Rocket

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3

Hybrid Chemical Rockets

Hybrid rocket engines use both liquid and solid fuels. Usually, the liquid oxidizer is injected onto the solid synthetic rubber fuel and ignited in the combustion chamber. Hybrid systems combine advantages of both solid- and liquid-fueled systems. Hybrid propellants are inexpensive, and their burn rate can be controlled by regulating the oxidizer flow. Hybrid rockets are still experimental and have not been widely used, but several rocket manufactures are testing hybrid systems. Hybrid propellants have specific impulses of around 2,900 N/kg/s (300 lb/lb/s), which is comparable to that of cryogenic liquid propellants.

B

Nuclear Rockets

Nuclear rockets are very powerful rockets that are theoretically capable of acting as launch vehicles and long-distance space travel systems. No nuclear rocket has yet made it into space, but experimental rockets have undergone tests on Earth. The complexity of building safe nuclear rockets and worries about using rockets that are carrying radioactive materials have limited the practical use of nuclear rockets.

Nuclear rockets generate thrust by using nuclear reactions to heat liquid hydrogen to a superheated gas, or a gas heated well beyond its boiling point, that shoots out of the rocket nozzle. In the nuclear reactions that occur, called fission reactions, heavy atoms such as uranium and plutonium split apart to produce lighter elements and energy. Nuclear rockets could produce much higher specific impulses than chemical systems, because nuclear rockets heat propellants to higher temperatures. Specific impulses of nuclear rockets are 7,800 N/kg/sec (800 lb/lb/sec) or more. In one form of nuclear rocket engine, a small nuclear reactor (similar to one used to produce electricity on the ground) superheats liquid hydrogen circulated through the reactor. Another type of nuclear rocket, called a gaseous fission nuclear rocket, offers specific impulses of 14,000 N/kg/sec (1,400 lb/lb/sec) or more. Gaseous fission rockets create an intensely hot fireball by splitting atoms of uranium-233 gas or a similar fuel. As before, liquid hydrogen is pumped in and converted into a superheated gas that exits the nozzle. See Nuclear Energy.

A fission reaction releases most of its energy in the form of heat, which helps power the rocket. Fission reactions also release other types of radiation in the form of gamma rays and fast-moving neutrons. Both gamma rays and these fast neutrons can be harmful to the rocket body and to any living things nearby. The intense heat of both kinds of reactors can also be quite destructive to the rocket’s structure. Engineers of nuclear rockets surround the reactor with heavy metals, such as lead, in order to contain radiation. Engineers also design extensive cooling systems—usually with circulated water or cold liquid hydrogen—to control the heat. The National Aeronautics and Space Administration (NASA) in the United States is investigating nuclear propulsion. This extremely powerful source of propulsion energy holds much promise for both piloted and unpiloted space exploration within and beyond the solar system.

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Electric Rockets

Electric rocket engines use batteries, solar power, or some other energy source to accelerate and expel charged particles. These rocket engines have extremely high specific impulses, so they are very efficient, but they produce low thrusts. The thrusts that they produce are sufficient only to accelerate small objects, changing the object’s speed by a small amount in the vacuum of space. However, given enough time, these low thrusts can gradually accelerate objects to high speeds. This makes electric propulsion suitable only for travel in space. Because electric rockets are so efficient and produce small thrusts, however, they use very little fuel. Some electric rockets can provide thrust for years, making them ideal for deep-space missions. Satellites or other spacecraft that use electric rockets for propulsion must be first boosted into space by more powerful chemical rockets or launched from a spacecraft.

Rocket manufacturers in the United States began experimenting with electric propulsion in the early 1960s. The first electric rocket engines shot a stream of cesium atoms through an electric field that was generated by an onboard battery. The electric field stripped off electrons from the atoms, creating ions—or atoms with a net electric charge. The charge of the ions made them more susceptible to being directed by electric fields. The stream of ions was directed into another electric field that accelerated them to very high speeds, then expelled them from the rocket nozzle. The steady stream of ions exiting the rocket produced the rocket’s thrust.

Plasma engines, another type of electric rocket engines, use a strong electric current to turn a normal gas into a plasma. Plasma is a state of matter in which many atoms have been ionized, or stripped of at least one of their electrons. This conversion turns the gas into a sea of ions, free electrons, and neutral atoms, with fairly equal numbers of positively charged ions and negatively charged electrons. The most common type of plasma rocket engines uses a cathode, or a positive electric terminal, that extends into a cylindrical chamber. One edge of the chamber is an anode, or negative electrical terminal. Injectors feed a neutral gas into the chamber. A strong electric current is put on the cathode. The current ionizes some of the gas (turning it into plasma) and uses the traveling ions to carry electricity between the cathode and the anode. This ionization sets up an electric field between the cathode and the anode, and a magnetic field around the cathode. These fields act to accelerate the charged particles out of the rocket nozzle. Collisions between the charged and neutral particles make the particles move faster and give the rocket even more thrust.

In 1992 Russian and American aerospace engineers began developing electric rockets called Hall thrusters, or Stationary Plasma Thrusters (SPTs). Hall thrusters act much like the plasma thrusters described above, except Hall thrusters have an external magnetic field. The chamber of a Hall thruster is surrounded by a magnet. A cathode extends into the chamber, and an anode forms the outer edge of the chamber. A neutral gas is fed through the back of the chamber. The electric field created by the cathode and the anode turns the gas into plasma, and the electric and magnetic fields accelerate the plasma out of the rocket. Hall rockets are especially useful for keeping satellites in the correct orbit, or station keeping. The electricity for most Hall thrusters comes from solar cells. Such rockets last for years and are much lighter and less expensive than chemical thrusters. Electric rockets work well for station keeping, but the amount of thrust they produce must be greatly increased if these rockets are to be used for primary propulsion systems or for long distance voyages.

The photon rocket is another potential means of rocket propulsion. Theoretically, photon rockets move by emitting a beam of light with an exhaust velocity of the speed of light. Photons (packets of light) have no mass, but their speed is so great that they could theoretically produce a tiny amount of thrust. The thrust of a photon rocket would be so small that such rockets would be of use only outside of the gravitational influence of the solar system.

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Rocket Flight

Rockets are used for many different applications, but they share some aspects of their flight profiles (the actions and the order of the actions that they perform during flight). All rockets require some structure or method with which they can be launched. They also require a design that provides stability and control in flight.

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Launching

Controllers can launch a rocket in a variety of ways, depending on the rocket’s use. For some applications, such as the military duty of missiles, rockets need to be protected from enemy detection and enemy attack while controllers ready them for launch. In the case of rockets carrying piloted spacecraft, the most important concern is the safety of the people aboard the spacecraft.