| Profil von Aleen Missiles FotosBlogListen |  Extras  Hilfe |
|
24 Februar Rocket BasicsRockets for Rookies
Popular terminology makes a distinction between jets and rockets: a jet takes in air from the atmosphere; a rocket needs no air supply, as it carries its own supply of oxygen. Both types of engines operate by expelling a stream of gas at high speed from a nozzle at the after end of the vehicle. Rockets are distinguished by the means used to produce exhaust material. The most common type of rocket engine obtains its high-pressure gases by burning a propellant. This propellant consists of both fuel and oxidizer and may be solid or liquid. The fuels and oxidizers used to power a jet/rocket engine are called propellants. The chemical reaction between fuel and oxidizer in the combustion chamber of the jet engine produces high-pressure, high-temperature gases. These gases, when channeled through an exhaust nozzle, are converted into kinetic energy creating a force acting in a direction opposite to the flow of the exhaust gases from the nozzle. This propulsive force, termed thrust, is a function primarily of the velocity at which the gases leave the exhaust nozzle and the mass flow rate of the gases. In order to develop a high thrust with a solid propellant, grains or charges of propellant are employed with large burning surfaces so that a high rate of mass flow is developed. The duration of burning of a propellant charge is determined by the web of the grain and the burning rate. Since the combustion chamber has fixed dimensions and capacity for propellant, the thrust may be either great, but of short duration, or low, but of long duration. Propellants are classified as either solid propellants or liquid propellants. Nearly all of the rocket-powered weapons in use by the United States use solid propellants. Liquid propellants are still used in some of the older ICBMS and will be used in future cruise missiles. Liquid fuels are more powerful than solid fuels; but other than this advantage, a liquid-fuel rocket is not ideally suited as a weapon-propulsion system. Because of their high volatility and corrosive nature, liquid fuels cannot be stored for long periods of time, which usually means the system must be fueled just prior to launch. This negates its ability to be a quick-reaction weapon, which is usually required in combat situations. The size and type of a missile selected for a particular function are based on the target, the launch vehicle or platform, range and maneuverability requirements, altitude envelope, and storage requirements. Minimum size and weight may not be the most efficient architecture, and it is often best to employ various types of structures for different sections of the missile to obtain certain design or maintenance advantages. The components of a missile are located in five major sections: the guidance section, warhead section, autopilot section, and control and propulsion sections. The functional systems of the missile are:
The Guidance System. The guidance system for a homing missile consists of an antenna assembly or electro-optical device protected by an optically transparent cover or a radome in the case of the radio frequency system, and electronic components that analyze signals from the target and compute orders for use by the autopilot. The sensor employed is usually a gimbal-mounted automatic tracking sensor (except the interferometer method) that tracks the target line-of-sight (LOS) and sends signals about the target's movement to the guidance electronics. The Warhead. The warhead consists of the fuze assembly, warhead, safety and arming device, and fuze booster. The fuze assembly usually contains a contact and proximity fuze. The contact fuze is enabled at all times, and the proximity fuze is actuated electronically. Its circuitry works in conjunction with the guidance section to ensure that the target detection device (TDD) remains unarmed until just prior to intercept, minimizing vulnerability to jamming. The safety and arming device prevents arming of the warhead until the missile is a safe distance from the firing platform. The Autopilot. The autopilot is a set of electronic instruments and electrical devices that control the electric actuators (motors) of aerodynamic control surfaces (fins). In the absence of signals from the guidance computer, the autopilot maintains the correct missile attitude and maintains the missile flight in a straight line. Called-for-acceleration signals from the guidance computer will cause the autopilot to command corresponding changes in flight path, while continuing to stabilize the missile. The Propulsion System. Any of the methods of propulsion previously described may be used as long as the missile has sufficient speed advantage over the target to intercept it. The propulsion system must accelerate the missile to flying speed rapidly to allow a short minimum range and achieve sufficient velocity to counter target maneuvers. Powered flight may occur for most of the operational range of the weapon or only at the beginning (boost-glide). Boost-glide weapons are limited in their ability to engage at long range targets that have significant altitude difference or perform rapid maneuvers. The Control System. The steering or control unit may be located forward, in the midsection, or aft on the missile, depending on where the control surfaces are located. Movement of control surfaces may be electrical or hydraulic, with electrical actuation becoming the dominant method. Some weapons are limited in allowable locations for the control actuators because of size limitations or difficulty in passing signals from the autopilot to remote points on the airframe.
