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→Auxiliary Power Unit and Hydraulics System
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As of June 2015, the APU and hydraulic system is modeled with a fair amount of detail and operated from a dedicated menu. APUs need to be started as part of the pre-launch checklist - refer to Help/Aircraft Checklists for the detailed procedure. <b>If the hydraulic system is not available during ascent, this will result in loss of the vehicle after SRB separation as there is no control over the Shuttle if the SSMEs can not be gimbaled.</b> | As of June 2015, the APU and hydraulic system is modeled with a fair amount of detail and operated from a dedicated menu. APUs need to be started as part of the pre-launch checklist - refer to Help/Aircraft Checklists for the detailed procedure. <b>If the hydraulic system is not available during ascent, this will result in loss of the vehicle after SRB separation as there is no control over the Shuttle if the SSMEs can not be gimbaled.</b> | ||
=== Main Propulsion System === | |||
Under the name Main Propulsion System (MPS), the various subsystems operating the SSMEs are summarized. This includes the SSME controllers (two per engine for redundancy), the propellant feeding system supplying liquid hydrogen and oxygen to the engines and the various hydraulically operated valves, a helium system to supply purge gas flows and emergency hydraulics power and finally the engines themselves. | |||
The SSME's feed high-pressure propellants into the combustion chamber. Power for the turbo pumps is provided by partial pre-combustion of the propellant, and ullage pressure in the external tank is maintained by branching off a small fraction of vaporized propellant back into the tank. The precise opening of the propellant feeding valves which throttles the engines is governed by the controllers which in turn receive throttle commands from the Shuttle's guidance computers. | |||
For the most part, the MPS settings are controlled on the ground prior to launch and not changed during ascent, however after MECO there are about 5,200 lb of propellant trapped in the feeding manifolds which need to be dumped. During this propellant dump, high-pressure helium is used to vent liquid oxygen through the thruster exhausts while hydrogen is allowed to boil off through the fill/drain valves. | |||
In case of a hydraulic failure, the SSMEs can neither be gimbaled nor can their valves be changed. Each of the three hydraulic systems operated the valves of one engine, and each engine gimbal is supported by two hydraulic systems (i.e. it takes two failures to disable gimbal on one engine, but each hydraulic failure will disable valves on one engine). | |||
If the valve settings can no longer be changed, the engine can still continue to run, but it can't be throttled any more, a condition known as 'hydraulic lockup'. It is still possible to shut down such an engine using pressure from the helium system though. Similarly, if sensors monitoring combustion chamber conditions or the command path from guidance computer to engine controllers fail, the engine is in a condition called 'electric lockup' - the controller will continue to operate it with the last known settings. Locked-up engines usually need to be shut down manually using the cutoff switches about 30 seconds prior to nominal MECO. | |||
As of June 2015, the MPS is modeled in a good amount of detail, including most of the relevant valve settings, hydraulic and electric lockup, power failures on the engine controllers and the propellant dump sequence. The in-sim checklists provide instructions on how to execute the propellant dump and how to safe the engines for orbital operations. | |||
== Avionics and guidance systems == | == Avionics and guidance systems == | ||