Engineering:SpaceX Starship development history

Short description: Aspect of the SpaceX Starship rocket and spacecraft

While the Starship program had only a small development team during the early years and a larger development and build team since late 2018, SpaceX CEO Elon Musk made Starship the top SpaceX development priority following the first human spaceflight launch of Crew Dragon in May 2020, except for anything related to reduction of crew return risk.

Flight testing of the Starship second stage with its new-design Raptor rocket engine began in 2019 and has continued into 2021. The initial high-altitude 10–15 km (6.2–9.3 mi) low-velocity testing phase of the second stage Starship had achieved successful controlled descents through the lower atmosphere and test landing by Q2 2021. An initial orbital test flight of the entire two-stage Starship system is planned for later in 2021 or early 2022.

Background

File:Tabella comparativa design starship.png

Historic Starship designs

The launch vehicle was initially mentioned in public discussions by SpaceX CEO Elon Musk in 2012 as part of a description of the company's overall Mars system architecture, then known as Mars Colonial Transporter (MCT). It was proposed as a privately funded development project to design and build a spaceflight system of reusable rocket engines, launch vehicles and space capsules to eventually transport humans to Mars and return them to Earth. Gwynne Shotwell mentioned that the payload capacity would be possibly 150–200 tons low Earth orbit.

As early as 2007 however, Musk had stated a personal goal of eventually enabling human exploration and settlement of Mars. Bits of additional information about the mission architecture were released in 2011–2015, including a 2014 statement that initial colonists would arrive at Mars no earlier than the middle of the 2020s, and SpaceX began development of the large Raptor rocket engine for the MCT before 2014.

Musk stated in a 2011 interview that he hoped to send humans to Mars' surface within 10–20 years, and in late 2012 that he envisioned the first colonists arriving no earlier than the middle of the 2020s.

In October 2012, Musk first publicly articulated a high-level plan to build a second reusable rocket system with capabilities substantially beyond that of the SpaceX Falcon 9 fleet, on which the company had by then spent several billion US dollars. This new vehicle was to be "an evolution of SpaceX's Falcon 9 booster ... much bigger [than Falcon 9]." But Musk indicated that SpaceX would not be speaking publicly about it until 2013. In June 2013, Musk stated that he intended to hold off any potential IPO of SpaceX shares on the stock market until after the "Mars Colonial Transporter is flying regularly."

In February 2014, the principal payload for the MCT was announced to be a large interplanetary spacecraft, capable of carrying up to 100 tonnes (220,000 lb) of passengers and cargo. Musk stated that MCT will be "100 times the size of an SUV". According to SpaceX engine development head Tom Mueller, concept designs at the time indicated SpaceX could use nine Raptor engines on a single rocket, similar to the use of nine Merlin engines on each Falcon 9 booster core, in order "to put over 100 tons of cargo on Mars." At that time, it appeared that the large rocket core that would be used for the booster stage to be used with MCT would be at least 10 meters (33 ft) in diameter — nearly three times the diameter and over seven times the cross-sectional area of the Falcon 9 booster cores—and was expected to have up to three rocket cores with a total of at least 27 engines.

By August 2014, media sources speculated that the initial flight test of the Raptor-driven super-heavy launch vehicle could occur as early as 2020, in order to fully test the engines under orbital spaceflight conditions; however, any colonization effort was then reported to continue to be "deep into the future".

Interplanetary Transport System

Interplanetary Transport System

Artist's conception of the ITS at liftoff
Function	Fully reusable orbital launch Multiplanetary transport Mars colonization
Manufacturer	SpaceX
Country of origin	United States
Cost per launch	US$430 million (expendable)
Size
Height	122 m (400 ft)
Diameter	12 m (39 ft)
Mass	10,500 t (23,100,000 lb)
Stages	2
Capacity
Payload to LEO	300 t (660,000 lb) (reusable) 550 t (1,210,000 lb) (expendable)
Payload to Mars	450 t (990,000 lb) (with refueling)
Launch history
Status	Cancelled (developed into Starship)
Launch sites	KSC LC-39A
First stage – ITS Booster
Length	77.5 m (254 ft)
Diameter	12 m (39 ft)
Empty mass	275 t (606,000 lb)
Gross mass	6,975 t (15,377,000 lb)
Propellant mass	6,700 t (14,800,000 lb)
Engines	42 Raptor
Thrust	128 MN (29,000,000 lbf)
Specific impulse	334 s (3.28 km/s)
Fuel	Subcooled LCH 4 / LOX
Second stage – ITS Tanker
Length	49.5 m (162 ft)
Diameter	12 m (39 ft) 17 m (56 ft) (incl. legs)
Empty mass	90 t (200,000 lb)
Gross mass	2,590 t (5,710,000 lb)
Propellant mass	2,500 t (5,500,000 lb)
Engines	3 Raptor 6 Raptor Vacuum
Thrust	31 MN (7,000,000 lbf)
Fuel	Subcooled LCH 4 / LOX
Second stage – Interplanetary Spaceship
Length	49.5 m (162 ft)
Diameter	12 m (39 ft) 17 m (56 ft) (incl. legs)
Empty mass	150 t (330,000 lb)
Gross mass	2,100 t (4,600,000 lb)
Propellant mass	1,950 t (4,300,000 lb)
Engines	3 Raptor 6 Raptor Vacuum
Thrust	31 MN (7,000,000 lbf)
Fuel	Subcooled LCH 4 / LOX

In January 2015, Musk said that he hoped to release details of a "completely new architecture" for the Mars transport system in late 2015, but this was later delayed to 2016. Musk stated in June 2016 that the first uncrewed MCT Mars flight could happen as early as 2022, to be followed by the first crewed MCT Mars flight departing as early as 2024. By mid-2016, the company continued to call for the arrival of the first humans on Mars no earlier than 2025. By 2016, the rocket had not yet been given a formal name by SpaceX, although Musk commented on a proposal on Twitter to name it "Millennium". In his September 2016 announcement, Musk referred to the vehicle versions as the "ITS booster", the "Interplanetary Spaceship", and the "ITS tanker".

In mid-September 2016, Musk noted that the Mars Colonial Transporter name would not continue, as the system would be able to "go well beyond Mars", and that a new name would be needed. The name selected was Interplanetary Transport System (ITS), although in an AMA on Reddit on 23 October 2016, Musk stated, "I think we need a new name. ITS just isn't working. I'm using BFR and BFS for the rocket and spaceship, which is fine internally, but...", without stating what the new name might be.

Musk unveiled details of the space mission architecture, launch vehicle, spacecraft, and Raptor engines that power the vehicles at the 67th International Astronautical Congress on 27 September 2016. The first firing of a Raptor engine occurred on a test stand in September 2016 as well.

