Relevant NASA-STD-3001 Technical Requirements

Volume One

• [V1 3003] In-Mission Preventive Health Care
• [V1 3004] In-Mission Medical Care
• [V1 5002] Crewmember Training
• [V1 5009] Physiological Exposure Mission Training

Volume Two

• [V2 4015] Aerobic Capacity
• [V2 6001] Trend Analysis of Environmental and Suit Data
• [V2 6004] Nominal Vehicle/Habitat Carbon Dioxide Levels
• [V2 6006] Total Pressure Tolerance Range for Indefinite Crew Exposure
• [V2 6020] Atmospheric Data Recording 
• [V2 6021] Atmospheric Data Displaying
• [V2 6022] Atmospheric Monitoring and Alerting Parameters 
• [V2 6107] Nominal Vehicle/Habitat Atmospheric Ventilation
• [V2 6108] Off-Nominal Vehicle/Habitat Atmospheric Ventilation
• [V2 7038] Physiological Countermeasures Capability
• [V2 7041] Environmental Control During Exercise
• [V2 11034] Suited Atmospheric Data Recording
• [V2 11035] Suited Atmospheric Data Displaying
• [V2 11036] Suited Atmospheric Monitoring and Alerting
• [V2 11037] EVA Suited Metabolic Rate Measurement
• [V2 11038] EVA Suited Metabolic Rate Display
• [V2 11039] Nominal Spacesuit Carbon Dioxide Levels

Background

Main Symptoms of CO₂ Toxicity (Hypercapnia)

These symptoms represent potential side-effects of hypercapnia, however it is important to note that most of these symptoms have never been observed during spaceflight.

• The most common symptoms experienced during spaceflight are headaches and vision changes (Spaceflight Associated Neuro-ocular Syndrome [SANS]).
• These two symptoms are commonly linked to fluid shifts and potential increase in intra-cranial pressure (ICP) due to the microgravity environment in spaceflight.

Effects of CO₂ on Cognitive Functioning

Prior research offers conflicting data on CO₂ exposure and the effects on cognitive functioning, including memory, concentration, decision-making, and task performance. However, several recent terrestrial studies have shown the CO₂ exposure at levels currently maintained onboard spacecraft and submarines does not reduce cognition and performance in astronaut-like subjects or submarine officers.

• Effects of acute exposures to carbon dioxide on decision-making and cognition in astronaut-like subjects (Scully et al., 2019). Findings state “There were no clear dose-response patterns for performance on either Strategic Management Simulation or Cognition”.
• Acute Exposure to Low-to-Moderate Carbon Dioxide Levels and Submariner Decision Making (Rodehe et al., 2018). Findings state “There were no significant differences for any of the nine Strategic Management Simulation measures of decision making between CO₂ exposure conditions”.
• Effects of -12° head-down tilt with and without elevated levels of CO₂ on cognitive performance: the SPACECOT study (Basner et al., 2017). Findings state “There were no statistically significant time-in-CO₂ effects for any cognitive outcome”.

Effects of −12° head-down tilt (HDT) with and without elevated CO₂ on cognitive performance

The chart represents a summary of results from a subset of tests from a widely used and validated neurocognitive battery, the Penn Computerized Neurocognitive Battery, as well as a number of additional tests that have either been used extensively in spaceflight or that assess cognitive domains of particular interest in spaceflight (including spatial orientation, emotion recognition, and risk decision making).

Displayed data reflect Standard Deviation (SD) units. Error bars reflect 95% confidence intervals. A confidence interval that does not include 0 indicates a statistically significant difference relative to baseline data collection (BDC) atP< 0.05. The only outcomes with a significant time-in-HDT main effect in both the discrete and the continuous exposure duration models were physical exhaustion (P= 0.0012 andP= 0.0043, respectively) and mental fatigue (P= 0.0328 andP= 0.0205, respectively). The severity of these symptoms increased with increasing exposure duration. 0.5% CO₂= 3.8 mmHg CO₂.

