If the reader is interested in the history of aerospace or rocketry, I’d like to recommend the book Saturn, by Alan Lawrie and Robert Godwin, Apogee Books, 2005 (ISBN 1-894959-19-4).  Lawrie is the author and Godwin produced the DVD that is included. The book is a comprehensive history of the Saturn V launch vehicle. 

Flipping through the book I happened to land on page 59 and a section on Ullage Motors.  If you have ever handled bulk liquids in drums or tankers, you know that there must be a certain fraction of the tank capacity left unfilled with liquid.  This gas space, or ullage, prevents rupture of the vessel by expansion of the liquid contents.

Ullage motors were used to get around certain problems associated with using liquid phase propellants in weightless conditions. The contents of a liquid propellant tank always include a volume of ullage.  In free fall it is possible for void spaces to form near the propellant transfer lines. 

Saturn V ullage motors were ammonium perchlorate solid rocket motors externally mounted on the lower part of the second stage.  Immediately after first stage engine cutoff, the first stage would separate and for a short time the upper stages would be in free fall.  To prevent transient fuel starvation problems during engine start it was desirable to force propellant to the bottom of the fuel tank prior to the engine start.  To accomplish this, a group of small solid rockets were fired to provide a bit of acceleration to force the liquid propellant to the bottom part of the tank. The eight ullage motors burned for 4 seconds and each generated a reported 22,500 lbs of thrust. 

Curiously enough, there are a reported 46 intact ullage motors in earth orbit.  The authors conclude that these remnants of the Proton 4 rocket represent explosion hazards.