eva-plss

EVA PLSS

Subsystem Hard import eva
TRL Mars
—
Energy intensity
—
Required by
0
Requires
4

Backpack-integrated life-support for EVA crew: O₂ supply + cabin-pressure control, CO₂ removal, water-based thermal management, in-suit comms, biomedical monitoring, and battery power. EMU PLSS (Skylab 1973 → ISS 2024) and xEMU PLSS (Artemis baseline) represent the production design space; Mars duty cycle and dust environment require harder reliability margins than either was designed for.

Last reviewed: 2026-06-09

Governing equations

PLSS O₂ supply rate = metabolic demand + suit-leakage makeup + intermittent vent purge. Typically 0.08–0.15 kg/h delivered. [1]

Sublimator cooling rate = water evaporation rate × latent heat of sublimation (2.84 MJ/kg at vacuum). At 350 W cooling, water consumption ~ 0.44 kg/h. [1]

Suit CO₂ concentration vs time, balancing crew production against sorbent uptake. Above 7 mmHg (NASA-STD-3001), cognitive impairment; above 50 mmHg, lethal. [2]

LiOH canister duration. Each kg LiOH absorbs ~ 0.92 kg CO₂ stoichiometrically. EMU PLSS uses 2 × 0.5 kg canisters for 7 h EVA. [3]

Key constants & quantities

Symbol Value Units Conditions Description
m_PLSS,EMU 65 ±5 % kg (Earth weight) — EMU PLSS mass without consumables — pumps, fans, regulators, sublimator, sorbent canisters, battery, electronics.[1]
t_EVA,EMU 7 h (designed) — EMU PLSS designed EVA duration. Limit set by water sublimator capacity + LiOH consumption + battery, in that order.[1]
t_EVA,xEMU 8 h (designed) — xEMU PLSS designed duration; achieved via regenerable RCA CO₂ scrubber + improved thermal management.[4]
ṁ_water,sublimator 0.3–0.6 ±15 % kg / h water evaporated — Sublimator water consumption rate at metabolic loads. Higher activity → higher rate.[1]
m_water,EVA 2.5–4.5 kg total per EVA — PLSS water consumed during one 7–8 h EVA. Includes sublimator + drinking + makeup.[1]
E_battery,PLSS 1.5 ±15 % kWh (battery capacity) — EMU PLSS Li-ion battery capacity. Powers fans, pumps, comms, display electronics, biomedical telemetry for 8+ h.[1]
p_O₂,emergency 1 h (emergency supply) — Secondary Oxygen Pack (SOP) emergency reserve. Bridge between primary failure and airlock abort.[1]

Operating envelope

ParameterRangeUnitsSource
O₂ delivery rate 0.08 – 0.2 kg/h [1]
CO₂ removal capacity 0.8 – 1.2 kg per EVA (4-crew) [1]
Cooling capacity 200 – 600 W metabolic + electronics [1]
Water consumption (sublimator) 0.3 – 0.6 kg/h [1]
Battery state-of-charge band 20 – 100 % [1]

Mass balance

Basis: 1 crew × 1 EVA × 8 h Mars surface excursion

Inputs

O₂ tank inventory 1.5 kg [1]
Water (LCVG + sublimator + drinking) 3.5 kg [1]
CO₂ scrubber capacity (RCA or LiOH) 1.2 kg CO₂ absorbed [4]
Battery energy 1.5 kWh [1]

Outputs

Breathing gas to crew 1 kg O₂ consumed metabolically [3]
Cabin pressure maintenance (leakage makeup) 0.4 kg O₂ leaked + purged [1]
Water vapor (sublimator vent) 3 kg to Mars vacuum [1]
CO₂ + heat (rejected to environment via PLSS) 1 kg CO₂ + 1.0 kWh heat [1]
TRL · Earth
9/ 9
TRL · Mars
7/ 9
EMU PLSS: TRL 9 — Skylab 1973, ISS 2000 onward, 250+ EVAs. xEMU PLSS: TRL 8 in integrated ground test (Artemis program). For Mars: TRL 6–7 — xEMU is directly transferable with dust-protection upgrades; multi-month duty cycle reliability unconfirmed in Mars conditions.[5]
Energy budget
0.19 kWhe / crew · EVA hour [1]

PLSS battery sized for 8 h with 25% margin. Includes pumps, fans, comms, lighting, helmet HUD, biomedical telemetry, control electronics.

