spaceflight far and wide, his primary goal is to cover humanity's ongoing efforts to expand beyond Earth to the Moon, Mars, and elsewhere.
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