Pressurization and Atmospheric Constituents

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Pressurization and Atmospheric Constituents

• Matt Estes
• Component Research Presentation

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Introduction

• The purpose of this presentation is evaluate the
  current means of gaseous pressurization and the
  particular elements used to do so.

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Introduction

• I researched this topic over the summer, reading
  through the bioastronautics data book, and this
  seemed like the natural progression for my field
  of research.

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Background

• Why do we need suit pressurization?
• Because in a low pressure environment the body
  would experience painful swelling or even death.

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Background

• Why do we need oxygen?
• Metabolism, its serves as the bodys primary
  electron receptor.
• Its a careful balance however
• Too much Oxygen Embolism
• Too little Oxygen Hypoxia.

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History

• The first pressurized suits were developed for
  divers, and this technology was later amended for
  use in high altitude flying and then for use in
  space.
• For most of their history contained atmospheres
  have used solely Nitrogen and Oxygen, the 8020
  gaseous mixture found here on Earth.

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History

• The pressure used was always that of Earths
  atmosphere.
• It is only recently that space suits have begun
  to use pressures under 101.4 kPa.

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Topics of Discussion

• I dissected the pressurization system up into 4
  categories I could use to compare it to
  alternative suit designs
• Advantages
• Entrenchment (huge subset of Advantages)
• Disadvantages
• Constants

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Advantages

• Sound attenuation
• Airlock time
• Leakage
• Energy

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Sound Attenuation

• If a lower pressure system was used, beyond a
  certain limit this would prevent all vocal
  communication.
• Helium is the frontrunner in the race to find an
  alternative means of gaseous pressurization.
• Unfortunately, it also makes you sound like Tiny
  Tim on methamphetamines.

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Sound Attenuation

• It has been shown that adding Neon to a
  Helium-Oxygen gas mixture can help curb the
  degree of sound attenuation, but this would
  lessen the amount of weight saved by using Helium
  in the first place.
• With Nitrogen and the current degree of
  pressurization we notice no difference at all
  because it is what we are used to.

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Airlock Time

• A switch to Helium at a lower pressure would
  require more time spent in the airlock before
  each EVA.
• This is not ideal for a mission requiring
  frequent trips on the surface.

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Leakage

• I am reviewing the mode of gaseous pressurization
  only, and while it is true that much leakage
  could be saved were a counter pressure suit used,
  that will not be discussed here.
• The comment I have to make is remarkably
  intuitive, but requires mentioning just the same.

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Leakage

• Helium is a much smaller gas than Nitrogen, and
  if used as a dilutent would escape much more
  easily than Nitrogen through micropuntures in the
  suit.
• This would require that more gas be brought along
  and more energy be expended pumping gas into the
  suit.

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Leakage

• Not only that, but it would increase the risk of
  contaminating the martian surface.

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Energy

• The air in the suit is also used to help cool the
  Primary Life Support System (PLSS).
• If the pressure were reduced, significantly more
  air would have to be pumped over the PLSS to
  achieve the same rate of cooling, requiring more
  energy.

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Entrenchment

• Perhaps the primary reason for retaining the
  current means of suit pressurization is that it
  is entwined with so many other facets of the
  mission.
• These include
• Metabolism
• Photosynthesis
• Physical Parameters
• Mission Parameters

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Metabolism

• Some of the metabolic factors that would be
  affected by a change in pressure or atmosphere
  are
• Antibody production
• Susceptibility to viral infection
• Recovery time from infection
• gas exchange

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Photosynthesis

• Some of the photosynthetic factors that would be
  affected by a change in pressure or atmosphere
  are
• Water loss by transpiration
• Production of toxic gases

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Physical Parameters

• Some of the physical parameters that would be
  affected by a change in pressure or atmosphere
  are
• Use of negative pressure as a means of material
  containment
• Solubility and/or chemical composition
• Acoustics

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Physical Parameters

• Combustion and chemical reaction (Increased
  flammability)
• Heat transfer through surrounding air

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Mission Parameters

• Some of the mission parameters that would be
  affected by a change in pressure or atmosphere
  are
• Materials selection
• Air cooling of equipment
• Sound levels
• Increased testing of materials
• Commonality with other space programs

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• Thus far the nitrogen-oxygen, high pressure
  system has appeared well suited to our needs, but
  this is not entirely the case

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Disadvantages

• Weight
• Nitrogen Bubbling
• Prebreath time
• Energy

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Weight

• The current EMU weighs 300 pounds on Earth,
  equivalent to 100 pounds in Martian gravity.
• This is not a tolerable amount for extended EVAs
  on a planetary surface.
• If Helium was used as the diluting gas, and the
  pressure was reduced, this would help lessen the
  load.

