ECLSS of NASA’s EMU

Abstract:

There is no doubt that human beings, as of yet, are the most versatile and adaptable experimenters we could count on, to explore microgravity, the Moon and Mars. In various space exploration missions, we can be sure that there are always going to many unknowns. Rovers and drones prove to be excellent for reliability and do not produce waste that needs to be managed. But they have one major limitation. Quick adaptability towards the numerous unknowns they come across, in terms of surprises in space environment, hardware malfunctions or both. Humans have proved their merit of solving problems innovatively[1]. Past lunar/microgravity explorers have shared their invaluable experiences thereby advocating the need for further human-space exploration. This document discusses about the life support system machinery that morphs the spacesuit into a habitable spacecraft. 

“The heavy spacesuits are spectacular to look at but very hot. Putting one on was like going from chilly London winter weather to the Bahamas in just minutes.”

-       Kathleen Quinlan (Actress from 1995 film Apollo 13)

Introduction:

To keep human space explorers alive, space suits have to shield them from the harsh external environment and provide pressure, oxygen, water and some food. As much as feasible, shielding against radiation, blinding light and micrometeorites potentially travelling faster than 17,500 mph is also appreciated. Furthermore, these spacesuits are required to remove Carbon dioxide, excess humidity, biological waste (urine/faeces) and maintain a comfortable range of temperatures to facilitate health, comfort and productivity. In addition to this, the spacesuits are expected to be reliable and ergonomically advanced. The NASA EMU can be used for Intravehicular Activities (IVAs) at lower levels of pressurization for scenarios such as take-off and landing. The EMUs can work in two different configurations defined by the source for consumables, which might either be an umbilical or an independent life support system.

Hence, we can encapsulate that humans require environmental protection along with consumable provision and waste management while functioning in the spacesuit. This serves as a classic scenario for inviting ECLSS[1]to save the day. The NASA EMU has an internal environment provision capability of 4.5 psi at 100% oxygen[2]. These spacesuits also provide thermal control via a liquid cooling and ventilation garment (LCVG) and remote powered electrically heated gloves[3].

NASA EMU ECLSS review:

Most of the ECLSS hardware for the NASA EMU is located in the Primary Life Support Subsystem (PLSS). The PLSS (weight: 101.4 lbs) provides a suitable breathing environment to the crew members via a set of fluid circuits. The Primary Oxygen circuit forms a closed loop with the pressure garment assembly by supplying Oxygen at a regulated pressure to facilitate suit pressurization for IVAs and EVAs, breathing, eliminating exhaled gases and water tank water expulsion (this tank supplies expendable water to the sublimator). This system has the capability to pressurise the spacesuit at 0.9+0.5 psid in IVA mode and 4.3+0.1 psid in EVA mode. It uses two O2 bottles with maximum operating pressure of 1050 psia and a volume of 240 cu.in. per bottle. 

Cooling of the crew member takes place via the LCVG (weight: 8.57 lbs). Water is pumped through the liquid transport circuit (including the LCVG) to a sublimator that is open to atmosphere (vacuum). Here, the water sublimates, thereby giving up the absorbed heat in the form of latent heat release. The LCVG absorbs body heat by convection via water flowing in an ethylene vinyl acetate tubing woven through spandex[4].

The exhaled CO2 is removed from the breathing environment by a Metox chemical contaminant control cartridge (CCC)[5]. It can absorb 1.48 lb of CO2. This is equivalent to metabolic rates of about 1000 Btu/Hr for 7 hours. Higher metabolic rates can be accommodated for by controlling the CO2 partial pressure. The CCC also houses a filter for trace contaminant collection and charcoal for odour control. Allowable pressure leakage limit for the CCC is 0.17 psid. 

There are a number of scenarios when the Primary Oxygen circuit might fail for example, pressure regulator failure, high O2 demand or opening of the purge valves due to trace contaminants, heat, humidity, etc. Hence, the PLSS also houses a Secondary Oxygen Pack (SOP) for redundancy. It weighs 23.2 lbs when charged with O2. It is capable of life support provision for 30 minutes with a flow range of 0.06 to 5.26 PPH O2.

For control and monitoring of the ECLSS systems, the EMU has a Display and Controls Module (DCM) and an Enhanced Caution and Warning Systems (ECWS). Allowable pressure leakage limit for the DCM (weight: 14.3 lbs) is 0.33 psid. The ECWS systems provides output in terms of caution/warning tones at different decibels across the 600-ohm earphones. The pressure transducer and CO2 sensor measurements are visible to the crew member on a Liquid Crystal Display (LCD).

