One of the main goals of a rocket is to speed up.
The amount by which a rocket can increase its speed,
after using up all its fuel, is called delta-vee.
Recall that the thrust of a rocket depends on two things:
the amount of fuel used per second, and the speed with which
the exhaust leaves the rocket:
dM
thrust = - ---- * u
dt
There are many different ways to design a rocket;
engineers may choose to emphasize one or the other of these
terms, depending on their needs.
The F1 engine was used in the first stage of the giant
Saturn V rocket in the Apollo missions. It was designed
to create stupendous thrust by dumping lots of exhaust,
quickly.
The ion engine aboard the Deep Space 1 spacecraft, on the other
hand, uses its fuel very slowly -- but throws the exhaust
out the back at a very high speed u.