In what ways is helium used on modern launch vehicles?

Question

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?

Answer 1

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:

• Shuttle Crew Operations Manual various pp.
• Space Shuttle News Reference page 2-40
• Environmental Control and Life Support System training manual
• Mass values: personal notes
• SODB: Buried in this directory of the old KSC shuttle history page

Answer 2

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.