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Weight

• The current suit utilizes the 101.4 kPa
  nitrogen-oxygen mixture found on Earth.
• Of that, only 21.4 is metabolically required
  Oxygen.
• The rest is Nitrogen, and prototype suits in
  development have supposedly worked the total
  pressure down to 57.2 kPa.

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Weight

• This is a substantial reduction in weight, but it
  might be possible with Helium to reduce the
  weight even more.

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Nitrogen Bubbling

• The transition from a high pressure habitat to a
  low pressure suit causes the nitrogen dissolved
  in the blood to return to a gas phase.
• The bubbling within the blood, frequently
  experienced by divers, is quite painful and can
  lead to death if the pressure drop is severe
  enough.

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Nitrogen Bubbling

• Helium has the advantage of being poorly soluble
  in blood.
• This means that less Helium would dissolve in the
  blood at high pressure, and in turn, less helium
  would gas out of the blood in low pressure.

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Prebreath Time

• The current means of dealing with the bends is to
  spend a significant amount of time prebreathing
  one hundred percent oxygen during a washout
  period before transitioning between the 101,4 kPa
  shuttle and the 29.6 kPa EMU suit.
• The transition from habitat to suit can be
  accomplished one of two ways

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Prebreath Time

• The first requires prebreathing pure oxygen for
  four hours before the EVA.

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Prebreath Time

• The second requires prebreathing one hundred
  percent oxygen for one hour at 101.4 kPa, then
  lowering the cabin pressure to 70.3 kPa for 24
  hours, and finally prebreathing one hundred
  percent oxygen in the EMU suit for forty minutes
  prior to EVA.

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Prebreath Time

• The current shuttle mission plan uses the latter
  method of successive drops in pressure.
• This has the advantage of being able to be
  carried out while the crew is active, rather than
  forcing them into an airlock for multiple hours
  at a time.

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Prebreath Time

• But the length of this cycle is not plausible for
  a Martian expedition requiring daily excursions
  to the planetary surface.

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Energy

• The use of Nitrogen within the suit requires that
  vast amounts be brought from Earth.
• This is extremely costly, as it necessitates the
  use of much more fuel.

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Energy

• If the suit were compartmentalized into body and
  head, the carbon dioxide rich atmosphere of Mars
  could be used for pressurization of the torso and
  extremities.
• This would save millions in fuel costs.

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Constants

• Diluting Gas
• Water Vapor
• Carbon Dioxide

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Diluting Gas

• As mentioned previously, one hundred percent
  oxygen atmospheres have been used, but only for
  short periods of time.
• Long term use is prohibited because of the risk
  of oxygen toxicity, or embolism.
• This is prevented by the use of a diluting gas,
  the current one being Nitrogen.

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Diluting Gas

• Current research is underway to determine the
  lower bounds of acceptable Nitrogen
  concentration.
• Diluting Gas is also required for proper lung
  ventilation.

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Diluting Gas

• Were a pure oxygen atmosphere used, poorly
  ventilated air sacs within the lungs might
  collapse because they absorbed all the oxygen
  they contained before new oxygen arrived.
• The diluting gas helps prevent this by providing
  volume that helps keep the ventilation pathways
  open.

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Water Vapor

• The pressure of water vapor with the suit is kept
  between one and two kPa.
• The minimum value is dictated by the amount of
  water required to moisten the lungs

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Water Vapor

• The upper bounds of water vapor pressure is
  necessitated by the fact that increased water
  within the system leads to an exponential
  increase in the rate of corrosion.

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Carbon Dioxide

• The acceptable pressure of carbon dioxide is
  quite variable and is allowed to fluctuate as
  long as it is kept below one kPa.
• This leads to some leeway on how stringent the
  filtration process must be, and it is generally
  accepted that it should be allowed at a near
  maximum because this reduces the energy costs of
  removing it.

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Disposal

• Both carbon dioxide and water are jettisoned
  from the suit into the environment.
• This wasteful, and and creates a dangerous risk
  of contamination
• If weight constraints allow, these should be
  returned to the habitat where they could help
  with plant growth.

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What This Means

• The use of the Nitrogen-Oxygen mixture at high
  pressure has some definite problems.
• But it has its advantages as well.
• I cant say if one is definitively better than
  the other, nor do I think can anyone else.

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Reccomendation

• I do think that the use of helium as a dilutent
  gas, and other alternatives such as suit
  compartmentalization, mechanical counter
  pressure, and closed cell foam should be explored
  for their potential advantages.

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Reccomendation

• It seems that any development costs incurred here
  on Earth would far outweigh the in situ costs of
  using second rate technology during the mission.