The US military is providing new details about its decade-long effort to develop the massive bunker-busting bombs used in recent strikes against Iranian nuclear facilities.
That raid saw seven Northrop Grumman B-2 stealth bombers from the US Air Force (USAF) drop 14 GBU-57 Massive Ordnance Penetrator bombs on two deeply buried sites associated with Tehran’s effort to enrich weapons-grade Uranium.
The 21 June mission was the first operational use of GBU-57s, new details about which the Pentagon revealed on 26 June.
Chairman of the US joint chiefs of staff General Dan Caine says the genesis of the GBU-57 began some 15 years ago, when in 2009 a defence intelligence analyst began observing Iranian efforts to construct the underground enrichment facility at Fordo.
“Along the way they realised we did not have a weapon that could adequately strike and kill this target,” Caine says. “So they began a journey to work with industry and other tacticians to develop the GBU-57.”
The Lockheed Martin F-16 pilot says that kicked off a process of “weaponeering” – a scientific enterprise of determining the right configuration of explosives and fuzing to achieve the desired effect against a particular target.
That process was so complex that at one point the GBU-57 development team was secretly the top user of supercomputer processing power in the USA, according to Caine. Those high-powered machines were being used to model and simulate the effects of different weapons configurations against the bunkers.
The lengthy campaign included hundreds of test shots, with multiple full-scale weapon prototypes demonstrated against targets built to represent the buried Iranian facilities, construction of which had been closely monitored by one Pentagon analyst.
“He studied the geology,” Caine says. “He watched the Iranians dig it out. He watched the construction, the weather, the discard material, the geology, the construction materials, where the materials came from.”
The final result was a 13,600kg (30,000lb) precision bomb purpose-built for its intended target, should the order ever come to strike the Iranian nuclear programme.
“[The] team understood with a high degree of confidence the elements of the target required to kill its functions,” Caine says.
The chairman also shared test footage from the GBU-57 development effort showing the massive explosive penetrating a rock tunnel, with the detonation delayed until the bomb passed through an earthen layer and into hollow space below.
When the order to strike Fordo did come, the process was not as straightforward as putting multiple GBU-57s down the ventilation shaft of the two underground facilities. Each of the seven B-2s carried two of the massive bombs, and each bomb had a specifically sequenced role in the chain of destruction.
“Each weapon had a unique desired impact angle, arrival [time], final heading and a fuze setting,” Caine says.
The primary strike points at Fordo were two ventilation shafts, which the Iranians had covered with concrete caps in the days before the bomber sortie – apparently tipped off to a pending American attack.
Speculation suggests that the increasing number of social media posts by US President Donald Trump regarding possible military action against Iran may have been the cause.
Caine says 12 GBU-57s were assigned to hit Fordo, with six allocated to each of the two ventilation shafts. The first bomb dropped on each vent was used to explosively remove those protective concrete caps. Bombs two through five entered the exposed shafts and detonated in what Caine calls the “mission space” of the underground complex.
The remaining two GBU-57s were back-ups in the case of failed detonation or aircraft malfunction.
The Pentagon has previously indicated that 14 of the Massive Ordnance Penetrators were employed on the mission, which would fit with the B-2’s maximum payload of two GBU-57s per aircraft.
Although the B-2’s official payload is just over 18,140kg, according to the USAF and Northrop Grumman, the Iran sortie would seem to indicate the stealth bomber can actually carry significantly more weight.
Public flight tracking data indicates the B-2s met with aerial refuellers shortly after taking off, possibly circumventing the payload restrictions by launching with a minimal fuel load.
Caine says that all of the GBU-57s dropped in the raid hit their intended target and functioned as designed. This was verified, he adds, via unspecified means of intelligence collection and the visual observation of US pilots in the trailing end of the B-2 formation.
“We know that the trailing jets saw the first weapons function,” Caine says. “The pilot stated, ‘This was the brightest explosion that I’ve ever seen. It literally looked like daylight’.”
President Trump was quick to hail the operation as an unequivocal success in the hours following the raid, describing the Iranian nuclear facilities as having been “completely and totally obliterated”.
Political appointees like defense secretary Pete Hegseth were quick to echo those claims, although Caine says the Pentagon does not conduct battle damage assessments, leaving that to outside agencies within the intelligence community.
Hegseth and Trump were quick to denounce a preliminary report leaked from within the US Defense Intelligence Agency suggesting the bombing strike may not have caused permanent damage to the Iranian nuclear programme.
Citing subsequent analysis from the US Central Intelligence Agency, Hegseth on 26 June reaffirmed the earlier assertions from administration officials that Iran’s nuclear facilities have been severely damaged.
The UN-run International Atomic Energy Agency has said the strikes likely produced “very significant damage” to three of Iran’s nuclear facilities.
In addition to the B-2 raid on Fordo, submarine-launched Tomahawk cruise missiles were used to strike sites at Natanz and Isfahan.
