Booster 4

Photo credit: Elon Musk


Type: Prototype
Class: Super Heavy
Status: Testing
Current Location: Boca Chica, Texas
Area: Orbital Launch Pad A
Year Built: 2021



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

It was reported by 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.


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.


All dates & times are local unless otherwise indicated.

Date Type Location
1 August 2021 Construction Boca Chica, Texas
3 August 2021 Moved to Pad Boca Chica, Texas
Rolled out to Orbital Launch Pad A
6 August 2021 Construction Boca Chica, Texas
Marks the first time a Super Heavy booster and Starship second stage had been stacked
7 September 2021 Moved to Pad Boca Chica, Texas
Rolled out to Orbital Launch Pad A

Raptor Engines

Engine Number Installation Year
RB7 2021
RB2 2021
RB3 2021
RB15 2021
RB19 2021
RB9 2021
RB21 2021
RB18 2021
RB4 2021
RB6 2021
RC74 2021
RC79 2021
RC58 2021
RB8 2021
RB11 2021
RB14 2021
RB20 2021
RC65 2021

Location History

Location Date From Date To
Production Facility (Boca Chica, Texas) 3 July 2021 3 August 2021
Orbital Launch Pad A (Boca Chica, Texas) 3 August 2021 12 August 2021
Production Facility (Boca Chica, Texas) 12 August 2021 7 September 2021
Orbital Launch Pad A (Boca Chica, Texas) 7 September 2021 Present


More Information

UTC Time

Monday, 06-Dec-21 18:36:13 UTC

Boca Chica Local Time

Monday, 06-Dec-21 12:36:13 CST