In October 2016, Musk indicated that the initial prepreg carbon-fiber tank test article, built with no sealing liner, had performed well in initial cryogenic fluid testing, and that a pressure test of the tank at approximately 2/3 of the design burst pressure was slated for later in 2016, with the very large tank placed on an ocean barge for the test. This test was successfully completed in November 2016.

In July 2017, Musk indicated that the architecture had "evolved quite a bit" since the 2016 articulation of the Mars architecture. A key driver of the updated architecture was to be making the system useful for substantial Earth-orbit and cislunar launches so that the system might pay for itself, in part, through economic spaceflight activities in the near-Earth space zone.

Design

The ITS stack was composed of two stages. The first stage would be the "ITS booster", while the second stage would have been either an "Interplanetary Spaceship" for crewed transport or an "ITS tanker" for on-orbit refueling.

Both stages of the ITS were to be powered by Raptor bipropellant liquid rocket engines in a full flow staged combustion cycle, with liquid methane fuel and liquid oxygen oxidizer. Both propellants would be fully in the gas phase before entering the Raptor combustion chamber. Both stages were intended to utilize a bleed-off of the high-pressure gas for autogenous pressurization of the propellant tanks, eliminating the problematic high-pressure helium pressurization system used in the Falcon 9 launch vehicle.

The overall launch vehicle height, first stage and the integrated second-stage/spacecraft, was 122 m (400 ft). Both stages of the ITS were to have been constructed of lightweight carbon fiber, even the deep-cryogenic propellant tanks, a major change from the aluminum-lithium alloy tank and structure material used in SpaceX Falcon 9 family of launch vehicles. Both stages are fully reusable and will land vertically, technology initially developed on the Falcon 9 launch vehicle first stages in 2012–2016. Gross liftoff mass is 10,500 tonnes (23,100,000 lb) at a lift-off thrust of 128 meganewtons (29,000,000 lbf). ITS would be able to carry a payload to low-Earth orbit of 550 tonnes (1,210,000 lb) in expendable-mode and 300 tonnes (660,000 lb) in reusable mode.

ITS booster

The ITS booster was a 12 m-diameter (39 ft), 77.5 m-high (254 ft), reusable first stage, to be powered by 42 sea-level rated Raptor engines producing some 3,024 kilonewtons (680,000 lbf) of thrust in each engine. Total booster thrust would have been approximately 128 MN (29,000,000 lbf) at liftoff, several times the 36 MN (8,000,000 lbf) thrust of the Saturn V.

The design engine configuration included 21 engines in an outer ring and 14 in an inner ring, with these 35 engines fixed in place. The center cluster of seven engines were to be gimbaled for directional control, although some directional control to the rocket was to be performed by utilizing differential thrust on the fixed engines. Design thrust on each engine was aiming to be variable between 20 and 100 percent of rated thrust.

The methane and oxygen propellants would also be used to power the reaction control thrusters, although the propellant would be in gas phase rather than the cryogenic liquid that fed the main engines. These thrusters would control booster orientation in space, as well as provide additional accuracy during landing.

The design was intended to achieve a separation velocity of approximately 8,650 km/h (5,370 mph) while retaining about 7% of the total initial propellant load to bring the booster back to the launch pad for a vertical landing, to be followed by inspection and relaunch.

The design called for grid fins to be used to guide the booster during atmospheric reentry. The booster return flights were expected to encounter loads lower than those experienced on the Falcon 9 reentries, principally because the ITS would have both a lower mass ratio and a lower density than Falcon 9. The booster was to be designed for 20 g nominal loads, and possibly as high as 30–40 g without breaking up.

In contrast to the landing approach used on SpaceX's mid-2010s reusable rocket first stages—either a large, flat concrete pad or downrange floating landing platform used with Falcon 9 and Falcon Heavy—the ITS booster was to be designed to land on the launch mount itself, where it may then be refueled with propellant and relaunched.

Interplanetary Spaceship

The ITS did not have a dedicated and single-function second stage in the way most launch vehicles have had. Instead, the upper stage function of gaining sufficient velocity to place a payload into Earth orbit is provided as a relatively short term role by a spacecraft that has all the requisite systems for long-duration spaceflight.

The Interplanetary Spaceship was a large passenger-carrying spacecraft design proposed by SpaceX as part of their ITS launch vehicle in September 2016. The ship would operate as a second-stage of the orbital launch vehicle, and would also be the interplanetary transport vehicle for both cargo and passengers. The Interplanetary Spaceship would be capable of transporting up to 450 tonnes (990,000 lb) of cargo per trip to Mars following refueling in Earth orbit.

The three sea-level Raptor engines would be used for maneuvering, descent, and landing, as well as initial ascent from the surface of Mars. In 2016, the first test launch of a spaceship was not expected until 2020 or later, and the first flight of the ITS booster was expected to follow a year or more later. Early Mars flights—in the mid-2020s or later—were expected to carry mostly equipment and few people.

ITS tanker

The ITS tanker is a propellant tanker variant of the ITS second stage. This spacecraft design was to be used exclusively for transporting up to 380 tonnes (840,000 lb) of propellants to low Earth orbit to refuel Interplanetary Spaceships. To fully fuel an Interplanetary Spaceship for a long-duration interplanetary flight, it was expected that up to five tankers would be required to launch from Earth, transferring a total of nearly 1,900 tonnes (4,200,000 lb) of propellant to fully load the spaceship for the journey. After refueling operations, the reusable tanker was to reenter the Earth's atmosphere, land, and be prepared for another tanker flight.

Reusability

2016 artist's concept of ITS booster returning to the launch pad

Both stages were to be designed by SpaceX to be fully reusable and were to land vertically, using a set of technologies previously developed by SpaceX and tested in 2013–2016 on a variety of Falcon 9 test vehicles as well as actual Falcon 9 launch vehicles.

Importantly, the "fully and rapidly reusable" aspect of the ITS design was the largest factor in the SpaceX analysis for bringing down the currently huge cost of transporting mass to space, in general, and to interplanetary destinations, in particular. While the transport system under development in 2016-2017 relied on a combination of several elements to make long-duration beyond Earth orbit (BEO) spaceflights possible by reducing the cost per ton delivered to Mars, the reusability aspect of the launch and spacecraft vehicles alone was expected by SpaceX to reduce that cost by approximately 2 1/2 orders of magnitude over what NASA had previously achieved on similar missions. Musk stated that this is over half of the total 4 1/2 orders of magnitude reduction that he believes is needed to enable a sustainable settlement off Earth to emerge.

