Reducing detectable vehicle presence during submerged operation using a biomimicry buoyancy-derived propless propulsion system

ABSTRACT

Submerged vehicles generally obtain movement and maneuvering thrust through the use of powered propeller thrusters that generate a significant audio presence in the vicinity of the vessel. This signature impacts the ability to observe marine life in a natural setting as the presence of the vehicle itself becomes disruptive, frightening animals into flight or hiding. In addition to research and commercial uses, this situation also impacts many military vessel designs that rely on stealthy progress underwater for mission success.

Recent developments have resulted in other forms of submerged propulsion such as the magnetohydrodynamic propulsion seen in the Red October film – a system being tested for surface propulsion as well. Drawbacks to this system include a significant magnetic field that may be as detectable in its own manner as the propeller sounds of traditional propulsion mechanisms, as well as ionization traces following its passage. Other factors include the use and maintenance of high-potential or superconductive magnets that generate their own detectable signatures and difficulties as well.

One alternative would utilize a vessel’s buoyancy in the same manner as a sailplane uses its height in order to develop forward movement, only more efficiently as a submerged vessel may use both positive and negative buoyancy in order to derive a continuous motion without the need to ‘land’. Borrowing from the method fish use in order to rise and fall in depth at will, a vehicle might utilize the mechanical equivalent of a swim bladder instead of a vented ballast system and further reduce the acoustic signature presented while conserving air for life support alone.

INTRODUCTION

Aristotle chronicled the first known use of a submersible in 322 BC. Since that time, many improvements have been made over the earliest crude diving bells. Simon Lake worked out the use of negative buoyancy for submergence of a submarine, prior to his death in 1945. Propulsion of submarines and submersible craft has taken many forms from early steam and then gasoline power to modern nuclear systems.

Most submersible forms that enjoy self-propulsive capability rely upon a mechanism or motor that in turn rotates a shaft penetrating the hull. To this shaft is affixed a screw-like propeller which, by turning its vanes, provides motive force to the vessel as it moves through the water. Control of submerged vehicles is managed through the use of diving planes and other control surfaces or using smaller propeller thrusters similar to those used for primary propulsion.

Magnetohydrodynamic propulsion systems are currently under development as well. These prop-less systems generate thrust through the use of powerful electromagnets that generate a stream of water that is then forced out the rear of the vessel to provide motive force. Their operation generates a significant magnetic signature and a trail of ionization after the vessel’s passage, while requiring support of the powerful magnets and equipment to provide the synchronization of high amperage power used in this system.

Submersibles are used for a variety of tasks including military, commercial, recreation, and research tasks. For both military and research tasks, reduction of the acoustic and other identifiable signatures produced by the operation of a submerged vessel is highly desirable. Stealth allows the observation of marine life in its natural state, while loud prop-noise or magnetic fields may disrupt the environment and prevent effective observation.

A system that provides enhanced stealth is therefore highly desirable for many purposes. One such system utilizes the same principles as a sailplane which does not have a primary mechanism for propulsion, but trades its height above the ground in return for forward movement due to the lift provided by the passage of air over its wings. Unlike the sailplane, which may only descend until it reaches the ground (ignoring for now the use of updrafts for additional time aloft), a submersible might use both positive and negative buoyancy in order to trade for forward motive force.

DESIGN

Due to the density of water, wing structures must be significantly smaller for a submersible than their aeroplane counterparts. Traditional lifting-wing structures are unnecessary for a submerged vessel using this form of propulsion, as a flat surface which resists direct ascent and descent is all that is necessary. Many children demonstrate this principle when playing in the pool with a buoyant foam kick-board. Sinking the board and then releasing it will cause it to rise at a rapid rate. If this rise is at an angle to the vertical, the board will move a significant distance as it rises due to the flat surface of the board resisting a direct ascent in favor of the more efficient rise at an angle.

Many underwater vessels use positive and negative buoyancy in order to submerge and rise to the surface by varying the amount of air and water present in their ballast tanks. Some vessels, such as the Trieste, carry heavy weights that are dropped in order to surface at the end of their excursion while others use ballast tanks in order to vary their depth more finely. Water is allowed into these tanks in order to displace less water than the vessel’s weight, allowing the vessel to sink. In order to rise, high-pressure air drives water out of the tanks, increasing the displacement of the vessel and causing it to rise once again. The release of this high-pressure air generates a detectable acoustic signature, degrading the stealthy capabilities of the vessel.

Many types of fish adjust their depth through the use of an internal swim bladder that provides varying displacement of water and alters the depth at which the fish may balance. Through the use of a mechanism similar to this, a vessel may vary its own buoyancy and depth.

Two forms of variable-buoyancy systems exist, one that relies upon a variable geometry configuration in order to provide the necessary change in displacement. The second form uses a simple pumping mechanism in order to move a liquid from an internal reservoir to an external flexible membrane in order to provide the change. Other alternatives might include fluids and waxes that change state easily when heated ot reduce complexity by eliminating the pump in favor of a heating element.

Both methods produce similar variance of displacement and therefore buoyancy of the vessel to which they are affixed. The variable geometry system may prove more susceptible to leakage than the sealed fluid-membrane system, while the fluid-membrane system may produce unacceptable levels of noise if the pumping mechanism is not acoustically shielded.

Reducing noise generated by propeller propulsion systems will allow submersible vehicles to interact with marine life with a lessened impact on the environment. Additional modifications may also be made which would render such vessels even less intrusive, such as the use of digitized natural biological sounds for acoustic range-finding equipment. The clicks of a dolphin or whale might provide sufficient acoustic illumination in situations where traditional ping-style active sonar might prove highly undesirable.

Submersible vehicles enjoying enhanced stealth through reduction of acoustic and magnetic signatures have many potential uses for marine research and military use. Vessels using buoyancy-derived propulsion systems may provide an efficient alternative to conventional propeller or magnetic systems, while gaining the advantage of being able to more fully mimic natural marine fauna and providing still lessened environmental disruption.

A vehicle utilizing a variable-geometry or fluid-membrane technique might sequentially gain the positive and negative buoyancy necessary in order to allow this prop-less form of propulsion to function without the need for high-pressure air systems and the noise these generate during operation. Air may then be used for life support and other functions alone, freeing up vital space within the vessel for human and equipment payloads.

Biomimetic designs borrowing from nature carry the advantages derived from many thousands of years of optimization. These designs allow us to improve our mechanisms and move within foreign environments with a significantly reduced presence. Becoming less intrusive may let us observe the natural marine ecology without having to be the ‘noisy neighbor’ frightening everything into hiding when we pass.

ORIGINAL POST: July 2000 on my http://khausman.wolf-song.com/ site