During the time interval t , a mass of fuel q t is expelled. Please note, our derivation neglects the effect of air resistance. Also note, -gt is directed vertically downward. For many spacecraft maneuvers it is necessary to calculate the duration of an engine burn required to achieve a specific change in velocity, v . As stated earlier, the equations derived in this section neglect the effect of air resistance.
Drag forces can have a significant influence on a rocket moving through an atmosphere.