Super Heavy BN1–B6

The series running from BN1 to B6 is not a version of Super Heavy in the commercial sense of the word: it is the phase in which SpaceX learned how to build one. None of these boosters flew. All of them were stacked, tested cold or hot, taken apart and, in most cases, scrapped at Starbase. The value of the series lies in what it left behind: a repeatable manufacturing method, a validated orbital launch mount and a catalogue of failures discovered on the ground before they could cost a mission.

BN1 was the first Super Heavy prototype and was never meant to fly: it was conceived as a pathfinder, an article whose job was to prove that the sections could be manufactured, moved and stacked. Its sections were built through the autumn of 2020 and stacking began that December. The vehicle was fully stacked inside the High Bay on 18 March 2021 and was scrapped barely twelve days later, on 30 March. Its working life was measured in weeks because its purpose was exhausted the moment it stood upright.

Booster 3 completed stacking in the High Bay on 29 June 2021 and was moved to the test stand. There it passed a cryogenic proof test on 13 July and, six days later, on 19 July 2021, performed a static fire: the first time a Super Heavy booster lit engines. On 15 August it was partially scrapped, although its liquid oxygen tank remained welded to the test stand until 13 January 2022, a detail that shows how far these vehicles were treated as expendable test rigs rather than flight hardware.

Booster 4 is the article from this series that stuck in the collective memory. It was fully stacked on 1 August 2021 and by the following day it had all twenty-nine engines installed, the configuration planned at the time and not an error against today's thirty-three. It received grid fins to support atmospheric reentry testing, and on 6 August 2021 Ship 20 was stacked on top of it for a fit check: the resulting stack was, for two years, the tallest rocket ever fully integrated. B4 passed its first cryogenic proof test on 17 December 2021, followed by a pneumatic proof test, another cryogenic test and a full-load cryogenic proof. Then it and Ship 20 were retired without flying. On 6 March 2024 its grid fins were removed, on 21 March it was moved to the Mega Bay and the next day it was scrapped.

Parts for Booster 5 were spotted as early as 19 July 2021 and its stacking was completed in November, but on 8 December it was retired alongside ships SN15 and SN16 and was later scrapped. The series is rounded out by a family of test articles that were never boosters: BN2.1 rolled out on 3 June 2021 for two cryogenic tests, on 8 and 17 June; B2.1 — not to be confused with the former — ran three cryogenic tests on 1, 2 and 3 December 2021; B6.1 was originally intended to be the third flight-worthy Super Heavy and ended up repurposed as a test tank, used in May 2023 to validate the modified flight termination system after the systems on B7 and S24 failed to destroy the vehicle; and B7.1 was cryogenically proof tested on 28 June and 19 July 2022 before suffering minor damage in a suspected pressurise-to-failure test two days later, moving to Massey's in September 2022 and being scrapped in December 2023.

The lesson of BN1–B6 is that Super Heavy was designed by breaking things on the ground. Tanks were taken to rupture, flight termination systems were validated on real structures and the stacking process was rehearsed until it became routine. When Booster 7 reached the launch mount in 2022, it did so on top of a proven method rather than a paper design.