Rocket

I

Introduction

Rocket, general term for a jet propulsion device propelled by the expulsion of gases generated in a combustion chamber (see Jet Propulsion). Because the combustible propellants contain both fuel and an oxidizer, a rocket develops thrust independent of its surroundings, unlike other types of jet engines that utilize oxygen from the atmosphere to burn fuel carried aboard (see Combustion). A rocket engine, therefore, is self-contained and is the only type of device suitable for flight propulsion into outer space.

Thrust to propel a rocket is based on Isaac Newton's third law of motion (see Mechanics), which states that for every action, there is an equal and opposite reaction. The principle of a rocket motor may be understood by considering the example of a closed container filled with a compressed gas. Within this container the gas exerts equal pressure on every point of its walls. If a hole is punched in the bottom of the container, however, the gas at the bottom escapes and the pressure against the top of the container is no longer equalized. The internal gas pressure then pushes the container upwards in reaction to the jet of air escaping downwards. The amount of thrust developed by a rocket motor depends mainly on two factors, the velocity with which the burning gases leave the combustion chamber, and the mass of the burning gases.

Rockets may be divided into two principal classes, solid-propellant rockets, such as intercontinental ballistic missiles (ICBM), and liquid-propellant rockets, such as the Saturn 5 space booster (See Guided Missiles). In both types of rockets, the combustion chamber in which the fuel burns is called the motor. In a liquid-propellant rocket, the propellants are carried in separate tanks and delivered to the rocket motor as required; in solid-propellant rockets, the propellant charge is stored and burned in the motor.

The term rocket has frequently been used to describe both the thrust-producing device and the whole rocket-powered vehicle. To avoid confusion, especially in the case of large vehicles such as missiles and space-launch vehicles, the propulsion device is now usually referred to as a rocket engine.

II

Solid-Propellant Rockets

Early solid-propellant rockets were powered by the combustion of a mixture containing the same ingredients as black gunpowder, but in different proportions. Gunpowder consisted of about 75 per cent saltpetre, 12 per cent sulphur, and 13 per cent charcoal by weight. Rocket charges consisted of 60 per cent saltpetre, 15 per cent sulphur, and 25 per cent charcoal. Because of this different composition, the rocket charge burned more slowly than gunpowder.

A

History

The solid-propellant rocket was invented by the Chinese in the early 13th century. The earliest recorded use of rockets took place in 1232 at the military siege of Kaifeng, former capital of Henan (Honan) Province, in which rockets were employed to set fire to tents and wickerwork fortifications impervious to arrows. A few years later, rockets were being used in military operations in Europe and in North Africa, but after the early 15th century they were used mainly as a device for setting fire to the rigging of enemy ships in naval battles. In 16th-century Europe, rockets were a major component of fireworks.

In East Asia, however, rockets were still used as weapons, for in the late 18th century, the army of the Muslim Indian prince Haidar Ali, the ruler of Mysore, had a standing corps of rocket throwers. These rockets, made of bamboo, were usually large and had a range of hundreds of metres. The rocket throwers won the first two battles at Seringapatam against the British forces in India.

When news of the unsuccessful campaigns reached Britain, a British ordnance officer, William Congreve, decided to investigate the suitability of the rocket as a weapon of war. Within a few years he had improved the fireworks rocket to such an extent that it had a range of about 3,279 m (3,000 yd). His rockets had a sheet-iron case carrying a 3-kg (7-lb) charge of incendiary material; the tail stick, used to stabilize its flight, was 4 m (15 ft) long, and the overall weight of the rocket was 14 kg (32 lb).

Congreve's rocket was first used in 1805, during the Napoleonic Wars, when Britain attacked the port of Boulogne, France, in an attempt to destroy or scatter the fleet of barges mustered by Napoleon for his contemplated invasion of Great Britain. The rocket and the attack failed, primarily because of stormy weather, and in the following year, Congreve's rockets were used with great success in the second attack on Boulogne. In 1807 Copenhagen and a large French fleet in its harbour were almost totally destroyed by a naval attack in which many thousands of rockets were expended. In 1813 the free city of Danzig (Gdańsk) was compelled to surrender when British rockets set fire to and destroyed the food supplies of the city. Rocket brigades were also formed in the land forces, and many of these brigades saw successful action against the United States in the War of 1812. Congreve rockets were used in the bombardment of Fort McHenry, Baltimore, Maryland, by the British ship Erebus. The same rockets were used in the Battle of Waterloo when Napoleon was defeated.

By 1825 nearly every country in Europe had copied Congreve's rockets and formed rocket brigades. In 1847 the British inventor William Hale developed a rocket that was spin-stabilized, eliminating the deadweight of the aerodynamic guidestick. The Hale rocket had a series of spin-jet holes, the later models had spin-jet vanes in the rear. Patent rights for these rockets were purchased by the United States, and rockets were made and used in the Mexican-American War and the American Civil War.

The use of rockets in warfare began to decline after 1850, however, as lighter-weight cannons were developed, and more accurate spin-stabilized explosive shells were produced. A peaceful application of rockets in the 19th century was the development of life-saving rockets. Before the age of steam power, sailing vessels often foundered during storms on the coasts of Britain and northern Europe. By using a modified Congreve rocket, a light line could be lofted from the shore over the ship in distress. By pulling out a heavier line, lifeboats could be pulled ashore or a breeches-buoy reserve system established, by which sailors could be moved from ship to shore on a hawser.

