How does the fancy bunker buster bomb work?

People are debating how much damage was done to the underground nuclear weapons facilities built and run by the noble Iranians (an analysis by the unfortunately named “ISIS”). Supposedly, the energy of an underground explosion weakens via an r^3 formula (i.e., falls off as the cube of the distance from the explosion), thus making it critical to place the explosion next to the entirely peaceful underground bunker that is the aggressor’s target. (By contrast, an explosion in air falls off as r^2 and an underwater explosion, e.g., from a depth charge, falls off even more gradually.) Putting the bomb into the desired latitude/longitude position is straightforward, but what control does an attacker have over the depth of explosion?

Despite my skepticism that precision weapons can achieve a decisive victory (as long as the Iranians have a functioning oil and gas industry they will, in my opinion, have the resources they need to develop whatever weapons they want to develop), I asked Prof. Dr. ChatGPT, PhD in Bomb Design: “What technical mechanisms does the GBU-57A/B MOP bomb have to ensure that it detonates at the proper depth to destroy an underground bunker?”

Answer (including an image generated by ChatGPT, which you can tell because “measurement” isn’t spelled correctly):

The GBU-57A/B Massive Ordnance Penetrator (MOP) uses a combination of inertial sensing, hardened fuzing, and time-delay logic to ensure detonation at the proper depth. While detailed specifications are classified, here are the known or inferred technical mechanisms it likely uses:

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1. Inertial Measurement Unit (IMU) / Accelerometers

• The bomb contains an IMU that tracks the weapon’s deceleration as it penetrates the ground.
• By analyzing deceleration patterns, the system can estimate penetration depth and determine when the warhead has reached the intended depth or structural resistance (e.g., reinforced concrete).

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2. Hard Target Smart Fuzing (HTSF)

• MOP likely employs multi-event, programmable fuzes, capable of:
  • Delaying detonation for a set duration after impact
  • Using deceleration signatures to detect when it strikes different material layers (e.g., soil, rock, concrete)
  • Triggering detonation based on penetration depth estimates or when further penetration slows/stops

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3. Time-Delay Fuze Programming

• Before release, operators can program a preset time-delay for detonation based on intelligence about bunker depth.
• This is used in combination with sensor data to refine detonation timing.

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4. Structural Stress Sensors

• Embedded strain gauges or piezoelectric sensors can detect physical resistance changes as the bomb passes through materials.
• These help distinguish when the warhead has entered the actual bunker structure versus overlying rock or soil.

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5. Robust Casing and Kinetic Penetration

• Although not a sensor, the MOP’s hardened steel casing and over 30,000-pound weight give it the ability to reach over 60 meters (200 feet) into concrete or earth before detonation—making precise depth control essential and achievable only with advanced fuzing.

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Summary

The MOP uses sensor-based inertial tracking, programmable fuzes, delay logic, and material sensing to control detonation at the correct depth. It does not rely on GPS during penetration, as signal loss is inevitable underground. Instead, it fuses inertial and physical resistance data to achieve its goal: destroying deeply buried, hardened targets like WMD bunkers.