If we keep the atmosphere at a right temperature, now
what else do we have to do so that the crew can actually
breath it?
Well, we have to provide the right breathing gas
mixture in the atmosphere.
On the earth at sea level, we have roughly 78%, almost 80%,
4/5 nitrogen, little over 1/5 oxygen,
little bit of water vapor depending
on the relative humidity, and then a lot of other materials,
a fair bit of argon almost 1%, then carbon dioxide, neon,
other rare gases and the like.
At sea level, we have a given atmospheric pressure,
which we call 1 bar which means 1 atmosphere.
And we can measure it in various ways,
in millimeters of mercury, pounds per square inch,
or the standard metric unit is the kilopascals-- roughly 101
kilopascals is 1 atmosphere.
By and large, most spacecraft, not all but most,
historically and at present, provide a sea level atmosphere.
So all of the Russian space stations, the space shuttle,
and the International Space Station
we have a sea level atmosphere.
Why do we have to do that?
Well, here's a somewhat complicated looking chart,
but let's take a look in detail.
I think you'll be able to understand
what we're dealing with here.
First of all to stay healthy, we need a certain amount
of pressure on the outside of our body.
If the pressure on the outside of your body
gets to be too little, then fluids inside your body
will start to vaporize.
Your body will swell up.
It would be very uncountable.
Just for a test once, I put my hand into a partial vacuum
chamber that we use for testing EVA gloves
and the blood rushes in to the hand
because it's not exposed to pressure on the outside.
Your hand swells up and it gets to be
fairly painful after a while.
So we have to provide pressure on the outside of your body,
and not just the pressure but we need enough oxygen to breathe.
What actually happens?
We talk about the amount of oxygen in the atmosphere
itself, but it's really the amount of oxygen in our blood
which we care about.
So the oxygen first has to come into our lungs,
down into the alveoli, and then it goes into our blood.
Too little oxygen and you become hypoxic.
We'll talk about that in a minute.
Too much oxygen is also not good,
particularly when you're at sea level atmospheric pressure.
If you try to breathe pure oxygen at sea level
pressure for a long time, the problem
is oxygen is absorbed very readily through your alveoli
and so the oxygen will go from your alveoli into the blood.
The alveoli in your lungs will be empty and they'll collapse.
So what you need is an inert gas,
a gas that doesn't react physiologically
but will keep your alveoli inflated.
That of course is what nitrogen provides in the atmosphere.
So what we have in the graph here
is a percentage of oxygen in the atmosphere on the x-axis.
On the y-axis we have the total pressure.
So as I said, we need to be above a certain minimum total
pressure, but we also need the right amount
of partial pressure of oxygen.
Now at sea level, here we are 760 millimeters of mercury.
That's at sea level, about 14.7 psi, 101.1 kilopascals
and about 21% oxygen.
So that's where we are here.
As you go up in altitude on the earth, the percentage of oxygen
doesn't readily change, but the overall pressure does.
So when you go up to, say, Leadville, Colorado,
here we are and we're only down to a partial pressure
now of about 520 millimeters of mercury.
That's the equivalent of being up at about 10,000 feet.
And if you go to the summit of Mount Everest, which is up
above just around 30,000 feet, you're down at just a little
over 200 millimeters of pressure of oxygen.
What we see here is that's too low for survival.
We want a minimum amount of oxygen, which
we call the partial pressure.
You can reduce the overall pressure of your atmosphere.
But if you do that, then you better
increase the percentage of oxygen
if you want to keep the same partial pressure of oxygen,
which is what we need to maintain consciousness.
You need enough oxygen to get your brain.
So we can come down this curve here,
which is called the unimpaired sea level equivalent, which
means as we basically go down in overall pressure, which
has the equivalent of going up in altitude, we in fact
have to increase the percentage of oxygen in the atmosphere
until by the time you get to 100% pure oxygen environment,
you can actually be down at about 1/5 of an atmosphere
and you'll still have enough oxygen to breathe.