Source: GlobalSecurity
05 Februar Nuclear BombNuclear Bomb
The first nuclear bombs were fission devices, and the later fusion bombs required a fission-bomb trigger. Following are the nuclear weapon categories.
An incredible amount of energy is released, in the form of heat and gamma radiation, when an atom splits. The energy released by a single fission is due to the fact that the fission products and the neutrons, together, weigh less than the original U-235 atom. The difference in weight is converted to energy at a rate governed by the equation e = m * c^2. In order for these properties of U-235 to work, a sample of uranium must be enriched . Weapons-grade uranium is composed of at least 90-percent U-235. In a fission bomb, the fuel must be kept in separate subcritical masses, which will not support fission, to prevent premature detonation. Critical mass is the minimum mass of fissionable material required to sustain a nuclear fission reaction. This separation brings about several problems in the design of a fission bomb that must be solved:
To bring the subcritical masses together into a supercritical mass, two techniques are used:
Neutrons are introduced by making a neutron generator. This generator is a small pellet of polonium and beryllium, separated by foil within the fissionable fuel core. In this generator:
Finally, the fission reaction is confined within a dense material called a tamper, which is usually made of uranium-238. The tamper gets heated and expanded by the fission core. This expansion of the tamper exerts pressure back on the fission core and slows the core's expansion. The tamper also reflects neutrons back into the fission core, increasing the efficiency of the fission reaction.
Click on image for an enlarged view of the actual nuclear weapons.
Source:HowStuffWorks
Fuel/Air BombsFuel/Air Explosive (FAE)
Fuel-Air Explosives [FAE] disperse an aerosol cloud of fuel which is ignited by an embedded detonator to produce an explosion. The rapidly expanding wave front due to overpressure flattens all objects within close proximity of the epicenter of the aerosol fuel cloud, and produces debilitating damage well beyond the flattened area. The main destructive force of FAE is high overpressure, useful against soft targets such as minefields, armored vehicles, aircraft parked in the open, and bunkers. Accidental vapor cloud explosion hazards are of great concern to the refining and chemical processing industry, and a number of catastrophic explosion accidents have had significant consequences in terms of injury, property damage, business interruption, loss of goodwill, and environmental impact. Almost all organic material in the form of a dust cloud will ignite at temperatures below 500 oC - approximately the same temperature as a newly extinguished match. Cotton, plastics and foodstuffs such as sugar, flour and cocoa can also, under the right conditions, act as explosives. In order for a dust explosion to take place, the dust particles must be of a certain size and the amount of finely granulated material per unit of volume must lie within certain critical values. There is generally a direct correlation between particle size and explosive hazard. The smaller the particle, the more reactive the dust. As the materials become smaller, they disperse and remain suspended more easily, increasing the potential for ignition and propagation of the reaction. Industrial explosion prevention measures include, where possible, providing nitrogen gas purging to ensure that the oxygen concentration is kept below that required for combustion. There are dramatic differences between explosions involving vapor clouds and high explosives at close distances. For the same amount of energy, the high explosive blast overpressure is much higher and the blast impulse is much lower than that from a vapor cloud explosion. The shock wave from a TNT explosion is of relatively short duration, while the blast wave produced by an explosion of hydrocarbon material displays a relatively long duration. The duration of the positive phase of a shock wave is an important parameter in the response of structures to a blast. Although the detonation combustion mode produces the most severe damage, fast deflagrations ( to burn suddenly, generally with flame and crackling noise - Chambers) of the cloud can result from flame acceleration under confined and congested conditions. Flame propagation speed has a significant influence on the blast parameters both inside and outside the source volume. The blast effects from vapor cloud explosions are determined not only by the amount of fuel, but more importantly by the combustion mode of the cloud. Significant overpressures can be generated by both detonations and deflagrations. Most vapor cloud explosions are deflagrations, not detonations. Flame speed of a deflagration is subsonic, with flame speed increasing in restricted areas and decreasing in open areas. Significantly, a detonation is supersonic, and will proceed through almost all of the available flammable vapor at the detonation reaction rate. This creates far more severe peak over-pressures and much higher amounts of blast energy. The speed of the flame front movement is directly proportional to the amount of blast over-pressure. A wide spectrum of flame speeds may result from flame acceleration under various conditions. High flame front speeds and resulting high blast over pressures are seen in accidental vapor cloud explosions where there is a significant amount of confinement and congestion that limits flame front expansion and increases flame turbulence. These conditions are evidently more difficult to achieve in the unconfined environment in which military fuel-air explosives are intended to operate. To watch fuel bomb in action, please refer the photo album with "Fuel Bomb" as its heading.
