12.gif (57938 bytes)

NUWC engineer in front of the full-sized elastomeric disk
prototype, seven feet in diameter.

"Water Balloon" Shoots
Weapons In Future
Launching System


by John Little


The same physical principles that govern the power of a slingshot or loosing the contents of a water balloon are being used to create an innovative new weapon-ejection system planned for future USS Virginia-class (SSN-774) submarines. The Elastomeric Ejection System (EES) will use the potential energy stored in a large rubber disk to accelerate a slug of water for ejecting weapons from submarine torpedo tubes. It is a simple and elegant approach whose basic feasibility has already been demonstrated.

There are two current tube-launched weapon-ejection systems in use on U.S. submarines, and like the EES, their basic function is to convert stored energy quickly into the kinetic energy associated with launching the weapon. The USS Los Angeles (SSN-688) class uses a Ram Pump Ejection System (RPES), and the Ohio (SSBN-726) and Seawolf (SSN-21) classes use an Air Turbine Pump (ATP) to push weapons out of the tubes. Although both approaches are effective and reliable, they require complex launcher hardware – mechanical pumps, turbines, gearboxes, etc. – that take up significant volume within the hull. The Seawolf ATP system represents the current state-of-the-art, but in a further attempt to reduce cost and complexity, the EES is being considered for Virginia-class construction.

EES is under development at Naval Undersea Warfare Center (NUWC), Newport Division, and General Dynamics’ Electric Boat Corporation and consists of two major subsystems – one for launching the weapon and the other for re-energizing the elastomer. Each EES can serve two torpedo tubes, so normally separate port and starboard systems would be found in a typical installation. The heart of the system is a 3,000-pound, seven-foot diameter natural rubber disk, one foot thick at the center, that is installed in the wall of an enclosure within the ballast tank. A charging pump draws water from the ocean through a sea valve and pressurizes this impulse tank so that the disk "inflates" and bulges outward into a hemispherical shape, displacing water out of the ballast tank through the flood grates. A check valve in the charging system holds pressurized water in the tank/diaphragm structure after the recharge pump has been shut down. Fully inflated, the highly strained disk is stretched by 100 percent and stores enough energy to generate 2,400 horsepower when released. In firing, a servo-controlled slide valve opens a channel from the impulse tank to the torpedo tube, and the energy stored in the elastomer disk forces pressurized water into the tube behind the weapon and pushes it out.

The EES is expected to satisfy Virginia-class launch requirements at much lower cost than earlier approaches. General Dynamics’ Electric Boat Corporation, the Virginia-class design yard, has estimated savings of $5 - 8 million per ship, which would recoup development costs after three or four hulls. Most of these savings come from eliminating complex and costly ATP hardware and also the need for high-pressure air. Acoustically, EES is expected to be as quiet as the best of today’s ATP systems.

Developmental Challenges
While the "pipes and pumps" of the EES system are fairly conventional, developing the elastomer disk was a major technical challenge for the materials scientists. Sophisticated simulation-based design models and new manufacturing technologies were brought to bear to produce a full-sized working prototype of the elastomer disk that was tested in the Submarine Launcher System Test Facility at NUWC. This prototype eventually demonstrated a fatigue life of 6,400 to 8,800 cycles at full inflation – far in excess of the operational requirement of only 2,200 cycles. Significant development resources were also expended on the closed-loop hydraulic servo controls that accelerate and decelerate the torpedo tube slide valve for most efficient performance, and the recharge pump used to inflate the rubber disk.

Operating Sequence for the
Elastomeric Ejection System

13a.gif (11353 bytes)

1. Charging pump draws water into the
impulse tank through a sea valve.

13b.gif (11356 bytes)

2. Rubber disk "inflates" as the pressure rises,
thus storing energy for expelling the weapon.

13c.gif (11997 bytes)

3. Torpedo tube slide valve opens,
releasing pressurized water behind the weapon.

13d.gif (14493 bytes)

4. As the disk deflates, the resulting
water "slug" drives the weapons fromthe tube.

13-2.gif (73640 bytes)


Past, Present, and Future
The idea of using the energy stored in an elastomer to pump water occurred originally to Larry Bissonnette, a former NUWC engineer, as a result of his spear-fishing experience. He conceived an improved spear gun that worked more like an underwater blowgun than a slingshot and extended that concept to include small-scale launch systems. These ideas were picked up in 1989 by NUWC’s Independent Research program, which funded several early demonstrations. The earliest elastomers were urethane sphere-like shapes, but in collaboration with Dr. Mike Roland, a polymer chemist at the Naval Research Laboratory, more appropriate materials were soon identified, and the basic shape was refined to that of the present configuration for a successful one-third scale prototype.

Failures during inflation of some of the first full-scale parts nearly killed the program, but perseverance on the part of both the NUWC engineers and supporters at the Naval Sea Systems Command (NAVSEA) kept it going, with substantial help from private industry and both Navy and private shipyards. At this point, successful launch performance and adequate fatigue life of full-scale disks have been realized.

Now, EES has progressed to the Advanced Development Model (ADM) phase under the sponsorship of the Submarine Advanced Research and Technology Office (NAVSEA 92R). At this stage of the development, Electric Boat Corporation has assumed the lead for all components except the elastomer disk, for which NUWC still retains cognizance. Newport Division will now perform further tests to quantify the resistance of the elastomer to temperature, shock, and long-term deformation over its expected life cycle. A major demonstration of Engineering Development Model (EDM) hardware is planned for the fall of 2000, with a Go/No-Go decision that December on forward-fitting EES into Virginia-class new construction.

Assuming no setbacks, the present planning schedules show availability of EES shipboard components starting in FY03, for installation on the fourth Virginia-class SSN, with at-sea system tests in 2006.

John Little is a mechanical engineer in the Launcher and Missile Systems Department at NUWC, Newport Division.