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To pressurize the liquid propellant tanks Helium is most widely used and after that Nitrogen. Helium is quite costly and Nitrogen is economic but it messes with the O/F ratio of the engine (ref). So, except these two we have other inert gasses as well like Argon, Xenon, etc.

Note: Self-pressurization is not being considered here.

Why other gases than helium and nitrogen are not being used to pressurize the liquid propellant tanks of rocket?

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    $\begingroup$ Hydrogen and oxygen are also used in autogenous systems. space.stackexchange.com/questions/6323/… $\endgroup$ – Organic Marble Nov 21 '18 at 15:09
  • $\begingroup$ @OrganicMarble, I understand that we can do self-pressurizing but I have came to know some problems in doing the same like flashing of the propellants. So, if we are considering that we are not self-pressurizing then which all gasses other than He and N $\endgroup$ – Amar Nov 21 '18 at 15:13
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    $\begingroup$ Autogenous is not necessarily self-pressurizing. Some systems (like Shuttle) heat the propellants in the engine and send them back to the tank. $\endgroup$ – Organic Marble Nov 21 '18 at 16:31
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A good pressurizing gas needs to satisfy a few basic properties:

  • it needs to stay gaseous at the temperatures and pressures your fuel and oxidizer are stored at, so that it won't just condense out when injected into the tanks;
  • it needs to be inert enough to be safe to mix with both the fuel and the oxidizer (since if you're going to have two different pressurizing gases anyway, you might as well just use the fuel and oxidizer themselves);
  • it should also be safe to store at high pressure, for obvious reasons; and
  • it should have a fairly low molecular mass, since that directly translates to less mass for a given pressure and volume.

As it happens, there are not that many gases that satisfy all those criteria. In fact, there aren't that many gases with low molecular masses to begin with, since they can only consist of a small number of atoms high up in the periodic table, and most of the ways to stick those together into a stable molecule end up failing one or more of the other criteria. Specifically, going roughly in ascending order by molecular mass, we have:

Period 1:

  • hydrogen (H2): lightest of all gases, good rocket fuel, very flammable and unsafe to mix with oxidizer;
  • helium (He): second lightest of all gases, moderately expensive, noble gas → very safe and inert, good pressurizing gas;

Single period 2 atom (+ hydrogen):

  • methane (CH4): also good rocket fuel, also very flammable and unsafe to mix with oxidizer;
  • ammonia (NH3): condenses easily, making it unsuitable for cryogenic or high-pressure fuels, also unsafe to mix with oxidizers;
  • water (H2O): cheap, safe and fairly inert, but the fact that it's liquid at room temperature makes it effectively useless for pressurization;
  • hydrogen fluoride (HF): highly corrosive and toxic, unsafe to store and handle;
  • neon (Ne): also a noble gas like helium, would be a perfectly good pressurizing gas except that it's both heavier and more expensive than helium, so there's no reason to ever choose it;

Two period 2 atoms (+ hydrogen):

  • acetylene (C2H2), ethylene (C2H4) and ethane (C2H6): all flammable and unsafe to mix with oxidizers (the first two are also chemically unstable when stored under pressure);
  • hydrogen cyanide (HCN): toxic, flammable, unsafe to mix with oxidizer, liquid at room temperature;
  • formaldehyde (H2CO): toxic, flammable, unsafe to mix with oxidizer, polymerizes under pressure;
  • fluoromethane: (H3CF): liquifies easily, flammable, unsafe to mix with oxidizer;
  • nitrogen (N2): fairly inert, cheaper than helium, safe to mix with most fuels and oxidizers, good pressurizing gas;
  • hydrazine (N2H4): toxic, good rocket fuel or monopropellant, unsafe to mix with oxidizer;
  • carbon monoxide (CO): toxic, about the same molecular mass as hydrogen but more reactive, somewhat flammable, probably not safe to mix with oxidizer;
  • nitric oxide (NO): heavier than nitrogen, reactive, probably not safe to mix with either fuel or oxidizer;
  • oxygen (O2): good oxidizer, unsafe to mix with fuel;
  • hydrogen peroxide (H2O2): good oxidizer or monopropellant, unsafe to mix with fuel;
  • fluorine (F2): good oxidizer, toxic, unsafe to store and handle, very unsafe to mix with fuel;

Single period 3 atom (+ hydrogen):

  • silane (SiH4): extremely flammable, ignites spontaneously in air, don't even think about mixing with oxidizer;
  • phosphine (PH3): toxic, flammable, unsafe to mix with oxidizer;
  • hydrogen sulfide (H2S): toxic, corrosive, flammable, unsafe to mix with with oxidizer;
  • hydrogen chloride (HCl): toxic, corrosive, unsafe to store and handle, probably unsafe to mix with fuel;
  • argon (Ar): noble gas, would be a good pressurizing gas except that it's heavier and more expensive than both helium and nitrogen.

I think that's pretty much every stable covalent molecule with a mass below about 40 Daltons, although I might have missed a few. Notably, out of that list, the only gases that meet all the criteria are the noble gases helium, neon and argon (of which helium is both the cheapest and the lightest) and nitrogen gas.

Going higher in molecular mass, the number of possible molecules to consider gets larger since we now have e.g. compounds of three period 2 atoms (and hydrogen) to consider. I'm not going to even attempt to list them all, but AFAIK there isn't really anything that could compete with helium and nitrogen there, either, especially as heavier gases also (all else being equal) tend to have higher boiling points than their lighter counterparts.

Carbon dioxide (CO2) probably gets closest, being cheap and fairly inert and only about 50% heavier than nitrogen, but its high freezing point makes it unsuitable for use with cryogenic fuels and oxidizers. Pretty much the same goes for sulfur hexafluoride (SF6) too. Some of the perhalogenated carbon compounds like tetrafluoromethane (CF4) might be inert and volatile enough to serve as pressurizing gases, but again, they're all heavier and more expensive than plain old nitrogen and helium, as are the heavy noble gases krypton and xenon. (Radon, the sixth noble gas, has its own unique issues due to being radioactive.)

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There aren't many atomic or diatomic gases at room temperature: H, N, O, F, Cl and the noble gases.

  • noble gases are expensive
  • F and Cl are corrosive
  • oxygen reacts with propellants, so can only be used to pressurize an oxygen tank
  • H is flammable/explosive
  • that leaves N and the cheapest of the noble gases, He.

Compound gases also exist (e.g. CO2) but they are heavier than He and N.

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  • $\begingroup$ Why not H to pressurize O? Is it an explosive mixture? $\endgroup$ – BlueCoder Nov 21 '18 at 15:33
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    $\begingroup$ yes, H+O is quite flammable, that's why that combination is commonly used as rocket propellant. $\endgroup$ – Hobbes Nov 21 '18 at 15:34
  • $\begingroup$ Thanks :) I would add that to the answer, for the non chemical/propellant-savy ones :) $\endgroup$ – BlueCoder Nov 21 '18 at 15:37
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    $\begingroup$ Argon is both cheap ($5/kilo) and inert, and fairly dense (about 1400 kg/m^3) when liquid. It's relatively dense as a gas though, so the residual argon left in a large tank would represent some significant waste mass. $\endgroup$ – Steve Linton Nov 21 '18 at 15:58
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    $\begingroup$ @OrganicMarble Real rough figures - a cylinder 40mx3.6m (F9S1 tankage) comes out to 407 m^3. 407 m^3 of helium masses 71kg and 407 m^3 of nitrogen masses 500kg, so that's a not insignificant change. $\endgroup$ – Saiboogu Nov 21 '18 at 20:28

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