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.
Siemens-Schuckertwerke was already occupied with remote controlled boats (the FL-boats or Fernlenkboote), and had some experience in this area. Flight testing was performed under the supervision of Dipl. Ing. Dorner from January 1915 onwards, using airships as carriers and different types of biplane and monoplane gliders airframes to which a torpedo was fitted. The last test flight was performed on 1918-08-02.
It was planned to use the R VIII bomber as a carrier craft, but the Armistice stopped the project.
The German solution was the development of a number of glide bombs employing radio control guidance. One was created by fitting a control package on the rear of an otherwise standard bomb, starting with their 100 kg armor-piercing bomb to create the Ruhrstahl SD 1400, commonly referred to as the Fritz-X. This bomb was designed specifically to pierce the heavy deck armor of heavy cruisers and battleships. The bomb aimer dropped the bomb from high altitude 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. This problem was solved by having the launch aircraft slow down and enter a climb to avoid overtaking the bomb as it fell.
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 8000 m, experienced bomb aimers could place half the bombs within a 15 m radius and 90% within 30 m.
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. HMS Warspite was also hit, had to be towed to Malta and was out of action for six months. The cruiser USS Philadelphia was very slightly damaged by several near misses of Fritz-X bombs. The light cruiser HMS Uganda was also hit and put out of action for almost the entire war as a result.
A more widely employed weapon was the Henschel Hs 293, which included wings and a rocket motor to allow the bomb to glide some distance away from the launching aircraft. This weapon was designed for use against thinly armored but highly defended targets such as convoy merchantmen or their escorting light warships. On release a small liquid fueled rocket fired to speed up the weapon and get it out in front of the bomber, which was flown 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, at least against slow-moving targets.
Design work started as early as 1939, and a version of the guidance package mounted to standard 500 kg bombs was tested in September 1940. It was found that the bomb was unable to penetrate a ship's 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 and RCN destroyers, sloops and frigates. It's combat appearance was made on August 25 1943 when HMS Bideford was slightly damaged by a missile which failed to fully detonate. HMS Landguard survived a near miss with slight damage. The Germans attacked again two days later, sinking HMS Egret on August 27, 1943 and seriously damaging HMCS Athabaskan.
Several defensive measures were implemented right away. Ships capable of manuevering at high speed were instructed to make tight turns across the missiles flight path in order to complicate the missile operator's efforts. Attacking aircraft were interdicted with air patrols and heavy-caliber anti-aircraft weapons, disrupting either the visual or radio links to the guided weapons. Smoke was used to hide ships at anchor. Allied bombing aircraft also attacked the home bases of the special German units equipped with these weapons (Gruppen II and III of Kampfgeschwader 100 and Gruppe II of Kampfgeschwader 40).
American, British and Canadian scientists also developed sophisticated radio jammersto disurpt the radio guidance signal. Ultimately nine different jamming systems were deployed in the European theater against these weapons. While early models proved inadequate, by the time the Allies were preparing for the invasion of France, more capable systems were deployed and the success rate of guided weapons declined considerably. Even more important to the defeat of the weapons was Allied command of the airspace and the interdiction of incoming bombers by Allied fighter aircraft.
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 Patton's advance, but this mission was unsuccessful. A similar mission against bridges on the Oder River, designed to slow the Soviet advance into Germany, was made in April 1945 but failed.
The Germans also experimented with television guidance systems on the Hs 293D models .It might sound like such systems 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. A wire-guided version was also developed, but this Hs 293B variant was never deployed.
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, GB-5, GB-12, and GB-13, which used contrast-seekers for anti-ship use, and the command-guided GB-8, Azon, Razon, as well as the infrared-guided Felix.
The US Navy also invested in glide bomb research during the war, and deployed two such systems, the Pelican and Bat. Both were based on a similar airframe. Pelican was intended to be launched from the PV1 Ventura, but during testing it proved to have too short a range to be effective, and work continued on the longer-range Bat. The Bat used an active radar seeker allowing it to be dropped at long range, up to , in a mode today known as fire-and-forget (or "launch and leave"); Felix was intended to have the same capability. Mounted on the PB4Y Privateer, Bat entered operational service in the Pacific in 1945. In practice it was found the seeker could not distinguish between targets in a cluttered environment, and could be easily spoofed by even simple radar countermeasures, even by the radar signals being broadcast by the target as a part of normal operations.
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 unnecessary for all but the most accurate of roles. Various TV based systems remain in limited service for super-accurate uses, but have otherwise been removed.
In the anti-shipping role, direct attack from an aircraft even at long range became more dangerous due to the deployment of anti-aircraft missiles on ships. Weapons like the Bat had ranges too short to keep the attacking aircraft out of range, especially in a force provided by air cover. This was addressed with the introduction of small jet engines that greatly extended the range, producing the anti-shipping missile class that remains widely used today.
Similarly, the need to attack highly defended targets such as airbases and military command posts has led to the development of newer generations of glide bombs. 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).