RS-25
The RS-25, also known as the Space Shuttle Main Engine, is a liquid-fuel cryogenic rocket engine that was used on NASA's Space Shuttle and is used on the Space Launch System.
Designed and manufactured in the United States by Rocketdyne, the RS-25 burns cryogenic liquid hydrogen and liquid oxygen propellants, with each engine producing thrust at liftoff. Although RS-25 heritage traces back to the 1960s, its concerted development began in the 1970s with the first flight, STS-1, on April 12, 1981. The RS-25 has undergone upgrades over its operational history to improve the engine's thrust, reliability, safety, and maintenance load.
The engine produces a specific impulse of 452 seconds in vacuum, or 366 seconds at sea level, has a mass of approximately, and is capable of throttling between 67% and 109% of its [|rated power level] in one-percent increments. Components of the RS-25 operate at temperatures ranging from.
The Space Shuttle used a cluster of three RS-25 engines mounted at the stern of the orbiter, with fuel drawn from the external tank. The engines were used for propulsion throughout the spacecraft ascent, with total thrust increased by two solid rocket boosters and the orbiter's two AJ10 orbital maneuvering system engines. Following each flight, the RS-25 engines were removed from the orbiter, inspected, refurbished, and then reused on another mission.
Four RS-25 engines are installed on each Space Launch System, housed in the engine section at the base of the core stage, and expended after use. The first four Space Launch System flights use modernized and refurbished engines built for the Space Shuttle program. Subsequent flights will make use of a simplified RS-25E engine called the Production Restart, which is under testing and development.
Components
The RS-25 engine consists of pumps, valves, and other components working in concert to produce thrust. Fuel and oxidizer from the Space Shuttle's external tank entered the orbiter at the umbilical disconnect valves and from there flowed through the orbiter's main propulsion system feed lines; whereas in the Space Launch System, fuel and oxidizer from the rocket's core stage flow directly into the MPS lines. Once in the MPS lines, the fuel and oxidizer each branch out into separate paths to each engine. In each branch, pre-valves then allow the propellants to enter the engine.Once in the engine, the propellants flow through low-pressure fuel and oxidizer turbopumps, and from there into high-pressure turbopumps. From these HPTPs the propellants take different routes through the engine. The oxidizer is split into four separate paths: to the oxidizer heat exchanger, which then splits into the oxidizer tank pressurization and pogo suppression systems; to the low-pressure oxidizer turbopump ; to the high-pressure oxidizer pre-burner, from which it is split into the HPFTP turbine and HPOTP before being reunited in the hot gas manifold and sent on to the main combustion chamber ; or directly into the main combustion chamber injectors.
Meanwhile, fuel flows through the main fuel valve into regenerative cooling systems for the nozzle and MCC, or through the chamber coolant valve. The fuel passing through the MCC cooling system then passes back through the LPFTP turbine before being routed either to the fuel tank pressurization system or to the hot gas manifold cooling system. Fuel in the nozzle cooling and chamber coolant valve systems is then sent via pre-burners into the HPFTP turbine and HPOTP before being reunited again in the hot gas manifold, from where it passes into the MCC injectors. Once in the injectors, the propellants are mixed and injected into the main combustion chamber where they are ignited. The ejection of the burning propellant mixture through the throat and bell of the engine's nozzle creates the thrust.
Turbopumps
Oxidizer system
The low-pressure oxidizer turbopump is an axial-flow pump which operates at approximately 5,150 rpm driven by a six-stage turbine powered by high-pressure liquid oxygen from the high-pressure oxidizer turbopump. It boosts the liquid oxygen's pressure from, with the flow from the LPOTP then being supplied to the HPOTP. During engine operation, the pressure boost permits the high-pressure oxidizer pump to operate at high speeds without cavitating. The LPOTP, which measures approximately, is connected to the vehicle propellant ducting and supported in a fixed position by being mounted on the launch vehicle's structure.Then, mounted before the HPOTP, is the pogo oscillation suppression system accumulator. For use, it is pre-and post-charged with and charged with gaseous from the heat exchanger, and, not having any membrane, it operates by continuously recirculating the charge gas. A number of baffles of various types are present inside the accumulator to control sloshing and turbulence, which is useful of itself and also to prevent the escape of gas into the low-pressure oxidizer duct to be ingested in the HPOTP.
