CEO Elon Musk says that SpaceX is on track to begin fabricating Starship’s first Super Heavy booster prototype later “this week” and even revealed plans to hop that booster in the very near future.
Taller than an entire two-stage Falcon 9 or Falcon Heavy rocket, Super Heavy will be the largest and most powerful liquid rocket booster ever built by a factor of two (or more). Measuring ~70m (~230 ft) tall, Super Heavy will weigh at least 3500 metric tons (7.7 million lb) when fully loaded with liquid oxygen and methane propellant. According to Musk, SpaceX’s thrust target for the booster is 7500 tons (~16.5 million lbf) – significantly more than twice the thrust of the Saturn V and Soviet N-1 rockets and more than three times the thrust of SpaceX’s own Falcon Heavy.
On paper, while multiple times larger and more powerful, Super Heavy will be substantially simpler than Falcon Heavy thanks to its single-core. Built out of the same simple steel rings used to assemble Starship prototypes, Super Heavy should also be substantially cheaper to build than Falcon Heavy. Thanks to the experience SpaceX has already gained through months of Starship production, testing, and iterative improvement, initial Super Heavy prototype production could have a much smoother start, but several major challenges remain.
SpaceX has structured its Starship development program in such a way that the hardest technical challenges are generally first in line. Raptor engine testing came first in September 2016, although SpaceX did simultaneously build and test a full-scale carbon composite liquid oxygen – a material choice that was ultimately made redundant by the move to steel in late 2018. Up next, Starhopper served as a sort of proof of concept for the assembly of a flightworthy steel rocket in an unprotected open-air tent.
Starship Mk1 came next and was built as a full-scale prototype in similarly spartan conditions – but with much thinner steel. Mk1 ultimately failed prematurely, serving as a catalyst for SpaceX to substantially upgrade its South Texas rocket production capabilities, as well as its manufacturing techniques. Beginning in January 2020, SpaceX completed a rapid-fire series of tests with three stout tank prototypes and five full-scale Starship tank sections over the next seven months, passing multiple challenging pressure tests, wet dress rehearsals, Raptor static fires, and even a 150m (500 ft) hop.
The biggest challenges still facing Starship (5+ minute Raptor burns, skydiver-style landings, heat shield qualification, orbital launch/reentry/reuse) are mostly unique to the orbital spacecraft. In other words, with all SpaceX has already accomplished so far with Starship development, it could very well be ready to build a fully-capable Super Heavy prototype right now.
Along those lines, Musk says that there’s a chance that SpaceX will be ready to hop a Super Heavy booster prototype as early as October 2020 – less than two months after the first prototype enters production. Musk also noted that the biggest technical challenge facing Super Heavy is its extraordinarily complex ‘thrust puck’ – a metal structure that must host up to 28 Raptor engines and transfer all of their thrust through the rest of the rocket.
Per past comments, SpaceX will begin booster testing – possibly up to and including the first few orbital launch attempts – with as few Raptor engines as possible. For Musk’s aforementioned booster hop test, Super Heavy could reportedly hop with as few as two Raptors installed. Beyond those early tests and Super Heavy thrust puck development, perhaps only other challenge facing SpaceX is finalizing Raptor’s design to the point that dozens of engines can be built in short order. As of now, SpaceX has completed 40 Raptor prototypes in 18 months, while every Starship/Super Heavy pair will need as many as 34 engines apiece.
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