Even as Washington emphasizes walking softly to pry Iran away from its nuclear ambitions, the Pentagon is speeding the manufacture of its own big stick.

    This month, the Defense Department awarded $51.9 million to McDonnell Douglas to more quickly adapt a 30,000-pound bunker buster to the B-2 stealth bomber.

    The GBU-57 bomb and the fleet of B-2s - stationed at Whiteman Air Force Base with occasional deployments to Guam and an outpost in the Indian Ocean - are widely seen as the likeliest U.S. military option for setting back Tehran's hopes for building nuclear weapons.

    "There is a certain amount of wise military planning in all this," said Robert Hewson, editor of Jane's Air-Launched Weapons, "and a certain amount of saber-rattling."

    The weapon is behind schedule. In 2007, officials at the bomber base east of Kansas City estimated the bomb would be B-2-ready in 2008.

    Budgetary hiccups pushed the delivery date to mid-2011.

    Now the testing of the bomb and the delicate job of outfitting it for any of the $2.2 billion planes is, as one Pentagon spokesman said, "back on track." It should now be ready, said Defense Department spokesman Geoff Morrell, "in the coming months."

    About 20 of the bombs are being made.

    "This has been a capability that we have long believed was missing from our - our quiver, our arsenal, and we wanted to make sure we filled in that gap," Morrell said at a press briefing this month. "I don't think anybody should read anything into it beyond what it is. And I don't think anybody can divine potential targets or anything of that nature."

    The Iranians have many uranium-enriching centrifuges at an underground location at Natanz. But on a visit to the United Nations last month, President Barack Obama announced the Iranians were building another secret nuclear facility near the holy city of Qum, this one deep in a mountain. Some speculate that the mountain facility is in response to a possible bunker buster.

    The disclosure of the new facility may have put the Tehran leadership on defensive. A tentative deal cut this week calls for Iran to ship about three-fourths of its known nuclear fuel stockpile to Russia. Once there, it would be converted into metal fuel rods practical for a nuclear power plant, but not for an atomic warhead.

    But the pact is shaky, and like less-successful efforts to stymie North Korea's nuclear program, short-term advances often see reverses.

    That's where the biggest-yet bunker buster comes in. By the reckoning of military analysts, the bomb is conventional - by which they mean it does not carry a nuclear warhead. But it is unconventionally large.

    The GBU-57 has the weight of about two elephants, stretches 20 feet and carries more than 5,300 pounds of explosive. Two will fit in the belly of a B-2.

    Previously, the biggest non-nuke in the U.S. arsenal was the MOAB, massive ordnance air blast, or "Mother of All Bombs." It explodes above its target.

    Four tons heavier, the GBU-57 is called a MOP, massive ordnance penetrator.

    Dropping such a behemoth is complicated.

    "When you let go, all of a sudden the plane is 10 percent lighter than it was a second ago," said John Pike of GlobalSecurity.org. "Planes have a tendency to pitch up when that happens."

    Some published reports suggest the new bomb can burrow through 200 feet of reinforced concrete before detonating, but many analysts are skeptical. The physics of bunker busting are tricky, and even nuclear bombs can't punch into the world's most hardened targets.

    Still, the bomb might be enough, if its shock can disturb the spinning centrifuges and make the sensitive devices wobble into self-destruction.

    "Once you shake it up, it becomes like the inside of an engine that has thrown a rod. It tears itself apart," said Owen Cote, a security analyst at the Massachusetts Institute of Technology. "But that's not your only issue."

    First, he said, Obama would have to consider if U.S. intelligence is good enough to find the right targets and whether a strike would do enough damage to the Iranian program.

    Then the White House would have to calculate the backlash. Iran would have the options of punishing U.S. troops in countries on either side of it, send missiles toward Tel Aviv, Israel, or sink tankers in the Strait of Hormuz.

    "If you can delay their nuclear program, that's good. Time is your friend," Cote said. "But you have to think about what (the Iranians) will do the day after you bomb them."

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