Key Historical CO₂ Concentrations

Anecdotal evidence from previous spaceflight missions have found the following associations between CO₂levels and symptomology:

• Headaches at levels between 2.8-4.5 mmHg; worsening with increasing levels of CO₂accompanied by fatigue and malaise flushing.
• Fatigue malaise, decreased sleep, and nausea reported at levels above 4.5 mmHg.
• 5^th cranial nerve dysesthesia at 4.5-5.0 mmHg.
• Chronic cough, poor sleep, blurred vision, and frontal headaches reported at 3.5 mmHg.

CO₂ Induced Headaches

Research and past historical data has revealed an association between reported headaches and CO₂levels among crewmembers.

• A study of data collected from Expeditions 2 to 31 (N=49) found that CO₂ level, age at launch, time in-flight, and data source were significantly associated with reported headaches on the ISS.
• 19 of the 49 astronauts reported experiencing headaches (~38.7%).
• To keep the risk of headache below 1%, average 7-day CO₂needs to be maintained below 2.5 mmHg (current ISS range: 1 to 9 mmHg; 3001 standard requires <3 mmHg).
• Most experienced headaches were moderate intensity, requiring the use of analgesics in many cases.
• A retrospective analysis of the case reports found a total of 46 days with reported headaches and 1,670 days of non-reports (no headaches observed). 
• Based on this data, the current NASA-STD-3001 Volume 2 Rev D [V2 6004] limits the average 1-hour CO₂partial pressure (ppCO₂) in the habitat volume to no more than 3 mmHg to keep incidence of headaches below 1.4%. 

Figure: Predicted probability of headache on the basis of average 7-day CO₂ levels

Some observations suggest that what appears to be increased CO₂sensitivity during spaceflight may actually be attributed to individual predisposition to CO₂retention, adaption to microgravity, and local fluctuations of CO₂that are not measured by fixed sensors (Law, Watkins, & Alexander, 2010). Fluid shift due to decreased venous and lymphatic drainage in microgravity largely contributes to symptoms similar to CO₂over-exposure, including headaches.

Reference Information

Contributors to CO₂ Levels During Spaceflight

• On earth, average indoor air contains CO₂ concentrations between 0.08% to 0.1% (0.608 to 0.76 mmHg).
• NASA-STD-3001 Volume 2 Rev D [V2 6004]: The system shall limit the average one-hour CO₂ partial pressure (ppCO₂) in the habitable volume to no more than 3 mmHg.

Crew-Induced Metabolic Loads for a Standard Mission Day with Exercise

Crewmember Activity Description	Duration of Activity (hr)	O2 Consumption kg/min 10-4 (lbm/min 10-4)	CO2 Output kg/min 10-4 (lbm/min 10-4)
Sleep	8	3.60 (7.94)	4.55 (10.03)
Nominal	.25	5.68 (12.55)	7.2 (15.87)
Exercise 0-15 min at 75% VO2 max	.25	39.40 (86.86)	49.85 (109.90)
Exercise 15-30 min at 75% VO2 max	.25	39.40 (86.86)	49.85 (109.90)
Recovery 0-15 min post 75% VO2 max	.25	5.68 (12.55)	7.2 (15.86)
Recovery 15-30 min post 75% VO2 max	.25	5.68 (12.55)	7.2 (15.86)
Recovery 30-45 min post 75% VO2 max	.25	5.68 (12.55)	7.2 (15.86)
Recovery 45-60 min post 75% VO2 max	.25	5.68 (12.55)	7.2 (15.86)
Total Per Day	24	0.82 (1.80)	1.04 (2.29)
Total Per Day (no exercise)	24	0.72 (1.59)	0.91 (2.00)

• Respiratory quotient (RQ) is used in calculations of basal metabolic rate when estimated from CO₂ production. It is calculated from the ratio of CO₂ produced to O₂ consumed by the body (RQ= CO_{2 eliminated}/ O_{2 consumed}). The RQ value is linked to intake of macronutrients (carbohydrates and fats). An RQ of 0.92 is assumed for the O₂ consumption and CO₂ output determinations in the table above. Modifying crew diet can help lower the environmental CO₂ output levels (example: consuming more fats and less carbohydrates). See Da Poian, et al. (2010). Nutrient Utilization in Humans: Metabolism Pathways.

