The Drive reports on the possible provision of a weapon system to Ukraine, the Ground Launched Small Diameter Bomb (GLSDB). (I am suggesting that we could launch it from ships, so Surface Launched might have been more appropriate).

This weapon might have a place as a replacement for the big guns that once provided Naval Gun Fire Support. It also has potential as an anti-ship weapon.

The system consists of a hybrid of a normally air launched, precision guided, winged bomb, the “small diameter bomb,” flung into the air by a rocket booster used in an early Multiple Launch Rocket System (MLRS) munition, the M26.

The M26 was the first rocket developed for the Multiple Launch Rocket System (MLRS). It is spin-stabilized by 4 fins, has a range of 32 km (20 miles) and is armed with 644 bomblets, anti-personnel/anti-materiel grenades. These bomblets have fallen out of favor because the dud rate creates potential for collateral damage that may occur long after the conflict that prompted their use. To create the Ground Launched Small Diameter Bomb, the grenades are replaced by a Small Diameter Bomb.

Small Diameter Bombs:

There are four different “Small Diameter Bombs”:

• The Boeing GBU-39B which is guided to a known geographic location by a GPS-aided inertial navigation system. Weight: 285 lb (129 kg). Length: 70.8″ (1.8 m). Width: 7.5″ (190 mm). Cost: $39,000 (2021)
• The GBU-39A/B, a “Focused Lethality Munition” (FLM) version intended to limit collateral damage.
• The GBU-39B/B Laser SDB which adds semi-active laser homing, the ability to home on a target identified by a laser designator.
• The Raytheon GBU-53B SDB II “StormBreaker” which also includes a tri-mode seeker (millimeter wave radar, infrared homing, and semiactive laser guidance) that allows it to be used against moving targets in any weather. Weight: 204 lb (93 kg). Length: 69″ (1.76 m). Diameter 7″ (180 mm). Cost $195,000 (2021).

“The bomb can use GPS/INS to guide itself into the general vicinity of a moving target during the initial search phase, with course correction updates provided using a Link 16 over UHF data link…The weapon is capable of fusing the information from the sensors to classify the target and can prioritize certain types of targets as desired when used in semi-autonomous mode.”

How does it compare to Naval Guns?:

These are small bombs, developed to increase the number of precision munitions an aircraft can carry in a single sortie. Four of these replace a single 2,000 pound bomb.

SDBs are small bombs but compared to most naval guns, they pack a pretty big punch. Because of their precision, the relatively small bomb is still adequate to destroy many targets including tanks, aircraft shelters, bunkers, and strong points.

 “Warhead penetration is 3 ft (1 m) of steel reinforced concrete under 3 ft of earth and the fuze has… selectable functions, including air burst and delayed options.”

The GBU-39’s 36 pound bursting charge is 50% larger than that of the last 8″ projectiles used by the US Navy and more than four and half times that of current 5″ projectiles. (The bursting charge in the 16″ High Cap projectiles fired by Iowa class battleships was only 153.6 lbs. (69.67 kg)).

Perhaps most importantly, this weapon out-ranges all existing naval guns with a range of 150 km / 81 nautical miles.

Why it will be difficult and expensive to shoot down:

Now anything can be shot down, from artillery and mortar rounds to ICBMs. Because these are glide bombs it might be assumed they would be easy to shoot down, but that is not necessarily the case. Their small size means they have a small radar cross section. Because they are a glide bomb, unlike aircraft or cruise missiles, they have little or no IR signature. That means they are not good targets for IR homing missile such as man portable air defense systems (MAPADS). Because the round is maneuverable, there may be opportunities to avoid heavy concentrations of AA.

It is probably going to require high quality AAW missiles to bring one of these down, meaning the cost exchange is likely to be favorable for the SDB. Being cheap they can be traded off against the more expensive missiles required to bring them down, depleting the enemies air defenses. That could result in making it safer for our manned aircraft.

Why not let Naval Air just drop the Small Diameter Bombs:

That is certainly an option, but if surface launched Small Diameter Bombs are available it can free aircraft for more demanding missions like air superiority and suppression of air defenses. Surface launched SDBs and aircraft could be complementary,

There is also the possibility that the carrier(s) may be called away or their flight deck might be damaged precluding air ops.

Where could we mount them?:

The video shows a six-tube launcher inside what is almost certainly a 20x8x8 foot container. That suggests that there are many options available including multiple launcher installations on Offshore Support Vessels, either manned or unmanned as well as many existing vessels.

As defensive weapons, the widespread use of 20x8x8 containers means that it is going to be very hard to single out those that mount these weapons. A “shell game” can make them very difficult to recognize and neutralize.

The Cost Exchange Ratio:

What makes these a game changer? It is the precision and range combined with its low price. The War in Ukraine has shown the rapid expenditure of munitions. There is a need for weapons with longer range and greater survivability, but they will cost much more. We cannot afford to expend weapons that cost millions on every target. There are times when it is necessary to expend an expensive weapon on a far less expensive target, but that can’t become the norm. We need weapons that can be produced in huge numbers at a reasonable cost.

Now About Taiwan:

If the Chinese are to invade Taiwan, it will be comparable in scope to the Normandy Invasion. The Chinese Navy can transport only a small percentage of the troops that would need to land on the first day of the invasion. They will need to mobilize a very large number of civilian craft including ferries and fishing boats to transport the number of troops that will be required.

If the Taiwanese are to stop the invasion, they are going to have to sink a very large number of craft as they transit the Taiwan Strait. (The Strait is 130km wide at its narrowest point.) Most of these craft will be relatively small and have little or no self-defense capability.

Using the GBU-53B, with its tri-mode seeker, the Ground Launched Small Diameter Bomb appears ideal for this purpose. Given the bombs, the Taiwanese could probably quickly devise an even longer-range booster and launcher.