The HPOTP consists of two single-stage centrifugal pumps mounted on a common shaft and driven by a two-stage, hot-gas turbine. The main pump boosts the liquid oxygen's pressure from while operating at approximately 28,120 rpm, giving a power output of. The HPOTP discharge flow splits into several paths, one of which drives the LPOTP turbine. Another path is to, and through, the main oxidizer valve and enters the main combustion chamber. Another small flow path is tapped off and sent to the oxidizer heat exchanger. The liquid oxygen flows through an anti-flood valve that prevents it from entering the heat exchanger until sufficient heat is present for the heat exchanger to utilize the heat contained in the gases discharged from the HPOTP turbine, converting the liquid oxygen to gas. The gas is sent to a manifold and then routed to pressurize the liquid oxygen tank. Another path enters the HPOTP second-stage pre-burner pump to boost the liquid oxygen's pressure from 30 to 51 MPa. It passes through the oxidizer pre-burner oxidizer valve into the oxidizer pre-burner and through the fuel pre-burner oxidizer valve into the fuel pre-burner. The HPOTP measures approximately. It is attached by flanges to the hot-gas manifold.
The HPOTP turbine and HPOTP pumps are mounted on a common shaft. Mixing of the fuel-rich hot gases in the turbine section and the liquid oxygen in the main pump can create a hazard and, to prevent this, the two sections are separated by a cavity that is continuously purged by the engine's helium supply during engine operation. Two seals minimize leakage into the cavity; one seal is located between the turbine section and the cavity, while the other is between the pump section and cavity. Loss of helium pressure in this cavity results in automatic engine shutdown.
Fuel system
The low-pressure fuel turbopump is an axial-flow pump driven by a two-stage turbine powered by gaseous hydrogen. It boosts the pressure of the liquid hydrogen from 30 to 276 psia and supplies it to the high-pressure fuel turbopump. During engine operation, the pressure boost provided by the LPFTP permits the HPFTP to operate at high speeds without cavitating. The LPFTP operates at around 16,185 rpm, and is approximately in size. It is connected to the vehicle propellant ducting and is supported in a fixed position by being mounted to the launch vehicle's structure.The HPFTP is a three-stage centrifugal pump driven by a two-stage hot-gas turbine. It boosts the pressure of the liquid hydrogen from 1.9 to 45 MPa, and operates at approximately 35,360 rpm with a power of. The discharge flow from the turbopump is routed to, and through, the main valve and is then split into three flow paths. One path is through the jacket of the main combustion chamber, where the hydrogen is used to cool the chamber walls. It is then routed from the main combustion chamber to the LPFTP, where it is used to drive the LPFTP turbine. A small portion of the flow from the LPFTP is then directed to a common manifold from all three engines to form a single path to the liquid hydrogen tank to maintain pressurization. The remaining hydrogen passes between the inner and outer walls of the hot-gas manifold to cool it and is then discharged into the main combustion chamber. A second hydrogen flow path from the main fuel valve is through the engine nozzle. It then joins the third flow path from the chamber coolant valve. This combined flow is then directed to the fuel and oxidizer pre-burners. The HPFTP is approximately in size and is attached to the hot-gas manifold by flanges.
Powerhead
Preburners
The oxidizer and fuel pre-burners are welded to the hot-gas manifold. The fuel and oxidizer enter the pre-burners and are mixed so that efficient combustion can occur. The augmented spark igniter is a small combination chamber located in the center of the injector of each pre-burner. Two dual-redundant spark igniters are activated by the engine controller and are used during the engine start sequence to initiate combustion in each pre-burner. They are turned off after approximately three seconds because the combustion process is then self-sustaining. The pre-burners produce the fuel-rich hot gases that pass through the turbines to generate the power needed to operate the high-pressure turbopumps. The oxidizer pre-burner's outflow drives a turbine that is connected to the HPOTP and to the oxidizer pre-burner pump. The fuel pre-burner's outflow drives a turbine that is connected to the HPFTP.The speed of the HPOTP and HPFTP turbines depends on the position of the corresponding oxidizer and fuel pre-burner oxidizer valves. These valves are positioned by the engine controller, which uses them to throttle the flow of liquid oxygen to the pre-burners and, thus, control engine thrust. The oxidizer and fuel pre-burner oxidizer valves increase or decrease the liquid oxygen flow, thus increasing or decreasing pre-burner chamber pressure, HPOTP and HPFTP turbine speed, and liquid oxygen and gaseous hydrogen flow into the main combustion chamber, which increases or decreases engine thrust. The oxidizer and fuel pre-burner valves operate together to throttle the engine and maintain a constant 6.03:1 propellant mixture ratio.
The main oxidizer and main fuel valves control the flow of liquid oxygen and liquid hydrogen into the engine and are controlled by each engine controller. When an engine is operating, the main valves are fully open.