Super Heavy (booster Rocket)

Super Heavy is the reusable first stage of the SpaceX Starship super heavy-lift launch vehicle, which it composes in combination with the Starship second stage. As a part of SpaceX's Mars colonization program, the booster evolved into its current design over a decade. Production began in 2021, with the first flight being conducted on April 20, 2023, during the first launch attempt of the Starship rocket.

The booster is powered by 33 Raptor engines that use liquid oxygen and methane as propellants. It returns to its launch site after propelling the second stage toward orbit, landing vertically by being caught by the launch tower.

Design

Super Heavy is tall, wide, and is composed of four general sections: the engines, the oxygen tank, the fuel tank, and the interstage. SpaceX CEO Elon Musk stated in 2021 that the final design will have a dry mass between and , with the tanks weighing and the interstage .

Tanks

The two cryogenic propellant tanks on Super Heavy are separated by a common bulkhead, a similar structural design to the S-II and S-IVB stages on the Saturn V rocket. After Starship's second flight test, the common dome's design was changed to be more elliptical, altering the propellant capacity of both tanks by a small amount. Each tank possesses roughly 74 stringers for structural reinforcement, attached to their interior walls. The booster's two tanks hold a combined of propellant: of liquid oxygen and of liquid methane. Fuel is fed to the engines via a single liquid funnel, and channeled into distribution manifolds of the engines. Block 1 and 2 boosters both have a single booster quick disconnect, along with multiple quick disconnects for the outer engines, while Block 3 boosters have two quick disconnects. One disconnect feeds liquid oxygen into the vehicle, the other liquid methane.

The oxygen tank ends at the thrust structure of the vehicle. While the outer twenty engines are mounted to the walls of the aft bay, the inner thirteen are mounted onto the thrust puck, a part of the aft dome. Large steel structures are attached to the bottom of the dome, reinforcing the puck sufficiently to fully support the inner thirteen engines, and at the same time providing pathways for methane and oxygen into the engines. In addition, large filters were added in this region beginning on Booster 10. Liquid oxygen is supplied by a header tank during landing burn for the inner thirteen engines. On Booster 15, the header tank had at least nine additional tanks attached, increasing capacity for the landing burn. The added tanks may have been present on Boosters 12, 13, and 14, though this was unconfirmed as of February 2025. Booster 5 was the only 29-engine booster to receive a header tank, mounted to the side of the oxygen tank instead of being integrated with the thrust puck.

The methane funnel is partially contained within the header tank, as the methane sump is directly below it. On Booster 7 and all subsequent vehicles, four aerodynamic chines are located on the outside of the oxygen tank, providing aerodynamic lift during descent, as well as housing batteries, composite overwrapped pressure vessels (COPVs), and tanks for fire suppression. On vehicles with hydraulic power units (HPUs), COPVs dedicated to engine ignition, batteries, and communication antennae are located within the HPU cover instead of the chines.

Propulsion

Super Heavy is powered by 33 Raptor engines housed within a dedicated shielding compartment. This compartment is not present before engine installation. Hence, boosters are roughly three meters shorter prior to engine installation. The outer 20 engines, arranged in a ring, are fixed in place. To save weight, the 20 engines are started using ground support equipment on the launch mount and cannot be reignited for subsequent burns. The inner thirteen engines are equipped with gimbal actuators and reignite for the boostback and landing burns. After Starship's first flight test, this gimbaling system was switched from a hydraulic system to an electric one, enabling the removal of the hydraulic power units. This change was made to the upper stage after the second flight test. During the ascent and boostback burns, the engines draw propellant from the main tanks, with the liquid oxygen being drawn from a dedicated header tank during the landing burn. Like the thrust vector control system, the engine shielding, which isolates individual engines in the event of a failure, was upgraded after Starship's first flight test, alongside the fire suppression system. The aft bay has eighteen vents visible on the outside of the booster, which are believed to be connected to the outer 20 engines, while the center engines vent directly below the launch pad.

