Definition: Entirely self-contained projectiles or vehicles that are self-propelled by jets of gas.
Significance: Because they are self-propelled and self-contained, rockets are capable of operating independently of any outside support equipment. Furthermore, because rockets do not need to take in air to operate, they are capable of operations outside of Earth’s atmosphere.
Nature and Use
A rocket is propelled forward by a jet of material coming from one of its ends. This jet is generally hot gases resulting from burning fuel in the rocket. The fuel is burned in a combustion chamber and the exhaust gases are expelled from the rocket. Frequently the exhaust gases are directed using a nozzle. Forcing the gases to be expelled in one direction pushes the rocket in the other direction. The amount of force pushing on the rocket as a result of expelling the jet of gas is called thrust.
An important consideration for a rocket is its thrust-to-weight (TTW) ratio. The higher a rocket’s TTW, the greater its acceleration. If a rocket has a TTW of less than one, it cannot lift off vertically from the surface of a planet or moon, though it can still fly horizontally with the aid of wings. As a rocket burns fuel, it has less mass and, thus, less weight. As the mass of the rocket decreases, its TTW increases. Rockets, therefore, tend to accelerate faster the longer they burn, unless the thrust is reduced. The Saturn V rocket that carried the Apollo missions to the Moon had an initial TTW of 1.25. The space shuttle was designed with a TTW of approximately 1.5.
A rocket is an entirely self-contained system. Jet engines also rely on the expulsion of hot gases in order to achieve thrust; however, jet engines take in air that mixes with the fuel and burns to provide the exhaust gases. Rockets, in contrast, carry everything they need with them and do not need to take in air to mix with the fuel. Depending on the fuel and the rocket design, rockets sometimes carry oxygen or another chemical that acts as an oxidizer to combine with the fuel in order to make it burn. The fuel and oxidizer taken together are called the rocket propellant. Other rockets contain self-oxidizing fuel, sometimes called a monopropellant, which does not need to be mixed with anything in order to burn. A few advanced rocket designs are able to expel gases without burning fuel at all and, thus, do not need an oxidizer to mix with the propellant.
Different propellants burn with different efficiencies and release different amounts of energy. As a consequence, not all combinations of propellants yield the same thrust even when used in the same rocket. Rocket engineers characterize the efficiency of a rocket propellant by its specific impulse. The specific impulse of a rocket propellant is determined by dividing the thrust provided by the propellant by the weight of propellant consumed per second. The amount of thrust produced depends not only on the propellant used, but also on the rocket design. Thus, the specific impulse of a propellant is valid only for that propellant used in a particular rocket, hence it is specific to the rocket.
Applications
Rockets have many uses. Some rockets are used in conjunction with other propulsion sources to provide additional thrust. Such rockets are called booster rockets. Some rockets are designed to carry cargo or scientific instruments. Anything carried by the rocket that is not part of the rocket itself is called the payload of the rocket. Many military rockets carry a payload of a bomb or other explosive weapon. In such cases, the rocket is called a missile, and the payload is called a warhead.
Rockets have been used with aircraft, either as the sole propulsion system or as strap-on boosters used to achieve extra thrust needed for heavy aircraft to lift off on short runways. The main use of rocket-assisted takeoff is for military transports that need to take off from airfields with runways that are too short to allow for takeoff with conventional engines.
Because rockets can continue to accelerate for as long as they have propellant, they are useful for achieving very high speeds. Such high speeds are needed to achieve orbit around Earth or to leave the vicinity of Earth. Rockets used to launch vehicles into space are called launch vehicles. Furthermore, because rockets are self-contained, they can operate outside of Earth’s atmosphere. All spacecraft use rockets for propulsion.
Sometimes rockets are used to propel other rockets. In such cases, the combined rockets are called multistage rockets. The first rocket is known as the first stage and is often called the rocket booster.
Types of Rockets
Rockets are often classified by the type of fuel that they use. Early rockets used a fuel composed of a paste made from gunpowder. Many modern rockets use a fuel that is a solid chemical. These are called solid-fueled rockets or, occasionally, simple solid rockets. Solid-fueled rockets have an advantage in that the fuel is often easy to manufacture and can be cast into the rocket casing itself. A single piece of solid fuel is called a grain or charge. The shape of the grain can be adjusted to yield different specific impulses as needed. The shape can even be adjusted to yield a different thrust at different times after the grain is ignited. Furthermore, solid fuel is often stable at normal environmental temperatures, and the rocket can be left fully fueled until needed. A disadvantage, however, is that once ignited, solid fuel burns by itself and cannot be easily controlled. A solid-fueled rocket is nearly impossible to turn off once it is ignited, and the amount of thrust cannot be much changed from the initial design considerations taken into account during rocket construction. Once the propellant of a solid-fueled rocket is ignited, it generally has to continue burning until it is all gone.
