Strela 2 - Description

Description

The missile launcher system consists of the green missile launch tube containing the missile, a grip stock and a cylindrical thermal battery. The launch tube is reloadable at depot, but missile rounds are delivered to fire units in their launch tubes. The device can be reloaded up to five times.

When engaging slow or straight-receding targets, the operator tracks the target with the iron sights in the launch tube and applies half-trigger. This action "uncages" the seeker and allows it attempt to track. If target IR signature can be tracked against the background present, this is indicated by green light and buzzer sound. The shooter then pulls the trigger fully, and immediately applies lead and superelevation. This method is called a manual engagement. An automatic mode, which is used against fast targets, allows the shooter to fully depress the trigger in one pull followed by immediate lead and superelevation of the launch tube. The seeker will uncage and will automatically launch the missile if a strong enough signal is detected.

Manufacturer lists reaction time measured from carrying position (missile carried at soldier's back with protective covers) to missile launch to be 13 seconds, a figure that is achievable but requires considerable training and skill in missile handling. With launcher on the shoulder, covers removed and sights extended, reaction time from fire command to launch reduces to 6–10 seconds, depending greatly on target difficulty and shooter's skill.

After activating the power supply to the missile electronics, the gunner waits for electricity supply and gyros to stabilize, puts the sights on target and tracks it smoothly with the launch tube's iron sights, and pulls the trigger on the grip stock. This activates the seeker electronics and the missile attempts to lock onto the target. If the target is producing a strong enough signal and the angular tracking rate is within acceptable launch parameters, the missile alerts the gunner that the target is locked on by illuminating a red light in the sight mechanism, and producing a constant buzzing noise. The operator then has 0.8 seconds to provide lead to the target while the missiles on-board power supply is activated and the throw-out motor ignited.

Should the target be outside acceptable parameters then the light cue in the sight and buzzer signal tell the gunner to re-aim the missile.

On launch, the booster burns out before the missile leaves the launch tube at 32 m/s, and rotating the missile at approximately 20 revolutions per second. As the missile leaves the tube the two forward steering fins unfold, as do the four rear stabilizing tail fins. The self-destruct mechanism is then armed, which is set to destroy the missile after between 14 and 17 seconds to prevent it hitting the ground if it should miss the target.

Once the missile is five and half meters away from the gunner, approximately 0.3 seconds after leaving the launch tube it activates the rocket sustainer motor. The sustainer motor takes it to a velocity of 430 meters per second, and sustains it at this speed. Once it reaches peak speed at a distance of around 120 meters from the gunner, the final safety mechanism is disabled and the missile is fully armed. All told, the booster burns for 0.5 second and the driving engine for another 2.0 seconds.

The missile's uncooled lead sulphide passive infra-red seeker head detects infrared radiation at below 2.8 μm in wavelength. It has a 1.9 degree field of view and can track at 9 degrees per second. The seeker head tracks the target with an amplitude-modulated spinning reticle (spin-scan or AM tracking), which attempts to keep the seeker constantly pointed towards the target. The spinning reticle measures the amount of incoming infrared (IR) energy. It does this by using a circular pattern that has solid portions and slats that allow the IR energy to pass through to the seeker. As the reticle spins IR energy passes through the open portions of the reticle. Based on where the IR energy falls on the reticle the amount or amplitude of IR energy allowed through to the seeker increases the closer to the center of the reticle. Therefore, the seeker is able to identify where the center of the IR energy is. If the seeker detects a decrease in the amplitude of the IR energy it steers the missile back towards where the IR energy was the strongest. Unfortunately, the seeker's design creates a dead-space in the middle of the reticle. The center mounted reticle has no detection capability. This means that as the seeker tracks a target as soon as the seeker is dead center, (aimed directly at the IR source) there is a decrease in the amplitude of IR energy. The seeker interprets this decrease as being off target so it changes direction. This causes the missile to move off target until another decrease in IR energy is detected and the process repeats itself. This gives the missile a very noticeable wobble in flight as the seeker bounces in and out from the dead-space. This wobble becomes more pronounced as the missile closes on the target as the IR energy fills a greater portion of the reticle. These continuous course corrections effectively bleed energy from the missile reducing its range and velocity.

The guidance of the SA-7 follows proportional convergence logic, also known as angle rate tracking system or pro-logic. In this method as the seeker tracks the target, the missile is turned towards where the seeker is turning towards - not where it is pointing at - relative to the missile longitudinal axis. Against a target flying in a straight-line course at constant speed, the angle rate of seeker-to-body reduces to zero when the missile is in a straight-line flight path to intercept point.