SpaceX Super Heavy

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.

Super Heavy was initially 71 m (233 ft) tall (in its Block 1 and Block 2 design, now retired), while Block 3 Super Heavy is 72.3 meters tall. It is 9 m (30 ft) wide, and is composed of four general sections, in ascending order: the engines, the oxygen tank, the fuel tank, and the interstage.

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 3,400 t (7,500,000 lb) of propellant: 2,700 t (6,000,000 lb) of liquid oxygen and 700 t (1,500,000 lb) of liquid methane. Fuel is fed to the engines via a single liquid funnel, and channeled into distribution manifolds of the engines. This system was upgraded on Block 3 boosters, featuring a substantially larger transfer tube connecting the engines and the methane tank. 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 feeds 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) for spin start, and CO₂ tanks for fire suppression. On vehicles with hydraulic power units (HPUs), COPVs dedicated to engine ignition, batteries, and communication antennae were located within the HPU cover instead of the chines.

Super Heavy is powered by 33 Raptor engines, which on Block 1 and 2 vehicles are housed within a dedicated shielding compartment. This compartment is not present before engine installation, thus 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.