Underwater Travel Becomes Superfast
Developments will give undersea craft new capabilities in speed and maneuverability.
The Los Angeles-class submarine USS Greeneville completed sea testing for the Advanced SEAL Delivery System (ASDS) in 2003. The ASDS is a 65-foot minisubmarine that rides atop large vessels. The supercavitation technology being studied in the Underwater Express program could one day increase the speed of the ASDS to more than 100 knots and sustain that speed for at least an hour.
People and materiel soon may be moving across the ocean much more quickly and outrunning torpedoes in the process. A developmental technology will use supercavitation to move underwater vessels at high speeds. In addition to the rapid rate, the project aims to sustain that pace over long periods of time and to maintain control and steering of the watercraft.
Supercavitation involves creating a bubble—the cavity—of gas around an object within a liquid, reducing drag and allowing the object to travel at high speeds. The process leads to a 60 percent to 70 percent reduction in drag resistance. Natural cavitation occurs when an object moves so quickly in a liquid that air emerges from the solution. Developers are using ventilation supercavitation in the Underwater Express program—an attempt to give the U.S. military an undersea vessel that travels with supercavitation—causing the craft to blow air out of its front to create the bubble around itself.
The Defense Advanced Research Projects Agency (DARPA),
DARPA’s requirements, outlined in a broad agency announcement, include using supercavitation to demonstrate stable and controllable high-speed underwater transport. The agency and developers hope to enable a new class of underwater craft for future littoral missions transporting high-value cargo and small units of personnel. According to DARPA, the large cavity and maneuvering control system required for this type of vehicle never has been created.
Phase 1 of the Underwater Express program focuses on stable cavity generation and sustainment by examining supercavitation physics and the interaction between the cavity and the body. Developers will generate a system concept for an 8-foot-diameter, 60-metric-ton notional superfast submerged transport (SST) craft. The concept will be used to design appropriate scale models for all three phases of the program, including a one-quarter to one-half scale of the notional SST for the Phase 3 demonstration. The contractors also must create an initial concept design of the control system for all vehicle operating conditions.
“The Underwater Express is a technology demonstration program from DARPA that is showing technologies that will allow us to build a vehicle that can go 100 knots for an hour at its most basic,” explains Jennifer B. Panosky, principal engineer, Underwater Express, General Dynamics.
By sustaining high velocity, the vessel will be able to outlast torpedo threats. Standard torpedoes run below the supercavitation range at approximately 40 to 60 knots and can be controlled, while nuclear-powered submarines travel at speeds greater than 25 knots. The Underwater Express Program will provide the advantages of higher speed along with the ability to control the vehicle. Rear Adm. John B. Padgett III, USN (Ret.), vice president for business development and strategic planning, General Dynamics, says, “In the demonstrations to date where you had any body in water going that fast, there is a tendency for it to be unstable, and the instability increases with the speed.” Adm. Padgett stresses that the importance of this project is not only the speed but also the ability to maintain the rapid pace and to control the vehicle at that rate.
The Underwater Express program has several possible applications, and DARPA says it potentially could be manned. Examples of manned uses include special operations such as the U.S. Navy SEALs would carry out. By using the supercavitation technology, the Navy would reduce the amount of time required for SEALs to reach their objective point. In combat situations, the technology would give
The project also has applications for logistics issues in the military and in the commercial world. “Instead of transiting the ocean at 50 knots, you could do it at 200 knots,” Adm. Padgett states. Besides delivering materiel to troops more quickly, this technology would allow shipping companies to invest in marine fleets instead of aircraft for their transport requirements. The military and corporations could load more cargo and deliver it to points of interest faster than current methods permit.
An official at Northrop Grumman shares that logistics will receive the greatest benefit from the new technology, whether organizations are transporting troops or cargo. Though the program has many potential finished-product applications, DARPA currently is aiming to demonstrate only the controllable, stable, sustainable supercavitation technology.
Franz R. Edson, director, submarine payloads, sensors and strategic weapons, General Dynamics, explains that one of the most interesting aspects of the Underwater Express program is the new thinking involved. He notes that many people have used supercavitation to move objects underwater quickly for years, including some torpedoes powered at high speeds for a sustainable period of time. The Russians began development on a torpedo with supercavitation capability called the VA-111 Shkval in the 1960s, and
|Sailors working in the sonar shop aboard the guided missile destroyer USS Mustin load an MK-46 Mod 5A[SW] torpedo into the launcher tube of an MK-32 Mod 15 torpedo launcher. Underwater Express program developers are studying the use of supercavitation to give undersea vessels the ability to outrun a torpedo at greater distances with increased speed and maneuverability.|
In this first phase of development, the General Dynamics team is building models and testing them in water tunnels. By the final phase of the program, models will be tried in the ocean. Once the capability to move relatively large objects at high speeds in the water has been demonstrated, DARPA can scale up the model to the necessary size. A particular platform for the technology has not been released, but in its earliest forms it likely will run on smaller underwater vehicles. Adm. Padgett postulates that the technology could be an integral part of platforms in future generations. However, because the goal in Phase 1 is to determine whether such supercavitation is feasible, developers are uncertain what the final form may be and believe it is premature to speculate how it might be fielded. Edson points out that even when in the third stage, the project still would be in development.
Integrating the technology onto a submarine platform is one of the program’s major problems. Making a piece of metal or plastic move quickly in water may be interesting but is fundamentally irrelevant, according to Adm. Padgett. Once the technology is demonstrated, developers have to meld it with the platform efficiently and effectively and demonstrate success again.
Adm. Padgett compares the process of the supercavitation technology to the advance of aircraft. From air travel that began with propellers, scientists have created jets that fly at speeds of Mach 3, and the space shuttle can attain speeds faster than that. In the same way, the Underwater Express program will move undersea vessels into the next realm of speed and capability, though rapidly moving objects through water is more difficult than moving them through air.
Work on the first phase of the program is being conducted at more than 10 locations across the country. The General Dynamics team expects to complete its work in October. The Northrop Grumman team plans to wrap up a month earlier. The entire project is scheduled to run for about three and a half years.
In the next phases of the project, developers will focus on supercavitation effects and eventually will work toward the final demonstration. Phase 2 includes continued examination of supercavitation physics, cavity-vehicle interactions and the development of vehicle control. At this stage, propulsion concepts will be integrated and maneuvering and body forces as well as gas expenditures will be characterized and measured. Work on the SST system concept will continue to progress.
During Phase 3, personnel will design and fabricate the demonstrator and system testing to guarantee safe and effective operations. The end of the program will include an at-sea demonstration of maneuvering the vessel at 100 knots for 10 minutes. Developers also will have completed the final system specifications for the full-scale SST notional concept craft.
Underwater Express program broad agency announcement: www.darpa.mil/sto/solicitations/underwaterexpress/index.htm
General Dynamics Electric Boat: www.gdeb.com
Northrop Grumman Electronic Systems: www.es.northropgrumman.com