The FGM-148 Javelin is an American-made man-portable anti-tank guided missile fielded to replace the Dragon antitank missileJavelin is a fire-and-forget missile with lock-on before launch and automatic self-guidance. The system takes a top-attack flight profile against armored vehicles (attacking the top armor which is generally thinner) but can also take a direct-attack mode for use against buildings or fortifications. This missile also has the ability to engage helicopters in the direct attack mode. The missile reaches a peak altitude of 150m (492 ft.) in top attack mode and 50m ( 164 ft.) in direct fire mode. The missile is equipped with an imaging infrared seeker. The tandem warhead is fitted with two shaped charges: a precursor warhead to detonate any explosive reactive armor and a primary warhead to penetrate base armor. The Javelin was used in the 2003 Invasion of Iraq, with devastating effects on the Iraqi version of T-72s and Type 69 tanks.
The missile is ejected from the launcher so that it reaches a safe distance from the operator before the main rocket motors ignite; a "soft launch arrangement". This makes it harder to identify the launcher and allows it to be fired from within buildings; however, back-blast from the launch tube still poses a hazard to nearby personnel. Thanks to this "fire and forget" system, the firing team may move on as soon as the missile has been launched. The missile system is carried most often by a two man team consisting of a gunner and an ammo bearer, although it can be fired with just one person if necessary. While the gunner aims and fires the missile, the ammo bearer scans for prospective targets and watches for threats such as enemy vehicles and troops.
Components
Missile
Warhead
The Javelin missile’s tandem warhead is a high-explosive antitank round.[2] This round utilizes an explosive shaped charge to create a jet of superplasticity deformed metal formed from trumpet-shaped metallic liners. The result is a high velocity jet that can penetrate armor.
The Javelin counters the advent of explosive reactive armor (ERA). ERA panels lie over a vehicle’s main armor and explode when impacted by a warhead. This explosion does not harm the vehicle’s main armor, but causes steel panels to fly into the path of the antitank round’s jet, so that the jet expends its most potent energy cutting through the panels, rather than through the main armor. The Javelin uses two shaped-charge warheads in tandem. The precursor charge sets off the ERA and clears it from the path of the main charge, which then penetrates the target’s primary armor.
A two-layered molybdenum liner is used for the precursor and a copper liner for the main charge.
To protect the main charge from the explosive blast, shock, and debris caused by the impact of the missile's nose and the detonation of the precursor charge, a blast shield is used between the main and precursor charge. This was the first composite material blast shield and the first that had a hole through the middle to provide a jet that is less spread out.
A newer main charge liner produces a higher velocity jet. This change makes the warhead more effective as a penetrator and smaller, with more room for propellant to increase the missile's range.
Electronic arming and fusing, called Electronic Safe Arming and Fire (ESAF), is used. The ESAF system enables the firing and arming process to proceed, while imposing a series of safety checks on the missile. ESAF cues the launch motor after the trigger is pulled. When the missile reaches a key acceleration point (indicating that it has cleared the launch tube), the ESAF initiates a second arming signal to fire the flight motor. After another check on missile conditions (target lock check), ESAF initiates final arming to enable the warheads for detonation upon target impact. When the missile strikes the target, ESAF enables the tandem warhead function (provide appropriate time between the detonation of the precursor charge and the detonation of the main charge).
Most rocket launchers require a large clear area behind the gunner to prevent injury from backblast, and thus cannot be fired from within a building. To address this shortcoming without increasing recoil to an unacceptable level, the Javelin system uses a soft launch mechanism. A launch motor using conventional rocket propellant ejects the missile from the launcher, but stops burning before the missile clears the tube. The flight motor is ignited only after a delay to allow for sufficient clearance from the operator. To save weight, the two motors are integrated together with a burst disc between them; it is designed to tolerate the pressure of the launch motor from one side, but to easily rupture from the other when the flight motor ignites. Both motors use a common nozzle, with the flight motor's exhaust flowing through the expended launch motor. Because the launch motor casing remains in place, an unusual annular (ring-shaped) igniter is used to start it; a normal igniter would be blown out the back of the missile when the flight motor ignited and could injure the operator.
In the event that the launch motor malfunctions and the launch tube is overpressurized -- for example, if the rocket gets stuck -- the Javelin missile includes a pressure release system to prevent the launcher from exploding. The launch motor is held in place by a set of shear pins, which fracture if the pressure rises too high and allow the motor to be pushed out the back of the
Seeker
As a fire-and-forget missile, after launch the missile has to be able to track and destroy its target without the gunner. This is done by coupling an onboard imaging IR system (different from CLU imaging system) with an onboard tracking system.
