RESEARCH PROGRESS REPORTS FROM THE NASA HUMAN RESEARCH PROGRAM
Thermoregulation While Operating
in Space Suits
Humans are homeotherms, meaning they have innate control systems that maintain their body temperatures within a narrow range—
around 37°±0.5°C. However, if excessive environmental stresses or workloads exceed thermoregulatory ability, core body temperature can
increase to unsafe levels. Thus, an astronaut working in any “space suit,”
including the Advanced Crew Escape Suit, is at risk and must be protected. The project reported here addresses this risk.
while wearing the advanced crew escape suit (ACES) and the liquid
cooled ventilation garment (LCVG).… The study has three phases. First,
the Human Thermal Model developed by Wissler…is subjected to
further validation.… Second, the Wissler model is used to study the relationship between workload and cabin temperature on astronauts’ body
temperature…. Third and finally, the model’s results are used to propose
mitigation strategies and possible solutions.”
Methods and Results
Phase 1
a. Firefighter test case (“Data obtained at the Naval Medical Research
and Development Command and provided by [R.D.] Hagan”): From
the complete data set, this validation test used data of one firefighter at
PUBLICATIONS
Pisacane VL, Kuznetz LH, Logan JS, Clark JB, Wissler EH. Thermoreg- 49°C, 50% relative humidity during two 20-min rest sessions and two
−1
ulatory models of safety-for-flight issues for space operations. Acta 20-min exercise sessions (treadmill walking at 1.1 m • s , 0% grade). A
complete fire-fighter ensemble (FFE) was worn during exercise and,
Astronautica 2006; 59: 531-46.
Pisacane VL, Kuznetz LH, Logan JS, Clark JB, Wissler, EH. Thermoreg- possibly, during rest. Results: Difference between actual data and model
ulatory models of Space Shuttle and Space Station activities. Aviat predictions for core temperature never exceeded 0.5°C (at 60 min), and
for skin temperature the maximum deviation was 1°C. These differences
Space Environ Med 2007; 78 (4, Suppl.):A48-55.
may be due to variations not included in the model, i.e., subjects
breathed compressed air, actual subject characteristics for input to the
SUMMARY
Dr. Vincent Pisacane and colleagues have used a mathematical model were not available, and the subject may have removed some garments during resting periods.
model of human thermoregulation to assess how well the currently used
b. ACES and LCVG chamber test (Data from “results of a human subAdvanced Crew Escape Suit and Liquid Cooled Ventilation Garment
ject test aimed at determining if the ACES and the LCVG could protect
(LCVG) system would protect crewmembers in the event high
against a simulated Shuttle reentry temperature profile higher than curworkloads were required and/or if there should be failures in elements
Delivered
by Ingenta
to: ? by NASA flight rules”): Four men and four women,
rently allowed
of the vehicle cooling system. They first tested the model’s
ability to
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On: in
Fri, 16
Jun the
2017
21:44:38
wearing
ACES
and LCVG, remained in a chamber for 5 h as the tempredict human thermal response against IP:
existing
data obtained
perature increased from 23° to 27°C, humidity increased from 40 to 42%,
stressful high-temperature environments, and it “achieved reasonable
inlet water temperature for the LCVG increased from 22° to 24.5°C, and
agreement.” They then used the model to predict thermal responses
of crewmembers wearing the suit under varying workload and water flow rate varied between 32.5 kg • h−1 and 37.5 kg • h−1 (controlled by the subject). Results: The model’s predictions were excellent,
environmental-temperature conditions and found that the LCVG
with mean differences as follows: rectal temperature 0.12°C; mean skin
system’s performance was marginal at moderately stressful workload/
temperature conditions and inadequate “at the potential upper limits of temperature, −0.39°C; water outlet temperature, 0.00°C. Model heart
metabolic rates.” Finally, they recommended that this problem could be rate values were also close to observed values with a mean difference of
mitigated by “lowering the LCVG inlet temperature from 25° to 20°C.” −1.9 bpm. Investigators attribute the differences “to uncertainties in
some of the inputs to the model (e.g., resting metabolic rate, muscle
This study has made a significant contribution to crewmember safety
and has also provided a tool that can be used in future evaluations of workload distribution, and subjects’ conditions at the start of the test).”
“space suit” safety.
