Gravity loss

[citation needed] Consider the simplified case of a vehicle with constant mass accelerating vertically with a constant thrust per unit mass a in a gravitational field of strength g. The actual acceleration of the craft is a-g and it is using delta-v at a rate of a per unit time.

A very large thrust over a very short time will achieve a desired speed increase with little gravity loss.

For instance, gravity loss on a rocket of mass m would reduce a 3mg thrust directed upward to an acceleration of 2g.

As orbital speeds are approached, vertical thrust can be reduced as centrifugal force (in the rotating frame of reference around the center of the Earth) counteracts a large proportion of the gravitation force on the rocket, and more of the thrust can be used to accelerate.

Using this perspective, when a spacecraft reaches orbit, the gravity losses continue but are counteracted perfectly by the centrifugal force.

Since a rocket has very little centrifugal force at launch, the net gravity losses per unit time are large at liftoff.