5.3 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| RS-25 | 3/9 | https://en.wikipedia.org/wiki/RS-25 | reference | science, encyclopedia | 2026-05-05T13:21:33.138292+00:00 | kb-cron |
==== 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.
==== Main combustion chamber ==== The engine's main combustion chamber (MCC) receives fuel-rich hot gas from a hot-gas manifold cooling circuit. The gaseous hydrogen and liquid oxygen enter the chamber at the injector, which mixes the propellants. The mixture is ignited by the "Augmented Spark Igniter", an H2/O2 flame at the center of the injector head. The main injector and dome assembly are welded to the hot-gas manifold, and the MCC is also bolted to the hot-gas manifold. The MCC comprises a structural shell made of Inconel 718 which is lined with a copper-silver-zirconium alloy called NARloy-Z, developed specifically for the RS-25 in the 1970s. Around 390 channels are machined into the liner wall to carry liquid hydrogen through the liner to provide MCC cooling, as the temperature in the combustion chamber reaches 3,300 °C (5,970 °F) during flight – higher than the boiling point of iron. An alternative for the construction of RS-25 engines to be used in SLS missions is the use of advanced structural ceramics, such as thermal barrier coatings (TBCs) and ceramic-matrix composites (CMCs). These materials possess significantly lower thermal conductivities than metallic alloys, thus allowing more efficient combustion and reducing the cooling requirements. TBCs are thin ceramic oxide layers deposited on metallic components, acting as a thermal barrier between hot gaseous combustion products and the metallic shell. A TBC applied to the Inconel 718 shell during production could extend engine life and reduce cooling costs. Further, CMCs have been studied as a replacement for Ni-based superalloys and are composed of high-strength fibers (BN, C) continuously dispersed in a SiC matrix. An MCC composed of a CMC, though less studied and farther from fruition than the application of a TBC, could offer unprecedented levels of engine efficiency.
=== Nozzle ===
The engine's nozzle is 121 in (3.1 m) long with a diameter of 10.3 inches (0.26 m) at its throat and 90.7 inches (2.30 m) at its exit. The nozzle is a bell-shaped extension bolted to the main combustion chamber, referred to as a de Laval nozzle. The RS-25 nozzle has an unusually large expansion ratio (about 69:1) for the chamber pressure. At sea level, a nozzle of this ratio would normally undergo flow separation of the jet from the nozzle, which would cause control difficulties and could even mechanically damage the vehicle. However, to aid the engine's operation Rocketdyne engineers varied the angle of the nozzle walls from the theoretical optimum for thrust, reducing it near the exit. This raises the pressure just around the rim to an absolute pressure between 4.6 and 5.7 psi (32 and 39 kPa), and prevents flow separation. The inner part of the flow is at much lower pressure, around 2 psi (14 kPa) or less. The inner surface of each nozzle is cooled by liquid hydrogen flowing through brazed stainless steel tube wall coolant passages. On the Space Shuttle, a support ring welded to the forward end of the nozzle is the engine attach point to the orbiter-supplied heat shield. Thermal protection is necessary because of the exposure portions of the nozzles experience during the launch, ascent, on-orbit and entry phases of a mission. The insulation consists of four layers of metallic batting covered with a metallic foil and screening.
=== Controller ===