Why use Helium

Quick Question. Why use helium to pressurise the tanks?- googling I found only a reference to its boiling point being lower than hydrogen, and therefor the only gas available for cryogenic H2 pressurisation. However, AA use it for LOX and ethanol not H2- surely there must be a cheaper inert gas than helium for LOX, and almost anything for ethanol (N2?)

James

I'll admit that this is just a blatant guess on my part, but Helium is good because:

1) Its cheap and plentiful and plenty of places sell it. I don't know that there's a less-expensive inert gas out there. Maybe nitrogen is inert enough; but while it is generally non-reactive, remember that it's not a true noble gas like Helium/Neon/Xenon/Argon...

2) Not reacting to the ethanol is not good enough. Remember that you may at some point get gas flow through to the injector/combustion chamber... So whatever pressurization gas you use, it shouldn't react poorly to ANY of the other elements in your fuel / engine system.

3) Lastly, remember that the idea is to have a reusable rocket (at least enough to make a powered landing!!) ...So all of your propellants and pressurization gases have to be compatible with all of the plumbing and valve materials and such.

Take care,

--Noel
P.S. Someone can bring up mass & acceleration versus plain weight - but it is also worth noting that Helium has a much lighter molecular weight than Nitrogen....

Primarily because helium has a gas constant of 2.0769 kJ/kg.K while nitrogen has a gas constant of 0.2968 kJ/kg.K.

Assuming ideal gas laws just for example purposes we can see the amount of gas required to fill the flight tanks at a certain pressure is:

m = (Pressure*Volume)/(Gas constant*Temperature)

All other things constant the mass of gas required to fill the tanks is linearly proportional to the gas constant. Therefore you'd need about 6.7 times as much mass of nitrogen as helium. Not that nitrogen can't or isn't used, especially in ground tests where weight isn't as important, but it's a no-brainer for flight vehicles.

In addition, do you guys use and sort of gauze covering the Fuel outlets to prevent helium gulping?

How far away are you from self-pressurising the Lox?

Would you then be able to pressurise the fuel with CNG?

Although helium made sense from the point of view of performance, it turned out that what made it compelling for all their work was the lack of solubility in LOX. When using nitrogen, performance was not consistent and this added variables that were not good when doing engine tests.

In the spacevidcast interview I think Carmack mentioned that the helium is actually the most expensive thing they use, making up 1/2 of the consumables cost per launch

It was the high cost of helium as mentioned in that interview that prompted me to ask the question...!!

Thanks for the answers guys.

What about using a separate pressurised oxygen tank for Lox pressurisation, and methane or some other similar gas for fuel pressurisation? It should give nearly the same performance while having a lot less cost.

Weight and complexity! as if things arent complex enough! any solution will add one the other or both. Unless you can simplfy something else? Or you have some other idea?

Monroe

I doubt switching from helium to O2 and methane would make things more complex.

Hopefully the extra weight is countered by having extra fuel to burn and thus a tiny bit more burn time.

What about "solubility"? O2 is highly reactive. dont you have to add another tank? Helium did not require another tank. You would need a siphon bottle and you would add to the slosh problem as well correct?

Monroe

What about just adding a heater to the lox tank to allow it to self pressurize. Or adding a heat sink and fan to the lox tank. The fan would turn on to boil off some LOX and turn off once the tank was pressurized.

Going through the fine interactive periodic table on http://www.chemicool.com/ we can go down through the list of inert gasses:

He, Helium - Density @ 293 K: 0.0001787 g/cm3, Specific heat: 5.193 J/gK, Boiling point: 4.2 K, cost, bulk: $/100g
Ne, Neon - Density @ 293 K: 0.0009 g/cm3, Specific heat: 0.904 J/gK, Boiling point: 27.1 K, cost, bulk: $/100g
Ar, Argon: Density @ 293 K: 0.0017824 g/cm3, Specific heat: 0.520 J/gK, Boiling point: 87.3 K, cost, bulk: $/100g
Kr, Krypton: Density @ 293 K: 0.003708 g/cm3, Specific heat: 0.248 J/gK, Boiling point: 119.7 K, cost, bulk: $/100g
Xe, Xenon - Density @ 293 K: 0.00588 g/cm3, Specific heat: 0.158 J/gK, Boiling point: 165 K, cost, bulk: $/100g
Rn, Radon - Density @ 293 K: 0.00973 g/cm3, Specific heat: 0.09 J/gK, Boiling point: 211 K, cost, bulk: $/100g

Noting that O2 has a boiling point (STP) @ 90.2 K knocks out everything heavier than argon. Comparing helium, neon and argon shows the bulk cost is roughly equal by mass. Comparing densities, 5 times more neon and 10 times more argon would be needed to do the same job. No bargain there.

N2 has a Boiling point of 77.4 K and has been used as a pressurant for LOX, but as Mr. Carmack and many others discovered, N2 is highly soluble in LOX and affects the performance during operation. Using GOX for pressurant has and is used for pump fed systems where one can tap the high pressure LOX feed line, heat the flow and pipe it back to the LOX tank. There are a number of issues with this. To suppress boiling the pressure in the LOX tank is raised just prior to engine start; the LOX goes from being self cooling by boiling to heating up just prior to operation. After engine shut down the LOX tank will go through a very dynamic pressure change called "pressure collapse" as the GOX cools and condenses into the residual LOX. This pressure can be significantly lower than atmospheric. Not an issue if you are in orbit and the tank is expended anyway, but this could be an issue if it happened right after landing.

So, what to do? He is the hands down best performer. N2 can be used, if performance and operational time constraints are understood and accounted for. GOX can also be used, if its own performance and operational time and complexity constraints are understood and accounted for.

Interesting that Arian 5 uses autogenous (GH2) for the hydrogen tank and He for the LOX tank, with the He stored supercritically in a Dewar and heated by the engine prior to use. The Shuttle uses autogenous pressurization for both tanks, but could save several thousand pounds due to the weight of the GOX in the tank at shutdown (even including eht weight of the required Dewar) and disappear a "crit 1" failure mode (GOX heat exhanger fire) if they had used Arianespace's concept instead.

As a member of another group that goes through a lot of helium, I can tell you that those gas prices are fairly wrong. Helium is a commodity item created from natural gas production, and it has a number of uses in industry. It's available from any industrial gas company. Neon through radon (except Argon) are used in progressively less situations. Given the radioactivity, I don't know if it's even possible for normal people to buy bottled radon.

Argon is used in welding, so it's fairly common and easy to get as well, though not really cheap.

You can't just pressurize with GOX on cold LOX, the gox will simply condense as you try to pressurize the tank. Self pressurizing the tanks works, as Armadillo has discussed, but you have to wait a while for them to get up to pressure. That also means the LOX is relatively warm, and you're more likely to run into two phase flow problems.

Comparing our pressurized tanks to the shuttle or Arianne is not really apt, as they only use a bit of pressure in the tanks to make sure the pumps have enough inlet pressure, whereas we're going direct into the engine.

One more thing about heating. It will lower the density of the O2 and could lead to varing density problems as well. Of course the higher the density the better for power.

Monroe