NASA Hedges On Critical Suit Battery Resupply

EMU with SAFER

EMU PLSS (top) with SAFER (bottom). Photo: NASA

NASA has confirmed to AmericaSpace that the Extravehicular Mobility Unit (EMU) Long Life Batteries (LLBs) will not be launching solely on SpX–4, as originally planned. Instead, according to NASA, two of the four LLBs have already been sent to Russia for launch aboard a Soyuz scheduled for late September, after SpX–4. Since August, ISS maintenance EVAs have been curtailed due to the current EMU battery issues, therefore making the resupply of new EMU batteries a top priority for NASA. Given the possibility of a launch delay of either SpaceX or Soyuz, NASA’s decision to hedge and split the payload between the upcoming SpaceX cargo flight and a later Soyuz flight appears prudent.Over a month ago, a failure was discovered in one of the astronaut Extravehicular Mobility Unit (EMU) Long Life Batteries (LLBs) that concerned one of the battery’s parts. All electrical parts in the life-support system, that is in the portable life-support system (PLSS) and secondary oxygen pack (SOP), are burned-in[1]. Since all LLB fuses use the same burn-in procedure and test setup, the concern was that other long-life batteries could have the same problem. In early August, the crew began removing the LLB’s from the onboard EMU’s to be returned to the ground, and replacement LLB’s were scheduled for launch on SpaceX–4 in September 2014.

The history of the EMU—and how it is powered—is interesting. Until the 1990s, the EMU had a silver-zinc battery that was 20.5 V and 45 Ah capacity. The EMU’s portable life-support system (PLSS) draws power from the battery during the entire period of an EVA. Due to the disadvantages of using the silver-zinc battery in terms of cost and performance, a new high-energy density battery was developed in the late 1990s. It was called the Long Life Battery (LLB).

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EMU Long Life Battery, showing detail of power cells. Photo: NASA

The current long-life battery is a Lithium-ion battery, or LIB, that consists of Li-ion polymer cells that provide power to the EMU suit. The battery design consists of five 8 Ah cells in parallel to form a single module of 40 Ah, and five such modules will be placed in series to give a 20.5 V, 40 Ah battery. Charging is accomplished on the Station using the LIB charger or the existing ALPS (Air Lock Power Supply) charger.

The LLB delivers a maximum of 3.8 A on the average, for seven continuous hours, at voltages ranging from 20.5 V to 16.0 V, and it should be capable of supporting transient pulses during start up and once every hour to support PLSS fan and pump operation[2].

The EMU provides the needs of life-support such as oxygen, CO2 removal, temperature control, and meteroid protection during an EVA. The EMU weighs approximately 225 lbs (102 kg) and has a overall storage envelope of 26 x 28 x 40 inches (66 x 71 x 102 cm). The EMU is pressurized to 4 psid (27 kpas). In 1978 the EMU had only a six-year lifespan. After several studies in 1986, 1991, 1992, and 1994 that was expanded to a 15-year lifespan with cleaning and drying between uses[3]. Items that do require more frequent replacement are the polyester/polyurethane pressure seals in disconnects and bearings, which have an eight-year lifespan[4]. The EMU has expendables for seven hours inside, including 15 minutes for checkout, six hours for EVA, 15 minutes for doffing, and 30 minutes in reserve.

The PLSS used today, part of the EVA suit life-support system (LSS), is an advanced version of the Apollo PLSS. It consists of two units: the primary and secondary units. The primary unit measures 80 x 58.4 x 17 cm[5]. The PLSS contains five major groups of components for live support: oxygen ventilation, condensate, feedwater, liquid transport, and primary oxygen circuits. The secondary unit, called the secondary oxygen pack (SOP), contains backup oxygen for breathing, ventilation, stable pressure, and cooling, in case the primary oxygen tanks or a suit leak. The SOP is located at the bottom of the PLSS[6].

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