Starship/Superheavy

2017 artist's concept of BFR with payload bay door open.

In September 2017, at the 68th annual meeting of the International Astronautical Congress, SpaceX unveiled the updated vehicle design. Musk said, "we are searching for the right name, but the code name, at least, is BFR."

Starship and Superheavy are 9-meter (30 ft) diameter carbon-composite launch vehicles powered by Raptor engines. The vehicles would initially be used for Earth orbit and cislunar operations, and later for flights to Mars.

The BFR was cylindrical and included a small delta wing at the rear end with split flaps for pitch and roll control. The delta wing and split flaps were said to be needed to expand the flight envelope to allow the ship to land in a variety of atmospheric densities (vacuum, thin, or heavy atmosphere) with a wide range of payloads in the nose of the ship. Three versions of the ship were described: BFS cargo, BFS tanker, and BFS crew. The cargo version would be used to launch satellites to Earth orbit, delivering "significantly more satellites at a time than anything that has been done before" as well as cargo to the Moon or Mars. After retanking in an elliptical Earth orbit, the spaceship could land on the Moon and return to Earth without further refueling.

The engine layout, reentry aerodynamic surface designs, and even the basic material of construction have each changed markedly since the initial public unveiling of the BFR in 2017, in order to balance objectives such as payload mass, landing capabilities, and reliability. The initial design at the unveiling showed the ship with six Raptor engines (two sea-level, four vacuum) instead of nine engines used in the previous ITS design.

By late 2017, SpaceX added a third sea-level engine to the conceptual design to increase engine-out capability and allow landings with greater payload mass, bringing the total number of engines to seven.

Additionally, the BFR was shown to theoretically have the capability to carry passengers and/or cargo in rapid Earth-to-Earth transport, delivering its payload anywhere on Earth within 90 minutes.

By September 2017, Raptor engines had been tested for a combined total of 1,200 seconds of test firing time over 42 main engine tests. The longest test was 100 seconds, which was limited by the size of the propellant tanks at the SpaceX ground test facility. The test engine operates at 20 MPa (200 bar; 2,900 psi) pressure. The flight engine is aimed for 25 MPa (250 bar; 3,600 psi), and SpaceX expects to achieve 30 MPa (300 bar; 4,400 psi) in later iterations. In November 2017, SpaceX president and COO Gwynne Shotwell indicated that approximately half of all development work on BFR was then focused on the Raptor engine.

Back in 2015, SpaceX had been scouting for manufacturing facility locations to build the large rocket, with locations being investigated in California , Texas , Louisiana, and Florida. By September 2017, SpaceX had already started building launch vehicle components: "The tooling for the main tanks has been ordered, the facility is being built, we will start construction of the first ship [in the second quarter of 2018.]" By early 2018, the first ship using carbon composite structure was under construction, and SpaceX had begun building a new permanent production facility to build the 9-meter vehicles at the Port of Los Angeles. Manufacture of the first ship was underway by March 2018 in a temporary facility at the port, with first suborbital test flights planned for no earlier than 2019.

2018 artist's concept of redesigned BFR at stage separation.

In March 2018, SpaceX announced that it would manufacture its next-generation, 9-meter-diameter (30 ft) launch vehicle and spaceship at a new facility the company is constructing in 2018–2019 on Seaside Drive at the Port of Los Angeles. The company had leased an 18-acre (7.3 ha) site for 10 years, with multiple renewals possible, and would use the site for manufacturing, recovery from shipborne landings, and refurbishment of both the booster and the spaceship. Final regulatory approval of the new manufacturing facility came from the Board of Harbor Commissioners in April 2018, and the Los Angeles City Council in May. By that time, approximately 40 SpaceX employees were working on the design and construction of BFR. Over time, the project was expected to have 700 technical jobs. The permanent Port of Los Angeles facility was projected to be a 203,500-square-foot (18,910 m²) building that would be 105 feet (32 m) tall. The fully assembled launch vehicle was expected at that time to be transported by barge, through the Panama Canal, to Cape Canaveral in Florida for launch.

The aspirational goal in 2017 was to send the first two cargo missions to Mars in 2022, with the goal to "confirm water resources and identify hazards" while deploying "power, mining, and life support infrastructure" in place for future flights. This would be followed by four ships in 2024, two crewed BFR spaceships plus two cargo-only ships bringing additional equipment and supplies with the goal of setting up the propellant production plant.

In August 2018, for the first time, the US military publicly discussed interest in using the BFR. The head of USAF Air Mobility Command was specifically interested in BFRs ability to move up to 150 t (330,000 lb) of cargo to anywhere in the world using the projected Earth-to-Earth capability in under 30 minutes, for "less than the cost of a C-5". They projected the large transport capability "could happen within the next five to 10 years."

In a September 2018 announcement of a planned 2023 lunar circumnavigation mission, a private flight called #dearMoon project, Musk showed a redesigned concept for the BFR second stage and spaceship with three rear fins and two front canard fins added for atmospheric entry, replacing the previous delta wing and split flaps shown a year earlier. The revised BFR design was to use seven identically-sized Raptor engines in the second stage; the same engine model as would be used on the first stage. The second stage design had two small actuating canard fins near the nose of the ship, and three large fins at the base, two of which would actuate, with all three serving as landing legs.

SpaceX Starship system

The two major parts of the re-designed BFR were renamed to "Starship" for the upper stage and "Super Heavy" for the booster stage, which Musk pointed out was "needed to escape Earth's deep gravity well (not needed for other planets or moons)."

In May 2019, the final Starship design changed back to six Raptor engines, with three optimized for sea-level and three optimized for vacuum. Also clarified was that the initial prototype Super Heavy will be full size, but was subsequently clarified that it would make initial test flights with less than the full complement of engines, perhaps approximately 20.

SpaceX began to refer to the entire two-stage-to-orbit, fully-reusable, super heavy-lift launch vehicle as the SpaceX Starship system in 2019, although they also continue to use "Starship" to refer to only the spacecraft (second stage).

As the Raptor engine design was iterated, and higher thrust versions tested well on the test stand, the number of engines in the Super Heavy booster stage changed. Super Heavy was initially announced to have as many as 37 Raptor engines on the first stage, and a design with 31 engines was the public plan as late as May 2020. However, in August 2020, Musk stated that the design had changed: "It might be 28 engines," as a result of engine design changes including increased chamber pressure and a higher thrust-to-weight ratio. In August 2020, Elon Musk expected a Super Heavy prototype for September or October. Musk clarified that SpaceX intends to fly hundreds of cargo flights with Starship before carrying human passengers.

