Bids are due within a month for a US Air Force contract to build an improved version of the 2,000lb BLU-109 penetrator and deliver at least 12,000 of the bunker-busting bombs over the next five years.

The improved BLU-109, which will be redesignated the BLU-137, and should correct known deficiencies and make the weapon more reliable against targets such as bunkers and hardened aircraft shelters, the service tells FlightGlobal.

Reliability is expected to improve with the next-generation weapon, though the bomb will use the same AFX-757 explosive fill as the legacy warhead. The US Navy has also expressed interest in BLU-137, though today the weapon remains with the USAF only, the service says.

Earlier this summer, the air force released a solicitation for BLU-137 production. The USAF will begin replacing the legacy 2,000lb BLU-109C/B as early as fiscal year 2018, according to air force budget documents. While the BLU-109 is employed on the laser guided bombs and the Joint Direct Attack Munition (JDAM), the incoming BLU-137/B is compatible only with the latter's GPS/INS-guidance and control system.

To improve the reliability of the BLU-109's fuze, the new warhead will use a modified JDAM tail kit that includes an Arming Generator Relocator Adaptor (AGRA). The adaptor sits within the JDAM aircraft mounting hardware and houses the system that provides power to the fuze.

In March, the air force awarded a $7.3 million contract to Boeing to integrate the AGRA into the JDAM tail kit.