For the second time in two months, SpaceX technicians have begun to install a Raptor engine on Starhopper, a full-scale Starship testbed theoretically capable of low-velocity, moderate-altitude ‘hops’.

Back in late March, Raptor and Starhopper were joined for the first time, enabling a lengthy series of attempted tests that were followed by two engine ignitions and tethered hops before Raptor was removed for inspection. In the two months since that first round of integrated testing, SpaceX has significantly upgraded Starhopper and its spartan launch facilities, all focused on transforming the odd vehicle from a largely fixed test stand into a giant, mobile Grasshopper.

All the way back in 2012, SpaceX began testing Falcon 9 recovery and reusability concepts with a low-fidelity prototype known as Grasshopper – essentially a minimalist Falcon 9 first stage with ad hoc legs and a single Merlin engine. It supported a series of 8 major test flights – all successful and a source of valuable data – before the vehicle’s 2013 retirement. An upgraded Grasshopper – known instead as Falcon 9 Reusable Development Vehicle (F9R Dev1) – began testing around the same time and continued even higher altitude vertical takeoff/vertical landing (VTVL) tests until its untimely demise in August 2014.

Starhopper is quite similar, although it is also serving as a testbed for a far more varied range of technologies due to the fact that it has been developed before the inaugural launch of its namesake (Starship/Super Heavy). By the time SpaceX started Grasshopper/F9R tests, Falcon 9 had already completed several successful launches. With Starhopper, SpaceX is building and testing its first 9m-diameter ‘flight’ hardware, its first propellant tanks built out of steel, its first flight-capable rocket fueled by methane and oxygen, and its first mobile Raptor testbed, among numerous other things. The challenges are inherently much greater, but SpaceX has the luxury of taking the opposite approach it took towards Falcon 9 and building a launch vehicle entirely around its intended reusability, rather than trying to squeeze a method of reusability around an already-flying rocket.

Saurid Oddities

As noted by NASASpaceflight.com in a June 2nd article, SpaceX seems to be juggling its growing selection of newly-produced and tested Raptor engines in pursuit of Starhopper’s return to flight. According to the publication’s reliable sources,

“Up until recently, [SpaceX] was planning to utilize Raptor SN4 for [Starhopper’s first] untethered hops. However, the company has now decided to utilize this engine only for fit checks, and will instead perform the hops with SN5 – the latest Raptor to come out of SpaceX’s factory in Hawthorne, California.” – NASASpaceflight.com, June 2nd, 2019

This indicates that the Raptor engine delivered to Boca Chica on June 1st and currently in the process of being installed on Starhopper is actually more of a stand-in* for a future Raptor, SN05. The reasons behind this Raptor shuffle elude detection, but it’s possible that the simplest explanation – also posed by NASASpaceflight – is the correct one. By shipping a Raptor that may not be ready for flight tests, SpaceX could likely save anywhere from a few days up to a few weeks by doing everything short of lifting off under the powered of Raptor SN04.

*By all appearances, SN04 is a flight-grade Raptor that has completed assembly and likely been test-fired in McGregor, Texas. Why it may currently be resigned to a “stand-in” role is unknown.

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It appears that the Raptor engine is not centered, could it be that they are going straight with the 3 engine test. (Idk honestly, I wonder why this is?) @elonmusk are things about to get epic?? pic.twitter.com/sne5v7SMhy— Austin Barnard? (@austinbarnard45) June 1, 2019

Very curiously, upon Raptor SN04’s South Texas arrival, it appears that SpaceX technicians have indeed rapidly installed the engine on Starhopper, but in a position that is decidedly off-center. Pictured above, the photo could have simply caught the engine while technicians were moving it to its actual installation spot, but it could also indicate that SpaceX is speeding towards Starhopper’s first triple-Raptor test flights.

Starhopper delays?

In line with the last-second switch from Raptor SN04 to Raptor SN05 as the engine-to-be for untethered hops, SpaceX has pushed the start of that test series from approximately May 31st to June 11th. More likely than not, the ~11-day delay is meant to allow time for Raptor SN05’s McGregor, Texas acceptance testing, given that – per CEO Elon Musk – the engine wasn’t even finished as of May 22nd.

On the other hand, with Raptor SN05 now scheduled to support Starhopper hop tests as early as mid-June, it begs the question of whether SpaceX is instead working towards expedited triple-Raptor testing. For unknown reasons, neither Raptor SN03 or SN04 are apparently ready to support flight operations, although both have been thoroughly hot-fired in McGregor. Perhaps each engine is a distinct prototype with a different level of experimental readiness, or perhaps SpaceX is just testing certain engines (like SN03) more extensively than others (SN05).

Regardless, SpaceX now seems to have 3-4 intact, functional Raptor engines (excluding SN01; destroyed during stress testing), 2-3 of which are actively testing or being worked on a day’s drive north of Boca Chica. SN02 – having successfully supported a brief duo of ignition tests with Starhopper – could still be intact and test-ready. SN03 is an unknown quantity, but SN04 is clearly in excellent shape and is probably close to flight-readiness if it isn’t already. This is to say that SpaceX likely already has three Raptors on hand that are capable of supporting multi-engine Starhopper testing, whether or not such a test regime would actually be valuable.

Musk has noted that both orbit-capable Starship prototypes will be far closer to finished products and will thus fly with “at least 3 engines” (3 sea level engines, as it would turn out) or even “all 6” (3 sea level, 3 vacuum-optimized). In the meantime, Starhopper stands with an off-centered Raptor, awaiting the arrival of a different Raptor to kick off a second hop test program. If nothing else, SpaceX’s Starship/Super Heavy development program is operating in a spectacularly hardware-rich fashion, lending itself to the breakneck-pace of iteration and improvement SpaceX is famous for.

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