The U.S. Navy is developing a variable explosive bomb (VEB) , based on technology demonstrated by weapons manufacturer ATK. Such a bomb could be set to a different amount of explosive force by the pilot, before dropping it. This is important for smart bombs, which can precisely hit a target. But often the targets have civilians nearby, and a smaller explosion will kill the enemy, and avoid civilian casualties.
The ATK demonstration bomb had three different settings, with the lowest creating a blast radius 40 percent less than at full power. This works by having the explosives split up in different compartments inside the bomb. For lower power, some of the explosives are ignited, using a lower power detonator that sets them to burn, like a rocket, rather than exploding. Modern explosives, like C-4, have long been used by troops as fuel, to boil water, by lighting them, rather than using an explosive detonator. Not recommended, but done anyway since before the Vietnam war. ATK has a bomb design that takes advantage of this. The navy is giving them $10 million, and several years, to produce a workable version of a variable explosive 223 kg (500 pound) bomb.
The U.S. Air Force and Navy have been working on lower "bang" 500 pound bombs for years. The objective was to create a bomb that can be used in urban areas, to destroy a single structure, or just part of a larger one, without injuring nearby civilians and the structures they are in. One recent development has been the FLM (Focused Lethality Munition) bomb, which uses a composite (carbon fiber) casing and replace some of the normal 127.2 kg (280 pounds) of explosives with 93 kg of explosives surrounded by high density filler (fine tungsten powder). A regular 500 pound bomb has a blast radius (injury from blast pressure) out to 13 meters, with dangerous fragments lethal out to 40 meters. The FAM would cut these distances by at least half. Meanwhile, the filler makes the bomb deadlier within the smaller blast radius. FAM, of course, is GPS or laser guided.
Three years ago, the U.S. Navy began using the LCDB (Low Collateral Damage Bomb) in Iraq. This is basically a 500 pound JDAM (GPS guided smart bomb) with 89 percent of the explosives removed, and replaced with non-explosive material (so the bombs flight characteristics remain the same.) The remaining 13.6 kg (30 pounds) of explosives give the bomb a much smaller bang, and thus less chance of nearby civilians getting hurt. Thus the LCDB has a bang that is closer to that of a 155mm artillery shell. What's interesting about that is the U.S. Army is currently using GPS guided Excalibur 155mm "smart shells" in Iraq. But Excalibur costs twice as much as an LCDB. So does the new air force SBD (121 kg/265 pound Small Diameter Bomb). Thus the demand for a cheaper, variable explosive 500 pound bomb.
The concept of the VEB, FAM and LCDB is not new. During the 1990s, the U.S. Air Force replaced all the 189 kg (416 pounds) of explosives (with concrete) in thousand pound laser guided bombs used against Iraqi anti-aircraft guns and missiles. This was because Saddam ordered his anti-aircraft weapons placed inside densely packed residential areas, in the hope that any American or British aircraft responding to fire from his anti-aircraft weapons, would also kill lots of civilians. That would make for a great photo op, as Saddam was trying to turn himself into a victim of American and British aggression. Dead civilians helped a lot. Concrete smart bombs took out the anti-aircraft weapons, but rarely hurt any nearby civilians. The LCDB was used against targets in buildings, or out in the open, who need at least a little bang, and bomb fragments, to take out the bad guys.
Variable yield nuclear explosives have been around for decades, because it is easier to limit the amount of nuclear material that will be turned into a nuclear explosion. But it has proved more difficult to do this with conventional explosives.