Development in BOMBS

                                                              Abstract 

Smart bombs are the weapons capable of destroying enemy targets without the need for launch aircraft to penetrate the envelopes of the air defense systems. These essentially comprise a terminal guidance unit that guides them in the last phase to achieve pinpoint accuracy. Increase accuracy means that a single, moderate size bomb can give a better result than multiple strikes with larger ,non-guided bombs.

Smart bombs are desirable both from ethical and tactical standpoints. On ethical grounds, the military desires that each warhead deployed should strike only its intended targets so those innocent civilians are not harmed by misfire. From tactical standpoint, it wants weapons with pinpoint accuracy to inflict maximum damage on valid military targets and minimize the number of strikes necessary to achieve mission objectives. Gravity bombs with laser or GPS/INS guidance is smart bombs that have changed the face of modern warfare.

You can see how lethal and merciless smart bombs can be a realistic battlefield scenario was amply demonstrated in Gulf War and recently reiterated during US attack on Iraq. This paper will explain you the operational mechanisms, salient feature and limitations of this rapidly grooming smart bomb

Introduction

"In World War II it could take 9,000 bombs to hit a target the size of an aircraft shelter. In Vietnam, 300.Today we can do it with one laser-guided munition from an F-117."
USAF, Reaching Globally, Reaching Powerfully: The United States Air Force in the Gulf War (Sept. 1991) The United States Army began experimenting with radio-controlled remotely guided planes in the First World War, but the program had few successes. The first successful experiments

with guided bombs were conducted during World War II when television-guided bombs, flare sighted bombs and other steerable munitions, such as the 450 kg (1000 lb) AZON bomb, were developed. The Germans developed several types of steerable munitions, such as the 1400 kg (3085 lb) Fritz X, the closest Axis equivalent of the US Army Air Force's AZON device. There was even an attempt to produce a glider bomb that was released from a larger plane over the target, but the program stopped with the nuclear attacks in Japan.

The programs started again in the Korean War, where the political ramifications of nuclear war would have been unthinkable. In the 1960s, the electro-optical bomb (or camera bomb) was introduced. They were equipped with television cameras and steerable flare sights, in which the bomb would be steered until the flare superimposed the target. The camera bombs transmitted a "bomb's eye view" of the target back to a controlling aircraft. An operator in this aircraft then transmitted control signals to steerable fins fitted to the bomb. Such weapons were used increasingly by the USAF in the last few years of the Vietnam War because the political climate was increasingly intolerant of collateral damage.

Laser Guided Bombs: -

The development of laser-guided weapons has dramatically improved the accuracy of weapon guidance and delivery. With the assistance of build-up guidance kits, general GP bombs are turned into laser-guided bombs (LGBs). The kits consist of a computer- control group (CCG), guidance canards attached to the front of the warhead to provide steering commands, and a wing assembly attached to the aft end to provide lift. LGBs are maneuverable, free-fall weapons requiring no electronic interconnect to the aircraft. They have an internal semi active guidance system that detects laser energy and guides the weapon to a target illuminated by an external laser source. The designator can be located in the delivery aircraft, another aircraft, or a ground source.

All LGB weapons have a CCG, a warhead (bomb body with fuze), and an airfoil group. The computer section transmits directional command signals to the appropriate pair(s) of canards. The guidance canards are attached to each quadrant of the control unit to change the flight path of the weapon. The canard deflections are always full scale (referred to as "bang, bang" guidance).

The LGB flight path is divided into three phases: ballistic, transition, and terminal guidance. During the ballistic phase, the weapon continues on the unguided trajectory established by the flight path of the delivery aircraft at the moment of release. In the ballistic phase, the delivery attitude takes on additional importance, since maneuverability of the UGB is related to the weapon velocity during terminal guidance. Therefore, airspeed lost during the ballistic phase equates to a proportional loss of maneuverability. The transition phase begins at acquisition. During the transition phase, the weapon attempts to align its velocity vector with the line-of-sight vector to the target. During terminal guidance, the UGB attempts to keep its velocity vector aligned with the instantaneous line-of- sight. At the instant alignment occurs, the reflected laser energy centers on the detector and commands the canards to a trail position, which causes the weapon to fly ballistically with gravity biasing towards the target.

