📅 8 April 2026 | Daily WOME Intelligence Briefing   [email protected]
Ordnance Employment Deep-Earth Penetrator Operation Epic Fury

GBU-57A/B Massive Ordnance Penetrator Employed Against Underground IRGC Command Facility in Tehran

The GBU-57 is widely characterised as an “unstoppable bunker buster” — but the weapon’s effectiveness against deeply buried hardened targets depends on geological conditions, target depth, and the interaction between a 2,400 kg AFX-757/PBXN-114 explosive fill and Eglin steel penetrator casing that together make it a uniquely specialised hard-target defeat system with significant operational constraints.

8 Apr 2026 6 min read ISC Editorial Team AI-assisted

Technical Summary

On 3 April 2026, United States Central Command (CENTCOM) directed B-2A Spirit stealth bombers from the 509th Bomb Wing, Whiteman Air Force Base, Missouri, to deliver GBU-57A/B Massive Ordnance Penetrator (MOP) munitions against an underground Islamic Revolutionary Guard Corps (IRGC) command compound near Tehran. The strike was conducted as part of Operation Epic Fury, the sustained US military campaign against Iran that commenced on 28 February 2026.

The GBU-57A/B is the largest conventional air-delivered munition in the US inventory. The weapon has an All-Up Round (AUR) mass of approximately 13,600 kg (30,000 lb) and a total length of 6.2 m. Only the B-2A Spirit and B-21 Raider possess the internal payload bay dimensions and structural capacity to carry the weapon. Each B-2A can carry two GBU-57A/B munitions simultaneously.

The reported target was a hardened underground facility. CENTCOM commander Admiral Brad Cooper ordered the strike while simultaneously directing a personnel recovery operation for a downed US airman inside Iranian territory — indicating fusion of time-sensitive intelligence with strategic deep-strike capability.

Analysis of Effects

The GBU-57A/B explosive fill comprises approximately 2,082 kg of AFX-757 and 341 kg of PBXN-114 (a polymer-bonded explosive based on an HTPB binder system), yielding a total Net Explosive Quantity (NEQ) of approximately 2,423 kg. AFX-757 is an advanced thermobaric/blast-enhanced explosive formulation developed by the Air Force Research Laboratory (AFRL) specifically for hard-target defeat applications.

The weapon’s penetrator casing is manufactured from Eglin steel (ES-1c), a high-strength, high-toughness alloy engineered to survive the extreme deceleration forces encountered during penetration of reinforced concrete and geological overburden. Assessed penetration performance is approximately 60 m through moderately compacted earth or 18 m through reinforced concrete at 5,000 psi (34 MPa) compressive strength. Penetration through higher-strength concrete (10,000 psi / 69 MPa) reduces to approximately 2.4 m.

The weapon employs GPS/INS guidance (tail-kit assembly) for precision delivery and a hard-target Smart Fuze (FMU-152 or derivative) with selectable delay to optimise detonation depth within the target structure. The fuze enables the weapon to count penetrated layers and detonate at a pre-programmed void or depth.

The fill ratio of approximately 17.8% (NEQ to AUR mass) is characteristic of deep-earth penetrator design — the majority of the weapon’s mass is in the high-strength steel casing required to survive penetration forces, not in the explosive fill.

Hazard Division and Compatibility Group classification for the GBU-57A/B depends on its packaging configuration and fuze state per STANAG 4123 / AASTP-3. In its delivered configuration with fuze fitted, the weapon would likely be classified HD 1.1, CG F (article containing a secondary detonating explosive substance with its own means of initiation). Without the fuze assembly, CG D would be more probable. Exact classification depends on whether the fuze meets the requirement of 2 independent safety systems per STANAG 4187.

Personnel and Safety Considerations

The GBU-57A/B is handled exclusively within USAF munitions maintenance and weapons loading operations at Whiteman AFB and designated forward operating locations. Given the AUR mass of 13,600 kg and NEQ of approximately 2,423 kg, Quantity Distance (QD) requirements under AASTP-1 / STANAG 4440 for storage and handling are substantial. Inhabited Building Distance (IBD) for a single weapon at this NEQ would be calculated using the QD tables in AASTP-1, yielding distances in excess of 600 m depending on the applicable exposure site category.

Post-strike battle damage assessment (BDA) in the target area will encounter unexploded ordnance (UXO) hazards from any weapons that achieved low-order detonation or failed to function. Given the weapon’s NEQ, any UXO GBU-57A/B represents an extreme hazard requiring specialist Explosive Ordnance Disposal (EOD) response. The AFX-757 fill and PBXN-114 booster are assessed as relatively insensitive under normal handling conditions, but the fuze state of any UXO item would be the primary safety concern.

The operational context — simultaneous strike and personnel recovery operations in the same theatre — raises Explosives Safety and Munitions Risk Management (ESMRM) considerations under ALP-16 / STANAG 2617 regarding coordination between strike planning and ground force safety.

Data Gaps

DATA GAP: Number of GBU-57A/B weapons delivered — the number of MOPs employed in this specific strike has not been confirmed by CENTCOM. Prior strikes on Iranian nuclear facilities in June 2025 employed 14 weapons across two sites.
DATA GAP: Target hardening specifications — the depth, construction method, and reinforcement of the IRGC underground facility are not publicly confirmed. Without this, penetration performance assessment remains generic.
DATA GAP: Battle Damage Assessment — no post-strike BDA imagery or functional assessment of the target has been released at time of publication.
DATA GAP: Fuze variant and delay settings — whether FMU-152 or a successor fuze was employed, and the programmed detonation parameters, are classified.
DATA GAP: AFX-757 published Velocity of Detonation (VoD) and detonation pressure — open-source performance data for this formulation is limited. Composition details beyond “thermobaric/blast-enhanced” are not available in unclassified sources.

Authoritative References & Evidential Record

  • Army Recognition — “U.S. B-2 Bombers Hit Underground IRGC Command Bunker in Tehran With 30,000-lb GBU-57 Bunker-Buster Bombs” — 7 April 2026. [C/2]
    Specialist defence media reporting based on Wall Street Journal primary source and CENTCOM confirmation of Operation Epic Fury strike operations.
  • The War Zone — “Did B-2s Just Drop GBU-57 Massive Ordnance Penetrators On Another Iranian Nuclear Site?” — April 2026. [B/2]
    Detailed technical analysis of GBU-57 employment with corroboration from multiple defence sources and seismic data.
  • Defence Blog — “B-2 bombers hit IRGC underground HQ near Tehran” — April 2026. [C/3]
    Corroborating report with operational context.
  • Wikipedia — GBU-57A/B MOP. [D/3]
    Aggregated technical specifications with cited sources including Boeing data and USAF fact sheets. Cross-referenced against GlobalSecurity and manufacturer data.
  • STANAG 4440 / AASTP-1 — NATO Guidelines for the Storage of Military Ammunition and Explosives. [A/1]
    QD and IBD calculation reference for weapons of this NEQ class.
  • STANAG 4123 / AASTP-3 — NATO Ammunition Classification and Hazard Assessment. [A/1]
    Reference standard for HD/CG classification criteria.
Corrections & Updates
Corrections and updates welcome. If you hold open-source data that refines or corrects any parameter in this article, please contact [email protected] citing the specific claim and your source. Verified corrections will be incorporated and credited in the revision history.