Navy, DARPA Seek Smaller Submarines

Sea Power, Feb 2005 by Hamilton, Robert A

Smaller May Be Better

DARPA and the Navy assess huge changes in sub design to find out if smaller is better.

* Sonar arrays might be fitted into the skin of the sub, eliminating the sonar dome.

* A smaller power plant - leading to elimination of reduction gears that weigh tons - is one key to success.

* External weapons stowage and retractable bow planes also will get a look.

The Navy and the Defense Advanced Research Projects Agency (DARPA) plan to pour $97 million during the next four years into a new joint project known as Tango Bravo, intended to lead to the design of a new attack submarine that would have all the capabilities of the current Virginia-class boats, but would be half the size and built at half the cost.

Begun in November, the project comes as the Navy continues its internal debate over the $2.2 billion unit procurement cost of Virginia-class subs and the future size of the attack submarine fleet, which now numbers 54 boats. An internal Navy study suggests that 37 boats would be sufficient, but some experts, such as retired Adm. Frank L. "Skip" Bowman, former director of naval nuclear propulsion, say the subs continue to be in high demand by the nation's combatant commanders.

Until now, however, the Navy has not officially focused on the prospect of cutting the size of its attack boats by half. The Virginia class is 377 feet in length and has a 34-foot beam.

Led by Naval Sea Systems Command's Program Executive Officer for Submarines, the Tango Bravo project is not designed to help rectify the Navy's near-term quandary. It is a demonstration project aimed at bringing fundamental change to future U.S. submarines while maintaining or improving their current capabilities. The two agencies will look at reducing or eliminating the nuclear power plant, for example, and fit future subs with a retraction system for the bow planes with no associated hydraulics, a major change from the current design.

"Submarines have always been sized around the propulsion plant, and if you get rid of the reduction gears and the shaft, you're talking about many tons of weight and cubic yards of space, which drive down the size that you need," said Nicholas L. Flacco, a former submarine commanding officer and now a senior V scientist at Areté Associates in Sherman Oaks, Calif.

Shaftless propulsion is one of the five key technology domains that will subjected to intensive scrutiny during the Tango Bravo project in hopes that innovations therein will foster reductions in cost and size of the nations subs.

This spring, the two agencies will fund up to $19 million in technology demonstration projects in external weapons stowage and launch, hull-adaptable sonar arrays, an automated attack center and general "radical ship infrastructure reduction" initiatives, in addition to shaftless propulsion.

Tango Bravo grew out of a joint Navy-DARPA study that was finished in May, which looked at a number of factors that affect the size and cost of hull, mechanical and electrical systems on a submarine.

"The study provided early indications that reduced size and cost would be feasible, so DARPA and the Navy embarked on the Tango Bravo effort which will conduct demonstrations to overcome selected technological barriers in order to enable design options for future submarines," said DARPA spokeswoman Jan Walker.

"DARPA and the Navy are excited about embarking on this new endeavor. We believe that the Tango Bravo technology demonstrations promise to go a long way toward providing new submarine capabilities for the Navy," she said.

Interest in the program has been running high on the outside as well, Walker said, with more than 140 people attending DARPA'S classified "Proposers Day Conference" Nov. 8 in Arlington, Va. DARPA hopes to be able to decide by late 2006 on what concepts to move into prototype production, which would be followed by full-scale demonstrations in early 2008, and at-sea demonstrations in 2009.

Flacco said another reason the power system will get a lot of attention is that Navy wants an efficient propulsion plant that will be at least as quiet as that of the Virginia class, and cost a minimum of 40 percent less.

In addition to looking at eliminating reduction gears and shafts - perhaps by relying on electric drive - "any shielding you would be able to eliminate from a smaller reactor will drive the space and weight down as well," Flacco said.

"You get an exponential decrease in the power requirements when you have weight reduction, so if there's anything you can do to achieve that, it pays big dividends," he said. "All of those features could pretty quickly move you down toward a half-sized ship."

External weapons stow-and-launch also holds out considerable promise for reducing the price of a submarine by making the pressure hull smaller. DARPA is seeking a method of stowing an unencapsulated Mk48 Advance Capability torpedo outside the pressure hull, and the capability to launch it at up to flank speed and test depth, a technique that could be expanded to multiple weapons.