SpaceX is making good progress on Starship’s first two Super Heavy boosters, both of which could potentially be ready for their first major test campaigns before the end of the year.

On November 19th, some ten weeks after the process began, SpaceX craned Super Heavy B5’s methane (LCH4) tank on top of its oxygen (LOx) tank, marking the end of major structural assembly for the 69m (~225′) tall booster. A team of welders has since been working around the clock to weld the two tanks together and complete a transfer tube that routes methane propellant down through B5’s oxygen tank.

Two days prior, CEO Elon Musk shared a photo of SpaceX’s other Super Heavy booster (B4) which has been slowly progressing towards test readiness for more than three months. It’s unclear why SpaceX has been so sluggish to prepare Super Heavy B4 for testing but with B5 finally approaching the finish line, the company will soon find itself in a position where it will need to decide which booster to proceed with towards the program’s near-term end goal: the first orbital Starship test flight.

Once Booster 5’s two halves are welded together, only a few things will set it and Booster 4 apart. In recent weeks, SpaceX’s slow progress on Super Heavy B4 relented a bit as technicians began closing out the booster’s raceway (a conduit for plumbing, wiring, and avionics) with basic covers. More importantly, SpaceX also began reinstalling Raptor engines and installing heat shielding around those engines for the first time. In the photo Musk published on November 17th, that heat shield is easily visible and there are signs that it will ultimately enclose the entire outer ring of 20 Raptor Boost engines above their nozzles.

Once complete, that shield will theoretically protect each engine’s nest of sensitive plumbing and wiring during static fires; ascent, boostback, and landing burns; and – most importantly – reentry. Unlike Falcon boosters, which always perform a ~30-second, three-engine ‘reentry burn’ to slow down and cushion the blow of reentry heating, SpaceX plans to recover steel Super Heavy boosters without reentry burns. In theory, that should making booster recovery more efficient, allowing another dozen or so tons of propellant to go towards sending Starship to orbit instead of landing.

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As of November 17th, SpaceX has reinstalled all 29 Raptor engines on Booster 4, partially finished the outer ring of Raptor heat shields, and set the stage for more heat shielding around its 9 center engines and the gap between those inner and outer Raptors. Shielding the Raptor Center engines in a way that still seals off Super Heavy’s aft will be even more challenging given that all nine need to be able to freely gimbal to vector their thrust, while the outer ring of 20 Raptor Boost (RB) engines are fixed in place. At pace of work established over the last few months, it will likely take SpaceX several more weeks to finish that heat shield and install seven ‘aerocovers’ over racks of sensitive equipment installed around Booster 4’s base.

Super Heavy Booster 5, on the other hand, has taken a slightly different path through assembly. Unlike Booster 4, which first rolled out as little more than a giant steel tank with Raptors half-installed, SpaceX appears to have installed most of Booster 5’s external plumbing, wiring, equipment racks, and maybe even the start of its Raptor heat shield during assembly instead of after. Perhaps as a result, SpaceX has taken more than ten weeks to stack Booster 5 versus 2.5 weeks for Booster 4. But given that Booster 4 still doesn’t appear to be complete some 18 weeks after its assembly began, there’s a chance that Booster 5 will ultimately take 4-6 weeks less to reach initial test readiness.

If SpaceX does complete Super Heavy B5 well ahead of B4’s schedule, it will soon find itself with two test-ready Starship boosters but only one orbital-class stand with which to test them, potentially forcing the company to make some interesting decisions.