This Is Rocket Science: True Stories of the Risk-taking Scientists who Figure Out Ways to Explore Beyond. Gloria Skurzynski

      IN THE 13TH CENTURY THE CHINESE DESIGNED FIRE ARROWS TO USE AS WEAPONS AGAINST INVADING MONGOL ARMIES.

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      FIRE ARROWS

      Some inventions happen by accident. Two thousand years ago, Chinese scientists searched for a way to make people immortal. They mixed chemicals with liquids and drank them, hoping this would keep them alive forever. Instead, they often died quickly because their experiments contained poisons.

      One of the main ingredients in those potions was saltpeter, a potassium nitrate compound found in dry caves in southern China. Alchemists (the scientists searching for eternal life) used saltpeter to dissolve ores and minerals. They learned how to make pure sulfur by heating iron pyrite, also known as fool’s gold. And they could easily obtain carbon from coal or charcoal. During centuries of experimentation, the alchemists discovered that if saltpeter, sulfur, and carbon were combined, the mixture would burn.

      By A.D. 1040 a Chinese official described three combinations of these ingredients to make three different kinds of weapons: a poison-smoke ball, a bomb that burned, and a bomb that exploded—but not violently. Later experiments showed that when extra saltpeter was added to the three main ingredients, the combination exploded with greater force. This became the earliest form of gunpowder, named huo yao, meaning “flaming medicine.”

      Hollow bamboo tubes filled with this early kind of gunpowder exploded spectacularly when thrown into a fire.

      These were the first fireworks. Chinese experimenters attached arrows to these tubes, sealed the tubes at one end, and left the tubes open at the other end. When the gunpowder was lit, a mixture of fire, smoke, and gas flew out through the open end and propelled the weapon forward. Called fire arrows, these weapons weren’t accurate enough to hit their targets very often, but their loud explosions frightened enemies. Chinese crossbowmen could shoot the arrows as far as 650 feet.

      AT FIRST, FIRE ARROWS FRIGHTENED MONGOL INVADERS AND TERRIFIED THEIR HORSES. THEN THE SOLDIERS LEARNED TO USE FIRE ARROWS AGAINST THEIR OWN ENEMIES.

      To the north of China lived nomadic tribes that moved their herds over vast distances of unbroken deserts and grasslands. Many of these tribes united to form the Mongol Empire under the command of the famous Genghis Khan. In A.D. 1232 an army of 30,000 Mongol warriors invaded the Chinese city of Kai-fung-fu, where the Chinese fought back with fire arrows. A book about this battle describes the destruction caused by a single fire arrow: “When it was lit, it made a noise that resembled thunder and extended [about 15 miles]. The place where it fell was burned, and the fire reached more than 2000 feet…. These iron nozzles, the flying powder halberds that were hurled, were what the Mongols feared most.” The explosions especially terrified the horses. Mongol leaders learned from their enemies and found ways to make fire arrows even more deadly as their invasion spread toward Europe.

      CHINESE CHARACTERS FOR “ROCKET” AND “FIRE ARROW”

      On Christmas Day 1241 Mongol troops used fire arrows to capture the city of Budapest in Hungary, and in 1258 to capture the city of Baghdad in what’s now Iraq. Soon the Arabs in Baghdad created their own fire arrows and used them against the army of French king Louis IX.

      By 1300 these weapons had moved farther into Europe, reaching Italy by 1500. There, the people enjoyed exploding them for the same reason the Chinese had at first: to make fireworks. In the old Italian language rocce was the word for a long, thin tube. As fire arrows advanced into explosive devices using tubes of iron filled with gunpowder, the word “rocce” evolved into the word “rocket.”

      AS THE MONGOL CONQUERORS MOVED WEST INTO THE ARAB WORLD, PEOPLE THERE LEARNED TO MAKE THEIR OWN FIRE ARROWS.

      Nearly two centuries after the Italians coined the term that became the English word “rocket,” British scientist Sir Isaac Newton formulated his famous laws of gravity and motion, now frequently used to explain how rockets and propulsion work. This statue (left) shows Newton standing above an apple. Tradition says that seeing an apple fall from a tree inspired his thoughts about gravity. Newton also built on the ideas of scientists who had come before him—Copernicus, Galileo, and Kepler—to formulate the three laws of motion. He published them in 1687 in a book titled Philosophiæ Naturalis Principia Mathematica (Mathematical Principles of Natural Philosophy).

      WHEN HE SAW AN APPLE FALL FROM A TREE, ISAAC NEWTON BEGAN TO STUDY GRAVITY.

      Newton’s first law states, “Every object persists in its state of rest or uniform motion in a straight line unless it is compelled to change that state by forces acting upon it.” In other words, if something isn’t moving, it will stay where it is until something moves it. And if it’s moving, it will keep moving at the same speed and in the same direction unless a force acts upon it to change its speed or direction.

      What are the forces in Newton’s first law as it applies to rockets? Gravity is a force. From the first millisecond of launch, Earth’s gravity pulls on the rocket. The force of gravity between two objects depends on the masses of the two objects and the distance between the centers of mass of the two objects. As the distance between two objects gets larger—for example, the distance between Earth and a moving rocket—the gravitational force between them gets smaller.

      Thrust

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