*Tsiolkovsky invented the rocket equation, the formula describing how much speed you gain from a rocket engine. It depends on the exhaust speed of the gas and the net change in mass of the rocket as fuel is expended. The rule of thumb is that the change in velocity ΔV = (exhaust gas speed) × (logarithm of the initial mass/ final mass). For example, if the rocket burns enough fuel to decrease the total mass by a factor of 3, then the velocity increase approximately equals the exhaust gas speed. If the mass decreases by a factor of 9, then the velocity increase approximately equals twice the exhaust gas speed. The takeaway is that you want to burn as much as quickly as possible, and expel it as fast as possible. The faster you burn, the lighter the rocket gets, and the easier it is to change the velocity.