DARPA Thinks Linking USVs Together Like Train Cars Can Increase Their Range At Sea.

The U.S. Navy is looking to increase its fleet of surface vessels to 500 or more with a mix of more affordable unmanned ships helping to reach that number. Like their manned counterparts, unmanned surface vessels (USVs) will have to operate across the strategically important the Indo-Pacific. While they offer certain advantages, including extended deployability, they come with a handicap – range.

The 3,500 nautical-mile range requirement for the Medium Unmanned Surface Vehicle (MUSV) which the Navy is pursuing is simply too short for the huge distances of Pacific operations. Their consequent need to refuel at sea or in port before reaching operational locations complicates already stretched logistics, increasing risk to individual ships and the Fleet according the Agency.

DARPA thinks it may be able to quadruple the range of MUSVs by putting them in a contiguous nose-to-tail (or bow-to-stern) line. Think of cyclists cutting their wind resistance in the Peleton at the Tour de France or of NASCAR racers “drafting” in line at the Daytona 500. Transpose that basic aerodynamic idea to the water DARPA’s thinking goes, and you may increase the range of an MUSV to as much as 14,000 nautical miles.

The Agency is investigating the possibility in a program it calls Sea Trains.

Resistance

Getting MUSV-sized platforms into theater without having to refuel could be as simple as overcoming resistance Sea Trains program manager, Andrew Nuss says.

One of the principal types of hydrodynamic resistance that affects water-borne vessels is wave-making resistance. “Wave-making resistance is equivalent to the amount of energy it takes for the boat to move the water out of the way,” Nuss explains. It’s visually apparent watching a bow cut through the water or observing the wake behind a boat’s stern.

Wave-making resistance is contingent on the length and speed of a vessel. Previous DARPA research shows that one way to reduce it is to place multiple vessels line astern with no gaps between them. Collectively, they form one long hull, reducing hydrodynamic drag. According to Nuss, four vessels in tight trail have approximately the same wave making resistance as one vessel alone.

DARPA envisions a four-ship Sea Train of MUSV-sized vessels convoying to their area of operations achieving up to 14,000 miles of travel unrefueled. Three companies – Gibbs & Cox, Applied Physical Sciences Corp. (APS) and InMar Technologies – will build scale models to test the concept in laboratory tanks in phase one of the program before moving on to small scale boats, then MUSV-representative vessels.

In a tank or on the water, the vessels will have to minimize “gap drag”. When the gap between the stern of one boat and the bow of the next widens, it creates eddies and turbulence that increase drag.

“You want imagine a nice lean hull line all the way from the first vessel to the fourth vessel with no breaks,” Nuss affirms.

Sea Trains will explore three ways to minimize the gap. APS will develop a “connected” approach literally linking the ships physically via a railroad-like coupling mechanism. Gibbs & Cox will work on a “connector-less” approach which sees the vessels in-contact but not physically held together. InMar Technologies will develop a vaguely-defined “collaborative formation” option.

The developers are allowed freedom in the hull and propulsion designs they choose though Nuss says the hull forms are “pretty traditional” as are the drivetrains. Departing too far from the Navy’s vision of an MUSV wouldn’t be practical anyway.

The need to eliminate gap drag could suggest notched sterns into which a following vessel’s bow could plug, an idea that Nuss confirms has been looked at in previous research.

Does DARPA think the physics will pan out? Previous lab studies “give us some confidence that this effect is realizable,” Nuss says.

Real World Resistance

Realizing big fuel savings by putting USVs into trains for transit to operational stations depends on more than the singular physics of a ship formation. Sea states, winds and other environmental factors could make a sea train less efficient or even dangerous.

“This has to work in conditions beyond a glassy sea state and no wind,” Nuss acknowledges. DARPA will evaluate feasibility on the water in the Chesapeake Bay and elsewhere.

That includes situations wherein there could be considerable relative motion between the vessels. One of the biggest challenges in Sea Trains is developing a more sophisticated autonomous control system solution than what has been demonstrated in other unmanned programs like DARPA’s Sea Hunter.

A workable control system will need additional sensing, behavioral predictability and decision-making acumen to separate Sea Train ships to maintain safety/integrity when necessary.

“If there are conditions where testing and physics that show that Sea Trains isn’t efficient, knowing that is beneficial. Maybe there’s an opportunity to avoid conditions like that,” Nuss says.

Of course, navigating around storms or typically rough sea regions could negate some of the benefits of the concept and ironically make such convoys more predictable. And sub-optimal sea conditions would likely reduce fuel efficiency gains though Nuss maintains that any gain is still a gain potentially giving the Navy “more operational reach and more flexibility.”

Sea Trains operational flexibility may be limited by other constraints like the detectability of such formations. DARPA won’t comment on the acoustic, radar, or thermal signatures a sea train might have. Though relevant and not ignored by the program, they’re ultimately considerations for the Navy Nuss says.

So too is the tension between the logistics and vulnerability of refueling shorter range USVs at sea and the likely operational patterns that come with longer range sea trains. If they’re too limiting, could they defeat the purpose?

“I don’t believe it defeats the purpose of the concept,” Nuss says. “First, we need to be able to demonstrate with in-water testing that those [increased ranges] are achievable.”