The U.S. Air Force is on the cusp of flight testing a new tail-kit assembly that will upend the old way of conducting tactical nuclear combat, should the United States ever enter a shooting match with a nuclear-armed state like Russia or China.

We're talking about an upgrade to the B61 thermonuclear bomb  —  the oldest nuclear gravity bomb in America's stockpile. The Boeing-built guidance unit adds range and accuracy, turning it into a "smart" nuke compatible with the F-35 Joint Strike Fighter and future Long-Range Strike Bomber.

The B61 entered production in 1968, and is still the Pentagon and NATO's go-to weapon for tactical and strategic nuclear combat.

The flying branch's weapons office at Eglin Air Force Base in Florida has been working with Boeing to design and develop the tail-kit assembly since 2012. The Air Force is scaling up the flight test program this year.

The tail kit will resemble a non-nuclear Joint Direct Attack Munition, with strap-on strakes for range, and tail fins guided by a GPS-aided inertial navigation unit.

The equipment will work even if America's navigation satellites are shot to pieces — a likely scenario during a nuclear war — and is hardened to survive the electromagnetic pulse generated by a high-altitude nuclear explosion.

Both the Air Force and Boeing have expressed confidence in the tail kit, and the $1 billion program is meeting its cost and schedule targets. But the program is marching toward a tougher phase of development, with a critical design review planned for later this year that will push Boeing's design to its limits.

"We don't anticipate any major risks associated with the tail kit assembly that can't be mitigated," Boeing spokeswoman Katie Kelly wrote in an email in January.

(U.S. Air Force/Courtesy War Is Boring)

Hans Kristensen, a nuclear weapons policy expert for the Federation of American Scientists, said he has serious reservations about the program. He added that cutting or canceling the B61 could save the government billions of dollars.

However, he believes the technology is there to produce a precision-strike gravity nuke  —  a concept the Pentagon first seriously considered in the 1990s.

"There might be individual challenges that are particular for this kind of weapon they have developed," Kristensen said. "But they have so much experience now with guided systems that I would be surprised if there were some real tough nuts that they wouldn't be able to crack or that would delay it significantly."

”There are so many precision weapons that have been produced and mated with so many different kinds of aircraft that I'd be surprised if there were major technical problems," he added.

According to the Air Force, developing the guidance kit might be the easy part. The National Nuclear Security Administration — which oversees America's nuclear warheads — has the tougher task of refurbishing the complete weapon. It's all part of an $8-billion life-extension program that will keep the nukes active until 2040.

The first fully-refurbished bomb won't be available until 2020, well after the tail kit enters service.

Today, there are five versions of the B61, including one ground-penetrating version to kill hardened bunkers. The government wants to cull the stockpile down to just two versions — a low-yield version, designated B61–12, and the bunker-busting nuke retained in its current form.

It falls to Sandia National Laboratories to consolidate 400 to 500 of these Cold War relics into the B61–12 variant.

"To date, Boeing and Sandia National Labs have completed several series of all-up-round level testing as well as fit checks and a captive-carry flight test with the launch aircraft," the Boeing spokeswoman wrote. "Each test helps reduce risk."

Along with the flight test program, 2015 will be a big year for integrating the bomb onto existing aircraft.

The program office has the money to begin pairing the smart nuke with other aircraft — the B-2, F-16, and F-35A as well as the European PA-200 Tornado.

This month, the NNSA confirmed that an F-16, F-15, and a B-2 armed with advanced measurement devices have been zipping around Eglin and Edwards Air Force Base to collect vibration and flight environment data for the B-61's new tail kit. Those flights took place between July and December 2014.

"This series is the first of many flight tests for the B6–12 life-extension program," a Feb. 9 press statement noted. "The testing is a key building block between ongoing system ground testing and the first development flight test drop scheduled in fiscal year 2015."

The Air Force will test the first complete tail kit on an F-15E Strike Eagle using an inert bomb. If the flight tests are successful, Boeing will pass the design review and proceed to the next round of development, with a plan to deliver the first production unit in 2018.

There are a series of smaller flight tests planned throughout the year to test different components of the weapon system. But Air Force plans the complete developmental test flight in November.

(U.S. Air Force/Courtesy War Is Boring)

So why turn a tactical nuke into a smart bomb? It's all about destroying the same target with a less powerful warhead.

"U.S. Strategic Command determined that with the accuracy provided by a tail kit, the yield provided by today's lowest yield B61 variant would be sufficient to meet all of the strategic and non-strategic requirements for gravity systems," Donald Cook, the NNSA's deputy administrator for defense programs, told Congress in 2013.

"As a result, there will no longer be any need to design, develop, certify, or maintain multiple variations of the B61."

But the larger context is the New START treaty signed by Pres. Barack Obama and then-Russian Pres. Dmitry Medvedev in 2010. The treaty limits each side to 1,550 deployed nuclear warheads by 2018.

To meet these arms reduction targets, U.S. Strategic Command advocated for what's still known as the "3+2 Strategy," where the U.S. would move toward a nuclear stockpile with three interoperable ballistic missile warheads — for the submarine and land-based legs of the nuclear triad — and two air-delivered warheads.

As part of this plan, the U.S. would eliminate the megaton-class B83 gravity bomb.

The U.S. has already cut its nuclear stockpile by 80 percent compared to its Cold War peak, and the Pentagon estimates it could still meet its nuclear deterrence strategy with one-third fewer weapons.

Whether those additional reductions are ever realized depends on the next round of arms control talks set to take place before the New START caps expire in 2021.

The Air Force's latest budget request includes $743 million during the next five years for B61 tail kit research and development. The NNSA has asked Congress for $643 million to support the B61 life-extension program in fiscal year 2016.

According to the flying branch's latest tail kit schedule, the critical design review should finish in early 2016, ahead of a contract award to Boeing for phase two of development.

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