In February 2021 SpaceX completed raising an additional US$3.5 billion in equity financing over the previous six months, to support the capital-intensive phase of Starship launch system development and the operational fielding of the Starlink satellite constellation. In April, SpaceX clarified that they continue to expect the "point-to-point transportation between two locations on Earth" use case to be operational and flying "large numbers of people" within five years.

The early atmospheric descent tests in 2020 through May 2021 provided SpaceX sufficient test data on the aerodynamics that by July 2021, Starship second stage body flaps were redesigned to be both narrower and lighter.

Starship prototypes

For a list of test flights of the system, see List of SpaceX Starship launches

Videos of test articles destruction and flight tests

From NASASpaceFlight.com and SpaceX

Starhopper 150m hop

Starship Mk1 failed cryogenic proof test

Starship SN1 failed cryogenic proof test

Starship SN3 failed cryogenic proof test

Starship SN4 failed static fire test

Starship SN5 150m hop

Starship SN6 150m hop

Starship SN8 failed third static fire test

Starship SN8 failed 12.5km test flight

Starship SN9 failed 10km test flight

Starship SN10 failed 10km test flight

Starship SN11 failed 10km test flight

Starship SN15 10km test flight

Name	First spotted	Rolled out	First static fire	Maiden flight	Decommissioned	Construction site	Cur. location	Status	Flights
Starhopper	December 2018	8 March 2019	3 April 2019	25 July 2019	August 2019	Boca Chica, Texas	Launch site	Repurposed	2
Mk1	December 2018	31 October 2019	N/A	N/A	20 November 2019	Boca Chica, Texas	N/A	Destroyed	0
Mk2	May 2019	N/A	N/A	N/A	November 2019	Cocoa, Florida	N/A	0
Mk3/SN1	c. October 2019	26 February 2020	N/A	N/A	28 February 2020	Boca Chica, Texas	N/A	Destroyed	0
Mk4	c. September 2019	N/A	N/A	N/A	November 2019	Cocoa, Florida	N/A	Template:NE	0
SN3	March 2020	29 March 2020	N/A	N/A	3 April 2020	Boca Chica, Texas	N/A	Partly reused	0
SN4	April 2020	24 April 2020	5 May 2020	N/A	29 May 2020	Boca Chica, Texas	N/A	Destroyed	0
SN5	April 2020	24 June 2020	27 July 2020	4 August 2020	February 2021	Boca Chica, Texas	N/A	1
SN6	May 2020	12 August 2020	23 August 2020	3 September 2020	January 2021	Boca Chica, Texas	N/A	1
SN8	July 2020	27 September 2020	20 October 2020	9 December 2020	9 December 2020	Boca Chica, Texas	N/A	Destroyed	1
SN9	August 2020	22 December 2020	6 January 2021	2 February 2021	2 February 2021	Boca Chica, Texas	N/A	Destroyed	1
SN10	September 2020	29 January 2021	23 February 2021	3 March 2021	3 March 2021	Boca Chica, Texas	N/A	Destroyed	1
SN11	September 2020	8 March 2021	22 March 2021	30 March 2021	30 March 2021	Boca Chica, Texas	N/A	Destroyed	1
SN12	September 2020	N/A	N/A	N/A	February 2021	Boca Chica, Texas	N/A	Template:NE	0
SN13	October 2020	N/A	N/A	N/A	February 2021	Boca Chica, Texas	N/A	Template:NE	0
SN14	October 2020	N/A	N/A	N/A	February 2021	Boca Chica, Texas	N/A	Template:NE	0
SN15	November 2020	8 April 2021	26 April 2021	5 May 2021	31 May 2021	Boca Chica, Texas	Display stand	Retired	1
SN16	December 2020	Not Yet	Not Yet	Not Yet	Unknown	Boca Chica, Texas	Display stand	Possibly retired	0
SN17	December 2020	N/A	N/A	N/A	May–June 2021	Boca Chica, Texas	N/A	Template:NE	0
SN18	January 2021	Not Yet	Not Yet	Not Yet	Unknown	Boca Chica, Texas	Unknown	Suspended	0
SN19	February 2021	Not Yet	Not Yet	Not Yet	Unknown	Boca Chica, Texas	Unknown	Suspended	0
SN20/Ship 20	March 2021	5 August 2021	21 October 2021	Not Yet	Not Yet	Boca Chica, Texas	Test stand	In use	0
Ship 21	3 July 2021	Not Yet	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	Build site	Under Construction	0
Ship 22	September 2021	Not Yet	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	Build site	Under Construction	0

Starhopper

Starhopper

Starhopper configuration as flown in August 2019

On 8 December 2018, nine months after starting construction of some parts of the first test article carbon composite Starship low-altitude test vehicle, Musk announced a "counter-intuitive new design approach" would be taken by the company: the primary construction material for the rocket's structure and propellant tanks would be "fairly heavy...but extremely strong" metal, subsequently revealed to be stainless steel. Musk revealed on 23 December 2018 that the initial test article—the Starship Hopper, Hopper, or Starhopper— had been under construction at Boca Chica for several weeks, out in the open on SpaceX property. The Starhopper was being built from a 300-series stainless steel. According to Musk, the reason for using this material is that "it's [stainless steel] obviously cheap, it's obviously fast—but it's not obviously the lightest. But it is actually the lightest. If you look at the properties of a high-quality stainless steel, the thing that isn't obvious is that at cryogenic temperatures, the strength is boosted by 50 percent." The high melting point of 300-series still would mean the leeward side of Starship would need no insulation during reentry, while the much hotter windward side would be cooled by allowing fuel or water to bleed through micropores in a double-wall stainless steel skin, removing heat by evaporation. The Starhopper had a single engine and was used for a test flight to develop the landing and low-altitude/low-velocity control algorithms.

From mid-January to early-March 2019, a major focus of the manufacture of the test article was to complete the pressure vessel construction for the liquid methane and liquid oxygen tanks, including plumbing up the system, and moving the lower tank section of the vehicle 3.2 km (2.0 mi) to the launch pad on 8 March 2019. Integrated system testing of the Starhopper—with the newly built ground support equipment (GSE) at the SpaceX South Texas facilities—began in March 2019. "These tests involved fueling Starhopper with LOX and liquid methane and testing the pressurization systems, observed via icing of propellant lines leading to the vehicle and the venting of cryogenic boil off at the launch/test site. During a period of over a week, Starhopper underwent almost daily tanking tests, wet dress rehearsals and a few pre-burner tests." In early 2019, a storm impacted the Texas site, blowing over the top nose cone of Starhopper and damaging it. It was thought that a rebuilt of the nose cone was required; however, in the end SpaceX decided to forgo the use of a nose cone altogether and use the Starhopper vehicle without a nose cone.

