Centaur Upper Stage external spherical tank

[caglayankutay]

Hi

What is this little balloon tank in Centaur for?

[Redclaws]

That’s not a balloon tank (a tank which cannot hold its own weight and must be pressurized at all times), it’s a tank which looks like a balloon.  That little guy is going to be a tank for a pressurant - I don’t know what centaur uses, probably helium?  Which is used to help pressurize some systems and possibly as a purge gas.

Here’s a quote on that, it’s helium:

Helium Supply and Pressurization System - Helium is the main pressurant used for tank pressurization. Typically, four 26-inch diameter high-pressure helium bottles mounted on the aft bulkhead are used for storage. They are made of a graphite/epoxy composite overwrap enveloping a stainless steel or aluminum liner. These bottles are charged to 4000 psia prior to liftoff. Helium in-flight purges are used for the sense lines, LO2 vent valve and the LO2 bubbler line.

Source:
https://www.ulalaunch.com/docs/default-source/upper-stages/the-centaur-upper-stage-vehicle.pdf (Right side of page 4)

[Jim]

Hi

What is this little balloon tank in Centaur for?

Two are for helium and the 3rd (in between) is hydrazine

[russianhalo117]

Helium tanks/bottles. Now days they use cylindrical COPV's. Others house  the hypergolic propellants which will be replaced with a hydrogox system.

[DaveS]

Hi

What is this little balloon tank in Centaur for?

One is helium and the other is hydrazine

Yes, the smaller of the three is the hydrazine fuel tank for the Rocket Engine Module (REM) thrusters used for attitude control during coast phases and settling of the propellants prior to Main Engine Starts (MES). The larger tanks are the two Gaseous Helium (GHe) tanks used to pressurize the propellant tanks.

[caglayankutay]

<3

 Well, how is this pressurization done? Is the helium sent directly to the fuel tank or is there a more complex system?

[ulm_atms]

<3

 Well, how is this pressurization done? Is the helium sent directly to the fuel tank or is there a more complex system?

Nope...that's about it.  He tank, plumbing, regulator/valve assembly, more plumbing, main tank( in from top...not bottom..don't want the engine to ingest He while running..that leads to a bad day).  I think there are some tap offs for engine purge but not 100% sure on that.