Application

Existing Technology

Carbon Dioxide Removal Assembly (CDRA) on the ISS (International Space Station):

• CDRA continuously removes six person-equivalent of CO₂, when operating with both CO₂ removal beds (dual beds functioning).
• Minimum ISS CO₂ removal rate is based on the equation for human equivalent unit (HEU) with the CO₂ partial pressure ranging from 2.0 to 3.9 mmHg.
• The CDRA houses a four-bed molecular sieve (4BMS) that utilize zeolite crystals composed of silicon, aluminum, and oxygen to remove excess carbon dioxide exhaled by the crew.

Process:

• Air enters CDRA, it passes through 1st bed where water is removed via desiccant portion of Bed 1.
• The Dry air is then routed to the zeolite portion of Bed 2 for CO₂ removal.
• The Scrubbed air flows over the previously saturated desiccant portion of Bed 2 absorbing previously removed moisture and returning it to cabin.
• Simultaneously, the isolated Adsorbent portion of Bed 1 is exposed to space vacuum and heated, causing the adsorbed CO₂ to be released and vented to space.
• When the adsorbent portion of Bed 2 becomes saturated the valves are reconfigured and the beds switch roles allowing the saturated Bed 2 to replace Bed 1 and vent CO₂ into space.

Advantages/Disadvantages:

• Pros –Closed loop (does not require consumables other than power); conserves water while removing CO₂.
• Cons –Significant volume, replacement parts, and maintenance required. Risk of zeolite dust potentially affecting seals and mechanical parts/function.

Existing Technology

VozdukhRussian CO₂ Removal System:

• Primary Russian segment CO₂ removal system aboard ISS (located in Service Module).
• Has two desiccant beds and three adsorbent beds.
• Cabin air is drawn into Vozdukh, passed through the desiccant beds, moisture is removed, and CO₂ is absorbed.
• As one bed becomes saturated, second bed is placed in line. The saturated bed is heated, and captured CO₂ is released to space. 
• If necessary, two beds can be used simultaneously and a third bed acts in CO₂ regeneration mode. 
• ISS utilizes both Vozdukhand CDRA in CO₂ removal capacity.

Lithium Hydroxide (LiOH):

• Primary CO₂ removal system aboard Apollo and the Space Shuttle, and currently used as backup CO₂ removal system on ISS (CDRA is primary CO₂ removal system on ISS).
• •Currently used as one option for CO₂ removal during space walks (regenerative Metal Oxide –METOX is the other method).
• •The removal of carbon dioxide is accomplished by a chemical reaction using lithium hydroxide (LiOH) adsorbent. This method relies on the exothermic reaction of lithium hydroxide with carbon dioxide gas to create more innocuous compound lithium carbonate (Li₂CO₃) solid, and water (H₂O). 
• LiOHmaterial arranged in air-permeable canisters such that cabin air flowing through canisters facilitates removal of CO₂.

Advantages/Disadvantages:

• Pros –Simple and effective; highly reliable.
• Cons –Canisters are not regenerated so may only be used once (mass, stowage volume).

Emerging Technology

Thermal Amine Scrubber (TAS):
CO₂ removal system based on thermally regenerated solid amine adsorbent. TAS cycles between one adsorbing bed and one desorbing bed to remove CO₂ and regenerate bed simultaneously. Desorbing bed is isolated from the process air and exposed to vacuum during thermal regeneration.

• Water locker has passive water save desiccant canister and can recover 90% of incoming humidity in process air stream, secondary desiccant wheel increases humidity recovery to 97%.
• CO₂ locker receives dry air and splits flow between 2 CO₂ removal beds.
• CO₂ is removed from one bed containing solid amine while the other bed is regenerated. 
• During regeneration cycle the non-adsorbing bed is exposed to vacuum to purge CO₂. 
• 96% of ullage air from adsorbing bed is evacuated using scroll compressor; removal rate of CO₂ is 3.7 kg/day at 2 mmHg partial pressure.