The gunner uses the CLU’s IR system to find and identify the target then switches to the missile’s independent IR system to set a track box around the target and establish a lock. The gunner places brackets around the image for locking.
The seeker stays focused on the target’s image continuing to recognize as the target moves or the missile’s flight path alters or as attack angles change. The seeker has three main components: 1) focal plane array (FPA); 2) cooling and calibration; and 3) stabilization.
1) Focal plane array (FPA) - The seeker assembly is encased in a dome which is transparent to the FPA long-wave infrared radiation. The IR radiation passes through the dome and then through lenses that focus the energy. The IR energy is reflected by mirrors on to the FPA. The seeker is a two-dimensional staring FPA of 64x64 MerCad (HgCdTe) detector elements [6]. The FPA processes the signals from the detectors and relays a signal to the missile’s tracker.
The staring array is a photo-capacitive device where the incident photons stimulate electrons and are stored in the detector as an accumulated charge. The electrons are discharged, pixel by pixel, as currents to a readout integrated circuits attached at the rear of the detector. A better photovoltaic mechanism in which a voltage signal is developed directly from the impact of the photons and charge storage is done in the readout rather than in the detector material.
2) Cooling/Calibration - The FPA must be cooled and calibrated. The CLU’s IR detectors are cooled using a Dewar flask and a closed-cycle Stirling engine. But in the missile there is not sufficient room. So prior to launch, a cooler mounted on the outside of the launch tube activates the electrical systems in the missile and supplies cold gas from a Joule-Thompson expander to the missile detector assembly while the missile is still in the launch tube. When the missile is fired this external connection is broken and coolant gas is supplied internally by an onboard argon gas bottle. The gas is held in a small bottle at high pressure and contains enough coolant for the duration of the flight of approximately 19 seconds.
The seeker is calibrated using a chopper wheel. This device is a fan of 6 blades: 5 black blades with very low IR emissivity and one semi-reflective blade. These blades spin in front of the seeker optics in a synchronized fashion such that the FPA is continually provided with points of reference in addition to viewing the scene. These reference points allow the FPA to reduce noise introduced by response variations in the detector elements.
3) Stabilization - The platform on which the seeker is mounted must be stabilized with respect to the motion of the missile body and the seeker must be moved to stay aligned with the target. The stabilization system must cope with rapid acceleration, up/down and lateral movements. This is done by a gimbal system, accelerometers, spinning mass gyros (or MEMS), and motors to drive changes in position of the platform. The system is basically an autopilot. Information from the gyros is fed to the guidance electronics which drive a torque motor attached to the seeker platform to keep the seeker aligned with the target. The wires that connect the seeker with the rest of the missile have no friction to keep the seeker platform balanced.
Tracker
The tracker is key to guidance/control for an eventual hit. The signals from each of the 4,096 detector elements in the seeker are passed to the FPA readout integrated circuits which reads then creates a video frame that is sent to the tracker system for processing. By comparing the individual frames the tracker determines the need to correct so as to keep the missile on target. The tracker must be able to determine which portion of the image represents the target. The target is initially defined by the gunner who places a configurable frame around it. The tracker then uses algorithms to compare that region of the frame based on image, geometric, and movement data to the new image frames being sent from the seeker, similar to pattern recognition algorithms. At the end of each frame the reference is updated. The tracker is able to keep track of the target even though the seeker’s point of view can change radically in the course of flight.
To guide the missile the tracker locates the target in the current frame and compares this position with the aim point. If this position is off center the tracker computes a correction and passes it to the guidance system which makes the appropriate adjustments to the four moveable tail fins, as well as six fixed wings at mid-body. This is an autopilot. To guide the missile the system has sensors that check that the fins are positioned as requested. If not, the deviation is sent back to the controller for further adjustment. This is a closed-loop controller.
There are three stages in the flight managed by the tracker: 1) an initial phase just after launch; 2) a mid-flight phase that lasts for most of the flight; and 3) a terminal phase in which the tracker selects the sweet spot for the point of impact. With guidance algorithms, the autopilot uses data from the seeker and tracker to determine when to transition the missile from one phase of flight to another. Depending on whether the missile is in top attack or direct attack mode, the profile of the flight can change significantly. The top attack mode requires the missile to climb sharply after launch and cruise at high altitude then dive on the top of the target (curveball). In direct attack mode (fastball), the missile cruises at a lower altitude directly at target. The exact flight path which takes into account the range to the target is calculated by the guidance unit.
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