What happens if we don't get enough oxygen?
Well, the critical thing is that you get starved of oxygen.
Your brain shuts down and we'll lose consciousness.
In spacecraft or high altitude jet flight,
we have to carry our atmosphere with us.
In a jet, you actually wear an oxygen mask.
In the spacecraft, we generally don't.
We have oxygen sensors that maintain
the right amount of oxygen.
But suppose something goes wrong with that,
it would be good if you knew what
your physiological symptoms are if you start
to become hypoxic so that you can do something about it
before you actually lose consciousness.
And so as part of the training for space flight
or high altitude jet flight, they
put test subjects in what we call altitude chambers.
You can see images of those here.
They basically put you in with a helmet and an oxygen mask.
Then, they pump the atmosphere down,
not to a vacuum of course, but to the equivalent
of maybe 25, 30,000 feet.
Then, you remove your mask.
At that level, you'll become hypoxic in a few minutes.
The idea is to observe your own personal syndromes or symptoms
so that hopefully if you get in a situation like this
and your oxygen sensors aren't working right,
you have this back up way, from your own physiological
symptoms, knowing that you're becoming hypoxic.
Different things can happen for different people.
Your lips and your fingertips can turn blue.
You can start to shake a little bit.
You don't remember things.
Your vision can go funny.
Eventually, you're going to lose consciousness.
The scary thing is before you lose consciousness,
there's often a sense of well-being, euphoria,
almost maybe like having had a little bit too much to drink.
You can feel real happy and have no idea
that you're about to pass out.
That can be dangerous.
I remember one run that I did in one of these altitude chambers.
We had to do it about once every three years
just to maintain currency.
I took my mask off and I was drawing pictures and things.
The next thing I knew, the instructor,
who of course had his mask on, was shaking me
and had basically put the mask on as apparently I was
just sort of off in la-la land.
People were yelling at me, Jeff, put your mask on.
Put your mask on.
I didn't hear them.
I was happily and I was going to write away
until finally I just kind of lost it
and they had to help me out.
That's why it's so dangerous.
Hypoxia you have to avoid.
You have to have the sea level equivalent amount of oxygen
even if you reduce the total amount of cabin pressure.
We have a choice as designers then, in a cabin atmosphere.
Just from an oxygen point of view,
we could run at normal sea level atmosphere with 20% oxygen.
That's what we do for the shuttle and Space Station.
Or in the early space flights, we actually
used a pure oxygen environment.
There are certain advantages to doing
that when you're going to be doing a lot of space walks.
We'll talk about that in another lecture
when I talk about EVA or space walking.
The other thing, if you're at a lower overall pressure,
of course, then the actual pressure
inside your spacecraft, which the structure has
to be able to hold in, is less and so
you can get away with a lighter structure.
But, of course, you have a problem with pure oxygen
you have much greater danger of fire.
Of course, we tragically lost three astronauts
in the Apollo 1 fire in a pure oxygen environment.
We don't want to get below this blue line
because that is the so-called hypoxic boundary.
Below that line, we are not happy campers.
We are going to be oxygen starved.
But we can move along this curve here,
and as long as we increase the percentage of oxygen
as the overall pressure goes down, we'll be OK.
In fact, in a space suit, we run our space suits at about 3/10
of an atmosphere with pure oxygen.
The reason we want to keep a lower overall pressure
is to increase mobility.
Again, I'll talk about that when I give a lecture on doing space
walks.
So that's the basic story on the composition of an atmosphere
and how much oxygen we need.
We can have a normal sea level atmosphere with sea level
pressure and about 21% overall oxygen
or we can lower the overall pressure
by increasing the percentage of oxygen
until we get down to a cabin atmosphere, which
is pure oxygen at a rather low pressure.
So that's basically the story on the composition
of the atmosphere.
And now we're going to look at how we actually
carry some of these gases with us and how we recycle them.