Varying the strength of a bomb just before dropping could be important in avoiding civilian casualties; often the danger of hitting civilians can’t be assessed until a pilot is almost on target. Plymouth, Minnesota-based weapons contractor ATK says it has tested a weapon that will fit the bill. According to New Scientist:
ATK’s approach is based on the principle that explosives can burn in two different ways. One is via detonation, in which the flame front moves at supersonic speed and produces a powerful blast. The other is deflagration, or subsonic burning, which causes little or no blast.
In ATK’s design the explosive can be ignited at both ends, one set to produce a detonation, the other deflagration. By varying the timing of these two ignitions, the proportion of explosive consumed by detonation can be altered in a controlled manner.
As Strategy Page points out, US troops in the field have used C-4 for years to boil water and perform other cooking tasks by igniting it, rather than detonating it. The brass aren’t always so stoked about this, but apparently it’s common practice.
Interestingly, the most common precedent for variable-yield bombs comes not from conventional weapon but from nukes:
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.