Some sounding rockets are very powerful, reaching altitudes of 1000 miles or more. The factor that keeps them as "sounding" rockets is that they never achieve orbital velocity (~17,400mph @ 100miles above sea level.) Below is a picture of a night launch of an Orion Rocket. These rockets achieve altitudes of 50-60 miles.

The gravitational force between two bodies is given as follows:

Where m₁ is the mass of the rocket, and m₂ is the mass of the Earth. R is the distance between the rocket and the center of the earth. G is the Newtonian Gravitational constant, equal to: 6.67 x 10^-11 N•m²/kg². Thus using this equation, scientists can determine the exact force of gravity acting on a rocket at all times. They then can solve the integral of this to get an average, and also a theoretical altitude.