Booster 4

History

Construction

Booster 4 is a Super Heavy booster that is currently under construction in Boca Chica, Texas.

It was reported by NASASpaceFlight.com that Ship 20 (previously known as SN20) with Booster 4 (originally planned to be BN3, which instead was incorporated into Booster 3) would attempt an orbital test launch by 1 July 2021. The launch scheduled was later pushed back into July/August.

This was later confirmed by Elon Musk who tweeted that Booster 4 would be the first one to fly, while Booster 3 would be used in ground tests. There would be significant design changes between Booster 3 and 4 as there had been difficulties in the construction of Booster 3.

A section of Booster 4 was spotted in the High Bay on 3 July 2021. By 21 July it had been stacked to twelve rings tall, with the Methane Transfer Tube (aka Downcomer pipe) being installed in the early hours of 27 July. The launch appeared to take on new urgency with Elon Musk ordering several hundred of SpaceX’s employees at Hawthorne to relocate to Boca Chica to speed up the development of Ship 20 and Booster 4, along with the Orbital Launch Platform to have the Starship system on the pad by 5 August.

The LOX section was stacked on 30 July. At the same time a grid fin was spotted arriving and installation started later that night. Around 3:30pm on 1 August final stacking operations began, by 4:27pm the stacking had been completed apart from the final welding. Raptor engines were then moved nearby and Elon announced that the engines were being installed, and observers kept note of the arriving engines as they arrived.

SpaceX engineers worked throughout the night so that by morning on 2 August, Super Heavy Booster 4 had received 29 raptor engines.

During a tour of SpaceX’s Boca Chica operations by Tim Dodd (Everyday Astronaut), Elon Musk stated that the grid fins mounted to Booster 4 would not retract, a decision made to cut down on the complexity of the build and speed up the production process.

Testing

On 3 August, Booster 4 was rolled out to the Orbital Launch Pad, marking the first time that an orbital capable Super Heavy had been completed and moved. The next day on 4 August Booster 4 was placed on the launch pad in preparation for having Ship 20 stacked on top of it.

After a delay a one day delay due to higher than expected winds, Ship 20 was successfully stacked onto Booster 4 on 6 August at 7:56am, marking the first time a Super Heavy Booster and the second stage Starship had been stacked, representing the largest rocket in human history to date. Booster 4 stayed on the Orbital Launch Pad until 12 August when it was rolled back to the High Bay, with Elon announcing that it would return to the launch pad on Monday 16 August (winds permitting).

After arriving in the High Bay, engineers began to remove the Raptor engines from the booster. The planned return to the launch pad did not end up taking place, with Booster 4 remaining in the High Bay for the rest of August as it was tended to by engineers. It was noted during the night of 23/24 August that more Raptor engines were spotted being moved nearby and several of them were reinstalled. By 7 September, Booster 4 had its 29 raptor engines installed and was rolled out again to the Orbital Launch Pad A and was lifted onto the launch mount for a second time a day later on 8 September.

On 10 September Elon stated on Twitter that the static fire testing for Booster 4 was planned for “hopefully next week“. 17 September saw the removal of Raptor 67 with it being replaced by Raptor 64 the next day. On 25 September Booster 4 was lifted off the Orbital Launch Pad and placed on a stand nearby, presumably to allow for more work on the launch tower including the installation of the arms that will be used to catch future Boosters.

Elon Musk gave an interview to the SSB & BPA Joint Fall Meeting on November 17 (watch here), revealing more details about the Starship program including a targeted launch date of January/February 2022 for S20/B4.

Testing was kicked up a gear when Booster 4 was returned to the Orbital Launch Platform on December 13, a few days later Elon Musk tweeted footage of the central Raptor engines undergoing a gimbal test. The first cryogenic pressure test was carried out on December 17, with another substantial test being observed on December 22.

Booster 4 was removed from the Orbital Launch Pad on December 30, being placed on a stand near by.

Milestones

All dates & times are local unless otherwise indicated.