By Elaine M. Grossman
Global Security Newswire

WASHINGTON – The U.S. Navy has toyed for years with the idea of improving the accuracy of its Trident D-5 nuclear payload, with an eye toward giving the weapon greater utility against a wider range of targets, according to defense officials and outside experts (see GSN, Aug. 1).

However, Congress has repeatedly thwarted efforts to launch an ambitious precision upgrade program for the submarine-launched missile’s Mk-4 re-entry body.

Lawmakers have cited concerns that a more accurate warhead could increase the risk of nuclear exchanges. A U.S. president, the theory goes, might be more tempted to order a nuclear strike if he had greater confidence that the weapon would very precisely destroy hardened ICBM silos or underground bunkers. Additionally, an adversary might pre-emptively launch nuclear weapons if its weapons or national leaders were thought to be at imminent risk of pinpoint attacks, critics on Capitol Hill have said.

Now, according to defense officials, the Navy faces an additional barrier to its latent nuclear upgrade ambitions: Technology that might have been used to add accuracy to a conventionally armed version of the D-5 will not fit on the nuclear variant.

If, in theory, policy issues on Capitol Hill posed no constraint, the Navy might have been able to transfer accuracy upgrades from its conventional Trident, currently under development, to the nuclear variant.

The Navy has neither acknowledged nor denied that it seeks to do so, and did not respond to a request for comment this week.

The current D-5 can currently deliver its warhead within 120 meters of its target, according to GlobalSecurity.org.

The Defense Department is working to improve the D-5 re-entry body’s targeting accuracy under an effort called “Conventional Trident Modification.” Even that program has proven controversial in Congress, with lawmakers slashing requested funds based largely on concerns about nuclear “ambiguity.” Congressional critics have said that U.S. strategic rivals could easily mistake a conventionally armed Trident for a nuclear-armed version and could order a nuclear counterattack (see GSN, May 16).

The conventional version of the missile is slated to modify a small number of Trident Mk-4 re-entry vehicles by altering the nose tip and adding a tail kit that boosts accuracy. The aft section would include guidance systems and control surfaces for maneuvering the weapon as it heads toward its target.

However, it appears developmental constraints would preclude the addition of accuracy upgrades to the nuclear-armed re-entry body, at least in the near term, defense officials told Global Security Newswire.

“You cannot do this with the nuclear payload,” said one defense official in an interview this week. “[It is] not technically possible.”

The official declined to offer specifics, citing concerns about classification.

This Defense Department source is among several officials interviewed for this article who spoke on condition of anonymity, because they were not authorized to discuss the issue publicly.

However, Gen. James Cartwright, who until recently served as the top commander for U.S. strategic operations, has alluded openly to such technical constraints.

Asked in March whether the Navy might apply accuracy upgrades intended for the conventional Trident to the missile’s nuclear-armed re-entry body, Cartwright said: “I don’t think you can do that. Physically that doesn’t work.”

Then head of U.S. Strategic Command, the Marine Corps general also declined in the brief interview to elaborate on the technical barriers. Early this month, Cartwright became vice chairman of the Joint Chiefs of Staff.

Missile experts speculate the Navy is running into problems accommodating a complex guidance package on the Mk-4 while remaining within overall size constraints dictated by a heat shield covering the re-entry body. The shroud protects the Mk-4 from debris, water and temperature swings as it descends from space in its ballistic trajectory.

“They [might] have run out of ‘real estate’ [because] the conventional tail kit has become much larger and [more] complicated than planned,” one industry expert surmised this week.

By all accounts, building an effective guidance unit for the conventional Trident is a tall order. To achieve a desired pinpoint accuracy, a Global Positioning System receiver and inertial measurement unit must direct meticulous corrections in heading to the vehicle’s control surfaces, all while diving at hypersonic speeds towards a target.

“There is a serious debate among engineers whether you can get the kind of accuracy this system requires,” one expert said.

That technological hurdle might well have resulted in a guidance package that extends from the tail kit into the re-entry body itself, the source said.

Assuming sizing issues are resolved on the conventional Trident, they would still block the Navy from adapting the same accuracy package to the nuclear re-entry vehicle, experts explained.

Engineers potentially could adjust the size or shape of the conventional warhead – albeit with a likely cost to the weapon’s destructive power. However, similar adjustments could not be made to a nuclear warhead, which has a fixed design.

The Navy has long sought greater accuracy for the Trident D-5 nuclear missile, despite being what Cartwright has called “the best we’ve got and more than capable.”

“That’s an old dream of the Navy,” said Hans Kristensen, director of the Nuclear Information Project at the Washington-based Federation of American Scientists.

