The second full-scale Super Heavy prototype to be constructed was initially slated for orbital flight, but is now set for ground testing according to Elon. There was some flux in the naming of the booster, but the latest confirmed name is Booster 3. The first stacking event occurred overnight 14-15 May, and the full stack was finished on 29 June, with rollout to Pad A only two days later.
Manufacturing is being documented below.
Sections
Forward dome section
The forward dome sleeve (a 3-ring section) was the first component of "BN3" to be spotted, on 28 March (NSF Mary).
Common dome section
A dome labelled "BN3 COMMON" is spotted on 30 March (NSF Mary), and a flyover shows it to have a hole for the descender (RGV Aerial Photography). The next month, the extension of Tent 3 allowed some great views inside, revealing what was later identifiable by welds to be a booster common dome quad sleeve (NSF Mary, RGV Aerial Photography), on 12 April.
The dome was sleeved with its quad on 26 April (NSF Mary).
Aft / thrust dome section
The aft dome sleeve was spotted in the ring yard on 2 April, revealed to be a tri rather than a double as on BN1; the welds show there are gaps in the stringers for the outer Raptors (NSF Mary). It appeared to have moved to the back of the yard in RGV's flyover of 8 April, may have appeared and disappeared in the dome yard around 12 April, and reappeared from Tent 1 for definite on 20 May (NSF Mary). It was flipped upside down on 21 April (NSF Mary). The thrust dome appeared on 23 April (NSF) and was sleeved on 24 April (NSF Mary). The resulting section was then reinforced in the tank region before flipping right way up on 4 May (NSF Mary).
A methane manifold, splitting the feed from the methane downcomer to the booster engines, was delivered on 19 May (NSF Mary). It seems likely this will go on Booster 3.
On 29 May Mary photographed the delivery of a machined thrust plate/puck with a ring of 8 Raptor mounts and a ninth offset from the centre. Elon confirmed on Twitter that boosters would start with 29 engines and add another three later in the year.
Aft tank / mid LOX sections
A quad labelled "BN.3 AFT TANK #5 BOOSTER WO#1626314" was spotted out front of Tent 1 on 3 April, without stringer welds yet (NSF Mary). It was seen again on 25 May fully reinforced, label snapped by Mary and an aerial view acquired by RGV. AT that stage it was in line to be the fourth quad incorporated into the stack.
A booster aft tank section (identifiable by welds) was seen outside the front of tent 1 on 4 May (NSF Mary), the label later revealed to say "Booster #3 Aft #2" (NSF Mary). It had disappeared again on 9 May (NSF).
The numbering and arrangement of the midsections became clear: the "aft tank #5" being the 4th midsection was consistent with top-down numbering starting with #2 - the dome section counting as #1.
Forward tank / mid methane sections
A plain quad was seen in the ring yard on 6 May labelled "BN-3 FORWARD TANK #2" (NSF Mary). On 27 May a booster forward tank quad recognisable by its spaced stringers was seen at the second back door of Tent 3 (NSF Mary); it emerged on 31 May and a clearer view was obtained on 1 June (NSF Mary). Seems likely this is the Booster 2 forward tank #2 quad.
On 12 May a plain tri with a booster graphic on the label was seen. (NSF Mary A closeup of the label was acquired on 15 May: "BN-3 FORWARD TANK #3" (Starship Gazer). It rolled into a tent for further work on 17 May (Starship Gazer).
On 23 May the label "BN-3 FWD TANK #4 BOOSTER WO# 1626309" was photographed on an undeveloped tri that had been in the ringyard for a couple of days (Starship Gazer).
2, 3 and 4 turned out to be the only midsections used in the forward tank.
Stacks
Aft / LOX tank stacks
Stacking began in the highbay on the night of 14 May, recorded from Mars Cam (NSF). The common dome section, now sporting a ring of external stringers, had been lifted onto an aft tank section, as seen the following day (NSF Mary). The result is what we may call the LOX 8-stack. The stack was rotated (NSF, Starship Gazer), revealing the common dome label and surrounds seen during the flipping of the common dome section. In the early hours of 22 May the stack was lifted to the left (LabPadre Sentinel Cam).
In the early morning of 23 May the stack was lifted to the right again, and stayed at a height consistent with stacking on an aft tank quad to create the LOX 12-stack (LabPadre Sentinel Cam, NSF Mary). The 12-stack got lifted to the left early on 25 May, but didn't increase in height (LabPadre Sentinel Cam, RGV).
Next, between 5 am and 6am on 26 May the 12-stack was lifted onto the third quad to create a 16-stack (LabPadre Sentinel Cam). This was lifted to the left again early on 3 June.
Early on 5 June the 16-stack was lifted onto the #5 quad next to SN16 (LabPadre Sentinel Cam).
The 20-stack was lifted onto the thrust section to create what we may call the aft 23-stack on 15 June (Elon).
Forward / methane tank stacks
With the aft tank complete and the forward dome sleeved, stacking of the forward tank occurred rapidly towards the end of June (detail to come). The forward stack was stacked onto the aft stack on 29 June.
