National Aeronautics and Space Administration
3.4 Extravehicular Mobility Unit (EMU) Operational Description The unanticipated introduction of a large quantity (1 to 1-1/2 liters) of water into EV2â€™s helmet focused immediate attention upon the operation of the EMU PLSS subsystems, as well as other elements of the system, including the water-bearing components of the Space Suit Assembly (SSA). In order to facilitate understanding of what items were investigated; how the investigations were carried out; and the meaning of results, some basic understanding of the EMU and its operation is necessary. The following sections seek to provide this understanding. The description of the EMU, as well as its overall performance requirements, are presented first. Subsequent sections delve into the subsystems and individual components themselves, with an emphasis on those featuring interaction with the various sources of water and potential points of entry into the ventilation loop.
3.4.1 System Level EMU Description The Extravehicular Mobility Unit (EMU) (Figure 3-1 and Figure 3-2) is an independent anthropomorphic system that provides environmental protection, mobility, life support and communication for ISS crewmembers to perform Extravehicular Activity (EVA) in Earth orbit. EVA is defined for EMU design considerations as activity that occurs in environmental pressure below 4.0 psia. The EMU system is comprised of two main assemblies, the pressure garment (also known as the Space Suit Assembly or SSA) and the Portable Life Support System (PLSS) with the attached Secondary Oxygen Package (SOP). As seen in Figure 3-1, the two assemblies are covered in an outer garment (the Thermal Micro-meteoroid Garment or TMG) that acts as a barrier both to the thermal extremes of space and to impacts due to micro-meteoroids, cuts, and punctures. The SSA provides the pressurized environment, thermal management, and pressurized mobility for the astronaut wearing the suit. This assembly is comprised of layers of materials which provide several functions. Innermost is a coated nylon bladder which retains the pressurized gas inside the suit. Surrounding the bladder is a pressure restraint garment which carries the load of the suit pressure. Outside the restraint are five layers of rip-stop scrim and aluminized Mylar, which provide thermal isolation. Finally, the TMG surrounds these inner layers. The thickness of these layers is approximately half an inch, and less on the gloves, which enables astronauts to perform complex tasks like the intricate and delicate repairs to the Hubble Space Telescope or replacement of large modules such as a Pump Module on the ISS. The PLSS and SOP provide the life support, power and communication systems. The main components that are found in the PLSS/SOP assembly are the space-to-space radio, the high-pressure primary and secondary oxygen tanks, the primary and secondary water tanks for cooling, the fan/pump/separator, the METOX canister for CO2 removal, and the water sublimator for cooling. These systems are monitored by the Enhanced Caution and Warning System (ECWS) and controlled by the EVA crewmember using the Display and Control Module (DCM). Astronaut Electrocardiogram (ECG) and EMU performance parameter data are telemetered to the ground via a Real Time Data System (RTDS). The EMU data signals are provided once every two minutes, at the end of the ECG transmission. This arrangement was a compromise, since the EMU data items were added after the system was designed to provide continuous ECG data.
Published on Feb 27, 2014
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