The Raptor engine uses a full-flow staged combustion cycle with oxygen and methane-rich turbopumps. Before 2014, only two full-flow staged-combustion rocket engine designs had advanced enough to undergo testing: the Soviet RD-270 project in the 1960s and the Aerojet Rocketdyne Integrated Powerhead Demonstrator in the mid-2000s. To improve performance, the engines burn subcooled propellant; i.e. the propellants are cooled below their respective boiling points to further increase their density and the engine mass flow rates.

The Block 1 version of the booster (used through November 2024) produces a total of just over twice that of the Saturn V first stage, with this total being expected to increase to for Block 2 boosters and later up to with the Block 3 vehicle. These later versions may have up to 35 engines. The thirty three engines collectively produce large shock diamonds in the exhaust jet, visible during ascent and descent.

During unpowered flight in the upper atmosphere, control authority is provided by cold gas thrusters fed with residual ullage gas. Four perpendicular vents are located within the interstage. Additional vents are located just below the common dome, pointing down toward the engines at a slight angle.

The Block 3 booster contains an updated aft dome, with metallic heat shield tiles mounted upon it.

Interstage

The interstage is equipped with four electrically actuated grid fins made of stainless steel, each with a mass of roughly . The fins remain extended during ascent to save weight, though this results in mild warping during stage separation. The interstage also has protruding hardpoints, located between grid fins, allowing the booster to be lifted or caught by the launch tower. The ability to lift a booster from these hardpoints was proven on August 23, 2022, when Booster 7 was lifted onto OLM A. The first catch of a booster occurred on October 13, 2024, using Booster 12.After the first Starship test flight, all boosters have an additional 1.8 m tall vented interstage to enable hot staging. During hot staging, Super Heavy shuts down all but the 3 center engines, while the second stage fires its engines before separating, thus the second stage "pushes off" from the first stage giving added thrust. The vented interstage contains a dome to shield the top of Super Heavy from the second stage's engines. Elon Musk in 2023 claimed that this change might result in a 10% increase in the payload to low Earth orbit. Beginning with Booster 11, the vented interstage is jettisoned after completion of the boostback burn, to reduce mass during descent. As of June 2024, SpaceX does not intend to jettison the interstage when flying Block 2 and Block 3 boosters, as the vented section will be directly integrated into the vehicle.

On Block 3 boosters, the interstage is directly integrated into the methane tank. Additionally, the number of grid fins is reduced from four to 3, in a 90/90/180 degree arrangment.

Manufacturing

The manufacturing process starts with rolls of stainless steel, which are unrolled, cut, and welded along an edge to create a cylinder of diameter, tall, and thick, and approximately kg () in mass. Thirty-three such rings are used in the Super Heavy Booster, while four rings are tall. These shorter rings are used exclusively in the aft section. A and a tall ring are used to construct the liquid oxygen header tank. These rings have a significantly smaller diameter than the main rings.

The forward dome is constructed out of two segments: the "dome knuckle" and the "dome frustum". The aft dome has a third component: the "thrust puck", which supports the inner thirteen engines, while the common dome is composed of a single part, and is more elliptical than the forward and aft domes.

These rings are stacked and robotically welded along their edges to form stacks of three to four rings in the Starfactory. Stringers are then added to the ring stacks, improving the structural strength of the booster. Cutouts are made for the grid fins and hardpoints for the forward section. Following this, the domes are installed within the forward, aft, and common ring stacks. The forward ring stack consists of three rings, and the common ring stack consists of four. The aft section is constructed solely from the four rings. Tank vents and external piping are added at this stage, followed by the COPVs and header tank.

Following the completion of each of the ring stacks, the stacking of these sections begins, beginning with the assembly of the methane tank.. This process occurs in Mega Bay 1. Once the methane tank is completed, the oxygen tank is assembled, already integrated to the common dome. Before assembly of the oxygen tank is finished, the methane downcomer is added, along with final stringers to the weld lines. When both tanks are complete, the methane tank is stacked onto the oxygen tank, completing the primary tankage assembly. Chines are added after this stage.

