# In what ways is helium used on modern launch vehicles?

Helium is a fairly rare material and does not remain in the atmosphere.

I'd like to know the uses of helium on modern launch systems?

Do most launch vehicles use helium in several different ways, or is there pretty much only one use for it?

• Do you mean this Spectrum rocket? Parabolic Arc and Isar Aerospace It might be hard to answer about this one if there isn't much information about it. Would the uses of helium on other rockets that are more widely discussed still be helpful?
– uhoh
Aug 21 '20 at 13:47
• Can you provide me an example with similar details for another launcher? Spectrum was just an example Aug 21 '20 at 13:49
• I've made an edit and adjusted the wording, is this still the question you'd like answered? Feel free to edit further or roll back.
– uhoh
Aug 21 '20 at 13:56
• Helium-3 is a specific isotope of helium that's more typically discussed in reference to fusion power; I'm going to change the helium-3 tag. Not sure yet if there's a helium tag but if there isn't I don't think that's really an issue. Aug 21 '20 at 21:43
• If you search this site for the falcon-9 tag (just for example) and the word helium, it returns over fifty posts! space.stackexchange.com/search?q=%5Bfalcon-9%5D+helium Searching for helium alone returns several hundred!
– uhoh
Aug 21 '20 at 23:36

Helium is used as a pressurant and purge gas. While no longer a "modern" launch system, the Space Shuttle made extensive use of helium to pressurize various systems and can perhaps serve as an example.

• Prelaunch pressurization of the External Tank liquid oxygen and liquid hydrogen tanks (on the order of 100 lbm of He for each tank)

During standby operations, the liquid oxygen and liquid hydrogen tanks are pressurized with gaseous helium to maintain a nominal positive pressure before loading and launch to avoid possible structural damage that could result from thermal and atmospheric pressure changes. Approximately 3 minutes before launch, the tanks are pressurized until lift-off with helium piped from a ground facility.

• The ammonia tanks for the ammonia boiler cooling system were pressurized with helium.

No bladder separates the NH3 from its helium (He) pressurant. If an NH3 controller is manually activated (SEC/ON) while in zero g, the He pressurant could leak out, making that system unusable

The location of helium tankage for the remaining systems to be discussed is shown in light green on this drawing from the Shuttle Operational Data Book (SODB).

• The main propulsion system had a significant helium system used to actuate pneumatic valves and purge the systems. It had three 17.3 ft^3 tanks and seven 4.7 ft^3 tanks which were pressurized to ~ 4000 psi prelaunch. (On the order of 225 lbm of He for the system)

• The Orbital Maneuvering System (OMS) was a pressure-fed system of 2 6000 lbf engines whose propellant tanks were pressurized with helium. There was a dedicated OMS helium tank in each of 2 OMS pods. (on the order of 80 lbm of He total)

• The Reaction Control System (RCS) was a pressure-fed system of attitude control jets whose propellant tanks were pressurized with helium. There were two helium tanks per RCS system for a total of six tanks.

Sources:

• Since many of the orbiter systems kept the helium in bladders and in theory would land with the same amount launched with, is there any information on if the systems were pumped down and recycled during post landing service or were they just bleed to atmosphere and replaced with known pure He for next launch? Aug 21 '20 at 23:27
• @GremlinWranger I am really not sure about the turnaround processing. Very interesting question. Aug 21 '20 at 23:47
• @GremlinWrangler I just realized you mentioned bladders. None of the systems I describe above used bladders, but that just makes your question more interesting. Aug 22 '20 at 2:44

If there is a rocket using cryogenic hydrogen tanks, you need helium for it.

Liquid hydrogen should not be mixed with air, oxygen or nitrogen. A mixture with oxygen is explosive and should be avoided for security. Nitrogen gas would liquify and even freeze at the temperature of hydrogen. Other noble gases like argon would liquify, even solidify at the temperature of liquid helium. So there is no alternative to helium.

A hydrogen tank filled with air from manufacture needs preparation before filling with hydrogen. So you need helium to purge out the air. You can not remove the air first, the resulting vacuum would destroy the tank. So you need much more gaseous helium than the tank volume to remove all air.

In theory it would be possible to purge the air in the tanks using nitrogen first. When there is no oxygen left, nitrogen may be purged by hydrogen. When there is no remaining nitrogen, liquid hydrogen may be loaded. All hydrogen mixed with nitrogen should be flared in some distance to the launch pad.

If helium is used for pressurization too, you need so much helium to maintain tank pressure of a nearly empty tank. Again multiple times the tank volume is needed (volumes at ambient pressure).

A heat exchanger to liquify hydrogen for pressurization may avoid helium for tank pressurization. But using gaseous hydrogen to purge the air before filling would be very dangerous.

• That means if we run out of Helium, we can say goodbye to space travel forever? Or is there a chance to find alternatives, just they are more expensive?
– vsz
Aug 22 '20 at 11:23
• @vsz.Jusy don't use hydrogen Aug 22 '20 at 11:37
• @vsz there is no other gas than helium that is gaseous at the temperature of liquid hydrogen. That is physics, no chance to change it. There are more expensive noble gases than helium but they are no altenative anyway.
– Uwe
Aug 22 '20 at 11:51