Explorers in earlier centuries traveled the world in modes of transportation made of wood and metal. Today, however, journeying into the unknown requires vehicles built with more durable materials. When those vehicles include components made from composite materials, they gain the toughness, strength and resiliency necessary for groundbreaking expeditions. Composites are proving their ability to survive the harshest, most challenging environments deep below the ocean surface, at the coldest place on earth and even on another planet.
Exploring the Ocean Depths
Submersible vessels designed for deep sea exploration have to withstand tremendous pressure. At sea level, the air pressure is 14.5 pounds per square inch (psi); at 4,000 meters (about 2.5 miles) below the surface, the deep water exerts a pressure of 5,800 psi. Few vessels can withstand that force, but the Titan, a five-person submersible developed and operated by OceanGate, has traveled safely to that depth several times. The Titan’s strong pressure vessel, made of a CFRP cylindrical hull capped by titanium domes on each end, makes it possible.
OceanGate had used a metal pressure vessel for its earlier submersible, Cyclops 1. Switching to CFRP for Titan provided some important advantages. “Carbon fiber is three times better on a strength-to-buoyancy basis than titanium, and underwater that’s what you care about,” says Stockton Rush, CEO and co-founder of OceanGate. “Strength-to-weight is not the issue, but strength-to-buoyancy is.”
One problem with using carbon fiber was the lack of information about how it would react under deep sea conditions. Most testing on large CFRP pressure vessels, like gas tanks, has focused on tensile loads. OceanGate, however, needed to understand how such vessels would perform at the uniform external pressure they’d encounter undersea.
OceanGate initially worked with Boeing to develop a pressure vessel manufactured using automated fiber placement with prepreg windings. But Boeing’s software, developed to model FAA-certified materials, had to include off-axis plies, and that’s not what OceanGate needed. “In a pressure vessel, there’s no torsional forces, no off-axis; it’s all orthogonal. So if you want to orient your fibers to the loads, you really only need it in the hoop and the axial – around it and longways,” Rush explains.
The company then turned to Spencer Composites, which had previously built a CFRP hull for a project for Steve Fossett, the late aviator/adventurer. “Spencer had done a lot of work, pushing the envelope with the 0/90 degree lay-up, which is what we really wanted,” Rush says. Spencer’s approach – build, test, modify and build again – also appealed to OceanGate.