The vehicle is then rolled to the Massey's test site and cryogenically tested twice. These tests fill both tanks with liquid nitrogen, which is nonflammable, though liquid oxygen may be loaded as well. After returning to the production site, the engines are installed, alongside their shielding, which forms the aft bay. This is followed by static fire testing at the launch site. Once this test is completed, the vented interstage is added to the vehicle.

History

Early concepts

Mars Colonial Transporter

In October 2012, the company made the first public articulation of plans to develop a fully reusable rocket system with substantially greater capabilities than SpaceX's existing Falcon 9. Later in 2012, the company first mentioned the Mars Colonial Transporter rocket concept in public. It was going to be able to carry of cargo to Mars and would be powered by methane-fueled Raptor engines. Musk referred to this new launch vehicle under the unspecified acronym "MCT", revealed to stand for "Mars Colonial Transporter" in 2013, which would serve the company's Mars system architecture. SpaceX COO Gwynne Shotwell gave a potential payload range between 150–200 tons to low Earth orbit for the planned rocket. According to SpaceX engine development head Tom Mueller, SpaceX could use nine Raptor engines on a single MCT booster or spacecraft. The preliminary design would be at least in diameter, and was expected to have up to three cores totaling at least 27 booster engines.

Interplanetary Transport System

On September 27, 2016, at the 67th International Astronautical Congress, SpaceX CEO Elon Musk announced SpaceX was developing a new rocket using Raptor engines called the Interplanetary Transport System. It would have two stages, a reusable booster and a spacecraft. The stages' tanks were to be made from carbon composite, storing liquid methane and liquid oxygen. Despite the rocket's launch capacity to low Earth orbit, it was expected to have a low launch price. The concept, especially the technological feats required to make such a system possible and the funds needed, garnered substantial skepticism. Both stages would use autogenous pressurization of the propellant tanks, eliminating the Falcon 9's problematic high-pressure helium pressurization system.

The ITS booster was to be a , , reusable first stage powered by 42 engines, each producing of thrust. Total booster thrust would have been at liftoff, increasing to in a vacuum, several times the thrust of the Saturn V. It weighed when empty and when completely filled with propellant. It would have used grid fins to help guide the booster through the atmosphere for a precise landing. The engine configuration included 21 engines in an outer ring and 14 in an inner ring. The center cluster of seven engines would be able to gimbal for directional control. However, some directional control would be achieved via differential thrust with the fixed engines. Each engine could throttle to between 20 and 100 percent of rated thrust.

The design goal was to achieve a separation velocity of about while retaining about 7% of the initial propellant to achieve a vertical landing at the launch pad.The design called for grid fins to guide the booster during atmospheric reentry. The booster return flights were expected to encounter loads lower than the Falcon 9, principally because the ITS would have both a lower mass ratio and a lower density. The booster was to be designed for 20 ''g'' nominal loads, and possibly as high as 30–40 ''g''.

In contrast to the landing approach used on SpaceX's Falcon 9—either a large, flat concrete pad or downrange floating landing platform, the ITS booster was to be designed to land on the launch mount itself, for immediate refueling and relaunch.

Big Falcon Rocket

In September 2017, at the 68th annual meeting of the International Astronautical Congress, Musk announced a new launch vehicle calling it the BFR, again changing the name, though stating that the name was temporary. The acronym was alternatively stated as standing for Big Falcon Rocket or Big Fucking Rocket, a tongue-in-cheek reference to the BFG from the Doom video game series. The vehicle was designed to be tall, in diameter, and made of carbon composites.

Starship

In December 2018, the structural material was changed from carbon composites to stainless steel, marking the transition from early design concepts of the Starship. Musk cited numerous reasons for the design change; low cost and ease of manufacture, increased strength of stainless steel at cryogenic temperatures, as well as its ability to withstand high heat. In 2019, SpaceX began to refer to the entire vehicle as Starship, with the second stage being called Starship and the booster Super Heavy. In September 2019, Musk held an event about Starship development during which he further detailed the booster.

Ground testing

In March 2021, SpaceX assembled the first Super Heavy prototype, BN1, a production pathfinder for future vehicles. It was scrapped on March 30. The next booster, BN3, was completed on June 29, 2021. It conducted the first cryogenic proof test of a Super Heavy on July 13, followed by the only static fire of a Super Heavy booster at the Suborbital Launch Site on July 19. It was partially scrapped in August, with the process concluding in January 2022.

