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31

Let's start with a Fermi estimate: The atmosphere has a mass of about 5.15×1018 kg, 20% of that is oxygen. A rocket launch uses on the order of 106 kg of oxygen. To use up all the oxygen (and assuming no oxygen is replenished by plants) requires 1012 rocket launches. Of course, rockets are only a tiny part of all oxygen consumption. We burn 4×1012 kg of ...


31

The same system was used on Shuttle - allow me to discuss that, the design philosophy applies to Apollo as well (Shuttle deleted the fans though, and had a special Avoid-Apollo-13-circuit in the O2 tanks). A supercritical fluid is any substance at a temperature and pressure above its critical point, where distinct liquid and gas phases do not exist. ...


28

There was no "slush" in the Apollo cryo tanks. The O2 and H2 in the tanks were stored at conditions that made them supercritical fluids. The critical pressure for O2 is ~ 730 psi, the Apollo tanks were at ~ 900 psi. (H2 critical pressure is ~187 psi). from Apollo Operations Handbook Block II Spacecraft , emphasis mine These supercritical fluids tended to ...


23

The sun is not "burning" in the sense you are used to: there is no chemical reaction going on. Instead, there is a very high pressure in the core of a star (like our sun) due to the high mass that starts/sustains a nuclear fusion process. In our sun, hydrogen is fused to helium and the energy that's released in this process is what makes the sun "glow" and ...


22

I would argue that no specific level of molecular or atomic oxygen in atmosphere is indicative of carbon-based life (i.e. life as we know it on Earth). A planet could have oxygen rich atmosphere which could be due to naturally occurring thermo-chemical reactions (e.g. Sabatier/Bosch reaction), loss of hydrogen in water vapor through atmospheric escape and ...


19

As suggested by OrganicMarble in a comment, nitrogen is miscible with oxygen (you can thus make liquid air). According to NASA Technical Paper 2464, this is a major concern because using "enriched air" instead of pure oxygen as the oxidizer degrades the performance of the engine: The transfer of liquid oxygen (LOX) from a storage vessel to a rocket engine ...


15

The higher plants are less efficient, so the entity will probably be not a bush, but a jar of algae. 30-40 liters of Сhlorella suspension could provide for a gaseous exchange of a single person. Grishin Yu. I. "Artificial Space Ecosystems" Cosmonautics, Astronomy 7/1989


15

It is easy enough to analyze this in terms of conservation of momentum, we'll assume the velocity through the atmosphere to be 7.8km/s, that is LEO orbital velocity - in actuality it'll be a little different due to rotation of the Earth and eccentricity of the atmosphere-grazing orbit, but not different enough to change the conclusion. Now assume the ship ...


14

There are certain plants that are more efficient in removing and adding Oxygen to the surrounding environment, but the main disadvantage is that they need soil with comfortable acidity otherwise the roots of the plants would die. That prevents them from being used on other planets where the acidic contents in the soil is more (but there are a few exceptions)....


12

As Andrew Thompson's comment hints, it's a matter of tankage mass vs the trifling extra mass of hydrogen. Tankage is no small thing; it's the main reason hydrolox ($H_2 / O_2$) rockets, despite their unparalleled* $I_{SP}$, don't have much better performance than other liquid fuels with considerably lower specific impulse but much better density impulse &...


11

Despite a higher risk of a fire, pure oxygen also has some advantages. First, the internal pressure of the vessel is only a fifth of a normal breathing mix, allowing less structural load on the hull of the spacecraft. The resupply system is also simplified, because a system including nitrogen must have an extra tank for the nitrogen. (If you had them mixed, ...


11

I guess the generic answer for any mass separation system would be that you spin it, but mass separation isn't really effective for electrolysis since that requires larger separation between anode and cathode which leads to protonic ohmic losses, i.e. increased internal resistance. So a more optimal species separation is done not by their mass but by their ...


11

Yes, in principle to both questions, why not? However we can calculate the maximum energy you could get from the generator. The vapour pressure of $CO_2$ at 0C is around 4MPa, so in a perfect world, you get a volume expansion of about $4 MPa/600Pa$ or 6400. The formula for the work per mole done by isothermal expansion of a gas is $$RT\mathop{\mathrm{ln}}...


11

From this NASA pdf: At 46:40:02 Mission Elapsed Time during the Apollo 13 mission, both oxygen tank fans were powered on, hopefully to get a more accurate reading of the tank pressure. In fact, the pressure is not influenced by fan operation. The reading of the capacitance tank gauge depends on a uniform temperature of tank contents. 56 hours into ...


10

Can electrolysis of water provide fuel for the generator where both water and liquid carbon dioxide is utilized in one self sustaining machine? NO. A 'self sustaining machine' would be a perpetual motion machine, and those are impossible. You have to provide energy from an external source. Electrolysis of water costs energy, about 50 kWh per kg of ...


