of gravity and pushing upward against another force: air resistance. These three forces work together, acting on the rocket at launch and during flight.
Newton’s second law says, “Force equals mass multiplied by acceleration.” As one rocket scientist says, “It’s a ridiculously simple and at the same time complex equation.” Mass is the amount of matter in an object. Weight is the gravitational attraction of the mass, and the weight stays constant unless the force of gravity is changing. Acceleration is the rate of change in the velocity (speed) of a moving body—a measure of how fast the object is changing its speed.
Thrust in a rocket depends on the rate at which the mass of the burning fuel inside the rocket is expelled through the nozzle at the end of the rocket and the speed at which it escapes. The force of the extremely hot gases escaping through the nozzle accelerates the rocket. The heavier the rocket, the more thrust/force will be needed to move it. If a rocket weighs a million pounds and only a million pounds of thrust is produced, the rocket won’t move. To launch it off the ground requires a thrust greater than a million pounds so it can overcome gravity and air resistance. To increase the thrust level requires burning more fuel, using a higher-energy fuel, or both.
Newton’s third law reads, “For every action there is an equal and opposite reaction.” With rockets, the action is the expelling of high-speed exhaust through the back end. The reaction is the movement of the rocket in the opposite direction. The same thing happens when you blow up a balloon and let it go. The air rushing out of the open end shoots the balloon away from you.
After Newton published his laws of motion, people began to think of rocketry as a science, which of course it had been all along. In the 1700s Germans and Russians experimented with rockets so powerful that, when lit and fired, their blasts blew holes in the ground. Gradually, as they understood and applied Newton’s laws of motion, scientists began to understand the forces in rocketry—how to control them and what to expect.
DURING THE WAR OF 1812, AS BRITISH WARSHIPS FIRED ROCKETS ON FORT MCHENRY IN MARYLAND FRANCIS SCOTT KEY WROTE ABOUT “THE ROCKETS’ RED GLARE.”
Just before the beginning of the 20th century, black
H. G. WELLS’S NOVEL THE WAR OF THE WORLDS INSPIRED YOUNG FUTURE SCIENTISTS TO DREAM OF SPACE TRAVEL.
2
THE FATHERS OF MODERN ROCKETRY
Born in 1857, Konstantin Tsiolkovsky was one of 18 children of a Polish patriot who’d been deported to Russia. When Konstantin was ten years old, he became deaf from scarlet fever. This kept him out of school, but he taught himself by studying as many books as he could borrow, including From the Earth to the Moon, the science-based imaginative space-travel adventure by French author Jules Verne. This novel was published at about the time young Konstantin learned to read.
Impressed by his enthusiasm for learning, Konstantin’s family sent him to Moscow when he was 16. There, a teacher, who also recognized the boy’s brilliance, tutored him at a library every day for three years. Tsiolkovsky not only studied mathematics and science but also became intrigued by rockets.
Later he recalled, “For a long time I thought of the rocket as everybody else did—just as a means of diversion and of petty everyday uses. I do not remember exactly what prompted me to make calculations of its motions. Probably the first seeds of the idea were sown by that great fantastic author Jules Verne—he directed my thought along certain channels, then came a desire, and after that, the work.” As early as 1865, when From the Earth to the Moon was published, Jules Verne already knew that escaping Earth’s gravity would require great speed. Tsiolkovsky began to think of ways to go fast enough to leave Earth behind. “Earth is the cradle of humanity,” he said, “but humanity cannot remain in the cradle forever.”
At the age of 21 Tsiolkovsky took a job as a math teacher in a small town south of Moscow, where he began to develop his ideas about space flight—not only how to blast rockets off the ground, but also how they could carry humans into space. He wrote scientific articles and several decades later published one titled “The Exploration of Cosmic Space by Means of Reaction Devices.” It encouraged readers to “[v]isualize…an elongated metal chamber…designed to protect not only the various physical instruments but also a human pilot…. The chamber is partly occupied by a large store of substances which, on being mixed, immediately form an explosive mass.”
KONSTANTIN TSIOLKOVSKY
Tsiolkovsky knew that the speed at which gas escapes from a rocket—called its exhaust velocity—results from the explosive force of the rocket fuel. The more powerful the explosion, the greater the thrust. The propellant mixture Tsiolkovsky had in mind was liquid oxygen (LOX) combined with liquid hydrogen (LH2), since LH2 provides high energy per pound.
Halfway around the world, just four years before Tsiolkovsky published his “Reaction Devices” article, 17-year-old Robert Hutchings Goddard climbed a cherry tree behind the barn of his Massachusetts home. A year earlier, in 1898, Goddard had read H. G. Wells’s science fiction novel The War of the Worlds. Perhaps its scenes about invaders from Mars lingered in his mind, because he later wrote that on that day, “I imagined how wonderful it would be to make some device which had even the possibility of ascending to Mars, and how it would look…if sent up from the meadow at my feet….”
ROBERT GODDARD BELIEVED THAT LIQUID-FUEL ROCKETS COULD FLY AS FAR AS THE MOON.
At Clark University Robert Goddard received a Ph.D. in physics and set up an experiment to prove that rockets could fly in a vacuum, which most people doubted. He built a chamber, removed all the air from it, put a small rocket inside, and fired it. This experiment convinced him that rockets could not only fly in a vacuum, but also achieve up to 20 percent more thrust in a vacuum than in air because there was no air resistance to reduce the rocket’s thrust.
Goddard had ideas about fuel, too. Unaware of Tsiolkovsky’s article, Goddard believed that liquid propellant was a higher-energy fuel that created more thrust than solid propellant did. In solid-fuel rockets the grain (the propellant charge) is densely packed and molded inside a casing. Goddard understood the disadvantages of liquid propellants—they’re harder to handle than solid propellants like gunpowder. Also, the two components, fuel and oxidizer, have to be kept in separate tanks until launch, and liquid oxygen (the oxidizer) must be stored at a very low temperature, near –300°F. But the advantage of liquid propellants is that during flight, the rate at which they’re injected into the combustion chamber can be increased or decreased or shut off if necessary, while solid fuels burn until they’re completely used up. Goddard would have liked to combine LH2 with LOX, but LH2 was hard to come by. Instead, he used gasoline combined with LOX for his rocket experiments.
“PROFESSOR GODDARD…SEEMS TO LACK THE KNOWLEDGE LADLED OUT DAILY IN HIGH SCHOOLS.”
NEW YORK TIMES
In