In 1926, the American physicist Robert H. Goddard designed and built a rocket that used a liquid rather than a solid propellant. The initial liquid propellant consisted of gasoline and liquid oxygen. Since that time, many other liquid propellants have been used. A major advantage of liquid-fueled rockets is that the amount of thrust can be easily controlled by simply adjusting valves that govern the amount of propellant that goes into the combustion chamber. Furthermore, liquid-fueled rockets can be turned off at any time by simply shutting off the propellant control valves. Most liquid-fueled rockets can even be turned on again by opening the valves again after they have been shut off. Although some liquid propellants need an ignition source to start burning the fuel, a few mixtures of fuel and propellant simply begin to burn on contact. These self igniting fuels are called hypergolic propellants. Although liquid-fueled rockets have some clear advantages over solid-fueled rockets, there are some serious disadvantages. Many liquid propellants are cryogenic liquids that must be kept at extremely low temperatures. These cryogenic fluids generally cannot be stored for extended periods of time in the rocket. The rocket, therefore, can only be fueled shortly before use. Furthermore, most liquid propellants are extremely dangerous to transport and to store. Liquid-fueled rockets tend to have complex valve and control systems and, thus, are usually more complicated to design and more expensive to construct than are solid-fueled rockets.
Most rocket designs require the burning of either solid or liquid fuels to provide the source of hot gas and energy that expels a jet of gas that powers the rocket. A few rocket designs do not require burning of the propellant to provide the exhaust jets needed to power the rocket. One such design would employ a reservoir of compressed gas, which would be released and expelled from the rocket. Alternately, the compressed gas could force another propellant, such as water, from the rocket. Some toy rockets are of this extremely simple and inexpensive design. A major disadvantage of this design, however, is that it is very inefficient and cannot provide very much thrust for very long.
Another rocket design calls for the use of a nuclear reactor to heat gases to cause them to be expelled from the rocket. Such nuclear-powered rockets are extremely efficient and powerful. A few nuclear-powered rocket motors have been constructed and tested on the ground or in very short flights, but, due to safety concerns, none have been used in extended flight.
Another technology used to provide the jet of gases is the acceleration of charged atoms or molecules, called ions, with electric fields. The ions are ejected from the rocket in the direction in which the electric field accelerated them. Ion-drive rockets are quite economical. The electric fields can be generated using solar power, and almost any gas can be used as a propellant. The major disadvantage of an ion rocket is that the gas must be very diffuse for the system to work, and this results in very low thrust. The thrust, however, can be sustained for extended periods, resulting in very high speeds after long periods of operation. Because ion-driven rockets have a very low thrust, they cannot be used to launch a vehicle into space, but they can be used once a rocket is already in space.
Parts of a Rocket
Different rocket designs obviously require different components. Some of the most complicated and diverse rockets are liquid-fueled rockets. There are general similarities among most liquid-fueled rockets, however. The bulk of the rocket’s volume holds tanks storing the propellant. For most propellants, there must be separate tanks for the fuel and the oxidizer. The location of the tanks does not really matter, but generally the oxidizer tank is located forward of the fuel tank. This placement allows for a shorter path for the fuel to travel from the tank to the rocket motor. If the rocket is designed to operate within the atmosphere, the rocket body may have fins that help stabilize the rocket in flight, but a true rocket does not use the fins as wings to fly. Some rockets are guided rockets, able to steer while in flight. The mechanisms for guidance and steering are also located within the rocket body. Generally, a rocket’s control mechanism is located away from the rocket motor in order to minimize any damage to the guidance system from the rocket motor’s heat or vibration.
The rocket motor consists primarily of the combustion chamber, which is where the propellant is burned. The rest of the rocket motor is generally composed of valves and plumbing to deliver the propellant and to mix the fuel and oxidizer as efficiently as possible. Frequently, because the fuel and oxidizer are cryogenic fluids, the fuel lines carry the propellant past the combustion chamber before injecting the fuel into the chamber. This has the advantage of helping to cool the combustion chamber and warm the fuel, generally resulting in a more efficient burning process.
