Every year, the U.S. Navy and Marine Corps conduct thousands of Maritime Interdiction Operations (MIOs) to enforce embargoes, intercept contraband, prevent drug and human smuggling, and fight piracy. These operations are usually conducted by Visit, Board, Search, and Seizure (VBSS) teams using rigid-hull inflatable boats (RHIBs) or helicopters. Key performance parameters were developed for a portable, throwable robot that can best support their missions. This robot can be used for advanced reconnaissance as the team is about to board a target vessel, to assist in compartment clearing, and for inspection of flooded compartments and bilges.

Figure 1. The Stingray prototype wearing a high-visibility Sling Flotation Device (SFD). The SFD is similar to a personal flotation device in color (fluorescent yellow) and material (closed-cell foam).

Subsequent user tests and demonstrations have revealed that its applicability is much wider than originally thought. The same characteristics critical to VBSS operations also make the system a useful tool for land-based tactical operations, especially for missions involving streams and culverts.

Design guidelines for a VBSS tactical robot were converted to explicit performance thresholds and objectives that required considerable research and development. Two prototype systems, each consisting of an operator control unit (OCU) and two amphibious Stingray robots, were developed. The areas that were the most challenging in the design of these robots include:

  1. Weight threshold
  2. Maximum volumetric envelope
  3. Flotation in seawater
  4. Mobility in water
  5. Traction on wet, oily surfaces
  6. Impact resistance

The maximum weight ceiling of 1.8 kg, when coupled to the other performance requirements, was a major challenge. The resulting design was a woven carbon-fiber monolithic chassis coupled to aircraft-grade aluminum sides and hardware, woven carbon-fiber wheels and internal brackets, and closed-cell foam for flotation purposes.

Figure 2. The micro-knobby paddle wheels performed the best overall, and were chosen as the final Stingray tread design.

The 4500-cm3 maximum volumetric envelope for the Stingray was determined by the requirement to fit in a Modular Lightweight Load-carrying Equipment (MOLLE) pouch. It had repercussions in terms of the wheelbase, width, and wheel diameter for the UGV, given that the wheels are the most prominent physical features of the robot. For practical purposes, the wheel diameter was dictated by the requirement to be able to cross a 5-cm-tall obstacle (i.e. the wheel diameter had to be approximately 10 cm to allow the wheel to climb over the 5-cm obstacle), and the width was mandated by the dimensions of the largest non-modifiable electronic component, which was the battery pack. As a result, the only free dimension was the overall length, which was set at 10 inches to provide an adequate amount of air inside the sealed UGV chassis for flotation, as well as to provide extra stability and better obstacle-climbing capabilities.

The Stingray had to float when immersed in seawater, both for recovery options and for operational reasons (capability of crossing standing water). Therefore, a passive, positively buoyant robot design was selected to accomplish both objectives. Instead of driving on the floor of a flooded space, the robot became a hybrid vehicle that can drive on the water surface as well as on land.

In order to achieve the desired results, the design team used a two-pronged approach where the UGV itself would be as buoyant as possible through the integration of custom-designed floats in the wheels, and the maximization of the internal volume of the UGV chassis (without sacrificing ground clearance), coupled with the custom development of a high-visibility Sling Flotation Device (SFD) that would be wrapped around the UGV when an in-water operating environment was expected, but would not impede ground operation. The SFD would be similar to a personal flotation device in color (fluorescent yellow) and material (closed-cell foam), with openings to accommodate the camera and the multipurpose high-intensity LEDs. Initial calculations showed that the system would have a density of 0.85 in saltwater, yielding a positive buoyancy of approximately 15%.

Once the UGV design had achieved the required goals of weight and buoyancy, the next challenge was to design a system that would be capable of mobility in the water. The original wheel design was a good starting point since the horizontal deep treads, initially designed mainly for traction and impact absorbance, had shown the ability to perform as rudimentary paddle wheels.

The requirement for Stingray traction specified that the UGV should be able to achieve sufficient traction on wet, oily metal surfaces up to sea states 5 (rough). To determine the performance of the Stingray in the operational environment, a test rig was created where a steel surface was left bare on one side and painted on the other (to simulate both scenarios) and wetted with oily water (to simulate conditions often found on a ship deck). The tread that performed the best overall was a microknobby design, which was chosen as the final Stingray tread design and implemented in the prototype units.

The capability of surviving 5-meter (threshold) and 10-meter (objective) drops onto a steel deck was one of the key requirements for the Stingray. Finite element analysis (FEA) was performed on all components and on the overall system, with safety factors always in excess of 10, which allowed the system to pass both the threshold and objective requirements once built.

This work was done by Hoa G. Nguyen of SPAWAR Systems Center Pacific and Cino Robin Castelli of Macro USA Corp. SPAWAR-0003

Aerospace & Defense Technology Magazine

This article first appeared in the May, 2015 issue of Aerospace & Defense Technology Magazine.

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