How a Rocket Works

Into Orbit

How a Rocket Works

The “Star-Spangled Banner” speaks of “the rockets’ red glare” at Fort McHenry in the War of 1812. Those were British rockets, fired in an unsuccessful attempt to destroy the American fort, located in Baltimore. Conceived and designed by Sir William Congreve 12 years earlier, the so-called ”Congreves” were not the first war rockets. Some six centuries before the War of 1812, the Chinese connected a tube of primitive gunpowder to an arrow, creating an effective rocket weapon to defend against the invading Mongols.

According to Chinese legend, a Chinese official named Wan Hoo designed a chair equipped with two kites and 47 gunpowder rockets in 1500 A.D. The rockets were ignited, presumably with “punk,” a dry, spongy material prepared from fungi and used to ignite firework fuses. When the ensuing explosion and smoke cleared, Wan Hoo and his innovative chair were gone.

Atlas V

Atlas V

Although Wan Hoo’s flight was likely his last, actual attempts at human spaceflight didn’t get off the ground until much later in history during the late 1950s and 1960s.

The early Chinese “fire arrows,” the British Congreves, and even our familiar Fourth of July skyrockets have certain things in common: they are basically tubes of gunpowder lighted at the bottom, where explosive thrust moves them in the opposite direction. They are similar in principle to the space booster rockets used to launch payloads into space today.

In both cases some type of fuel is burned with an oxidizer, or propellant, in a combustion chamber, creating hot gases accompanied by high pressure. In the combustion chamber these gases expand, pushing equally in every direction. A relatively small hole, called the throat, exists at the bottom of the chamber and leads to an exhaust nozzle that flares out like a bell. The expanding gasses pushing in all directions inside the chamber push through the throat and out against the widening sides of the nozzle.

As a given volume of gas reaches the edge of the nozzle, it spreads out over a much greater area than at the throat, so that the pressure on the edge of the nozzle is less than that within the combustion chamber.

The sum of all the pressures inside the chamber acting on the chamber and nozzle skirt in a forward or upward direction is more than the sum of all the pressures acting in a rearward or downward direction. This net difference makes the rocket engine and the vehicle attached to it—its fuel and oxidizer (propellant) tanks, plus payload or spacecraft—lift from the launch pad.

You can demonstrate this principle by blowing up a balloon.

After you blow up the balloon, hold the opening tightly closed. The air inside the inflated balloon is made up of gases that are pushing equally in all directions. Let the balloon go and it will fly across the room or up to the ceiling. The opening you have released is the exhaust nozzle, where the gases escape and expand. Enabling the gases to pass through the opening at the rear of the balloon reduces the pressure inside the balloon. The pressure opposite the nozzle at the front of the balloon remains unchanged, however, and the higher pressures being released at the rear of the balloon push it forward.

The balloon’s flight will be erratic because it has no guidance system, fixed shape, or exhaust control.

The balloon’s flight demonstrates Sir Isaac Newton’s Third Law of Motion: to every action, there is an equal and opposite reaction.

Rocket Principles

In the inflated balloon, the pressure of the gases inside is the action. The counter-pressure of the balloon walls holding the gases in is the opposite reaction. When these two factors are in balance, the gas in the balloon remains at rest. The balloon also remains at rest because its weight is balanced by the force you exert to hold the balloon shut and because the force exerted by the internal gas on each part of the balloon is balanced by a force of the gas outside of the balloon.

Newton’s First Law of Motion says in essence: a body at rest will remain at rest…unless an unbalanced force is exerted upon it.

When an opening or nozzle is provided for the gases inside the balloon, an imbalance occurs because the internal gases can escape through this low-pressure area. There is no longer an equal pressure maintaining the balance. The internal pressure of the gases creates an unbalanced force that drives the gases through the opening.

The side of the balloon opposite the opening is now experiencing a force that is no longer balanced by an equal and opposite force on the side of the balloon where the opening is. This unbalanced force on the balloon in a direction opposite to the side of the balloon where the internal gases are escaping through the hole makes the balloon move forward.

Because Newton’s laws were postulated in the 17th century, some people might say, “All the basic scientific problems of spaceflight were solved 300 years ago. Everything since has been engineering.”

Many scientific developments in mathematics, chemistry, and other disciplines were needed, however, before man could apply Newton’s basic principles to achieve controlled spaceflight.