Delta V
Delta-v (literally "change in velocity"), symbolised as ∆v and pronounced delta-vee, as used in spacecraft flight dynamics, is a measure of the impulse that is needed to perform a maneuver such as launch from, or landing on a planet or moon, or in-space orbital maneuver. It is a scalar that has the units of speed. As used in this context, it is not the same as the physical change in velocity of the vehicle.
As a simple example, take a conventional rocket which achieves thrust by burning fuel. Delta-v is the change in velocity that can be achieved by burning that rocket's entire fuel load.
Delta-v is produced by reaction engines, such as rocket engines, and is proportional to the thrust per unit mass and the burn time. It is used to determine the mass of propellant required for the given maneuver through the Tsiolkovsky rocket equation.
For multiple maneuvers, delta-v sums linearly.
For interplanetary missions delta-v is often plotted on a porkchop plot, which displays the required mission delta-v as a function of launch date.
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