Space Launch System core stage
The Space Launch System core stage, or simply core stage, is the main stage of the American Space Launch System rocket, built by The Boeing Company in the NASA Michoud Assembly Facility. At tall and in diameter, the core stage contains approximately of its liquid hydrogen and liquid oxygen cryogenic propellants. Propelled by 4 RS-25 engines, the stage generates approximately of thrust, about 25% of the Space Launch System's thrust at liftoff, for approximately 500 seconds, propelling the stage alone for the last 375 seconds of its burn. The stage lifts the rocket to an altitude of approximately before separating, reentering the atmosphere over the Pacific Ocean.
The core stage originated in 2011, when the architecture of the Space Launch System as a whole was defined. In the aftermath of the end of the Space Shuttle program and the cancellation of its prospective replacement the Constellation program, the SLS emerged, a super-heavy lift launch vehicle intended for human spaceflight to the Moon. The core stage is the first newly-developed stage of the SLS; the ICPS and five-segment boosters are adaptations of existing hardware, to be replaced by the Exploration Upper Stage and BOLE boosters respectively.
Production of core stages began by 2014, but was beset by numerous difficulties in production and testing which delayed the readiness of the first core stage by several years. The core stage first flew on November 16, 2022, on the Artemis I mission, in which it performed successfully. As of 2024, the second core stage is completed, with the third and fourth core stages in production and while work has begun for the fifth and sixth, their production pending the transfer of SLS operations to Deep Space Transport, the vehicle's future operator.
Design
The core stage comprises five major sections: the engine section, the liquid hydrogen tank, the intertank, the liquid oxygen tank, and the forward skirt. These elements can be further divided into ten barrel sections, four domes, and seven rings, together forming the structure of the rocket stage.Main propulsion
The core stage is powered by 4 RS-25 engines housed inside the engine section at the base of the stage. The engines are associated with the main propulsion system, which support the engines in their operation, allowing them to gimbal, or deflect, to control the rocket, supply them with liquid hydrogen and liquid oxygen propellants, and keep the propellant tanks pressurized. In service of this role, the main propulsion system is outfitted with hydraulic systems that move the engine bells to allow them to gimbal, pneumatics to actuate the numerous valves within the rocket, a pressurant system to feed gaseous propellants into their tanks, and large amounts of ducting. The pneumatic system is kept pressurized by helium stored in five composite-overwrapped pressure vessels within the engine section, while hydraulic power is provided by an auxiliary power unit called the CAPU, which on the first two core stages is directly reused Space Shuttle hardware. The CAPU is a turbine which is spun by pressurized gaseous helium during vehicle startup, then by hydrogen gas, as opposed to its Space Shuttle usage when it was powered by the flow of hydrazine. The hydraulic system powered by the CAPU also includes the gimbal actuators which themselves deflect the RS-25 engines. These actuators, like the CAPUs, are directly reused Space Shuttle parts on early-production core stages. The main propulsion system works to reduce the risk of fire in the engine section: while staged for work and servicing, the engine section is purged with clean air; on the launch pad, during flight preparations, the space is filled with nitrogen gas supplied from ground support equipment to mitigate the buildup of hazardous gases like hydrogen or oxygen. Before flight, the core stage also receives all of its supplies through the MPS, with propellants and helium pressurant flowing through quick-disconnect connections of the tail service mast umbilical, interfacing with the vehicle on a plate located on the engine section.Thrust structures
The engine section and intertank of the core stage both feature large thrust structures, which transmit thrust forces through the vehicle. The engine thrust structure also enables the stage's RS-25 engines to be gimballed. Each engine is mounted an attachment point at the base of the thrust structure, while its hydraulic thrust vectoring system is installed on top of that same structure. The engine section thrust structure is bolted together and attached inside the cylindrical engine section barrel. The intertank thrust beam, mounted with the intertank much higher up on the vehicle, is a single beam, which, in conjunction with the thickened and strengthened bolted structure of the intertank itself, allows the thrust of the solid rocket boosters to be transmitted through the stage.Propellant tanks
The largest structures of the core stage are its propellant tanks, built to carry approximately 987 tonnes of cryogenic propellants, liquid hydrogen and liquid oxygen. The extremely low cryogenic temperatures of these fluids – for liquid oxygen and for liquid hydrogen – causes substantial shrinkage in the propellant tanks. The liquid hydrogen tank shrinks about in length and in diameter after being filled, while the liquid oxygen tank's size decreases by lengthwise and across. Therefore, all hardware attached to the propellant tanks must be mounted using bellows that allow them to flexibly adjust to the shifting size of the propellant tanks.Space Shuttle heritage and differences
The design of the core stage was intended to make use of knowledge and experience gained from the Space Shuttle program, similarly to the rest of the Space Launch System. This is reflected in aspects such as the rocket's diameter, identical to that of the Space Shuttle's external tank, while feedlines and ducts are designed to make use of existing valve designs and connectors. However, the core stage is also significantly different from the external tank. There is no structure on the external tank comparable to the core stage's main propulsion section, which is analogous to the main propulsion section which occupied the tail of the Shuttle orbiter. The core stage is also made from a different, harder aluminum alloy than that in the definitive version of the external tank, which was lighter but more difficult to work with. Structural construction of the core stage's propellant tanks is also dissimilar to that of the Shuttle external tank, partly through more extensive use of friction-stir welding, while the core stage's stringers are milled out of the workpiece instead of riveted in.Manufacturing
The SLS core stage is primarily manufactured by Boeing in the NASA-owned Michoud Assembly Facility in New Orleans, previously the site of Space Shuttle external tank and Saturn V S-IC manufacturing.A number of tools are used in the primary manufacture of the core stage. These include the Circumferential Dome Weld Tool and Gore Weld Tool, both used in conjunction with the Enhanced Robotic Weld Tool, the Vertical Weld Center, Segmented Ring Tool, and Vertical Assembly Center. These tools are generally designed to enable the friction-stir welding, both self-reacting and conventional, circumferential and linear, of the 2219 aluminium-copper alloy of much of the core stage. Additional tools include the floor assembly jig and Intertank Final Assembly Tool.