Target designators are semi-active illuminators used to "tag" a target. Typical laser guided bomb receivers use an array of photodiodes to derive target position signals. These signals are translated into control surface movements to direct the weapon to the target. An airborne detector can provide steering information to the pilot, via his gun sight, for example, and lead him on a direct heading to the target, finally giving him an aim point for a conventional weapon. Alternatively, a laser guided "smart" bomb or missile may be launched when a pilot is satisfied that the detector head has achieved lock-on and launch envelope requirements are satisfied. In either of these cases, the pilot may never see the actual target, only the aim point as indicated by the laser.

Laser designators and seekers use a pulse coding system to ensure that a specific seeker and designator combination work in harmony. By setting the same code in both the designator and the seeker, the seeker will track only the target designated by the designator. The pulse coding is based on Pulse Repetition Frequency (PRF). The designator and seeker pulse codes use a truncated decimal system. This system uses the numerical digits 1 through 8 and the codes are directly correlated to a specific PRF. Dependent upon the laser equipment, either a three digit or a four-digit code can be set. Coding allows simultaneous or nearly simultaneous attacks on multiple targets by a single aircraft, or flights of aircraft, dropping laser guided weapons (LGWs) set on different codes. This tactic may be employed when several high priority targets need to be expeditiously attacked and can be designated simultaneously by the supported unit(s).

Fire control laser systems are laser rangefinders (LRFs) and laser designators (LDs). These laser systems can be far more harmful to the eye than laser training devices such as MILES and Air-to-Ground Engagement System/Air Defense (AGES/AD) laser simulators. Consequently, fire control lasers require control measures to prevent permanent blindness to an unprotected individual viewing the laser system from within the laser beam.

                                                                        Development:-

Laser-guided weapons were first developed in the United States in the early 1960s. The USAF issued the first development contracts in 1964, leading to the development of the Paveway™ series, which was used operationally in Vietnam starting in 1968. Although there were a variety of technical and operational problems, the results were generally positive. LGBs proved to offer a much higher degree of accuracy than unguided weapons, but without the expense, complexity, and limitations of guided air-to-ground missiles like the AGM-12 Bullpup. The LGB proved particularly effective against difficult fixed targets like bridges, which previously had required huge loads of "dumb" ordnance, and large numbers of sorties, to destroy.

It was determined that 48 percent of Paveways dropped during 1972–73 around Hanoi and Haiphong achieved direct hits, compared with only 5.5 percent of unguided bombs dropped on the same area a few years earlier.^[1] The average Paveway landed within 23 feet of its target, as opposed to 447 feet for gravity bombs.^[1] The leap in accuracy brought about primarily by laser guidance made it possible to take out heavily defended, point objectives that had eluded earlier air raids. The most dramatic example was the Thanh Hoa Bridge, 70 miles south of Hanoi, a critical part crossing point over the Red River. Starting in 1965, U.S. pilots had flown 871 sorties against it, losing 11 planes without managing to put it out of commission. In 1972 the “Dragon’s Jaw” bridge was attacked with Paveway bombs, and 14 jets managed to do what the previous 871 had not: drop the span, and cut a critical North Vietnamese supply artery.

In the wake of this success, other nations, specifically the Soviet Union, France, and Great Britain, began developing similar weapons in the late 1960s and early 1970s, while US weapons were refined based on combat experience.

The USAF and other air forces are now seeking to upgrade their LGBs with GPS guidance as a back-up. These weapons, such as the USAF Enhanced Guided Bomb Unit (part of the Paveway™ family), use laser designation for precision attacks, but contain an inertial navigation system with GPS receiver for back-up, so that if the target illumination is lost or broken, the weapon will continue to home in on the GPS coordinates of the original target.