Following initial integrated system testing of the Starhopper test vehicle with Raptor engine serial number 2 (Raptor SN2) in early April 2019, the engine was removed for post-test analysis and several additions were made to the Starhopper. Attitude control system thrusters were added to the vehicle, along with shock absorbers for the non-retractable landing legs, and quick-disconnect connections for umbilicals. Raptor SN4 was installed in early June for fit checks, but the first test flight that is not tethered was expected to fly with Raptor SN5, until it suffered damage during testing at SpaceX Rocket Development and Test Facility, in McGregor, Texas. Subsequently, Raptor SN6 was the engine used by Starhopper for its untethered flights.

On 3 April 2019, SpaceX conducted a successful static fire test in Texas of its Starhopper vehicle, which ignited the engine while the vehicle remained tethered to the ground. The firing was a few seconds in duration, and was classed as successful by SpaceX. This was the first firing of Starhopper, the first firing of a rocket engine in the Texas launch site and the first tethered hop/flight (according to Musk) in the Starship programme. The vehicle might have lifted off the ground, but this would have only been to the height of few inches, and it is not possible to see the lift off in public video recordings of the test. A second tethered test followed just two days later, on 5 April 2019. This time the vehicle rose off the ground to hit tether limit of about 1 metre altitude.

By May 2019, SpaceX was planning to conduct flight tests both in South Texas and on the Florida space coast. The FAA issued a one-year experimental permit in June 2019 to fly Starhopper at Boca Chica, including pre-flight and post-flight ground operations. By late May 2019, while the Starhopper was preparing for untethered flight tests in South Texas, they were building two high-altitude prototypes simultaneously, Mk1 in Texas and Mk2 in Florida. The two ships were constructed by competing teams—that were required to share progress, insights, and build techniques with the other team, but neither team is required to use the other team's techniques. The larger Mk1 and Mk2 test vehicles featured three Raptor methalox engines meant to reach an altitude of no more than 5 km (3.1 mi), and the initial flight was expected no earlier than the first half of 2019. Construction of a Mk3 prototype began in late-2019. A first orbital flight was not expected until Mk4 or Mk5 in mid 2020. The build of the first Super Heavy booster stage was projected to be able to start by September. At the time, neither of the two orbital prototypes yet had aerodynamic control surfaces nor landing legs added to the under construction tank structures, and Musk indicated that the design for both would be changing once again. On 21 September 2019, the externally-visible "moving fins" began to be added to the Mk1 prototype, giving a view into the promised mid-2019 redesign of the aerodynamic control surfaces for the test vehicles.

On 25 July 2019, the Starhopper made its initial flight test, a "hop" of approximately 20 m (66 ft) altitude, and a second and final "hop" on 27 August, reaching an altitude of approximately 150 m (490 ft) and landing approximately 100 m (110 yd) from the launchpad, demonstrating the first use of the Raptor engine in real flight. Starhopper remains situated next to launch area.

Mk1, Mk2, Mk3, Mk4

Starship Mk1

Mk1 and Mk2 were early prototypes of the final design for Starship. SpaceX completed the external structure of the Starship Mk1 in time for Musk's public update in September 2019. Watching the construction in progress before the event, observers circulated photos online and speculated about the most visible changes, including a move to two tail fins from the earlier three. During the event, Musk added that landing would now be accomplished on six dedicated landing legs, following a re-entry protected by ceramic heat tiles. Updated specifications were provided: when optimized, Starship was expected to mass at 120,000 kg (260,000 lb) empty and be able to initially transport a payload of 100,000 kg (220,000 lb) with an objective of growing that to 150,000 kg (330,000 lb) over time. Musk also hinted at an expendable variant capableof placing 250,000 kg into Low-Earth Orbit. Musk suggested that an orbital flight might be achieved by the fourth or fifth test prototype in 2020, using a Super Heavy booster in a two-stage-to-orbit launch vehicle configuration, and emphasis was placed on possible future lunar missions.

At the September 2019 presentation, Elon Musk unveiled Starship Mk1. The Mk1 prototype was 9 m (30 ft) in diameter and approximately 50 m (160 ft) tall, with an empty mass of 200 t (220 short tons). It was intended to be used for testing the flight and reentry profiles, with the end goal of a successful suborbital flight. This was a new test concept, having never been done on a launch vehicle second stage. It was briefly equipped with three sea-level Raptor engines, two fins each at the front and back, and a nose cone containing cold-gas reaction control thrusters for attitude control. All of these were removed after the presentation.

On 20 November 2019, the Mk1 test article came apart during a pressure test. Mk2 was never completed. The same day, SpaceX stated they would stop developing Mk1 and Mk2 and move on to work on the Mk3 and Mk4 articles. Construction began on the Starship Mk4 in Florida by mid-October 2019. A few weeks later, the work on the vehicles in Florida paused, with Mk4 being scrapped. Some assemblies that had been built in Florida were transported to the Texas assembly location in Boca Chica; there was reportedly an 80% reduction in the workforce at the Florida assembly location as SpaceX paused activities there.

Starship SN1 (Mk3)

In December 2019, Musk announced that the Starship Mk3 would be redesignated "Starship SN1" and there would be at least minor design improvements at least through Starship SN20. Musk also explained that there was a change in the production of Starship. Some parts are now stamped and TIP TIG welded vs bump-formed and flux core welded. The new production process guarantees stronger joints and a mass reduction of 20%.

In January 2020, SpaceX performed pressurization tests on two test article tanks in Boca Chica. One such test took place on 10 January 2020, when a test tank was intentionally destroyed by over-pressurizing it; the tank achieved pressure of 7.1 bar (710 kPa). Later, another test tank underwent at least two pressurization tests; in the first experiment, on Monday 27 January 2020, the test tank withstood a pressure of 7.5 bar (750 kPa) before springing a leak. The leak was welded and the tank subjected to cryogenic pressure test on 28 January 2020, when the tank was intentionally pressurized until it ruptured and was destroyed at the pressure of 8.5 bar (850 kPa) The test was however considered a success despite the destruction of the tank, as the pressure reached 8.5 bar (850 kPa), the pressure the tank needed to hold to be considered safe for human spaceflight; that is, the tank demonstrated a safety factor of 1.4 (1.4 times the operational pressure).

Starship SN1 (originally known as Starship Mk3) was stated to be "designed for orbit" according to SpaceX. Later on, it was unclear whether this was the case (that SN1 would fly to orbit), and whether Starship SN1 would be used only for static fire testing (with one or more Raptor engines installed) and perhaps for one or more suborbital flights taking the vehicle to a 20 kilometer altitude with a soft landing back to Boca Chica.