Main structural elements
The core stage's 2 propellant tanks – the "wet' structures – are each built up from a number of barrels, 2 rings, and 2 domes. A barrel consists of eight vertically, linearly-joined "panels", welded in the VWC. Each dome is manufactured from 12 gore panels, first joined into a gore dome in the GWT, a Y-ring, and an end cap, joined by conventional circumferential friction-stir welds. These elements are assembeld into whole domes in the CDWT. Segmented rings, used to connect domes and barrels and to provide stiffness, are produced using the SRT. These items are then welded together circumferentially in the VAC, starting with a forward dome, then barrels, one by one – 5 for a LH2 tank, 2 for a LOx tank – then the aft dome. With the stiffening rings installed, these elements make up a propellant tank. After initial assembly, tanks undergo testing and remediation. X-ray radiography is used to inspect welds for quality. Any defects in welds are corrected using automated and manual techniques. Substantial technology development was required to enable the self-reacting friction stir welding of the core stage's propellant tanks, as the metal making up their walls was thicker than any previously joined using the technique. The core stage Intertank is manufactured rather differently to the other elements. A "dry" structure, it is required not only to handle the loads of the core stage itself but also the thrust of the twin Solid Rocket Boosters of the SLS, transmitting these loads. Additionally, as a dry structure, it is not pressurized like the propellant tanks. Accordingly, it features external stiffener ribs and thicker structure, which prohibit the use of welding for assembly; therefore, the intertank is bolted together from eight panels using more than 7500 fasteners. By the time the intertank panels are bolted together, they are pre-painted with protective primer.A Forward Skirt adapts the core stage to the Launch Vehicle Stage Adapter on SLS Block 1 and the Exploration Upper Stage interstage on SLS Block 2. Considered the least complicated and complex of the "dry" structures, its assembly is more straightforward and takes less time than any of the other main elements of the core stage. The ring panels that make it up its structure, together with adapter ring flanges, are covered in anti-corrosion primer prior to structural assembly.
The Engine Section is the most complex element of the core stage, as well as the Space Launch System as a whole. Procurement of its parts and assembly are the longest-lead items in the manufacture of a core stage, and beginning with CS-3, engine sections are integrated in the Space Station Processing Facility at Kennedy Space Center. Structural assembly of all engine sections takes place in the Michoud Assembly Facility. The structural elements consist of a welded barrel, a mating ring, used to adapt the engine section to the LH2 tank, and a thrust structure, whose structure is bolted. These structures are then aligned in the FAJ, together with certain elements such as brackets for equipment, and bolted together with approximately 2000 fasteners. Once primary structural assembly of the engine section is complete, it then enters the "clean" phase of manufacture, during which it is kept in a controlled environment. For engine sections integrated at Michoud Assembly Facility, small enclosures with airlocks were set up surrounding the article, torn down and rebuilt as needed. In contrast, for assembly at the SSPF, the stage is kept in a controlled building with airlock. Here, installation of hardware takes place. Piping for hydraulic and pneumatic systems is joined, as well as ducting for the rocket's cryogenic propellants. Electrical wiring is added and installed to harnesses, and avionics hardware and instrumentation installed. After these line installations are complete, larger subassemblies such as the thrust vector control platforms and helium tanks are lifted into the engine section from above and installed. Additionally, a boattail, the base of the engine section, is installed.