I already knew about the Apollo I capsule fire, of course. But recently, I saw something about how the oxygen rich atmosphere caused the astronauts’ mouths to be dry when they landed on the moon. I didn’t know breathing oxygen would do that. So, why don’t they just bottle up air instead of oxygen? Or simulate it with mixture of oxygen and nitrogen? Is air hard to package?
It takes up too much space. If you run an oxygen-enriched atmosphere, then you can take less gas with you and still breathe. Common air runs, what, about 26-28% oxygen? That means you are hauling arounf 72% wasted gas. There are other advantages as well. The less nitrogen in the capsule, the less nitrogen in the blood. This would lessen the chances of the “bends” in case of loss of cabin pressure.
It’s more like 21% at sea level, but yeah, I think you have the basic idea there.
They used oxygen until the fire in Apollo I, then switched to air. SCUBA tanks usually carry air. (Oxygen-enriched air very popular, but most people just use air.) The air is filtered and dehumidified so that contaminants will not be inhaled and cause (possible fatal) problems to the diver, and so that moisture will not attack the inside of the cylinder. Cylinders must be visually inspected for corrosion every year, and they must be hydro tested every five years.
Anyway, the air divers breathe is rather dry. I get a touch of “dry mouth” when I dive.
Astronauts also breathe air that is stored in cylinders, and the air must also be dehumidified. So dry mouths are a symptom of the dry air, rather than breathing pure oxygen.
No, they used oxygen thoughtout the Apollo program. They switched to air (or more specifically, an oxygen-nitrogen mix) for ground testing. The thing is that the Apollo program ran under very low pressures – under 5 psi – while in space. When pressure gets that low, humans need extra oxygen to function; an air-like mix would not have done the trick.
manny’s nailed it. NASA only switched to an air like mixture with the advent of the shuttle, however, the astronauts do use oxygen at about 4 psi in their spacesuits.
I stand corrected.
The point of using pure oxygen is that it requires a lot less pressure to support human life. Our lungs ability to transfer oxygen is based on the partial pressure of oxygen. In our atmosphere of 14 psi and 21% oxygen that is about 3 psi. An atmosphere of 100% oxygen therefore only needs to be around 3 psi to support human life. In a space capsle, the difference between 14 and 3 psi in the amount of structural reinforcing required is tremendous.
Very, very nearly, but not quite. After Apollo 1, they also used an oxygen-nitrogen mixture for launch. As the capsule reached orbit this could safely be shifted to pure oxygen.
On the pad, either during testing or during launch, it was structurally advantageous to have the capsule at atmospheric pressure, but once in space it becomes easier to have it at a much lower pressure. That’s much easier to contain.
As noted, humans need to be breathing pure oxygen at these lower pressures. It’s also safe from a fire danger point of view. It was pure oxygen at atmospheric pressure, such as they’d been initially using on the pad, that wasn’t safe.
I read a fascinating NASA web page that compared the advantages and disadvantages of the various atmosphere options, but I can’t find it now, unfortunately.
This would apply to space suits as well. Higher pressure also inflates the suit harder and makes it more difficult to bend your joints.
And if you go from normal pressure air in the spacecraft to lower pressure oxygen in the suit, you have to decompress to avoid the bends. This complicates an EVA. You can’t just jump into a suit and immediately go out the hatch.
I was under the impression it had to do with the way the air-exchangers worked.
You have a set amount of nitrogen, the astronauts remove O2, the scrubbers remove CO2, and the O2 is replaced by tanks. Putting air in the tanks seems like you would end up with too much nitrogen (leading to an increase in pressure and/or screwed up gas ratios).
I just wanted to add that higher pressures (1 atm vs .21-.28 atm) require sturdier structures to contain the pressure,a nd this means more weight. Further, you’re carrying about 3.5 lbs of Nitrogen for evey pound of oxygen in use. You don’t need to carry 3.5 lbs of nitrogen for every lb of oxygen in storage, because the nitrogen isn’t consumed the way the oxygen is, but it’s still extra weight.
You can see why (after doing suitable human testing) they decided to cut all that useless nitrogen on moon missions, where every lb of payload weight required hundreds of lbs of fuel at liftoff (you need fuel to boost the extra mass, then you need more fuel to boost the extra fuel, etc. – It really adds up!) It must have seemed like a no-brainer. Air is safer than pure oxygen on the ground (where everything is at 1 atm anyway), and all that inert nitrogen exerts a moderating thermal effect on flame, but those effects might not have seemed too critical when they were facing the challenge of getting the dang thing to the in the first place. The ground phase must have seemed pretty ‘safe’ and ‘routine’ – it’s where we live every day. Nothing scary about that, right … Right??
Had the Soviets been more open about their space program, the Apollo 1 fire might never have happened. The Soviets had a similar accident years before, but kept it quiet, so no one ever knew about it.
Of course, because of the Apollo 1, there were vast improvements in fire retardant gear (NASA spent lots of money researching better materials so that something similar couldn’t happen again.), so had they not died, we might not have the fire retardand gear we have now.
You’re quite right of course – I failed to address the time between testing and space, and launches did indeed have a mixture. I should have in GQ of course.
Here’s one I don’t know: Did they switch back for reentry, or did they just let the capsule naturally repressurize with air and let the mixture take care of itself?
Nothing I’ve read suggests that they switched back or let the capsule “naturally repressurize.” Reentry was pretty damn quick, and once they hit the water, one of the first things they did was pop open the hatch. Certainly, they could have switched over on the way back, but why bother? They’re only going to be in the sealed capsule for about 30 minutes after they hit the top of the atmosphere, and as soon as they stop bobbing around, they’re going to have the hatch open, and the oxygen in the capsule’s going to mix with air immediately.
That’s a good follow-up question, manhatten. A little poking around threw up a relevant incident during the re-entry on the Apollo-Soyuz mission, which is described in this NASA history.
At about 24,000 feet, Brand was expecting to have seen the parachutes deploy. When they didn’t:
The manual deployment of the drogue chutes caused the CM to sway, and the reaction control system thrusters worked vigorously to counteract that motion. When the crew finally armed the automatic ELS 30 seconds later, the thruster action terminated.
The gas was nitrogen tetroxide fumes from thrusters outside the capsule.
Thus it seems that the pressure relief valve had correctly operated, with the intention of allowing air into the capsule as it descended. What hadn’t been anticipated was that the thrusters might still be firing when this happened.
I presume that the valve that had caused the loss of Soyuz-11 was intended to serve a similar purpose.