10

It can be made to work, but it would be quite different from the typical SF depiction. As Blake points out, you can't gain anything from mass that is already moving faster than rocket exhaust. However, you do not have to collect the gas at orbital velocity. In the extreme case you could slow to a complete stop and refuel while landed, as long as your fuel ...


10

There is oxygen and many other elements on the Sun. The "burning" you refer to is fusion, it does not require oxygen. Oxygen is created by: 12 C + 4 He ------> 16 O + energy. In case you are wondering how the carbon for that reaction is created it comes from: 3 (4He) ------> 12C + energy, with the helium derived from: 4 (1H) ------> 4 He + 2 e+ + 2 ...


9

Even at the high atmosphere level, far below 'space' it is barely possible. Look at Skylon, whose core focus is to inject enough oxygen from the very thin air, cool it and compress it very fast down to LOX to then use it as oxidizer, since normally oxygen at high atmosphere is not usable for this. I.e. Extend ability to run 'air-breathing'. And Skylon is ...


9

This is what Kenneth S. Thomas and Harold J.McMann have to say about it in U.S. Spacesuits: Operating pressures: The Shuttle extravehicular mobility unit (EMU) has an operating pressure of 4.3 psi (30 kPa) and the Shuttle crew escape/launch/entry suits operates at a maximum of 3.5 psi (24 kPA). All Russian spacesuites, in comparison, operate at 5.8 ...


9

The melting point of hydrogen isn't much lower than its boiling point (6K), so the temperature isn't necessarily that much of an obstacle. However, using solid fuel requires either melting it, or burning it in place. Trying to burn solid hydrogen would likely result in the whole thing flash-boiling from the radiated heat, so that's a bit of a non-starter. ...


9

There are two different effects of oxygen toxicity, the Lorrain-Smith-effect and the Paul-Bert-effect. See Wikipedia. The Lorrain-Smith-effect may occur at a partial oxygen pressure above 0.5 bar for more than about 24 hours. It is a lung toxicity. The Paul-Bert-effect may occur at a partial pressure above 1.6 bar for minutes to a few hours. It is a ...


8

Okay, first things first: You can take as much oxygen as you're willing to pay for. One liter of LO2 at 1.41kg converts to 810 liters of oxygen at 1 bar, so space isn't that much of a problem; the weight is - but you only need it for the trip, because you can manufacture as much as you desire on Mars from the atmosphere. Now that means question 2 only ...


8

Assuming you could extract all the water from the soil, which apparently has roughly 2% of it by weight (by the way, that's really dry and even the driest of desert sands contain more), then you'd need at least 189.27 kg (417.27 lb) of Martian soil to produce one gallon (≈ 3.79 liters) of water. This would be extremely difficult to do though and would ...


8

The first third of Earth's own existence had neglible atmospheric oxygen. (Simple) life existed during a few hundred million years of this period. Thus, atmospheric oxygen does not correlate with life on a planet.


7

As @TidalWave said, no specific level of oxygen is by itself sufficient to potentially indicate life. However, based on this article, what astronomers are looking for is the combination of both oxygen and methane in a single atmosphere. Both oxygen and methane can be created independently by nonliving processes, so their individual presence is of ...


7

Theoretically yes. But it would take hell of a lot of time. In all phases, the interstellar medium is extremely dilute by terrestrial standards. In cool, dense regions of the ISM, matter is primarily in molecular form, and reaches number densities of 106 molecules per cm3. In hot, diffuse regions of the ISM, matter is primarily ionized, and the density ...


7

The ISS does receive regular shipments of oxygen as a gas in specially built, very high pressure (about 6000 psi or 400 atmospheres), scary looking tanks, as pointed out in this interesting answer. They are called NORS (Nitrogen/Oxygen Recharge System). You can read more about them here. They require careful handling and high pressure feed connections. A ...


7

I'm not a chemist but I'll go out on a limb and suggest a way to resolve some issues in comments. It looks to me as though as long as you are above both the critical pressure and critical temperature at the same time, it's a supercritical fluid; thus the name. So as long as the pressure is above 50.4 bar and the temperature is above 154.5 K (-118.6 C) it's ...


6

Providing sufficient oxygen for a long journey is not the challenge. If you have enough to meet the demands for a week, you have enough to meet the demands forever. Oxygen is not destroyed or created in respiration Animals (people) take in oxygen where there are two atoms joined together and exhale Carbon Dioxide where there are is a carbon between the two ...


6

Yes, at least in some Centaur applications, the LOX pump shaft is splined, and in fact has a gear drive attached to it. This drive provides mechanical power to a hydraulic pump which in turn supplies hydraulic fluid to the Centaur's Thrust Vector Control (TVC) actuators. Source is this document, I quote the relevant paragraph (emphasis mine): During ...


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