A recent UAV missile attack on an Islamic terrorist leader in Syria revealed that the U.S. had used a Hellfire with a non-explosive warhead to kill the al Qaeda second-in-command while the he was in a moving vehicle. Pictures of the automobile were released and showed the large hole in the roof but no signs of an explosion. Some saw this as a new development, but it isn’t.
There is nothing high-tech about this either. The Israelis have frequently used Hellfire missiles with practice (non-explosive) warheads that will kill a few people travelling in an automobile and not cause additional damage. Users of Hellfire and ATGMs (anti-tank guided missiles) had long known that a practice warhead could still do a lot of damage because the practice warhead weighed the same as the explosive one, was heavy, and travelling at high speed. Troops noted the damage these practice rounds could do to a variety of targets. Thus with the U.S. and Israel became unofficial policy in some units to carry a practice TOW missile with them on some missions where it would be important to avoid casualties to nearby civilians. The Israelis later used the same technique with Hellfires.
Meanwhile 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. This eventually led to development of the FLM (Focused Lethality Munition) bomb. This uses a composite (carbon fiber) casing and replaces 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 cut these distances by at least half. Meanwhile, the filler makes the bomb deadlier within the smaller blast radius. FAM, of course, was GPS or laser guided.
Meanwhile in 2007, 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. 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 had begun using GPS guided Excalibur 155mm "smart shells" in Iraq at the same time. But Excalibur costs twice as much as an LCDB as does the new (then) 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 goes back even further. 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. Despite that in 2010 the U.S. Navy began working on 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 spent $10 million to find that it was possible to produce a workable version of a variable explosive 223 kg (500 pound) bomb but in the meantime other solutions had turned up.