The U.S. Air Force has produced an innovative dud with its new SDB (small diameter bomb). Turns out that while the SDB was being developed, a lot of cheaper competition showed up. Work began on the 250 pound SDB smart bomb, seven years ago. It was set to enter service in 2006, and did so. But the only aircraft equipped to drop it is the F-15E. The SDB was tested in the F-22 (PHOTO...), but there are no plans to deploy F-22s carrying SBD, for several years.

SDB (small diameter bomb) is a completely new smart bomb design, weighing only 250 pounds. This weapon has a shape that's more like that of a missile than a bomb (70 inches long, 190 millimeters in diameter), with the guidance system built in. The smaller blast from the SDB is still pretty substantial (51 pounds of explosives). A new SDB design has a Focused Lethality Munition (FLM) warhead, which reduces the number of metal fragments created when the bomb explodes, and increases the blast effect. This is meant to reduce casualties to nearby civilians. But there are cheaper solutions to the $50,000 SDB.

Basically, if you add high precision to a bomb or missile, you increase its cost by $25-50,000. But while cost is a consideration, it's not the only one. You need just enough explosives to do the job. Too much bang, and you just endanger your own people. More important is availability. The infantry need their explosion when they need it, not when the air force gets around to it. Thus the army prefers to rely on precision weapons they control. One of the first, widely successful precision weapons to show up was the fifty pound TOW anti-tank missile. It has a 13 pound warhead, and, when wars broke out, was mainly used for taking out rooms in buildings where enemy gunmen were hiding. It was a TOW that got Saddam Husseins two sons four years ago. Every mech infantry unit has plenty of TOW missiles, and very few enemy tanks to use them on. So the TOW has become a very popular precision weapon for the ground troops. Since the 1990s, a more portable ground combat missile, and just as accurate as TOW, came along in the form of the 26 pound Javelin (PHOTO...), with its nine pound warhead. These two missiles are expensive, with TOW costing $25,000 each, and Javelin $75,000.

For a smaller bang, there's the AT4 rocket launcher, and its four pound warhead. It's not laser guided, and you have to be pretty close to use it. But at the normal ranges its used (a hundred meters or so), it's very accurate, and it's cheap ($2,700). The LAW is similar, smaller (2.2 pound warhead) and cheaper ($2,000).

Helicopters and UAVs use Hellfire missiles, which weigh 100 pounds, and have a 20 pound warhead. A little less than half of a missile warhead is explosives. Hellfire is laser guided, and good for taking out vehicles full of bad guys, or small buildings. Hellfire costs $50,000 each. For about the same price you can use the 44 pound Viper Strike, and its four pound warhead. Even cheaper ($25,000 each), and smaller, are the new, laser guided 70mm rockets. There weigh 25 pounds and have a six pound warhead. The Viper Strike is a laser guided glide bomb that basically comes straight down. The 70mm rocket has a range of about six kilometers.

The army also has 155mm GPS guided 155mm shells (Excalibur ) . Each hundred pound shell has about 20 pounds of explosives (PHOTO...). This makes for a bigger bang than Hellfire or Tow, but much less than smart bombs. There's also a 227mm MLRS GPS rocket (or GMLRS). But this carries over 150 pounds of explosives. About half the bang of a 500 pound JDAM. The GPS guided 155mm shell and MLRS rocket each cost over $50,000 each.

The big advantage of these GPS artillery munitions is that they are available to the troops 24/7, and the need for fewer rounds per mission means there are fewer problems with running out, or low, on supplies.

Price is not really a decisive factor when it comes to these weapons. The whole point of smart (much more accurate) munitions is to reduce the number of explosions, and to only blow up what needs to be destroyed. The proliferation of rockets, smart bombs and missiles, from those with a pound of explosives (LAW) to 500 pound bombs (with 280 pounds), gives troops a lot of flexibility on the battlefield. This makes American troops much more lethal, and greatly reduces friendly, and civilian, casualties.

Although the air force had smart (GPS guided) bombs since the late 1990s, these came in only two sizes; half ton and one ton. This proved to be too much blast for urban fighting. The need for less firepower compelled the air force to quickly modify its GPS guidance kit to fit on a 500 pound bomb. But that's still 280 pounds of explosives. The troops wanted precision, and less bang.