Part Two of a Three-Part Series

By Gregory Quinn, Special to the Port Authority

Imagine energy reserves increasing during flight! To make this dream a reality, we had to make maximum use of every single watt supplied by the sun and store it in our batteries. We tracked down every possible source of energy efficiency. Today, Solar Impulse is the first solar airplane flying through night and day, the first aircraft to come close to perpetual flight.
–André Borschberg, from http://www.solarimpulse.com

  Solar Impulse 2, which is vying to be first sun-powered plane to circle the globe, represents the culmination of 12 years of intense study, design, preparation and construction.

Hundreds of scientists, engineers and technicians have labored for untold hours toward one ultimate goal: Creating  an aircraft that can fly for five consecutive nights and days without using any fuel. Only such an airplane could dream of solar-powered circumnavigation, with enough reserve power to cross Earth’s vast oceans.

The genesis of Solar Impulse 2 (SI2) traces back to 2003 and the design and construction of its predecessor, Solar Impulse 1. That plane, which itself made history in 2013 when it landed at John F. Kennedy International Airport to complete the first transcontinental solar flight, served as the prototype for SI2.  For eight years, teams of workers led by Swiss aeronauts Andre Borschberg and Bertrand Piccard, designed, built and tested the original Solar Impulse 1.

Once that craft proved capable of extended solar flight, work began on the development of the second plane, one that would have an even larger power reserve capable of ocean crossings. Construction on SI2 began in 2011; she flew her maiden test flight in 2014, before embarking on her historic journey in March. Borschberg, Piccard and their team spent years solving the basic riddle of power retention:  How to build a plane that stored enough battery power to traverse the Pacific Ocean, but still aerodynamic enough to fly.  It was a riddle no one had ever answered, much less put into practice.

At the risk of grossly oversimplifying the process, the answer lies in the wingspan of the Solar Impulse, or more accurately, it’s preposterously gigantic wing.

The SI2’s wingspan reaches a staggering 236 feet. That’s longer than five school buses lined end-to–end! The Airbus A380, the largest passenger airliner in the world, has a wingspan only 20 feet longer than the SI2.  But while the A380 weighs 500 tons, the SI2 is a lean 2.3 tons, a little less than your garden-variety SUV.

The oversized wing made of superlight carbon fiber allows the SI2 to fly at much slower speeds than a fuel-powered jetliner (a cruising speed of about 90 km/h for the SI 2 compared to around 900 km/h for a standard commercial aircraft). The slower speed enables the craft to rely on reduced battery power (particularly at night, where it slows to a crawl at 60 km/h) and thus haul less battery weight. And that, in a very simplified nutshell, is how the Solar Impulse 2 is able to undertake the world’s first solar-powered circumnavigation.

Of course, it’s not just the extended wing that makes the Solar Impulse 2 soar; the energy cells onboard are nothing short of scientific marvels. The four electric motors are powered by 17,000 solar cells; during daytime hours, the cells recharge lithium batteries with up to 340kWh (kilowatt hours) enabling the plane to fly at night with virtual unlimited autonomy —as the designers attest.

All the energy collected each day is stored in lithium polymer batteries insulated by high-density foam and stored in four engine nacelles (those large, cylindrical things you see mounted to the bottom of the wings on commercial jets).  All told, the cumulative weight of the batteries takes up just over a quarter of the SI2’s total mass.

Once the science of the SI2 was built and perfected, the logistics of the flight—particularly the burden being placed on the pilots—had to be addressed. After all, it’s one thing to build a plane theoretically capable of circumnavigation, and another thing entirely to find pilots capable of flying the craft.

And while the SI2 is being flown by two of the most talented pilots on Earth—Borschberg and Piccard—accommodations still had to be implemented to make the flight possible. The SI2 features advanced avionics—autopilot—allowing for longer trips between stops, as well as supplemental oxygen and other environmental support systems for a cruising altitude of 12,000 meters or about 33,000 feet.  Additionally, the two pilots are alternating legs of the trip to minimize wear-and-tear on their bodies, and they are followed on their journey by a 60-person support team.

Solar Impulse 2 is currently holding in Nagoya, Japan, its journey from Asia to Hawaii temporarily interrupted due to weather concerns. There, Borschberg and Piccard, along with their roving team, await optimal weather and wind conditions.  Once that occurs, the SI2 will be back in the air, heading east toward JFK, before crossing the Atlantic Ocean and returning to Abu Dhabi, where the flight began and then into the annals of aviation history.

Tomorrow:  The Visionary Men Behind Solar Flight — Borschberg and Piccard