Fueling up: to travel from the Earth to the sky requires propulsion. Propulsion requires energy. Energy requires fuel

Natural History, June, 2005 by Neil deGrasse Tyson

In daily life you rarely need to think about propulsion, at least the kind that gets you off the ground and keeps you aloft. You can get around just fine without booster rockets--simply by walking, running, rollerblading, taking a bus, or driving a car. All those activities depend on friction between you (or your vehicle) and Earth's surface.

When you walk or run, friction between your feet and the ground enables you to push forward. When you drive, friction between the rubber wheels and the pavement enables the car to move forward. But try to run or drive on slick ice, where there's hardly any friction, and you'll slip and slide and generally embarrass yourself as you go nowhere fast.

For motion that doesn't engage Earth's surface, you'll need a vehicle equipped with an engine stoked with massive quantities of fuel. Within the atmosphere, you could use a propeller-driven engine or a jet engine, both fed by fuel that burns the free supply of oxygen provided by the air. But if you're hankering to cross the airless vacuum of space, leave the props and jets at home and look for a propulsion mechanism that requires no friction and no chemical help from the air.

One way to get a vehicle to leave our planet is to point its nose upward, aim its engine nozzles downward, and swiftly sacrifice a goodly amount of the vehicle's total mass. Release that mass in one direction, and the vehicle recoils in the other. Therein lies the soul of propulsion. The mass released by a spacecraft is hot, spent fuel, which produces fiery, high-pressure gusts of exhaust that channel out the vehicle's hindquarters, enabling the spacecraft to ascend.

Propulsion exploits Isaac Newton's third law of motion, one of the universal laws of physics: for every action, there is an equal and opposite reaction. Hollywood, you may have noticed, rarely obeys that law. In classic Westerns, the gunslinger stands flat-footed, barely moving a muscle as he shoots his rifle. Meanwhile, the ornery outlaw that he hits sails backward off his feet, landing butt first in the feeding trough--clearly a mismatch between action and reaction. Superman exhibits the opposite effect: he doesn't recoil even slightly as bullets bounce off his chest. Arnold Schwarzenegger's character the Terminator was truer to Newton than most: every time a shotgun blast hit the cybernetic menace, he recoiled--a bit.

Spacecraft, however, can't pick and choose their action shots. If they don't obey Newton's third law, they'll never get off the ground.

Realizable dreams of space exploration took off in the 1920s, when the American physicist and inventor Robert H. Goddard got a small liquid-fueled rocket engine off the ground for nearly three seconds. The rocket rose to an altitude of forty feet and landed 180 feet from its launch site.

But Goddard was hardly alone in his quest. Several decades earlier, around the turn of the twentieth century, a Russian physicist named Konstantin Eduardovich Tsiolkovsky, who earned his living as a provincial high school teacher, had already set forth some of the basic concepts of space travel and rocket propulsion. Tsiolkovsky conceived of, among other things, multiple rocket stages that would drop away as the fuel in them was used up, reducing the weight of the remaining load and thus maximizing the capacity of the remaining fuel to accelerate the craft. He also came up with the so-called rocket equation, which tells you just how much fuel you'll need (assuming you won't be stopping at any filling stations en route) for your journey through space.

Nearly half a century after Tsiolkovky's investigations came the forerunner of modern spacecraft, Nazi Germany's V-2 rocket--"V" for Vergeltungwaffen, or "Vengeance Weapon." The V-2 was conceived and designed for war, and was first used in combat in 1944, principally to terrorize London. The brainchild of Wernher von Braun and hundreds of other scientists and engineers working with the Nazis, the V-2 was the first ballistic missile and the first rocket to target cities that lay beyond its own horizon. Capable of reaching a top speed of about 3,500 miles an hour, the V-2 could go a few hundred miles before plummeting back to Earth's surface in a deadly free fall from the edge of space.

To achieve a full orbit of Earth, however, a spacecraft must travel five times faster than the V-2, a feat that, for a rocket of the same mass as the V-2, requires no less than twenty-five times the V-2's energy. And to escape from Earth orbit altogether, and head out toward the Moon, Mars, or beyond, the craft must reach 25,000 miles an hour. That's what the Apollo missions did in the 1960s and 1970s to get to the Moon--a trip requiring at least another factor of two in energy.

And that represents a phenomenal amount of fuel.

Because of Tsiolkovsky's unforgiving rocket equation, the biggest problem facing any craft heading into space is the need to boost "excess" mass in the form of fuel--most of which is the fuel required for transporting the fuel it will burn later in the journey. And the spacecraft's weight problems grow exponentially. The multistage vehicle was invented to soften this problem. In such a vehicle, a relatively small payload--such as the Apollo spacecraft, an Explorer satellite, or the space shuttle--gets launched by huge, powerful rockets that drop away sequentially or in sections when their fuel supplies become exhausted. Why tow an empty fuel tank when you can just dump it and possibly reuse it on another flight?