Variants & trade-offs

EMU PLSS (ISS heritage, LiOH CO₂)

[1]

Skylab-derived architecture: high-pressure O₂ + water sublimator + LiOH single-use CO₂ canister + Ag-Zn battery (later Li-ion). The 1973–present design baseline for U.S. spacewalk.

EVA duration
6.5–7.5 h
CO₂ removal architecture
0–0 LiOH single-use
Stack lifetime
60–200 EVA hours per PLSS
Materials: Aluminum pressure shell · Sublimator porous plate (sintered nickel) · LiOH granular canister · Li-ion battery (post-2020 retrofit) · Centrifugal fan + water pump
  • Largest cumulative space flight experience
  • Robust to varied EVA workloads
  • Multiple-EVA refurbishable in habitat
  • LiOH consumable mass is non-recoverable
  • Sublimator water consumption (~ 0.4 kg/h) demands large water budget
  • Parmitano-style separator vulnerability requires post-2013 mitigations

xEMU PLSS (Artemis baseline, RCA CO₂)

[4]

Modernization with rapid-cycle amine (RCA) regenerable CO₂ scrubber, Spacecraft Water Membrane Evaporator (SWME) instead of porous-plate sublimator, redundant water pump, Li-ion battery, enhanced biomedical telemetry.

EVA duration
7–8.5 h
CO₂ removal architecture
0–0 RCA regenerable amine
Stack lifetime
200–500 EVA hours per PLSS
Materials: Composite pressure shell · Amine-functionalized sorbent (Sorbead R) · SWME hollow-fiber membrane · Magnetic-bearing fans + pumps · Li-ion battery (NASA-qualified)
  • Regenerable CO₂ scrubber — no consumable
  • SWME water-evaporator more leak-tolerant than sublimator
  • Redundant pumps and sensors
  • Designed for higher cadence than EMU
  • RCA sorbent uses Pt-group catalysts — hard imports on Mars
  • Higher complexity = more potential failure modes
  • Less flight-hours than EMU

Soviet/Russian Orlan PLSS (rear-entry, alkaline scrubber)

[6]

Single-piece PLSS integrated into rear hatch of suit; KOH-based CO₂ scrubber, copper-mesh + cobalt-rare-earth fan, NiMH battery.

EVA duration
5–7 h
CO₂ removal architecture
0–0 KOH-based regenerable
Stack lifetime
40–150 EVA hours
Materials: Aluminum + steel pressure shell · KOH-canister scrubber · NiMH battery · Single-fan ventilation
  • Decades of operational flight (Mir + ISS)
  • Simpler architecture — fewer wear items
  • Lower per-EVA consumable mass than LiOH
  • Lower power output limits EVA flexibility
  • KOH carbonation parallels alkaline-electrolyzer issues
  • Heavier per kg/EVA than xEMU