Carbon Dioxide & Moisture Removal Amine Swing-bed (CAMRAS):

• Pair of interleaved-layer beds filled with SA9T, a sorbent system, of highly porous plastic beads coated with an amine. SA9T is an effective CO₂ sorbent and has an affinity for water vapor.
• A linear multi ball valve rotates 270 degrees back and forth to control flow of air and vacuum to adsorbing and desorbing beds.
• Air flows into the Amine Swing bed, a regenerable desiccant wheel dries and heats the air; air is later re-cooled by noncondensing heat exchangers.
• Scrubbed air comes out of the CAMRAS and flows back into the double locker; flows through a blower, through an electric heater, and through the opposite side of the desiccant wheel for cooling and rehydration.
• Air is returned to cabin through another filter and long hose that routes return air away from supply air inlet to prevent short circuiting of process air.
• CAMRAS sorbent beds are regenerated by exposure to space vacuum via direct connection to the ISS vacuum system.

Existing Technology

Extravehicular Mobility Unit (EMU) CO₂ Removal:

• When astronauts go on a Spacewalk/EVA (extravehicular activity), they build up CO₂ levels from breathing into a small closed space. 
• The spacesuit has a backpack system aboard called the PortableLife Support System (PLSS), which controls the environment of the spacesuit.
• One of the key functions of the PLSS is to remove and control CO₂ delivered to the crewmember. 
• CO₂ washout is a method in which CO₂ levels are controlled within the spacesuit helmet to limit the amount inhaled by a crewmember.
• The PLSS utilizes LiOHor METOX cartridges to absorb and eventually disperse the excess CO₂. 
• The lithium hydroxide cells are not regenerable and are exchanged after each EVA. 
• The METOX cells are regenerable. They contain CO₂ absorbent material. After each EVA they are placed into a vehicle oven that heats off the CO₂ from adsorbent beds and vents it to the vehicle. 
• The CDRA then absorbs the CO₂ from the vehicle and vents into space.
• The spacesuit helmet also has an emergency purge valve to vent CO₂ in emergency situations. This can adversely affect the O₂ /CO₂ balance.

EMU Helmet Assembly

• Consists of: Transparent shell, Neck Ring, Vent Pad, Purge Valve and adjustable Valsalva Device.
• The Vent assembly diffuses incoming gas over the astronaut’s face.
• Under the EMU helmet, astronauts wear the communications carrier assembly, A.K.A. Snoopy Cap. 

CO₂ Washout System

• Carbon Dioxide washout describes the process of removing CO₂ in an enclosed space. For the purpose of this brief, we are discussing CO₂ washout in a spacesuit and space helmet.
• CO₂ is a significant concern as the consequence of inadequate CO₂ removal can result in hypercapnia and related symptoms ranging from headache and fatigue to impaired cognitive function and death.
• CO₂ removal in a space helmet is challenging as it is difficult to ensure the quantity of CO₂ that is removed from the suit and not forming pockets in any area of the suit or helmet. It is also important to ensure when CO₂ is purged, it does not disrupt the critical breathable oxygen balance.
• Fresh air outlet from the PLSS is located at the rear base of the helmet. Air flow is directed from the
  back of the crewmember’s head over their head and face into the suit where it exits the suit and flows into the PLSS for filtration. This provides a continuous airflow, so the exhaled CO₂ is entrained with the fresh air loop from the PLSS. This mixed air then flows down into the suit to where it exits the suit and flows back into the PLSS.
• Exhaled air including CO₂ is flowed through an adsorbent charcoal bed and contamination control cartridges composed or either LiOH or METOX. This removes CO₂, trace gases, and odors. The LiOH cartridges are stored on the vehicle and replaced on the ground, the METOX cartridges can be regenerated on orbit. Either of these are installed in the PLSS prior to EVA.
• To address the life support challenges, especially when considering future, longer space missions, NASA is developing an advanced portable life support system (APLSS) spacesuit for exploration. The ventilation loop of the APLSS is designed to assist with CO₂ washout using a regenerable rapid cycle amine (RCA) removal system, efficient fan and heat exchange systems, and trace contaminant control (TCC) unit placed inside the suit hatch that allows for easy filter exchange.
• NASA has also developed a new system called the Integrated Ventilation Test System (IVTS) designed to study CO₂ washout.
• The IVTS has a ventilation test loop as found in APLSS and suited manikin test apparatus (SMTA). The purpose is to supplement human testing, optimize CO₂ removal efficiently, validate CO₂ washout, evaluate spacesuit nitrogen efficiencies, and optimize the rapid cycle amine (RCA) performance for scrubbing CO₂.

