Week 3: Tsiolkovsky's Vision

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For the next century after Congreve, rocketry stayed almost entirely a weapon of war. Then a mostly self-taught Russian schoolteacher had an idea that turned it into a way to reach other worlds.

History

Konstantin Tsiolkovsky went deaf as a child after a bout of scarlet fever, which cut him off from formal schooling. He taught himself mathematics and physics from books, and — without ever building or testing a real rocket — worked out the fundamental equation that governs how far a rocket can go. It's one of the great examples in science of a correct answer reached almost entirely through pure thought.

Forces

The key idea this week is momentum and mass ratio. The more propellant mass a rocket can throw out the back relative to how much rocket is left, the bigger the velocity change it can achieve. A rocket that's mostly fuel at launch and mostly empty tank at burnout can pick up a lot of speed; a rocket that barely changes mass can't.

Math

Δv = ve·ln(m0/mf)

Worked example (metric): with an exhaust velocity of 2,000 m/s and a rocket that's 3 times heavier fully fueled than empty, Δv = 2000 × ln(3) = 2000 × 1.0986 ≈ 2,197 m/s of velocity change from the propellant alone.

Key Date

In 1903, Tsiolkovsky published “Exploration of Cosmic Space by Means of Reaction Devices,” laying out the rocket equation and the basic case for space travel using rockets — decades before anyone would fly one.