INTRODUCTION
- ROCKET, general term for a jet propulsion device propelled by the expulsion of gases generated in a combustion chamber. Since 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. 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 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. - 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, we also call the propulsion device as a rocket engine.
SOLID PROPELLENT ROCKET:
Early solid-propellant rockets were powered by the combustion of a mixture containing the same ingredients as black gunpowder, but in different proportions.
Gunpowder - 75 % saltpetre,
12 % sulphur,
13 % charcoal by weight.
Rocket charges consisted of - 60 % saltpetre,
15 % sulphur,
25 % charcoal.
Because of this different composition, the rocket charge burned more slowly than gunpowder.
The principal parts of the solid-propellant rocket are
(i) payload(consisting of the warhead or scientific instruments)
(ii) 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,
(i)those with unrestricted burning charges
(ii) with restricted burning charges, known also as wall-fitting charges. - 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 . The two solid rocket boosters (SRBs) on the space shuttle weigh more than half a million kg 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.
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