The **Tsiolkovsky rocket equation** or **ideal rocket equation** is an equation used to calculate the impulse imparted by a rocket which other object ejecting mass, such as a hose. The equation is

- $ \Delta v = v_\text{e} \ln \frac {m_i} {m_f} $

where ∆v is the change in velocity, v_{e} is the velocity of the ejected material, m_{i} is the initial mass and m_{f} is the final mass. In a gravitational field, this becomes

- $ \Delta v = v_\text{e} \ln \frac {m_i} {m_f} - gt $