Failure modes

Mode Cause Detection Mitigation
Sublimator water-flood (Parmitano mode)[7] Porous-plate clogged by contaminants; cooling water back-flows into ventilation loop; water collects in helmet. Crew reports water in helmet; cabin H₂O monitor; humidity spike. Helmet absorbent pad / snorkel; pre-EVA loop flush; SWME architecture (xEMU) replaces porous plate with hollow-fiber membrane.
CO₂ scrubber breakthrough[2] LiOH canister fully consumed early (high work-rate); or RCA bed contamination prevents regeneration. In-suit CO₂ sensor crosses threshold (5.3 mmHg WARN, 7 mmHg ABORT). Secondary CO₂ canister; immediate abort protocol; pre-EVA bed-saturation check.
Primary fan failure[1] Bearing wear or contamination; ventilation loop loses flow. Flow sensor in ventilation circuit; crew reports stale air. Redundant fan with manual changeover; auto-failover (xEMU); abort if both fail.
O₂ regulator stuck closed / open[1] Mechanical jam in pressure-regulating valve; can over- or under-pressurize suit. Suit pressure monitor; redundant pressure transducers. Manual O₂ supply backup; immediate abort if regulator failure indicated; SOP (Secondary Oxygen Pack) deployed.
Battery thermal runaway[8] Internal short, mechanical damage, or charging excursion; Li-ion cell ignites in low-pressure / high-O₂ environment. Battery T sensor spike; pack voltage drop. Cell-level fusing; ceramic-fiber inter-cell barrier; thermal-runaway propagation testing (xEMU); abort to airlock immediately.
Comms loss in-suit[5] Transceiver failure; antenna dust occlusion; relay station out of view. Telemetry dropout from ground / habitat side. Dual-redundant radio; tethered backup; abort-if-loss-exceeds-N-minutes protocol.
Dust contamination of PLSS air intake[9] Mars dust ingress through suit-port seal or helmet vent; abrades fan + clogs sorbent. Fan current rises; pressure drop across sorbent climbs. HEPA pre-filter at all intake points; suit-port architecture eliminates intake exposure to interior dust; periodic field-replaceable filter cassettes.

Mars adjustments

Water-vapor sublimator works better on Mars than ISS vacuum[1]

Impact: Mars ambient ~ 600 Pa is well above the triple point but well below the water vapor pressure at 0 °C. Sublimator water evaporates effectively at higher temperatures (lower sublimation pressure differential) compared to true space vacuum.

Mitigation: Real benefit — Mars sublimator operates with somewhat reduced water consumption per cooling rate. SWME variant on xEMU even better-suited.

PLSS dust ingress at fan + valve + filter interfaces[9]

Impact: Mars dust contaminates PLSS at every air interface. Across 400+ EVAs per Mars stay, cumulative particle ingress degrades sorbent, fouls valves, abrades fans.

Mitigation: Suit-port architecture eliminates dust exposure to PLSS interior; HEPA pre-filters on all intake; programmed PLSS service intervals matched to dust accumulation.

Long-duration cumulative use vs ISS[5]

Impact: ISS EVA: ~ 10/year per PLSS unit. Mars: 50–100/year. Bearing + fan + sorbent wear scales linearly with operating hours.

Mitigation: Higher TRL spares stockpile (4× ISS); modular PLSS architecture (xEMU); on-site refurbishment of CO₂ sorbent via Sabatier waste heat.

PLSS-to-Sabatier integration opportunity[10]

Impact: CO₂ captured from EVA sorbent can feed the habitat Sabatier reactor. Per EVA: 1.2 kg CO₂ → 0.4 kg methane + 0.5 kg water (recoverable). Multiplied across Mars stay: ~ 1000 kg water + 400 kg propellant.

Mitigation: Architect PLSS desorb cycle for direct transfer to Sabatier feed buffer; integrate with habitat ECLSS plumbing.

PLSS battery cold-soak before EVA[8]

Impact: Suit-port architecture parks suit outside between EVAs. PLSS battery sees Mars night cold-soak (−90 °C) before each EVA — Li-ion charging at low T causes plating damage.

Mitigation: Pre-EVA battery warm-up cycle from habitat power; insulated battery enclosure inside PLSS; LFP chemistry where Mars-compatible.

Alternatives & substitutes

Umbilical-tethered life support (no PLSS)[5]

  • Eliminates PLSS mass on the crew
  • Unlimited duration if hose flexible enough
  • No CO₂ or O₂ inventory on suit
  • Range limited by umbilical length (typically < 100 m)
  • Tether-snag risk on Mars terrain
  • Tether breach = catastrophic suit decompression

When preferred: Construction tasks adjacent to airlock; emergency suit-up; never wide-area surface ops.

