QUESTION:
Could you tell me how much force was required for the launch vehicle 
(for the Cassini flight system) to break earth's gravity?

ANSWER from Trina Ray on October 18, 1999:
What a terrific question!  Cassini had to use the nations largest and
most powerful expendable launch vehicle, the Titan IV, because we are the
heaviest spacecraft ever to be launched into deep space . At liftoff, the
total stack height (Titan IV + Centaur + Cassini) was 56 meters (184 feet),
which is approximately the height of a twenty-story building; and the total
mass was 941,658 kilograms (1038 tons), of which only 5586 kg (12,220 lb.)
was Cassini (roughly 1/150th of the total mass).

The solid rocket motors provided a thrust of 3.4 million lb. (15.2
million Newton (N)).  The first stage provides 548,000 lb. (2.4 million N)
of thrust and the second stage provides 105,000 lb. (467,000 N). The upper
stage Centaur, which is designed to fly atop the Titan IV second core
stage, is 4.3 meters (14.1 feet) in diameter and 8.8 meters (29 feet) high,
and provides 33,000 lb. (147,000 N) of thrust.

I hope that answer's your question, check out our website
http://www.jpl.nasa.gov/cassini/ for even more details about the Titan IV,
our launch, or Cassini's mission.

Extra info from our website:

Why do we need the largest rocket for this mission? There are two very good
reasons for this. First, coming in at a mass of 5586 kg at launch (compared
to about 825 kg for Voyager), Cassini is among the heaviest spacecraft ever
to be launched on an interplanetary mission. Second, Saturn being so far
away requires a great amount of launch energy to hurl a spacecraft towards
it. These two formidable factors combine to require the largest rocket to
be used to accomplish the job. As a matter of fact, the Titan IV/Centaur
rocket is not powerful enough to impart adequate energy to send the massive
Cassini spacecraft on a direct path to Saturn. But it is the largest rocket
we have!

Fortunately, the mission analysts at JPL have designed a unique trajectory
path which requires the rocket to only send Cassini towards Venus at first.
Thereafter, a little help from subsequent gravity assists from Venus
(twice), Earth, and Jupiter flybys will impart enough energy for Cassini to
arrive at Saturn to accomplish its science objectives.

Did you know...? It will take a Titan IV launch vehicle with two solid
rocket motor strap-ons, a centaur upper stage, and about 3000 kilograms
(6600 pounds) of propellant to complete Cassini's mission at Saturn. Well
over 99 percent of Cassini's trip, however, will bean unpowered coast
through space!

Did you know...? The Titan IV launch vehicle, including the Centaur upper
stage, weighs a total of 940,000 kilograms (2 million pounds), of which
840,000 kilograms (1.8 million pounds) is propellant -- all required to
launch a spacecraft weighing about 6,000 kg (13,000 pounds, or about
1/150th the mass).