One nuclear warhead that flies on the Mk-4 re-entry vehicle – the W-76 warhead – makes up more than 80 percent of the Trident missile fleet, with the significantly more powerful W-88 warhead comprising the remainder.

Even at 100 kilotons, the W-76’s yield offers only limited utility against the full array of potential targets. While the W-76 is the mainstay of the Trident D-5 fleet, it is incapable of penetrating hardened targets, such as ICBM silos or deeply buried leadership bunkers, according to a 1993 Congressional Budget Office study.

By contrast, the W-88 warhead can be used against the hardest of targets. However, only one in five D-5 warheads is a W-88, following a production halt in the early 1990s, according to data compiled by Kristensen and Robert Norris of the Natural Resources Defense Council.

As the United States continues to reduce its nuclear stockpile in accordance with U.S.-Russian arms control treaties, pressure mounts on the Navy to boost the utility of its W-76 warhead, in the eyes of some (see GSN, March 2).

“It is the dirty little secret in warhead reductions: When we go to fewer warheads, the ones left must be capable of doing more types of missions,” Kristensen told GSN. “So the Navy has been trying to improve the effectiveness – they call it ‘flexibility’ – of the W-76 so it can cover a wider range of potential targets and scenarios than before.”

From a technical perspective, improved accuracy could be an effective fix for the W-76 limitations. Boosting a weapon’s accuracy by 50 percent would make it three times as likely to destroy a hardened target as would a 50 percent increase in yield, the budget office stated in its 1993 report.

Naval officials began working on precision upgrades for the Mk-4 re-entry vehicle several years ago. Starting in 2000, the Navy helped develop a precision re-entry system and penetrator warhead for the Army Tactical Missile System, in an effort called “TACMS-P.”

The Navy followed up with a fiscal 2003 budget request to Congress that would allow it to explore how an “Enhanced Effectiveness” modification of the Trident re-entry body – regardless of payload – could improve targeting accuracy.

Service budget materials said a three-year effort would allow officials to “demonstrate a near-term capability to steer a sea-launched ballistic missile warhead to GPS-like accuracy.” The Global Positioning System likely would not be used for a nuclear weapon because it is not believed survivable in such an environment, but an inertial measurement unit could be, according to experts.

“With the conventional version of the D-5, the [Global Positioning System] is necessary,” said one industry source. “With the nuclear version, the [inertial navigation system] should be more than adequate.”

However, lawmakers nixed funding for the enhancement, citing long-standing concerns about giving a ballistic re-entry vehicle maneuvering capabilities that might encourage nuclear exchanges.

Still, the Navy was able to carry out enough research and development in the TACMS-P and Enhanced Effectiveness efforts to draw initial conclusions on how modifications to the Mk-4 re-entry body might work, a defense official told GSN this week.

The Pentagon is now capitalizing on initial development tests in these two programs to design changes to the Mk-4 in the Conventional Trident Modification effort, according to officials.

While this latest conventional Trident development effort has similarly proved controversial on Capitol Hill, the Navy is moving forward with it in incremental steps. The service is using $20 million in fiscal 2007 funds Congress appropriated for the exploration of technologies common to all options for conventionally armed “prompt global strike,” including conventional Trident, defense officials say.

This year’s work on conventional Trident missile payload and engineering could also benefit a number of other potential land- and sea-based weapons platforms for the mission, under which the Pentagon seeks the ability to strike any target around the globe within 60 minutes, according to defense officials. Such weapons would be used sparingly against urgent targets, such as a terror leader pinpointed at a safe house or a rogue-nation missile on a launch pad, defense officials say.

Cartwright, the former strategic commander, has acknowledged in Pentagon meetings and on Capitol Hill that the modified Trident missile is not an optimal solution for the prompt global strike mission. However, he says it is the system that could be developed and deployed most quickly. Lacking such a conventional weapon, the only long-range and fast-flying tool available for attacking important-but-fleeting targets must come from the nuclear arsenal, which is much less likely to be used, the general has said.

Speaking last year, Cartwright would not rule out the possibility that he might support future efforts to develop more precise nuclear weapons, perhaps to include the D-5.

“The merit that you might be able to realize would be like what we realized with conventional munitions, in that the number of weapons associated with a specific target could be less,” he said in a July 2006 interview. “Is there a desire to have fewer to accomplish the same job? … That’s a debate that could be had.”

Other defense officials explained that future “spiral” modifications to the D-5 might allow for accuracy upgrades to the nuclear-warhead-bearing re-entry vehicle, if current physical limitations are surmounted.

This spring, though, Cartwright voiced some skepticism about the need for a more precise, nuclear-tipped D-5. He harkened back to earlier comments about the improbability that the United States would ever use its powerful arsenal.

“Think about what you’re talking about here,” Cartwright said after the March hearing. “It’s a nuclear weapon.”

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