Re: Ullage

John C. Broman, Jr. (
Mon, 26 Jul 1999 19:40:22 -0400

At 03:56 PM 7/26/99 -0700, Randy John wrote:

>	I noticed in the Globalstar web pages, reference to
>an 'ullage' event over Australia.  A trip to the dictionary
>didn't satisfy me so I am appealing to the list.

Basically, the ullage is the gaseous part of a container when partially
filled with a liquid.  A sealed soda bottle has a small ullage of C02 at
the top.  Rocket tanks often contain a pressurized ullage of helium to help
deliver the propellant to the engine under pressure.  And helium is often
chosen because it's light and non-reactive.

In zero-G, there is no "top" for the ullage to rise to, so prior to an
engine restart, small jets give the rocket a small boost forward which
allows the liquid propellant to settle back to the bottom of the tank.

John Broman
Beginner satellite observer
Professional rocket builder