Source: GlobalSecurity
RADAR Guided missileACTIVE AND SEMIACTIVE RADAR MISSILE GUIDANCE
Radar guided air to air missiles currently represent the best of what state of the art technology can offer, both in terms of range, accuracy and resistance to countermeasures. This reflects in the fact, that these weapons are only used by the world's frontline air forces, the maintenance of the complex fire control systems required being beyond the abilities of the average Third World country.
Radar guidance systems detect and home in on their targets by sensing electromagnetic energy reflected from the target's surface. The source of the reflected radiation is a radar transmitter; in the instance of weapons with active radar guidance, this transmitter is situated within the missile; in the case of semiactive guidance, it is carried by the launch aircraft. In either case the transmitter must beam electromagnetic radiation at the target, this radiation must travel to the target, reflect, travel back to the receiving antenna of the missile, be amplified, demodulated and analysed to determine the direction of the target, this information then enables the guidance computer to steer the weapon toward the target to achieve a kill. An effective weapon must have the ability to discriminate between the target's return and reflections from its background, i.e. the surface of the Earth or ocean, it should also be capable of resisting jamming or deception and be able to penetrate through adverse weather conditions.
Courtesy of Carlo Kopp
Cruise MissilesCruise Missiles
First of the cruise missile family to enter service and essentially the only dedicated strategic weapon in the family, the AGM-86B is a generic development of the early seventies Boeing SCAD program. That vehicle subsequently developed into the AGM 86A, which was supplanted by the longer ranged AGM-86B which became the operational version of the missile.
The Current AGM-86B is equipped with the basic TERCOM (Terrain Contour Matching) inertial guidance system and this also reflects in the vehicle's mission profile. A TERCOM equipped vehicle will employ an inertial navigation system (INS) to find its way to the target, however the INS will accumulate a position error with time and this error must be eliminated or reduced if the weapon is to have any sort of useful accuracy. TERCOM does exactly that. (View the pictures attached with this article.)
The TERCOM system uses a radar altimeter and barometric altimeter to measure the profile of the terrain beneath the aircraft. This information is then compared with stored terrain profile information within the memory of the TERCOM computer to yield a position update. The update is then fed into the INS. The exercise of measuring and matching the terrain profile is carried out only several times during the weapon's flight, this provides sufficient accuracy while limiting the size of the computer memory required to store the terrain maps. The missile is loaded with its position and a file of terrain maps for its flight prior to launch, it is then fully autonomous. It will hug terrain at several hundred to a thousand feet, following a very indirect flightpath through hostile territory, this serves to confuse the defenders as to the exact nature of the target, alternately the flight planner may route the flightpath to avoid known early warning and air defence facilities. The AGM-86B is a 20ft 10.5 in (6.3m) vehicle weighing in at 3,1501b (1,420kg) at launch. Most of the airframe is occupied by the four fuel tanks which feed the powerplant. That is a Williams International F107 600 lb thrust class low bypass ratio turbofan, weighing about 1501b (68kg) and occupying the last three feet of the airframe. The engine mounts above the set of actuators for the vehicle's folding elevons, the tailpipe is shielded by a tailcone which conceivably blocks infrared emissions from the engine hot end. The use of a turbofan is advantageous in that it does provide better fuel consumption than a turbojet, with a reduced infrared signature. The penalty to be paid is in the thrust limitation, which will force the vehicle to follow terrain with larger clearance. More Photos with detailed description is available in the photo album under "Cruise Missile" heading.
