SpaceX plans Starship’s first triple static raptor fire test

Technically, it’s wrong to say that Starship serial number 8 (SN8) is the first prototype to receive three Raptor engines. At the end of September 2019, in advance of Musk’s promised Starship update, the company installed three Raptors on the first full-size prototype, the Starship Mk1. The engines were only installed as an obvious fit test or even as a photo opportunity, however – evidence that they weren’t actually built into the spacecraft’s fuel tanks.

Even then, as of September 2019, Starship Mk1 was far from ready to use Raptor engines and was more than a month away from doing its first print and cryo-proof tests – tests that quickly failed. As such, the Starship SN8 – having passed its “cryo-proof” more or less successfully by October 9th – is undoubtedly the first ship to attempt to ignite multiple Raptor engines at the same time.

Oddly enough, SpaceX stayed quiet for a few days after the Starship SN8 passed its first major test. While SpaceX has often submitted test plans (in the form of road closures) on previous Starship prototypes before the current test phase is complete, the company waited until Tuesday, October 14th to submit closure notifications for “SN8 Static Fire” tests.

Like the starships SN4, SN5, and SN6, all of which have successfully transitioned from cryo-proof to static fire testing (and even flight tests for the latter two), SpaceX kicked off the Starship SN8 test campaign with a cryo-proof. It took three days and at least as many attempts, but SN8 ultimately passed “cryo-proof,” according to Elon Musk, which likely meant the ship was reaching sustained pressures of 7.5 bar (~ 110 psi) or more.

SpaceX installed the Starship SN8 engines – the first time multiple Raptors have been fully integrated into a rocket or dyno – in preparation for yet another Raptor: static fires with multiple engines. While modern computational fluid dynamics (CFD) and modeling mean that the great unknowns of rocket propulsion are rarely as opaque as they used to be, the first test of several powerful engines in close proximity is still a guaranteed recipe for surprises.

Thanks to the expertise gained from nearly 100 Falcon 9 and Falcon Heavy launches, SpaceX is likely the world’s leading expert on the challenges and dynamics of proximity operations of more than two rocket engines. At the same time, the Raptor is a dramatically different engine from the Merlin 1D, and while Starship will have a maximum of six engines, those six engines will produce thrust that is almost the equivalent of two full Falcon 9 boosters.

In other words, even with a (relatively) simple static three-raptor fire, SpaceX will break new ground and almost certainly learn one or more things about the design and functioning of the Raptor. Most likely, SpaceX will begin the Starship SN8 static fire test campaign with a wet dress rehearsal (like a cryo-proof, but with real liquid methane and oxygen propellant) and a Raptor spin prime (turbo-pump spin-up) or pre-burner test (turbo-pumps Spin-up) skip a turbo pump spin-up, but with partial combustion) if the WDR runs smoothly. If all three Raptor engines appear healthy, SpaceX can recycle the first static fire and try just an hour or two later.

Starship SN8’s triple Raptor static fire test window opened at 9 p.m. CDT on October 14 and closed at 6 a.m. on the 15th, with identical backups from 9 p.m. to 6 a.m. on October 15 and 16. LabPadre (below) continues to offer 24/7 views of Starship, including static fire testing, while NASASpaceflight.com will likely have live coverage once the testing begins in earnest.