kb/data/en.wikipedia.org/wiki/RS-25-3.md

title	chunk	source	category	tags	date_saved	instance
RS-25	4/9	https://en.wikipedia.org/wiki/RS-25	reference	science, encyclopedia	2026-05-05T13:21:33.138292+00:00	kb-cron

Each engine is equipped with a main engine controller (MEC), an integrated computer which controls all of the engine's functions (through the use of valves) and monitors its performance. Built by Honeywell Aerospace, each MEC originally comprised two redundant Honeywell HDC-601 computers, later upgraded to a system composed of two doubly redundant Motorola 68000 (M68000) processors (for a total of four M68000s per controller). Having the controller installed on the engine itself greatly simplifies the wiring between the engine and the launch vehicle, because all the sensors and actuators are connected directly to only the controller, each MEC then being connected to the orbiter's general purpose computers (GPCs) or the SLS's avionics suite via its own engine interface unit (EIU). Using a dedicated system also simplifies the software and thus improves its reliability. Two independent dual-CPU computers, A and B, form the controller; giving redundancy to the system. The failure of controller system A automatically leads to a switch-over to controller system B without impeding operational capabilities; the subsequent failure of controller system B would provide a graceful shutdown of the engine. Within each system (A and B), the two M68000s operate in lock-step, thereby enabling each system to detect failures by comparing the signal levels on the buses of the two M68000 processors within that system. If differences are encountered between the two buses, then an interrupt is generated and control turned over to the other system. Because of subtle differences between M68000s from Motorola and the second source manufacturer TRW, each system uses M68000s from the same manufacturer (for instance system A would have two Motorola CPUs while system B would have two CPUs manufactured by TRW). Memory for block I controllers was of the plated-wire type, which functions in a manner similar to magnetic core memory and retains data even after power is turned off. Block II controllers used conventional CMOS static RAM. The controllers were designed to be tough enough to survive the forces of launch and proved to be extremely resilient to damage. During the investigation of the Challenger accident the two MECs (from engines 2020 and 2021), recovered from the seafloor, were delivered to Honeywell Aerospace for examination and analysis. One controller was broken open on one side, and both were severely corroded and damaged by marine life. Both units were disassembled and the memory units flushed with deionized water. After they were dried and vacuum baked, data from these units was retrieved for forensic examination.

==== Main valves ==== To control the engine's output, the MEC operates five hydraulically actuated propellant valves on each engine; the oxidizer pre-burner oxidizer, fuel pre-burner oxidizer, main oxidizer, main fuel, and chamber coolant valves. In an emergency, the valves can be fully closed by using the engine's helium supply system as a backup actuation system. In the Space Shuttle, the main oxidizer and fuel bleed valves were used after shutdown to dump any residual propellant, with residual liquid oxygen venting through the engine and residual liquid hydrogen venting through the liquid hydrogen fill and drain valves. After the dump was completed, the valves closed and remained closed for the remainder of the mission. A coolant control valve is mounted on the combustion chamber coolant bypass duct of each engine. The engine controller regulates the amount of gaseous hydrogen allowed to bypass the nozzle coolant loop, thus controlling its temperature. The chamber coolant valve is 100% open before the engine start. During engine operation, it is 100% open for throttle settings of 100 to 109%. For throttle settings between 65 and 100%, its position ranged from 66.4 to 100%.

=== Gimbal ===

Each engine is installed with a gimbal bearing, a universal ball and socket joint which is bolted to the launch vehicle by its upper flange and to the engine by its lower flange. It represents the thrust interface between the engine and the launch vehicle, supporting 7,480 lb (3,390 kg) of engine weight and withstanding over 500,000 lbf (2,200,000 N) of thrust. As well as providing a means to attach the engine to the launch vehicle, the gimbal bearing allows the engine to be pivoted (or "gimballed") around two axes of freedom with a range of ±10.5°. This motion allows the engine's thrust vector to be altered, thus steering the vehicle into the correct orientation. The comparatively large gimbal range is necessary to correct for the pitch momentum that occurs due to the constantly shifting center of mass as the vehicle burns fuel in flight and after booster separation. The bearing assembly is approximately 290 by 360 mm (11 by 14 in), has a mass of 105 lb (48 kg), and is made of titanium alloy. The low-pressure oxygen and low-pressure fuel turbopumps were mounted 180° apart on the orbiter's aft fuselage thrust structure. The lines from the low-pressure turbopumps to the high-pressure turbopumps contain flexible bellows that enable the low-pressure turbopumps to remain stationary while the rest of the engine is gimbaled for thrust vector control, and also to prevent damage to the pumps when loads were applied to them. The liquid-hydrogen line from the LPFTP to the HPFTP is insulated to prevent the formation of liquid air.

=== Helium system === In addition to fuel and oxidizer systems, the launch vehicle's main propulsion system is also equipped with a helium system consisting of ten storage tanks in addition to various regulators, check valves, distribution lines, and control valves. The system is used in-flight to purge the engine and provides pressure for actuating engine valves within the propellant management system and during emergency shutdowns. During entry, on the Space Shuttle, any remaining helium was used to purge the engines during reentry and for repressurization.

== History ==

=== Development ===