In 1814, during the War of 1812 between the United States and Great Britain, the British invaded the United States and attacked Fort McHenry, near the city of Baltimore. The poet Francis Scott Key watched the battle and wrote “The Star-Spangled Banner”: “The rockets' red glare…Gave proof through the night that our flag was still there.” Those British missiles resembled skyrockets that today are fired to celebrate the Fourth of July.

Those missiles used gunpowder for the propellant, with this powder being packed into tubes. When ignited, the gunpowder did not explode like a bomb. Instead it burned rapidly but in a controlled manner, producing a strong flow of hot gas. This flow gave the rocket its thrust. To increase the thrust, the tube had a nozzle or constriction at its end. This partially blocked the flow of gas and brought an increase in the pressure of the gas within the tube. In turn, this greater pressure gave more thrust.

Today's solid-propellant rockets follow similar principles. They no longer use gunpowder; modern propellants have much more energy. However, these propellants continue to resemble gunpowder because they mix an oxidizer with fuel. The oxidizer is a chemical that breaks down to release oxygen. The oxygen makes the fuel burn, and when it burns, it produces the hot gas. As in 1814, this gas again flows through a nozzle.

Today's nozzles do more than raise the internal pressure. They are carefully designed to increase the speed of the hot-gas flow, which gives a further increase in the thrust. The largest such nozzles are 14 feet (four meters) across and are used with the Space Shuttle. These nozzles swivel; they can point in different directions to steer the rocket in flight.

 
A view of a Saturn 5 rocket just after engine ignition.

Solid-propellant rockets are simple in design and give plenty of thrust. However, they fall short in a critical area: exhaust velocity. For high performance, as when flying to orbit, rockets need the highest possible speed of the hot gas as it blasts from the nozzle. The reason is that high exhaust velocity greatly reduces the amount of propellants that a rocket must carry. The Solid Rocket Boosters of the Space Shuttle have an exhaust velocity of 8436 feet per second (2,571 meters per second). By contrast, the main engines of the Shuttle produce 14,590 feet per second (4,447 meters per second).

These main engines use liquid propellants: liquid oxygen and liquid hydrogen. Both of them are “cryogenic,” which means “formed by cold.” Liquid oxygen has a temperature of -300 degrees Fahrenheit (-184 degrees Celsius). Liquid hydrogen is colder still: -423 degrees F (-253 degrees C). Indeed, liquid hydrogen is only 36 degrees F above absolute zero, the coldest that anything could possibly be. This fuel can be stored and handled safely, but it demands great care. Even a small heat leak would cause liquid hydrogen to boil, making it useless for a rocket.

Hydrogen and oxygen are gases at ordinary temperatures. But it is not possible to store them as gases for use in a rocket. They would have to be compressed to carry them in quantity, and these compressed gases would have to be held in thick-walled tanks to withstand their pressure. These tanks would add weight, which is a rocket designer's enemy, for rocket builders always seek the lightest possible weight. When these gases are liquefied at low temperatures, the rocket can carry the largest possible quantities, and the tanks are light in weight.

These propellants must be pumped from the tanks to the rocket engines. The Saturn V, which carried astronauts to the Moon, had five main engines. Together, they burned propellants at a rate of 15 tons per second. Each engine had its own set of pumps, and each pump developed as much as 60,000 horsepower (44,742 kilowatts). This meant that the pump alone had as much power as a dozen diesel locomotives. The rocket engines, of course, developed far more power. At full thrust, they had the total power of a string of locomotives more than 200 miles (322 kilometres) in length, extending from New York City almost to Washington, D.C.

 
Developed in the 1970s by NASA's Marshall Space Flight Centre in Huntsville, Alabama, the Space Shuttle Main Engine is the world's most sophisticated reusable rocket engine.

The main fuel pumps of the Space Shuttle are also rated at 60,000 horsepower (44,742 kilowatts). Each pump is four feet (1.2 meters) long and two feet (0.6 meter) across; it would fit on a kitchen table. It not only produces, but also uses, this power within this small space. It does this by using turbines. A turbine is a disk made of heat-resistant metal, with many small blades fitted around its edge. When a strong flow of hot gas strikes the blades, this gas flow produces power by forcing the disk to rotate very rapidly. This power then drives the pump.

There are several ways to obtain the hot gas for the turbines. The Saturn V used a “gas generator cycle.” It tapped off small quantities of fuel and liquid oxygen, burning these propellants in a small chamber to produce the hot gas. After driving the turbines, this gas simply went overboard and did no further work.

The Space Shuttle uses the more demanding “staged combustion cycle.” It uses “pre-burners,” which amount to rocket engines in their own right. Hydrogen fuel burns with a limited supply of oxygen within a pre-burner, producing a hot fuel-rich flow of gas that drives the turbines. But this gas does not go overboard. Instead it goes into the engine's main combustion chamber, where it burns with the rest of the oxygen to produce the engine's thrust. When hydrogen burns with oxygen, it produces very hot steam. The Space Shuttle's rocket engines thus amount to high-tech steam engines.

Within the Shuttle's oxygen pumps, the hydrogen-rich gas that drives the turbines is hotter than a blowtorch. Two feet away, within the pump, is liquid oxygen that is more explosive than gasoline. Hence it is essential to keep the hot gas separate from the oxygen. To do this, the pump has a zone between them that is filled with high-pressure helium, which does not burn. Neither the hot gas nor the oxygen can leak past this zone, and so they do not mix.

Hydrogen and oxygen propellants give the highest energy and the best exhaust velocity. However, they cannot be stored for long periods because they evaporate readily. Some rocket engines therefore use storable propellants, which can be held in tanks at room temperature. The Shuttle uses such propellants for on-orbit manoeuvres; it can fly in orbit for days while keeping these propellants ready for use. The fuel is a form of hydrazine; the oxidizer is nitrogen tetroxide.

When propellants burn within an engine's combustion chamber, they produce temperatures hot enough to boil iron. Hence the chamber must be cooled. It uses “regenerative cooling,” in which the fuel itself serves as the coolant. The chamber is constructed using a large number of thin tubes fastened side by side, with metal bands encircling them to provide strength. The fuel absorbs the heat as it flows through the tubes. Then, being hotter, it burns in the chamber with more energy. In this fashion the rocket engine recovers its own heat and puts it to good use.

Large rockets, built for flight to orbit, typically have two or three stages. The first stage ignites to produce lift-off and accelerates to its highest speed as its propellants all burn up. It carries the second stage, which ignites as the first stage falls away, flying on to higher velocity. A third stage can fly onward to still higher speeds, placing a spacecraft in a high orbit or launching a mission to one of the planets.

The world's launch vehicles use this principle. The Space Shuttle, the Air Force's Titan IV, the Delta 2, and Europe's Ariane rockets all rely on solid-propellant boosters. These deliver high thrust for the initial boost, with liquid-propellant engines driving the stages that fly to orbit. The top stage, which carries the spacecraft, may use either solid or liquid propellants. The Russians also use multi-stage rockets, and their most important launch vehicles use liquid propellants exclusively.