You Can't Be Too ThinI'm very excited about the Boeing GBU-39 Small Diameter Bomb (SDB), which has begun operational trials by the US Air Force at Eglin Air Force Base in Florida. The SDB is a 250-pound-class unpowered strike weapon that is guided with a GPS-aided inertial navigation system (INS). The SDB program builds on previously proven technology, enhances the effectiveness of practically the entire existing US inventory of strike aircraft, enables the F/A-22 to become a strike aircraft without compromising its stealth characteristics, is an enabling technology for the Joint Unmanned Combat Air System (J-UCAS), and offers combat commanders a weapon to support forces on the ground that can be employed under the very restrictive environments imposed by urban warfare.
Physically, the SDB cuts an almost missile-like profile. Dropped from an aircraft, it deploys small wings that give it the appearance of a flying crossbow. The wings enable the glide-weapon to achieve a published stand-off range of about 60-70 nautical miles when released from high altitude. The speed of the terminal descent impart the penetration capabilities of a 2,000-lb bomb with only a 50-lb warhead, greatly reducing blast damage. Because its INS has a new 12-channel GPS receiver, the SDB is a "fire-and-forget" weapon similar to the Joint Direct Attack Munition (JDAM) from which much of its underlying technology was derived, but with a circular error probable (CEP) accuracy of about seven meters compared to about a 14 meter CEP for JDAM.
To understand just how revolutionary the advent of GPS-aided systems are, it is important to see bombing from the standpoint of people who fight wars. The conventional wisdom for tactical strikes prior to the 1991 Persian Gulf War was that it required six iron bombs to kill a particular "aim point." This is about the entire load of a typical strike fighter. And a given target might have more than one aim points, say an air base, depot, or missile site. So multiple aircraft would be tasked with striking the target. A percentage of these aircraft might be expected not to reach their aim points because of mechanical trouble or enemy action. Therefore, particularly valuable targets might have backup aircraft assigned for such eventualities. Compound this situation by dozens or scores of individual targets that need to be hit within a period of hours -- hundreds within days, thousands within weeks -- and the complexities and logistics required to mount and sustain an air campaign become clear.
The six-bomb rule was not because targets were so well protected, but because iron bombs tend to miss by such a margin that you need a six pack on average to get one good hit. The exception to this rule was for those few, special targets that rated a precision-guided weapon. But these were relatively few and far between, as were the numbers of such weapons in inventory. For a long time, since the Vietnam War at any rate, this was the accepted accounting: Drop six, kill one. Except that over time air defenses improved to the point where over-flying most any defended target was risky business. And then people started to become more concerned about where the other five bombs went.
So being able to hit a target, or even multiple targets, with a single aircraft at stand-off ranges satisfied three key concerns: First, fewer sorties needed to be flown to achieve mission goals, saving wear and tear on equipment and personnel. Second, aircraft could attack targets from outside the effective range of air-defense systems, saving lives and reducing the possibility of crew capture. Third, collateral damage to civilian lives, property, and infrastructure could be reduced.
While the JDAM -- an impressive and hugely successful program in its own right -- was a tail kit assembly that could be attached to existing iron bomb warheads to make them guided weapons -- the SDB was built from the drawing board as a new weapon system. The SDB was originally developed to provide the F-35 Joint Strike Fighter with the ability to carry useful bomb loads in internal bays that would not break the low-observable lines of its fuselage features as externally carried ordnance would. It was, however, always understood that the weapon would be usable by essentially all US jet combat aircraft with a strike role. Moreover, the lightness of the SDB means that four bombs can be carried in place of a single 1,000-lb. bomb. This means the number of potential aim-points attacked per sortie quadruples also. In the case of the B-2 bomber, up to 160 SDBs can be carried, each of which can be independently targeted in flight to attack a different aim point.
In addition to multiplying the effectiveness of existing aircraft as strike platforms, the SDB is also enabling the evolution of the F/A-22 into a strike aircraft. Conspicuously during its development, the F-22 Raptor was billed as an air-dominance fighter. The US Air Force still insists that it needs the hugely expensive aircraft in this role. However, in an attempt to broaden its mission statement and hence its appeal in Congress, the Air Force ostentatiously added the "A" label to the aircraft's designation, a la the F/A-18. Since up to eight SDBs will fit in the Raptor's weapons bays, the aircraft will be able to carry out strike missions without compromising its stealth characteristics. Thus, the SDB is will help justify the cost of the fighter program.
The SDM is an important component of the X-45 J-UCAS program, which is seeking to develop a semi-autonomous unmanned strike aircraft. The SDB is the strike weapon of the J-UCAS, which also carries the weapons in an internal bay. Any deployed unmanned combat air vehicle (UCAV) developed from the J-UCAS technology demonstrator program will almost certainly be armed with SDB. The development of jet-powered unmanned combat aircraft will certainly be one of the main storylines of 21st century military aviation.
A final word on the SDB is that it clearly supports the ways that the US is likely to have to make war in the future. The relatively small warhead size and high expected accuracy will enable to SDB to be employed as a close-support weapon in fairly tight proximity to friendly forces and with a minimized effect outside the target area. To be clear, a semi-active laser weapon is more accurate than one that is GPS/INS guided. However, most laser-guided bombs, such as the Paveway family, are much heavier and larger warheads.
So, in all, the SDB program is one to watch. It will make existing aircraft more effective and enable new generations of strike aircraft to be fielded, including revolutionary unmanned ones. The weapon will be an important one for the "small wars" in the US' future, as well as any larger conflicts that may arise.