Rover-mounted external life support[5]

  • Crew suit acts as just a pressure bag; bulk LSS in rover
  • Longer effective EVA range than tethered PLSS
  • Shared infrastructure across crew
  • Restricted to rover proximity
  • Cannot disengage from rover quickly
  • Single rover failure = multiple EVA loss

When preferred: Combined excursion / sample-collection ops; field reconnaissance with rover; not standalone EVA replacement.

Requires

Inputs

References

  1. Hamilton Sundstrand (2008). EMU Data Book. Hamilton Sundstrand / NASA Johnson Space Center. EMU-DB-005. — NASA Extravehicular Mobility Unit (EMU) reference: pressure, mass, PLSS architecture, consumables, operational data.
  2. National Aeronautics and Space Administration (2023). NASA Space Flight Human-System Standard, Volume 2: Human Factors, Habitability, and Environmental Health. NASA. NASA-STD-3001 Vol. 2 Rev. C. — Cabin CO₂ partial-pressure limits; crew habitat environmental health standard.
  3. Wieland, P. O. (1998). Living Together in Space: The Design and Operation of the Life Support Systems on the International Space Station. NASA Marshall Space Flight Center, NASA/TM-98-206956. NASA/TM-98-206956. — NASA Baseline Values & Assumptions (BVAD); LiOH, amine, and zeolite scrubber trade study.
  4. Watts, C. R., Conger, B., Anderson, M. S., & Vonau, W. (2015). Rapid Cycle Amine Swing Bed System for Carbon Dioxide and Humidity Control. 45th International Conference on Environmental Systems, ICES-2015-141. — xEMU PLSS RCA (Rapid Cycle Amine) CO₂ scrubber architecture; performance data + heritage.
  5. Larson, W. J., & Pranke, L. K. (Eds.) (1999). Human Spaceflight: Mission Analysis and Design. McGraw-Hill. ISBN 978-0-07-236811-4. — Standard reference for crewed-mission engineering: EVA architectures, life support, mission design, system trades.
  6. Abramov, I. P., & Skoog, A. I. (2003). Russian Spacesuits. Springer-Praxis. ISBN 978-1-85233-732-9. — Soviet/Russian Orlan + Krechet engineering history; 40 kPa pressure architecture.
  7. NASA EVA-23 Mishap Investigation Board (2014). International Space Station EVA Suit Water Intrusion High Visibility Close Call. NASA Johnson Space Center, JSC-EVA-13-001. NASA JSC-EVA-13-001. — Luca Parmitano helmet water flood incident (EVA-23, July 2013); sublimator separator failure analysis + corrective actions.
  8. Reid, C. M., Manzo, M. A., & Logan, M. J. (2007). Performance Characterization of Lithium-Ion Cells for Aerospace Applications. NASA Glenn Research Center, NASA/TM-2007-214958. NASA/TM-2007-214958. — NASA Glenn Li-ion testing at low temperature, cold-soak performance, aerospace cycling models.
  9. Davila, A. F., Willson, D., Coates, J. D., & McKay, C. P. (2013). Perchlorate on Mars: a chemical hazard and a resource for humans. International Journal of Astrobiology, 12(4), 321-325. doi:10.1017/S1473550413000164 — Biological reduction of perchlorate as pre-treatment for ISRU water.
  10. Bagdigian, R. M., Dake, J., Gentry, G., & Gault, M. (2015). International Space Station Environmental Control and Life Support System Mass and Crewtime Utilization in Comparison to a Long Duration Human Space Exploration Mission. 45th International Conference on Environmental Systems, ICES-2015-094. — ISS ECLSS state-of-the-art review, mass and energy budgets, projection to long-duration Mars-class missions.