                                                              Problems And Limitations:-

While LGBs are highly accurate under ideal conditions, they present several challenges for successful use, making them somewhat less than the "silver bullet" sometimes suggested.

The first problem is designation. To ensure accurate guidance, the target must be illuminated for several seconds before launch, allowing the weapon's seeker to obtain a positive lock, and the target must remain illuminated during much of the weapon's transit time. If the designator's "sparkle" is turned off, blocked, or moved, the weapon's accuracy will be greatly reduced.

For an accurate attack against a small target, uninterrupted designation is essential. But, the guidance controls of many LGBs (such as the American Paveway™ II) cause large deflections (visible as a noticeable wobble) which reduce the bomb's range. To compensate, crews will often release their weapons in an unguided, ballistic arc, activating the designator only to refine the bomb's final impact point. This is more demanding of crew and aircraft, requiring a high standard of basic, unguided bombing accuracy and more attention to the bomb's flight.

Laser designation is very sensitive and vulnerable to weather conditions. Cloud cover, rain, and smoke often make reliable designation impossible. In war conditions, many attacks have been aborted due to poor visibility.

In the 1970s and 1980s it was common for aircraft to rely on a separate designator, either carried by ground forces, operated by the forward air controller, or carried by another aircraft in the strike group. It was often deemed more practical for one aircraft to designate for its comrades. Modern conflicts and a growing emphasis on precision-guided weapons have pointed to the need for autonomous designation, and many fighter-bomber aircraft are now being fitted with designator pods to self-designate for laser-guided munitions.

Even if the launch aircraft is capable of autonomous designation, problems remain. Laser illumination can be interrupted by smoke, fog, or clouds, limiting the usefulness of LGBs in poor weather or very dusty conditions. In desert warfare, such as the 1991 Gulf War, laser designation sometimes reflected off the sand, causing weapons to home on false targets. Furthermore, the need to provide designation may leave the aircraft dangerously exposed to ground fire or enemy air support.

An additional concern is the limited "launch envelope" of an unguided weapon. The reflected laser "sparkle" can be described as a basket into which the weapon must be steered to hit the target. If the weapon is released too low or to far from the target, or in a trajectory that puts the weapon outside the seeker's field of view, it is likely to miss. Optimum altitude for an effective LGB attack is relatively high, increasing the aircraft's vulnerability to surface-to-air missile (SAM) attacks.

For these reasons, while all modern air forces have put an increasing emphasis on LGBs and other precision-guided munitions, some tacticians still see an important role for the accurate delivery of unguided bombs. During their 1981 raid on the Iraqi nuclear reactor at Osirak, the Israeli Air Force chose to use unguided Mark 84 bombs rather than laser-guided weapons because they felt the need to designate the target would leave the attackers unacceptably vulnerable.

 

Figure 1 A laser-guided GBU-24 (BLU-109 warhead variant) strikes its target

Figure 2 BOLT-117, the World’s  first laser guided bomb & Carrier

Specifications
Mission	Offensive counter air, close air support, interdiction
Targets	Fixed hard
Class	4,000 lb. Penetrator, Blast/Fragmentation
Service	Air Force
Contractor	Lockheed (BLU-113/B), National Forge (BLU-113A/B),
Program status	Production
First capability	1991
Weight (lbs.)	4,414
Length (in.)	153
Diameter (in.)	14.5
Explosive	6471bs. Tritonal
Fuze	FMU-143 Series
Stabilizer	Air Foil Group (Fins)
Guidance method	Laser (man-in-the-loop)
Range	Greater than 5 nautical miles
Development cost	Development cost is not applicable to this munition.
Production cost	$18.2 million
Total cost	$18.2 million
Acquisition unit cost	$145,600
Production unit cost	$145,600
Quantity	125 plus additional production
Platforms	F-15E, F-111F