Phase 2
“Parametric study of astronauts clothed in the LCVG and the ACES under
Introduction
the conditions of a degraded ATCS (Active Thermal Control System) during
The investigators point out that “small increases in body core
temperature can impair the ability to carry out complex tasks…affect reentry”: Parameters for the simulated astronaut (e.g., mass and metabolic rate), for environmental conditions, and for LCVG maximal
short-term memory and slow perceptual and motor skill…(and cause)
coolant flow rate and coolant inlet temperature—as well as initial condisignificant performance decrements in perceptual motor tasks.” Greater
tions—were input to the model. The time dependence of rectal and
elevations in core temperature can cause heat stroke and death.
mean skin Association
temperature at various cabin temperatures (from 20° to 40°C)
Copyright: Aerospace
During launch and re-entry, Shuttle crewmembers
wear the Medical
Advanced Crew Escape Suit (ACES) over the Liquid Cooled Ventilation was predicted by the model. Results: Selected results of the model simGarment (LCVG); and during re-entry, they also wear an anti-gravity ulation indicate the following:
“Shivering occurs at cabin temperatures of 20°C and metabolic rates
suit that protects against orthostatic intolerance. The ACES, which
at and below 150 kcal • h−1.
includes body suit, gloves, boots, and helmet, is pressurized and air and
“An
increase in rectal temperature approaching conditions for
water tight. The LCVG, which “covers the body limbs and
‘decreasing manual dexterity’ and ‘loss of tracking skills’ for motor
torso…contains a network of plastic tubing that circulates water from
tasks… can occur for metabolic rates above 250 kcal • h−1 and cabin
the ICU (Individual Cooling Units).”
temperatures
above 25°C.
Mathematical models of human thermoregulation have been in use,
“An increase in sweat rate greater than 0.25 kg • h−1 can occur for
improved, and refined over 70 yr. Approximately 20 yr ago, Eugene
metabolic rates above 250 kcal • h−1 and cabin temperatures above
Wissler developed a model of thermoregulation “that covers a broad
25°C with the rate increasing significantly with increases in either
range of human experiences from immersion in cold water to exercise in
cabin temperature or metabolic rate.”
a hot, humid environment.… This model was modified to simulate an
From these results, the investigators conclude, “The performance of
automatic control system for a liquid cooling garment to provide therthe LCVG-ICU system is marginal at temperatures greater than 25°C
mal comfort to astronauts.”
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It is important to know whether the ACES/LCVG suit has sufficient and metabolic rates greater than 200 kcal • h ; for cool conditions, the
astronaut
can
restrict
the
flow
of
the
coolant;
and excessive sweating at
cooling capability to protect working crewmembers in the unlikely, but
higher cabin temperatures and/or higher metabolic rates could intropossible, event of failures in both primary and backup components of
the vehicle cooling system during periods of high power requirements
in the vehicle. Normally, the primary mechanism by which the human
These HRP summaries are written for NASA and prepared for the
thermoregulatory system induces body heat loss to protect against
journal by Mary Anne Frey, Ph.D., with the support of the NASA
increased core body temperature is sweating and evaporation of the
Human Research Program Office and are intended to alert the sciensweat. Under the failure mode postulated above, however, the sweating
tific community to ongoing work in space medicine. Original sources
mechanism is not adequate in a space suit without a cooling garment.
for the material may include journal articles, NASA Technical Reports,
The LCVG provides additional cooling, but we do not know whether its
internal NASA Documents, and personal communications from Princooling power is sufficient to protect crewmembers under these condicipal Investigators.
tions.
Reprint and Copyright by the Aerospace Medical Association,
“The specific purpose of this study is to evaluate the effect of elevated
Alexandria, VA.
Shuttle cabin temperatures and workloads on astronauts during reentry
DOI: 10.3357/ASEM.27005.2009
666
Aviation, Space, and Environmental Medicine • Vol. 80, No. 7 • July 2009
HRP REPORT SUMMARIES—FREY
duce dehydration and electrolyte loss.” These results are “optimistic,”
since the simulations were at the maximum coolant flow rate.
Phase 3
Mitigation strategies: 1) The model predicts that lowering the LCVG
inlet temperature from 25° to 20°C will decrease rectal temperature,
decrease heat storage, and mitigate sweating and dehydration. At high
metabolic rates, this would make the difference between reaching a
body core temperature at which performance is degraded and remaining at a safe temperature. Lowering the inlet temperature can be accomplished by either increasing “the cooling capability of the ICU” or
rerouting “the LCVG directly to the Shuttle Water Coolant Loop System.” 2) Reducing heat production by lowering metabolic rate could
also be helpful; but it would be less effective and less practical than lowering the LCVG inlet temperature.
Discussion
This project has tested and validated a modification of the Wissler
model of thermoregulation that can provide an important tool for future
safety testing of “space suits.” In addition, the investigators used this
model to test the current ACES/LCVG system and learned that it allows
increases in body temperature and heat storage that are marginal (with
regard to astronaut safety) at moderately-stressful work loads and inadequate at the upper limits of crewmember metabolic rates. Using the
model, the investigators also recommended strategies to mitigate this
problem.
ACKNOWLEDGMENTS
This work was supported by funding from the National Aeronautics
and Space Administration (NASA) and the American Society of Engineering Education.
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Copyright: Aerospace Medical Association
Aviation, Space, and Environmental Medicine • Vol. 80, No. 7 • July 2009
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