PROPULSION

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Propulsion — the rocket equation is the boss fight.

Spaceflight has one universal currency: ∆v, pronounced "delta-v," measured in kilometres per second. Every burn costs you some. Every move on a trajectory has a price tag. The mission's ∆v budget — sum of everything you'll spend, end to end — is the single number that says whether the mission can fly.

The cruel constraint is the rocket equation, written down by Konstantin Tsiolkovsky in 1903. It says ∆v scales LOGARITHMICALLY with how much of your rocket is fuel. Want twice the ∆v? You need exponentially more fuel. This is why a Saturn V is 90% propellant. It's why landing on Mars and coming back is so much harder than landing on the Moon. Every gram of mass that isn't fuel gets fought over.

Six sections here. Start with ∆v as mission budget — that's the framing for everything. Then Tsiolkovsky to see why the math is so unforgiving. Specific impulse explains the engine side; C3 and V∞ are the launch-energy and arrival-energy numbers you'll see all over the FLIGHT tab in /missions. The Oberth effect at the end is a rare piece of free magic — burn deep in a gravity well and your fuel does more work.