Mishaps

Apollo 13 –April 11-17, 1970

• Apollo 13 began their mission to land on and discover new information about the moon on April 11, 1970.
• Two days into their mission, a stir of the Service Module (SM) oxygen tank resulted in an electrical short causing the tank to explode, leaving the crew without adequate life support.
• The Lunar Module (LM), originally supplied for 2 of the crew to use for 2 days on the lunar surface, became a lifeboat supporting 3 crew for 4 days due to the SM’s inability to power the CM.
• CO₂ levels quickly rose (14.9 mmHg) and it became apparent the LM did not have enough LiOH cylinders to support the crew for 4 days.
• Ground support identified that the CM had usable remaining LiOH cartridges, but they were not the same type/shape as the LM filters.
• The astronauts were instructed to use plastic bags, duct tape, and a suit hose to modify the filters, enabling them to absorb CO₂ and support the 3- person crew. The CO₂ levels dropped to 1 mmHg.
• The crew rationed supplies and water and used the minimum amount of power possible, allowing them to survive in the cold, dark LM for 4 days during their return to Earth.
• The objective of visiting the moon was aborted, but with significant ground support planning, the crew was able to transfer back to the CM and safely land.

Relevant Technical Requirements

[V2 6004] Nominal Vehicle/Habitat Carbon DioxideLevels The system shall limit the average 1-hour CO₂ partial pressure (ppCO₂) in the habitable volume to no more than 3 mmHg.

[V2 9001] Crew Interface Commonality Hardware and equipment performing similar functions shall have commonality of crew interfaces.

[V2 10005] Crew Interface Consistency The system shall provide crew interfaces that are consistent in appearance, arrangement, location, and operation throughout systems.

From: NASA-STD-3001 Volume 2, Rev D

Soyuz 23 –October 14, 1976

• The Soyuz 23 had an automatic docking system malfunction during the final approach to Salyut 5 due to an electronics failure.
• Cosmonauts had less than two days of battery power remaining and missed the landing opportunity for the day, so they powered down systems to conserve power.
• A blizzard was active in the landing zone forcing a lake rather than land landing.
• The descent module lowered in the dark on a single parachute which rocked as it entered the high winds of the landing area.
• They splashed down into the freezing water of Lake Tengiz, making recovery efforts extremely difficult.
• The cosmonauts stayed in the capsule with systems shut off to save power.
• As the capsule floated, the pressure equalization valve above the waterline provided air.
• The salt water from the lake caused the secondary chutes to deploy by shorting out the sensors, the parachute filled with water and dragged the capsule below the surface.
• The ventilation had to be closed to prevent water entering the vehicle.
• Finally, a helicopter, which could not lift the capsule due to water weight, was able to drag Soyuz 23 to the lake edge.
• The crew was found alive but incapacitated due to high CO₂ levels

Relevant Technical Requirements

[V2 6004] Nominal Vehicle/Habitat Carbon DioxideLevels The system shall limit the average 1-hour CO₂ partial pressure (ppCO₂) in the habitable volume to no more than 3 mmHg.

[V2 6006] Total Pressure Tolerance Range for Indefinite Crew Exposure The system shall maintain the pressure to which the crew is exposed to between 34.5 kPa < pressure ≤ 103 kPa (5 psia < pressure ≤ 15.0 psia) for indefinite human exposure without measurable impairments to health or performance.

Referenced List

https://www.NASA.gov/sites/default/files/atoms/files/NASA-std-3001_vol_2_rev_b.pdf

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