Less Is More: The GBU-39 Small Diameter Bomb

small diameter bombs
Small Diameter Bombs on an F-15E

Since aerial bombing was first used during the First World War there have been constant efforts to make bombs more accurate. The main reason for this is economy; if you can put a bomb closer to the target you can destroy it with a smaller load of explosives. That means a smaller bomb, of course, and smaller bombs can be carried in greater numbers. A stores hardpoint that can carry a single Mk 84 2,000 pound bomb can carry a rack of three Mk 82 500 pounders; if they can be dropped accurately enough that lets three targets be attacked rather than one. Of course, in conventional warfare a smaller bomb means less damage, and that can be a problem. Realistically a 500 pound bomb will wreck most things except for large structures, but if you’ve gone to the trouble of dropping a bomb in the middle of an enemy installation or unit you might as well do as much damage as possible. That’s why the drive to make bombs more accurate has always worked in parallel with the desire to make them more destructive for the same weight. In fact it’s attempts to squeeze the biggest possible bang out of size-limited bombs and shells that’s spurred the development of high-powered military explosives.

Wars aren’t all conventional, though, and weapons that might be ideal for bombing enemy tank divisions can be a liability in today’s low-intensity conflicts. In an age of human rights lawyers and embedded journalists a high priority has to be avoiding collateral damage. Unfortunately a bomb isn’t exactly a surgical instrument. Modern targeting packages like the laser-guided Paveway series or the GPS-based JDAM let pilots place a bomb on target with near-pinpoint accuracy, of course. The GBU-24B Paveway III is precise enough that it can be directed to land within a yard of a targeted terrorist’s feet. It can be flown through a selected window or down a ventilation shaft. It can strike a targeted vehicle directly over any seat the laser operator chooses. It’s rubbish at minimising collateral damage, though, because what’s delivered to the target with such phenomenal accuracy is 945 pounds of Tritonal high explosives. That’s enough to blast a crater 50 feet wide and over 30 feet deep, and spray lethal fragments nearly a quarter of a mile away. It’s hard to avoid collateral damage with a bomb like that. Even its little brother, the GBU-12D, has a lethal radius of over 100 yards. If your aim is surgical attacks on individual targets, with minimal or no unintended victims, bombs may not be the tool you’re looking for.

Mk 84 bomb explosion
Mk 84 2,000lb bomb exploding. This is not surgical.

Now Boeing are trying to change that. The GBU-39 Small Diameter Bomb weights just 250 pounds and a rack of four can be carried instead of a single larger bomb. Despite being a small package, though, it manages to squeeze in a lot of technology. Integrated Diamondback folding wings give it a significant glide range – over 70 miles from high altitude – while a GPS-backed inertial navigation system will drop it within eight yards of the target. Pinpoint accuracy is required, because this little bomb carries only 38 pounds of explosives. It does carry it in a heavy steel case, so penetration is excellent for a weapon this size – three feet of reinforced concrete. The case presents a fragmentation hazard out to several hundred feet but, even so, the SDB’s small charge gives a huge reduction in the risk of collateral damage.

To reduce that risk even more Boeing have developed a Focused Lethality Munition (FLM) version of the SDB. This replaces the steel case with lightweight composites and incorporates a bursting charge laced with heavy metal dust. The detonation of the bomb throws out the dust at extremely high velocity, guaranteeing the instant death of anyone within about 15 feet. However the dust loses energy very rapidly and will come to a complete stop within 25 feet, limiting the dangerous radius of the weapon to much less than any previous bomb – in fact it’s less than most hand grenades.

A combination of increased accuracy and smaller warheads will continue to reduce collateral damage from precision air strikes. It’s no longer acceptable for civilised militaries to launch a 500 pound bomb or 155mm shell into an urban area, so future wars will increasingly be fought with highly focused killers like the SDB.