Delta-v
The currency of spaceflight: how much you can change your velocity, in metres per second.
Every manoeuvre in space costs velocity change, and a spacecraft's total budget is fixed at launch by how much propellant it carries and how fast its engine throws it. Getting to low Earth orbit costs about 9.4 km/s of delta-v; the Moon's surface another 6; Mars's surface roughly the same again. The cruel part is the rocket equation: because propellant must itself be accelerated, the mass required grows exponentially with delta-v, not linearly. This one relationship is why rockets are almost entirely fuel tank, why staging exists, and why a launch site nearer the equator — where the Earth's rotation is a free contribution — is worth having.
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Questions
What is Delta-v?
The currency of spaceflight: how much you can change your velocity, in metres per second.
Why does it matter?
The cruel part is the rocket equation: because propellant must itself be accelerated, the mass required grows exponentially with delta-v, not linearly. This one relationship is why rockets are almost entirely fuel tank, why staging exists, and why a launch site nearer the equator — where the Earth's rotation is a free contribution — is worth having.