Source: AirPowerAustralia
Rocket EngineRocket Engine
The following article describes the spaces shuttles rocket engine. This is somewhat analogous to the one employed in missiles.
A rocket engine is generally throwing mass in the form of a high-pressure gas. The engine throws the mass of gas out in one direction in order to get a reaction in the opposite direction. The mass comes from the weight of the fuel that the rocket engine burns. The burning process accelerates the mass of fuel so that it comes out of the rocket nozzle at high speed. The fact that the fuel turns from a solid or liquid into a gas when it burns does not change its mass. If you burn a pound of rocket fuel, a pound of exhaust comes out the nozzle in the form of a high-temperature, high-velocity gas. The form changes, but the mass does not. The burning process accelerates the mass.
The idea behind a simple solid-fuel rocket is straightforward. What you want to do is create something that burns very quickly but does not explode. As you are probably aware, gunpowder explodes. Gunpowder is made up 75% nitrate, 15% carbon and 10% sulfur. In a rocket engine you don't want an explosion -- you would like the power released more evenly over a period of time. Therefore you might change the mix to 72% nitrate, 24% carbon and 4% sulfur. In this case, instead of gunpowder, you get a simple rocket fuel. This sort of mix will burn very rapidly, but it does not explode if loaded properly. Here's a typical cross section:
On the left you see the rocket before ignition. The solid fuel is shown in green. It is cylindrical, with a tube drilled down the middle. When you light the fuel, it burns along the wall of the tube. As it burns, it burns outward toward the casing until all the fuel has burned. In a small model rocket engine or in a tiny bottle rocket the burn might last a second or less. In a Space Shuttle SRB(Solid Rocket Boosters) containing over a million pounds of fuel, the burn lasts about two minutes.
For more details visit: HowStuffWorks
03 Februar Stinger MissilesStinger Missile
The Stinger missile, officially known as the FIM-92A, is designed to give ground troops a way to deal with low-flying airplanes and helicopters. From the perspective of soldiers on the ground, low-flying enemy aircraft are normally a problem because they are either bombing or strafing, doing surveillance work or inserting, extracting and resupplying enemy troops. Shooting down these aircraft is the easiest way to eliminate the threat.
Here are the basic parts of a Stinger missile:
And here are the basic parts of the launching system:
While the missile is flying, the image of the airplane that it is trying to hit may become off-center on the image sensor. When it does, that tells the missile that it is off-course, and the guidance system in the missile has to decide how to get back on course. This is where proportional navigation comes in. The missile looks at the angle of off-centeredness and changes its angle of flight proportionally. In other words, it uses a multiplier. If the multiplier is 2, then if the guidance system thinks it is 10 degrees off course, it will change its flight direction by 20 degrees. Then, a tenth of a second later it will look at the angle again, and change again. By over-correcting this way, it lets the missile anticipate the path of the moving plane in the same way that you anticipate the path of a moving object. If you are a quarterback trying to throw a ball to a receiver running across the field, you would not throw the ball toward where the receiver is -- you would throw it toward where he will be when the ball arrives. Obviously, the guidance system is more involved than this. The image sensor is riding on a rotating, gyrating missile at Mach 2, and it is trying to hit a small target that may be flying at Mach 1. It is not a simple problem. But this is the general idea.
Source: HowStuffWorks
|
||||||||||||||||||
|
|