Most rockets contain a nozzle to help direct the exhaust gases from the combustion chamber. A nozzle is not strictly necessary, because a properly designed combustion chamber tends to direct the exhaust gases away from the chamber as a jet through an opening at one end of the chamber. The exhaust gases, however, tend to expand as soon as they are out of the combustion chamber and the jet of exhaust gas becomes less directional after leaving the combustion chamber. A nozzle will direct the gas jet in the desired direction. The more directional the jet of gas leaving the rocket motor, the higher the thrust that the rocket motor will have. Thus, a properly designed rocket nozzle is an important part of a rocket motor. The most efficient type of nozzle is the bell-shaped Venturi nozzle, which is narrow at the point where it connects to the combustion chamber and flares out to a much larger diameter farther from the combustion chamber. Some nozzles are fixed in position, whereas others are capable of tilting slightly, thus changing the direction of the jet of gases and, consequently, the direction of the rocket’s thrust. Such movable rocket nozzles are important in guided rockets. For rockets designed to operate outside Earth’s atmosphere, the motion of the rocket nozzle is the chief mechanism for steering the rocket.
Solid-fueled rockets are more simple in design than are liquid-fueled rockets. Both types of rocket have similar rocket bodies and nozzles; the main difference between the two types is in the design of the combustion chamber and the propellant storage. Often, the solid-fueled rocket contains a single grain of fuel. The combustion chamber is often located inside the grain, with the grain itself forming the walls of the combustion chamber. As the grain burns, the combustion chamber expands outward. Alternately, the grain fills the combustion chamber and burns from one end to the other. This is generally a less efficient design. The design and shape of the grain can be adjusted to yield variable thrust according to a predetermined formula, but the thrust variations are determined by the manufacture of the grain and cannot be changed after the rocket is ignited.
The Physics of Rockets
One of the fundamental laws of physics is the law of conservation of momentum. Momentum is defined as the mass multiplied by velocity. The conservation of momentum law says that the total momentum of a system does not change unless an external force acts on the system. Rockets operate using this principle. Jets of material streaming away from one end of the rocket carry momentum. This may be thought of as negative momentum since it is in a direction opposite to the direction of the rocket’s flight. As a consequence, the rocket must have momentum in the opposite, or positive, direction. The sum of the two yields zero. As the jet of gas leaves the rocket carrying negative momentum, the rocket must have an increase of positive momentum. This means that the rocket must increase its forward speed as the rocket’s mass decreases.
Thus, the more mass that is expelled from the rocket, the more momentum it carries. Likewise, the faster the mass leaves the rocket, the more momentum that it carries. The rate at which negative momentum is carried from the rocket determines the thrust of the rocket. For a highly directional jet of gas from a rocket, the thrust is given by the first rocket equation:
In this equation, F is the thrust, u is the speed at which the jet of gas leaves the rocket, and R is the rate of propellant used, measured in mass per time.
As a rocket continues to burn, its mass decreases, and, thus, the acceleration increases if the thrust remains constant. The final speed of a rocket operating outside of the influence of other forces can be determined by the second rocket equation:
In this equation, vf is the final rocket velocity, and vt is the initial rocket velocity. The In indicates the natural logarithm function. The initial rocket mass is given by m. and the final rocket mass is given by mf. The u term represents the exhaust velocity of the gas as it leaves the rocket.
Both of these equations describe idealized rockets. Real rockets are generally not ideal, and, thus, modifications to the equations must be made. Generally, the adjustments to the equations are to yield an effective thrust less than the thrust determined from the first rocket equation and to yield a final velocity less than that determined by the second rocket equation. These equations, therefore, provide the maximum thrust and maximum possible final velocity for a rocket. The goal of rocket engineers is to construct rockets that are as close to ideal as possible.
Much of a rocket consists of storage space for the propellant. After propellant has been used, the portion of the rocket used in storing the propellant becomes dead weight. As indicated in the second rocket equation, if the final mass were reduced, then the final velocity could be increased. This mass reduction could be accomplished by jettisoning the propellant storage spaces. Rather than building rockets that jettison used propellant storage areas, rockets are often designed to propel other rockets, called stages. The first rocket, or first stage, fires until it has used up its propellant. After the first stage has finished using its propellant, it drops off, and the second stage begins operation. A rocket can have as many stages as needed. However, the difficulty and expense of designing and constructing multiple stages, each with its own rocket motors, makes it generally economically unfeasible for a rocket to have more than two or three stages.