For consumables replenishing, the EMU can be connected to the Service and Cooling Umbilical (SCU) after the EVA. The SCU (weight: 15.7 lbs) also supplies cooling and expendables during umbilical operations such as take off and landing. Furthermore, they facilitate wastewater drainage. The CCC and the diapers used for waste collection are one of the very few consumables that cannot be recharged via the SCU. The Metox CCCs (weight: 32 lbs) are regenerated in the ISS after each use. The SCU is 252 inches long when fully extended and weights about 30 pounds.

With the absence of atmosphere in space environments, sunlight can be intense and blinding. To avoid visor burn, the EMU has an Extravehicular Visor Assembly (EVVA). EVVA (weight: 7.2 lbs) also protects the crew members against visual, thermal and MMOD hazards. 

In terms of drinking water provision, the EMU has the Disposable In-Suit Drink Bag (DIDB) mounted within a reusable Restraint Bag. The DIDB (weight: 2.48 lbs) can provide up to 32 0z of potable water during the EVA via a flexible drink tube that can be actuated by applying oral suction. The bacteria control and water filters used in the potable water supply can be used for up to 36 EVAs.

The 3000 series battery used in the EMUs is rechargeable for about 50 cycles over 5 years. It can provide power of 26.6 amp-hrs over 20.6 to 17.5 Volts for 7 hours. The charged life for this battery is 600 days and it weighs 15.5 lbs.

For maintainability purposes, the On-Orbit Replaceable Units (ORUs) such as the PLSS, SOP, DCM and the Hard Upper Torso (HUT) are 100% interchangeable with spare EMUs onboard. 

Discussion:

With the complication of fitting all the fluid/electronic circuits in such a flexible spacecraft such as the EMU, complications with respect to leaks can use high levels of ingenuity. Historically, crew members have had the risk for drowning in space twice due to the helmet having water leaks[6]. This can be attributed to the ventilation breakdown at the fan-pump-separator. New cooling and ventilation concepts such as the Spacesuit water Membrane Evaporator (SWME), Space Evaporator Absorber Radiator (SEAR) and Lithium Chloride Absorber Radiator (LCAR) are being investigated for mitigating known risks and increasing efficiency[7]. This might also solve the problem of limited cooling surface area in the sublimator plate and the loss of water in the current cooling system[8]. Also, as spacesuits will need to be functional in dust prone partial gravity environments sustainably, newer cooling systems might pave the way towards achieving that goal. 

Conclusive remarks:

The EMU has flown in Apollo and Shuttle missions for the development of Skylab, Hubble and the ISS. It has kept hundreds of crew members alive and safe for several hundreds of work hours. To assemble the ISS alone, 162 EVAs spanning 1021 work hours were performed. Unforeseen events such as the Skylab emergency and Shuttle EMU fire have led to serious redesign[9]. In spite of the EMUs limitations, mankind could extend its limits to the moon and back and this alone is the reason to celebrate the EMU’s achievements before it retires. 

References:

1.     T. Clark, ASEN 5016: Space Life Sciences (Lecture 2, Jan. 2021), University of Colorado Boulder

2.     A. Anderson, ASEN 6519-002B: Extravehicular Activity (Lecture 2, Jan. 2021), University of Colorado Boulder

3.     Book: US Spacesuits, 2ndedition by K. Thomas and H. McMann Chapters 3 & 10

4.     NASA EMU Data Book: UTC Aerospace (Sept. 2017), sections 1,2 & 3.

5.     Larson, Kipp A., and J. Nabity at the 47thICES, July 2017

6.     J. Stroming and D. Newman in ICES-2019-338

7.     C. Massina, J. Nabity, D. Klaus in the Journal of Spacecrafts and Rockets: Thermal Vacuum Evaluation of Simulated Spacecraft Radiators with Discretized Emissivity Surface Properties: DOI: 10.2514/1.A33654 

8.     C. Chullen, J. McMann, K. Thomas, J. Kosmo, C. Lewis, R. Wright, R. Bitterly and V. Oliva in “U.S. Spacesuit legacy: Maintaining it for the Future” in the 43rdInternational Conference on Environmental Systems (July 2013).