Liquid-fuel rocketry

A rocket moves by throwing mass. It burns propellant, and the hot gas rushing out the back at high speed pushes the rocket forward — the exact reaction Newton described, thrust equal and opposite to the exhaust. What makes a rocket different from every other engine is that it carries its own oxidiser as well as its fuel, so it needs no outside air. That is why it alone can accelerate in space, and why the newspaper editors who thought it needed air to "push against" had it precisely backward.

Solid rockets — packed gunpowder — had existed for a thousand years, but they burn all at once, can't be throttled or shut off, and hold too little energy for their weight to reach space. Liquid propellants changed the ceiling. Fuel and oxidiser held in separate tanks can be fed steadily, burned far hotter, and metered and controlled. Robert Goddard laid out the physics in his 1919 Smithsonian paper *A Method of Reaching Extreme Altitudes* and, on 16 March 1926, flew the first liquid-fuel rocket. It rose only 41 feet in two and a half seconds, but it proved the principle that put humans on the Moon.

Liquid-fuel rocketry is the only means humans have ever had of leaving the planet. Goddard's small flame led, through decades of engineering, to the artificial satellite, the crewed spacecraft, and the interplanetary probe — and to the intercontinental missile that shadowed the same era. Everything in orbit, every image of Earth from space, and every footprint beyond it descends from the reaction Goddard demonstrated on a Massachusetts farm, and from the equation that says a machine can indeed push against nothing at all.