- A B-1B bomber was sighted over the Mojave desert armed with the new GBU-72 Advanced 5K penetrator.
- The GBU-72 is designed to smash through reinforced concrete and destroy underground facilities.
- The sighting comes as Iran speeds up its underground refinement of enriched uranium, bringing it closer to nuclear weapons.
An Air Force bomber was recently spotted with one of America’s newest munitions: the GBU-72 Advanced 5K penetrating bomb. The bomb is designed to help penetrate hardened underground defenses, destroying targets buried under earth and concrete. The weapon appears just as the UN’s nuclear agency warns that Iran is accelerating efforts to manufacture enriched uranium—an essential component of nuclear bombs—at underground facilities.
A TORCH Sighting
According to The Aviationist blog, aviation photographer Ian Recchio was in California’s Mojave Desert earlier this year when he overheard chatter on his scanner that referred to an aircraft with the TORCH call sign. Recchio looked up and saw a KC-135 Stratotanker “dragging” a B-1B Lancer bomber—a term used by enthusiasts when an aerial refueling tanker is seen in tandem with another aircraft, typically in the lead. The two planes continued to loiter in the area for two hours.
Recchio took several pictures, not suspecting anything that was unusual. He later went back and noticed that the B-1B had a munition on one of the mounting points of its fuselage. The munition appeared to be a GBU-31—a 2,000 pound (1 ton) GPS guided bomb—but upon further examination, the bomb carried by the B-1B was discovered to have a second set of fins along the length of its body. That matched the GBU-31’s bigger brother, the much larger GBU-72.
A Brief History of Bunker Busters
In the 1980s, the U.S. Air Force began developing earth-penetrating bombs to reach Soviet underground command centers in wartime. In the event of a conventional war in Europe, earth-penetrating bombs could reach targets such as the underground bunker complex headquarters of the Group of Soviet Forces in Germany (GSFG) located in Zossen-Wunsdorf, East Germany. Destroying these bunkers could slow a Warsaw Pact attack without the use of nuclear weapons.
In 1991, the United States and its Coalition allies commenced Operation Desert Storm, an air-ground campaign designed to eject the Iraqi military from Kuwait. The U.S. made extensive use of what came to be known as “bunker busters”—bombs designed to penetrate earth and concrete—to destroy underground Iraqi targets. The Air Force even rushed into production a new bunker busting bomb: the 5,000 pound GBU-28, a bomb which was so hastily put together that the casings were made from deactivated Army howitzer barrels.
By the early 2000s, the threat from Iraq was replaced with a new threat from Iran, who were beginning to assemble a nuclear program that the country claimed was for peaceful energy-generation purposes. Others, however, suspected that a nuclear energy program was in fact a cover for a nuclear weapons program—especially when nuclear facilities were being built underground, which offered some protection from conventional weapons. The Air Force continued to hone its bunker buster arsenal, which included the 30,000 pound, 20-foot-long Massive Ordnance Penetrator (MOP) bomb reportedly capable of penetrating 200 feet of reinforced concrete.
Two and a Half Tons of Bomb
The GBU-72 replaces the GBU-28 in Air Force service. The bomb entered development sometime around 2017, with a projected cost of $36 million for 125 bombs. The first test was in October of 2021, during which the bomb was released from a F-15E Strike Eagle at Eglin Air Force Base. At the time, the Air Force revealed that the GPS guidance kit was actually the same one from the 2,000 pound GBU-31 bomb. The Air Force has not disclosed how improved the GBU-72 is over the GBU-28, but at the time of the tests, the program manager said he expected the new bomb’s lethality to be “substantially higher” than the old bomb.
The GBU-72 should be as accurate as other satellite-guided bombs that rely on military-grade GPS signals, which are substantially more precise than GPS signals civilians have access to in their cell phones. As a result, the bomb has a circular error probable, or CEP, of five meters (CEP is a standard means of determining weapon accuracy of weapons delivered against the ground, from bombs to ICBM warheads). In the case of GBU-31, it means that fifty percent of GBU-31s dropped should impact within 16.4 feet or less of the target, with the rest falling outside that radius. If the bomb encounters GPS jamming, it relies on a backup inertial navigation system that (although less reliable) should ensure the bomb impacts within 98.4 feet of target.
The GBU-72 is a glide bomb,—it is released from midair and glides to the target, its tail fins making minute corrections to compensate for wind and other factors that might blow the bomb off course. The higher the bomb’s release, the farther it can glide to target, allowing an aircraft to deliver ordnance on targets while remaining out of range of short-range air defenses. The GBU-72’s actual range is unknown, but the GBU-31 bomb can glide up to 15 miles. The presence of additional fins on the bigger bomb’s body may indicate that it is capable of a greater release range, which would be useful, as a GBU-72 would typically be used against a heavily defended target.
A Not-so-Subtle Sign?
The B-1B bomber is one of three heavy bombers in the U.S. Air Force’s inventory. The Air Force has 45 B-1Bs, and although the “Lancer” originally had a nuclear mission, the fleet was stripped of its ability to carry nukes to comply with U.S.-Russian nuclear treaty obligations. The B-1Bs are purely conventional aircraft which will be retired in the 2030s and replaced by the new B-21 Raider.
The B-1B has three internal bomb bays, each of which can carry eight 2,000 pound bombs or eight AGM-158A Joint Air-to-Surface Standoff Missiles. So, with all that room internally, why was the bomb captured in the recent photos on the outside of the aircraft? The B-1B is probably forced to carry the bomb attached to its fuselage because it is just too long: the GBU-28 is 19 feet long, and the B-1B’s bomb bays are 15 feet long. If the new bomb is as long as the old bomb, it would need to go on one of the B-1B’s original eight external hardpoints.
This flight test has also taken place as the U.S. and Iran are trading blows in the Middle East, and Iran is drawing closer to manufacturing weapons-grade uranium. From the U.S. standpoint, it is useful for Iran to see a B-1B bomber—an aircraft with the range to strike virtually anywhere in Iran—flying around with a weapon that can hold its underground facilities at risk. Having the press reveal the incident can be perceived as less directly threatening than if the Air Force did, but Iran sees it all the same. This is known as signaling, and is done frequently by many countries. Will we see more signaling in the future? Almost certainly.
Kyle Mizokami is a writer on defense and security issues and has been at Popular Mechanics since 2015. If it involves explosions or projectiles, he's generally in favor of it. Kyle’s articles have appeared at The Daily Beast, U.S. Naval Institute News, The Diplomat, Foreign Policy, Combat Aircraft Monthly, VICE News, and others. He lives in San Francisco.
