Glide bomb
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A glide bomb is an aerial bomb that is modified with aerodynamic surfaces to modify its flight path from a purely ballistic one, to a flatter, gliding, one. This extends the range between the launch aircraft and the target. Glide bombs are often fitted with control systems, allowing the controlling aircraft to direct the bomb to a pinpoint target.
The first glide bombs were developed by the Germans during World War II as an anti-shipping weapon. Ships are typically very difficult to attack: a direct hit is needed to do any serious damage, and hitting a target as small as a ship was difficult during WWII. The US felt the solution was massive fleets of bombers dropping a huge numbers of bombs, leading to the development of the B-17, but the Germans instead concentrated on making the bombing itself much more accurate. At first they used dive bombers with some success in this role, but their successes were countered by ever increasing anti-aircraft defenses placed on the Royal Navy ships they were attacking. By 1941 such attacks were still just as difficult as ever, but the added defenses made such attacks almost suicidal as well.
The German solution was the development of a number of radio controlled glide bombs. These were constructed by fitting a control package on the rear of an otherwise standard bomb, starting with their 100kg armor-piercing bomb to create the Ruhrstahl SD 1400, commonly referred to as the Fritz-X. The bomb aimer dropped the bomb while the aircraft was still approaching the ship, and guided it into an impact with the target by sending commands to spoilers attached to the rear of the bomb. This proved to be difficult to do, because as the bomb dropped toward the target it fell further behind the launch aircraft, eventually becoming difficult to see. In addition it proved difficult to properly guide the bomb to impact as the angle of descent changed, and if the bombadier didn't "get it right" and end up with the bomb roughly right over the target, there was little they could do at late stages to fix the problem.
Nevertheless the Fritz X proved useful once crews were trained on its use. In test drops from 8000m, experienced bomb aimers could place half the bombs within a 15m radius and 90% within 30m. Following the capitulation of Italy in 1943, Germany damaged the Italian battleship Italia and sank the Roma with Fritz X bombs. Attacks were also made on the USS Savannah, causing heavy damage, and on several transports off Anzio. HMS Warspite was also hit, had to be towed to Malta and was out of action for six months. Fritz X attacks also damaged the cruiser USS Philadelphia and sank the British light cruiser Spartan.
A more useful weapon was the Henschel Hs 293, which fitted a set of small wings to the guidance system to flatten the trajectory and make the system easier to guide. On release a small liquid fueled rocket fired to speed up the weapon and get it out in front of the bomber, which was aimed to approach the target just off to one side. The bomb then dropped close to the water and glided in parallel to the launch aircraft, with the bomb aimer adjusting the flight left or right. As long as the bomb was dropped at roughly the right range so it didn't run out of altitude while gliding in, the system was easy to use.
Design work started as early as 1939, and a version of the guidance package mounted to standard 500kg bombs was tested in September 1940. It was found that the bomb was unable to penetrate a ships armor, so changes were made to fit an armor-piercing warhead before the system finally entered service in 1943. The basic A-1 model was the only one to be produced in any number, but developments included the B model with a custom armor-piercing warhead, and the C model with the conical warhead that was designed to hit the water short of the ship and then travel a short distance underwater to hit the ship under the waterline.
The Hs 293 was first used operationally in the Bay of Biscay against RN destroyers, sinking the HMS Egret on August 27, 1943. Full scale use started along with the Fritz X in the Mediterranean, where its smaller warhead limited it to attacks on destroyers and transport ships. No defense was obvious, so the British instead attacked the system by capturing the radio control systems using commando raids and installing radio jammers on their ships. The Hs 293 was also used in August 1944 to attack bridges over the River See and River Selume at the southern end of the Cherbourg peninsula in an attempt to break the Patton's advance, but this mission was unsuccessful.
The US also developed glide bombs for similar reasons as the Germans, but in this case the primary target was heavily defended German industrial sites. A series of glider units was constructed, the GB-1 by Aeronca being the first to see service. Unlike the German systems the GB-1 had no guidance system, and was released at a specific point so its glide path would bring it to its target. The system was used in only one major raid, on Cologne in May 1944, and generally proved to be useless. A number of more advanced models in the GB series included the TV guided GB-4 and GB-5, GB-12, GB-13 that used contrast-seekers for anti-ship use.
The Germans had also experiments with television guidance systems on the Hs 293D models. It might sound like such a system would make bombing almost trivial, but in fact TV guidance is quite difficult. As the bomb approaches the target, even tiny amounts of control input would cause the target to jump around the TV display, so much of the difficulty was in developing control systems that would become progressively less sensitive as the pilot required.
After the war the increasing sophistication of electronics allowed for these systems to be developed as a practical device, and starting in the 1960s air forces deployed a number of such systems, including the USAF's AGM-62 Walleye. Contrast seekers were also steadily improved, culminating in the widely used AGM-65 Maverick missile. Both were standard systems until the 1980s when the development of laser guidance and GPS based systems made them unessesary for all but the most accurate of roles. Various TV based systems remain in limited service for super-accruate uses, but have otherwise been removed.
The need for "stand off" range between the launch platform and ground (or ship) based defenses remains just as important today as it did in WWII, and the air forces of the world continue to develop glide bombs for this reason. European air forces use a glide package with a cluster bomb warhead for remotely attacking airbases, and the US is in the process of deploying their own similar system based around the GPS guidance system. It appears likely that the falling cost of such systems will eventually lead to almost all bombs being fitted with some sort of guidance package (such as the Paveway and JDAM systems).