SpaceX began construction of internal components for the SN1 vehicle in December 2019. The company began stacking SN1 in February 2020 after a series of pressurization tests on propellant tank prototypes. The weld quality of the rings had been improved, but SN1 was destroyed during a cryogenic pressurization test on 28 February 2020 due to a design failure in the lower tank thrust structure. The structure ruptured from the bottom up, with most of the top part sent flying in the air and crashing into ground. At the time of the rupture, the SN1 vehicle had no nose cone, flight control structures, or Raptor engines installed, and was positioned on a test stand. The loss of SN1 was similar to the loss of Starship Mk1 in November 2019, leaving little of the vehicle intact. There were no injuries.

Starship SN3, SN4

Static fire of SN4.

In March 2020, Musk discussed SpaceX's future plans for Starship prototype tests. Starship SN3 was planned to be used for static fire tests and short hops, while SN4 would be used for longer flights.

Starship SN3 was destroyed during testing on 3 April 2020. The cause of the failure was a testing configuration error. The liquid oxygen tanks housed in the lower part of the prototype were pressurised with nitrogen in order to keep them structurally capable of withstanding the weight of the full methane tanks undergoing testing. A valve was inadvertently commanded to open resulting in pressure loss in a section. The section suffered a structural failure as it crumpled under the weight of the heavy methane and caused the top section to fall off. SN4 was built reusing parts of SN3 not damaged during the mishap.

Starship SN4 passed cryogenic pressure testing on 26 April 2020, making it the first prototype since the smaller SN2 test tank to do so. On 5 and 7 May 2020, SN4 passed two static fires: One using the main tanks, while the other used the fuel header tank. Three nights later after uninstalling the engine, a new cryogenic pressure test was conducted. On 19 May 2020 during the third test firing of the engine, vibrations shook loose the methane fuel piping in the engine causing a leak which ignited and spread to flammable insulation, the fire caused significant damage to the base of the rocket and destroyed the control wiring leaving SpaceX unable to command the depressurization of the fuel tanks for two days. SN4 was destroyed on 29 May 2020 after a successful static fire test of its single Raptor engine, due to a failure with the Ground Support Equipment's quick-disconnect function.

Starship SN5, SN6

File:SpaceX SN5 Starship 150m Hop & Powerslide.jpg

SN5 during its 150m test flight.

In March 2020, Musk had set "an aspirational goal" of using SN5 or SN6 to conduct an orbital flight of Starship before the end of 2020. After a successful static fire test on 30 July 2020, SN5 completed a 150-meter flight on 4 August 2020 with a single Raptor engine, SN27. After the success of SN5, SN6 completed a static fire on 24 August 2020. On 3 September, Starship SN6 was tested in a 150-meter hop test flight with a single Raptor engine, SN29.

In March 2021, SN5 was moved to the scrapyard. Meanwhile, SN6 was scrapped in January 2021.

Starship SN8

SN8 shortly after taking off during its test flight

In July 2020, Starship SN8 was planned to be built out of 304L stainless steel. However it is believed there were still some parts made of 301 steel. It was the first proof-of-principle prototype to include a nose cone fairing, aerodynamic control surfaces, and three Raptor engines. In late November, Musk gave the odds of SN8 landing in one piece at 1/3. The body flaps and nosecone with front flaps were installed at the pad after the first static fire. In early October, SN8 conducted three cryogenic proof tests. In late October and November, SpaceX conducted four static fires with the vehicle. During the third one, on 12 November 2020, debris from the pad caused the vehicle to lose pneumatics. On 3 December 2020, SpaceX had lowered the altitude of a planned 15 km (9.3 mi) flight to 12.5 km (7.8 mi). The flight was postponed from 8 to 9 December due to a "Raptor auto-abort". The launch took place on 9 December 2020 at 22:45 UTC. Launch, ascent, reorientation, and controlled descent were successful, but due to low pressure in the methane header tank, the engines failed to produce enough thrust for a successful landing burn, resulting in SN8 being destroyed by impact forces.

Starship SN9

SN9 on Suborbital Pad B, with the production facility in the background.

On 11 December 2020 the stand beneath the fully-built SN9 deformed causing the vehicle to tip and contact the walls inside the High Bay. SN9 was subsequently secured vertical again on 14 December 2020, revealing damage to one of its canard fins. On 20 December 2020, a new forward flap replaced the damaged flap on SN9. SN9 rolled out to the launch site and was mounted onto Suborbital Pad B on 22 December 2020, followed by a cryogenic proof test. It underwent final integration operations over the Christmas holiday and began system tests on the launch stand on 28 December 2020. SN9 conducted 6 static fires in total, all in the month of January. On 13 January 2021, SN9 underwent three separate static fires, just hours apart. After some issues detected, it was decided to swap out two of its Raptor engines, engines 44 and 46. After struggling to gain permission from the FAA to launch, SN9 conducted a 10 km (6.2 mi) flight test on 2 February 2021. Similar to SN8, the ascent, engine cutoffs, reorientation and controlled descent were all stable, but one of the engines had an issue with the oxygen pre-burner and failed, causing the vehicle to lose control and crash into the landing pad. After this, the landing pad was reinforced with an additional layer of concrete.

Starship SN10

On 29 January 2021, SN10 was moved to the launch site on suborbital pad A, concurrently with SN9. As such, SN10 was present when SN9 crashed, but was not damaged. After the failure of SN9 due to a Raptor engine ignition issue, Musk stated that future missions would light all three Raptors to perform the belly flop landing sequence, instead of only two. This acts as a failsafe in the case that one engine fails to ignite. SN10's first cryogenic proof test occurred on 8 February, with a static fire following on 23 February. After an engine was swapped out, another static fire occurred on 25 February. On 13 February, Musk gave the probability of a successful landing as being about 60%.

Two launch attempts were conducted on 3 March. The first launch attempt at 20:14 UTC was automatically aborted after a single Raptor engine produced too much thrust while throttling up. The expected launch was delayed by 3 hours after increasing the tolerance. The day's second attempt resulted in a successful launch with ascent, engine cutoffs, flip maneuver, descent, flap control, and landing burn. After following the same flight profile as SN8 and SN9, SN10 became the first Starship prototype to land intact after a high-altitude test. However, the vehicle had a hard landing as it impacted the landing pad at 10 m/s, most likely due to partial helium ingestion from the fuel header tank. Three of the landing legs were not locked in place, causing a slight lean after landing. Although the vehicle remained intact upon landing, the impact crushed the legs and part of the leg skirt. The prototype suffered an explosion 8 minutes later, which sent the prototype flying in the air before crashing into the ground (similar to SN1). A methane leak may have been the cause of the explosion.