Booster 4 was the first vehicle intended to fly on Starship's Flight Test 1. It was the first Super Heavy to be stacked with Starship, and conducted multiple cryogenic tests before being retired in favor of Booster 7 and Ship 24.

Flight testing

Booster 7 and Ship 24 conducted several static fire and spin prime tests before launch, with the first such test doing significant damage to Booster 7 on July 11, 2022. After a launch attempt aborted on April 17, 2023, Booster 7 and Ship 24 lifted off on April 20 at 13:33 UTC in the first orbital flight test. Three engines were disabled during the launch sequence and several more failed during the flight. The flight concluded when the booster lost thrust vectoring control of the Raptor engines, resulting in the rocket spinning out of control. The flight termination system (FTS) was activated, though the vehicle tumbled for another 40 seconds before disintegrating.

After the first test flight, SpaceX began work on the launch mount to repair the damage it sustained during the test and to prevent future issues. The foundation of the launch tower was reinforced and a water-powered flame deflector was built under the launch mount. Ship 25 and Booster 9 were rolled to the suborbital and orbital launch sites in May to undergo multiple tests.

On November 18, 2023, Booster 9 and Ship 25 lifted off the pad. All 33 engines continued to function until staging, where the second stage separated by pushing itself away from the first stage using a hot-staging technique. Following separation, the Super Heavy booster completed its flip maneuver and initiated the boostback burn before exploding following multiple successive engine failures. Three and a half minutes into the flight at an altitude of ~90 km over the Gulf of Mexico, blockage in a liquid oxygen filter caused one of the engines to fail in a way that resulted in the destruction of the booster.

IFT-3 launched from the SpaceX Starbase facility along the South Texas coast around 8:25 CDT on March 14, 2024, coincidentally the 22nd anniversary of its founding. Like IFT-2, all 33 engines on the booster ignited and stage separation was successful. B10 conducted a boostback burn, however, the planned landing in the Gulf of Mexico was not successful, as it exploded at above the surface.

The fourth integrated flight test of the full Starship configuration launched on June 6, 2024, at 7:50 am CDT. The goals for the test flight were for the Super Heavy booster to land on a 'virtual tower' in the ocean. Super Heavy achieved a soft splashdown, before being destroyed after tipping over.

In April 2024, Musk stated one of the goals was to attempt a booster tower landing based on successful booster performance in flight 4. Vehicle testing commenced in May 2024. SpaceX claimed that B12 and S30 were ready to launch in early August, in advance of regulatory approval. SpaceX flew S30 and B12 on October 13, 2024, with B12 returning to the launch site for a catch. After B13 aborted its catch attempt during flight 6, B14 was caught during flight 7, and was later reused.

Mission profile

Super Heavy and Starship are stacked onto their launch mount and loaded with fuel via the booster quick disconnect (BQD) and ship quick disconnect (SQD) arm. At the T – 19:40 mark, engine chill begins on the booster.. This is to protect the engine's turbopumps from thermal shock. At three seconds before launch, the 33 engines startup sequence begins.

After liftoff, the engines burn for approximately 159 seconds before Super Heavy cuts off all but three of its center gimbaling rocket engines at an altitude of roughly . It throttles down the remaining engines, before Starship ignites its engines while still attached to the booster, and separates. The booster then rotates, before igniting ten additional engines for a "boostback burn" which stops all forward velocity and reverses the trajectory towards the launch site. After the boostback burn, the booster's engines shut off with Super Heavy on a trajectory for a controlled descent to the launch site using its grid fins for minor course corrections. At six minutes after launch, shortly before landing, it ignites its inner 13 engines, then shuts off all but the inner 3, slowing it sufficiently to be caught by a pair of hydraulic actuating arms attached to the launch tower.

Notes

References

{{SpaceX

Proposed reusable launch systems
Reusable spaceflight technology
Reusable launch systems
Rocket stages
SpaceX Starship