By 1880 whaling rockets were developed, using a rocket-propelled lance that was discharged from a small boat. An explosive charge in the nose of the rocket killed the whale and fixed a toggle attached to the trailing rope leading back to the small boat. Rockets were widely used in signalling at sea. By the end of the 19th century, rockets were little used by the military. A few scientists, however, such as the Russian physicist Konstantin Eduardovich Tsiolkovsky, were suggesting the use of rockets to power space vehicles for interplanetary flight.

Rockets were used in World War I primarily for signalling and were also fired from French aircraft against hydrogen-filled observation balloons. The American physicist Robert Goddard was at this time experimenting with solid-propellant rockets and developed a sounding rocket to make scientific measurements in the upper atmosphere at altitudes higher than a balloon could reach. At the time the United States entered World War I in 1917, Goddard offered his services to the US Army. Preliminary trials of high-velocity rockets took place a few days before the end of the war in November 1918. Goddard had improved rocket design by use of smokeless powder instead of black powder. Also, he had added a properly designed convergent-divergent nozzle that greatly improved efficiency of the rocket motor.

Some 20 years later, further developments were made on this small rocket concept by one of Goddard's assistants, Clarence N. Hickman. The result was the anti-tank rocket, the bazooka. The feature of the bazooka rocket that made it very powerful was the addition of a shaped-charge warhead. Fired, without recoil, by an infantry soldier from a shoulder-held tube launcher, the bazooka had an effective range of 182 m (200 yd). A 0.22-kg (0.50-lb) explosive charge was capable of penetrating tank armour of up to 17 cm (7 in) in thickness. Later modifications and improvement of this 5.99-cm (2.36-in) diameter weapon increased the range to 640 m (700 yd). Post-War development of the so-called superbazooka yielded a weapon with double the penetration and a range of 731 m (800 yd).

Rockets of 11.3-mm (4.5-in) calibre were developed by the United States for artillery rockets, fired from multiple launchers; for individual armament, carried by individual soldiers, and fired from the shipping tube or crate; and for aircraft rockets, fired from single or multiple launchers mounted on the wings of aircraft. They varied in length from 76 cm (30 in) for the spin-stabilized artillery rocket, with a range of 4,752 m (5,200 yd), to 1.90 m (6.25 ft) for a fin-stabilized aircraft rocket that was capable of high accuracy. The model most used in aircraft-rocket firing was the 12-cm (5-in) High Velocity Aircraft Rocket (HVAR), which carried a 21-kg (46-lb) high-explosive warhead at a velocity of 410 m (1,350 ft) per second to ranges in excess of 4,570 m (5,000 yd).

German scientists originated two types of bombardment rockets, the 15-cm (6-in) Nebelwerfer and the 20.9-cm (8.25-in) Wurfgerät. In spite of its name, which means “smoke thrower”, the Nebelwerfer carried a high-explosive warhead, whereas the Wurfgerät had incendiary warheads. The Nebelwerfer rocket was adapted subsequently as a powerful air-to-air weapon.

B

Construction

After World War II solid-propellant rockets were developed for many purposes, mostly as boosters for guided missiles. The principal parts of the solid-propellant rocket are the payload, consisting of the warhead or scientific instruments, and the combustion chamber, or motor, containing the fuel charge and nozzles to expel the combustion gases. Fins may be added to stabilize its flight.

Solid-propellant rockets today are divided into two categories, those with unrestricted burning charges and those with restricted burning charges, known also as wall-fitting charges. A typical example of the unrestricted-burning charge rocket is the HVAR utilized in World War II, which was charged with a single stick of powder having a cruciform cross section and suspended in the centre of the rocket motor. This charge burned at all its surfaces, except at the two ends, which were covered by non-flammable plastics. An unrestricted burning charge also may be shaped in the form of a thick-walled hollow tube, which burns at both its inside and outside surfaces. No matter what its size or shape, the charge is called a grain and the devices that hold it in place are known as traps. Unrestricted burning charges have burning times of less than one sec.

For longer burning times a wall-fitting charge is used. This type of charge either burns across its cross section, or it may be hollowed out at the centre so that it burns from the inside towards the rocket wall. The latter method permits a reduction in the thickness of the wall of the outer metal tube of the rocket, because for virtually all of the burning time, the metal tube is reinforced by what is left of the charge.

Modern solid-propellant charges are of very large size. For example, the take-off weight of the solid-propellant submarine-launched Trident-II D5 missile is about 59,000 kg (about 130,000 lb). The two solid rocket boosters (SRBs) on the space shuttle weigh more than half a million kg (more than 1 million lb) each. Made of 11 steel segments, the SRB is the largest solid-propellant rocket ever built in the United States. As a result of the Challenger space shuttle disaster, the seals between the segments were redesigned to prevent a recurrence of the problem that caused the destruction of the spacecraft.

The problem of developing a defensive antiballistic missile (ABM) to intercept approaching ICBMs requires very quick reaction times and high acceleration. The solid-propellant rocket meets these requirements best, and, therefore, the Safeguard ABM system used solid propellants, which included the Sprint, a low-altitude 24-40 km (15-25 mi) intercept missile, and the Spartan, a high-altitude (over 160 km/100 mi) antiballistic missile.

Modern types of solid propellants are synthetic rubbers with an oxidizer such as ammonium perchlorate mixed in during the manufacturing process. Synthetic rubbers are good fuels, and they also have the advantage of staying somewhat flexible so that they do not develop cracks when handled roughly. The synthetic rubber and ammonium perchlorate mixture can be made more powerful by the addition of powdered metals such as aluminium.