Starship SN11

SN11 was moved to Suborbital Pad B on March 8, 2021, to begin its testing campaign. On March 12, 2021, SN11 successfully underwent a cryogenic proof test also including testing of the RCS (Reaction Control System). On March 15, 2021, SN11 attempted a static fire test. However, immediately after ignition of the Raptor engines, the test was aborted. In March 2021, the nose cone and other components of SN12 were repurposed for a structural testing unit.

Few parts were made for SN13 and SN14, and SpaceX did not complete the two prototypes. They were both scrapped.

Starship SN15

Elon Musk referenced major upgrades to the design for SN15 and later prototypes. These include an improved avionics software suite, an updated aft skirt propellant architecture, and a new Raptor engine design and configuration. A Starlink antenna on the side of the vehicle has been identified as a new feature.

Four days before rollout on 4 April 2021, a thrust simulator was installed on Suborbital Pad A to test new thrust puck design of SN15 before Raptor installation. On 8 April 2021, SN15 was moved to the launch site where it was later mounted on Suborbital Pad A. On 9 April 2021, SN15 successfully underwent an ambient temperature pressure test. A cryogenic proof test of SN15 was conducted on 12 April 2021, and a header tank cryogenic proof test was conducted on 13 April 2021. The tests could have possibly made use of the thrust simulator installed on Suborbital Pad A. On 14 April 2021, the thrust simulator attached to SN15 and Suborbital Pad A was removed. A static fire was conducted on 26 April 2021, and a header tank static fire was conducted on 27 April 2021. A 10 km (33,000 ft) high-altitude flight test was successfully conducted in heavy cloud on 5 May 2021. The launch saw successful ascent, engine cutoffs, flip maneuver, flap control and soft touchdown. A small fire near the base occurred shortly after landing, but was later extinguished. SN15 was placed onto Suborbital Pad B on 14 May 2021. After its Raptor engines were removed, it was rolled back to production site on 26 May 2021. On May 31 2021 SN15 was moved and placed onto a display stand, and officially retiring and holding the honour of first Starship prototype to fly, land and successfully be recovered

Starship SN16

As of August 2021, SN16/Ship 16 was fully stacked inside the Highbay for several weeks before it was rolled to out of the Highbay and placed next to SN15 on June 17, 2021. SN17 scrapping began around 6 June 2021. Since March 2021, there has been speculation, but no known confirmation or denial, that SN18 and SN19 have been suspended.

Static Fire Test of SpaceX Starship 20 on 10-21-2021 19:16:00

Starship SN20/Ship 20

SN20 getting its heatshield inspected

SN20/Ship 20 is the first test article that is being planned for testing as a second stage, where the ship will be launched atop a booster rocket. It is also the first ship to include the heat shield tile system covering much of the vehicle. If the two-stage vehicle is successful in leaving the vicinity of the launch mount, SpaceX is hopeful that the tests will continue on ascent through the upper atmosphere, the ship accelerate to orbital velocity, and then execute a test of the body flaps, vehicle attitude control and the heat shield at hypersonic speeds as the ship reenters the atmosphere over the Pacific Ocean to a splashdown north of Hawaii.

SN20 was rolled out to the launch mount on 5 August 2021 and placed on booster 4 for a fit test. The thermal protection system required further tiles and remedial work to many tiles was also needed. It has not yet completed any pressure tests or static fire test. SN20 is currently the only starship to have Raptor Vacuum engines mounted on the thrust puck. A few subassemblies for the prototype were first spotted around 7 March 2021. NASASpaceflight announced via Twitter on 15 March 2021, that the prototype may fly atop of the next Super-Heavy prototype BN3/Booster 3 as part of the first Starship Orbital Flight Test, but this was changed to SN20/Ship 20 flying atop Booster 4. FCC filings in May 2021 by SpaceX stated that the orbital flight will launch from Boca Chica. After separation, Starship will enter orbit and around 90 minutes later attempt a soft ocean landing around 100 km off the coast of Kauai in Hawaii.

SN20 conducted a cryogenic pressure test on 27 September 2021, marking its first test. Venting was seen from SN20, but there was no accumulation of frost. Two days later, SN20 conducted another cryogenic pressure test, and a lot of frost accumulation was seen on SN20.

On 18 October 2021, SN20 conducted a pre-burner test on it's raptor vacuum engine at 23:39 CDT. Two small fires broke out on the stand, where the first one went out very quickly, and the second one went out at 23:42. The engine configuration was one SL Raptor, and one Vacuum Raptor. On 21 October 2021, SN20 conducted two static fire tests with the engine configuration being unchanged. The first static fire occurred at 19:16 CDT, with the Vacuum Raptor engine being fired. Significant venting and frost was observed after that. At 20:18 CDT, the second static fire was conducted. Both the Vacuum Raptor and Sea-level Raptor engines were fired SN20 then began detanking.

Future prototypes

Ship 21 is currently under construction, with the thrust dome being spotted in July 2021. On August 20, Ship 21's Aft Dome was spotted being sleeved.

Test tanks

Name	First spotted	Rolled out	Decommissioned	Construction site	Cur. location	Status	Tests
TT1	January 2020	9 January 2020	10 January 2020	Boca Chica, Texas	N/A	Template:Intentionally destroyed	1
LOX HT	January 2020	23 January 2020	25 January 2020	Boca Chica, Texas	N/A	Template:Scrapped	2
TT2	January 2020	27 January 2020	28 January 2020	Boca Chica, Texas	N/A	Template:Intentionally destroyed	2
SN2	February 2020	7 March 2020	March 2020	Boca Chica, Texas	Production site	Retired	1
SN7	May 2020	12 June 2020	23 June 2020	Boca Chica, Texas	N/A	Template:Intentionally destroyed	2
SN7.1	July 2020	8 September 2020	22 September 2020	Boca Chica, Texas	N/A	Template:Intentionally destroyed	2
SN7.2	December 2020	20 January 2021	22 May 2021	Boca Chica, Texas	Production site	Retired	2
BN2.1	20 April 2021	3 June 2021	25 June 2021	Boca Chica, Texas	Midbay	2
GSE 4.1		23 August 2021	Not Yet	Boca Chica, Texas	Sanchez site	Potentially retired	1

TT1, LOX HTT, and TT2

The Test Tank 1, abbreviated as TT1, was a subscale test tank consisting of two forward bulkheads connected by a small barrel section. TT1 was used to test new materials and construction methods. On 10 January 2020, TT1 was tested to failure as part of an ambient temperature test reaching a pressure of 7.1 bar (710 kPa) before bursting.

The Liquid Oxygen Header Test Tank, known as LOX HTT, was similar to TT1, but this time based on the LOX Header tank inside a nosecone section. On 24 January 2020, the tank successfully underwent a pressurization test which lasted several hours. The following day it was tested to destruction.

The Test Tank 2, abbreviated as TT2, was another subscale test tank similar to TT1. It consisted of two forward bulkheads connected by a small barrel section just like TT1. On 27 January 2020, TT2 underwent an ambient temperature pressure test where it reached a pressure of 7.5 bar (750 kPa) before a leak occurred. Two days later, it underwent a cryogenic proof test to destruction, and burst at 8.5 bar.

SN2

The SN2 test article was a half-size test tank used to test welding quality and thrust puck design. The thrust puck is found on the bottom of the vehicle where in later Starship tests up to three sea-level Raptor engines would be mounted. SN2 passed the pressure test on 8 March 2020.

SN7, SN7.1, and SN7.2

SN7 was a pathfinder test article for the SpaceX manufacturing process to switch to type 304L stainless steel from the type 301 stainless steel used for the earlier prototypes. A cryogenic proof test was performed in June 2020, where it achieved pressure of 7.6 bar (760 kPa) before a leak occurred, which was repaired. During a pressurize to failure test on 23 June 2020, the tank burst at an unknown pressure and briefly lifted itself off the ground.

SN7.1 was the second 304L test tank, with the goal of reaching a higher failure pressure than they achieved with SN7. The tank was tested several times in September, and tested to destruction on 23 September 2020. The tank burst at a pressure of 8 bar (800 kPa) near the top of the tank where the tank metal separated.

SN7.2 was another test tank, this time with the intention of testing a design with thinner walls. It is believed to be constructed from 3 mm steel sheets rather than the 4 mm thickness of its predecessors. On 26 January 2021, SN7.2 passed a cryogenic proof test. On 4 February 2021, during a pressurize to failure test, the tank developed a leak, which was repaired by workers throughout the days. On 15 March 2021, SN7.2 was rolled back to the production site. SN7.2 seems to be retired as of August 2021.

BN2.1

BN2.1 was rolled out on 3 June 2021 and cryogenic tests were carried out on 8 June 2021 and 17 June 2021.

Super Heavy booster prototypes

Name	First spotted	Rolled out	First static fire	Maiden flight	Decommissioned	Construction site	Status	Flights
BN1	September 2020	N/A	N/A	N/A	30 March 2021	Boca Chica, Texas	Template:NE	0
BN3/Booster 3	March 2021	1 July 2021	19 July 2021	N/A	14 August 2021	Boca Chica, Texas	Template:NE	0
Booster 4	3 July 2021	3 August 2021	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	0
Booster 5	19 July 2021	Not Yet	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	Stacking	0
Booster 6	22 August 2021	Not Yet	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	Under construction	0
Booster 7	29 September 2021	Not Yet	Not Yet	Not Yet	Not Yet	Boca Chica, Texas	Under construction	0

Booster BN1

BN1 was the first Super-Heavy Booster prototype, designed to be a pathfinder and not intended to be flight-tested. Sections of the ~70 m (230 ft) tall test article were manufactured throughout the fall, and stacking of the first prototype began in December 2020, inside the incomplete high bay building. BN1 was fully stacked inside the high bay on 18 March 2021. On 30 March 2021, Elon Musk stated that BN1 would be scrapped in favour of BN2 and will not roll out to the launch site and perform testing. On 13 April 2021, the scrapping of BN1 commenced.

BN3/Booster 3

Booster 3, also known as BN3 or B3 had once been suggested it could be the first to make an orbital flight, but it will actually only be used for ground tests. (Cryo test conducted on 13 July 2021) Booster 3 completed stacking in the High Bay on 29 June 2021, and moved to the test pad location on 1 July 2021. Boosters do not have an engine skirt so when rolled out to the launch site without engines, boosters are about 3 meters shorter than a full size Super Heavy. Three engines were subsequently added to Booster 3, making the vehicle a full-length booster.

A static fire test of the booster was conducted 19 July 2021 with those three engines. CEO Musk stated that a further static fire with 9 Raptor engines installed could happen depending on progress with Booster 4. However, BN3/Booster 3 was scrapped on 15 August 2021.

Booster 4

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 with a goal to have the Starship system on the pad by 5 August. However, due to high winds, stacking of Ship 20 on top of Booster 4 was delayed until early morning August 6. Booster 4 was fully stacked on 1 August, with it full complement of 29 engines—four less than the 33 planned in the operational design—installed by 2 August. Grid fins have been added to support atmospheric reentry testing, but notably, the grid fins on the Booster 4 test article will not fold down for launch, as they do on the Falcon 9. Moreover, Musk indicated in late July that future optimizations in the iterative design process could result in further changes, perhaps even removal, to the grid fins from the Super Heavy design.

Booster 4 may fly with SN20/Ship 20 as part of the first Starship Orbital Flight Test. The booster is planned to perform a soft water landing in the Gulf of Mexico after the orbital launch. It was moved to the launch complex on 3 August 2021. On 4 August 2021, Booster 4 was moved from the stand to the Orbital Launch Table and was mounted in place. Booster 4 has 29 Raptor engines installed. Starship SN20 was successfully stacked on top of Booster 4 on 6 August 2021 for a fitting test, making it the largest rocket ever stacked in the history of mankind. Booster 4 was then brought back to the high bay for secondary wiring. On 9 September 2021, Booster 4 was brought back to the launch site and was put on top of the newly modified Orbital Launch mount. A static fire of all 29 raptor engines is about to be attempted. After it was delayed, the booster was taken off the pad and onto the booster transfer vehicle. It sat on it for two weeks to make the booster catch arms (Mechazilla) to be installed. After that was done on 22nd of October 2021, the booster was starting to be fitted with atmospheric engine shielding.

Booster 5

Parts for Booster 5 have been observed at least as early as 19 July 2021. On September 11 Booster 5's Common Dome Section has been spotted in the highbay, later on Booster 5's Forward Dome was seen being placed on the sleeving stand.

Booster 6

Parts for Booster 6 have been observed at least as early as 22 August 2021 Booster 6's Common dome has been spotted on September 13 2021.

Booster 7

A part for Booster 7 was spotted on 29 September 2021, and delivered to the production facility on 2 October.

See also

• Launch vehicle system tests
• List of Starship flights
• SpaceX Starship
• SpaceX Mars transportation infrastructure