Solar Impulse set to soar on U.S. tour (pictures)

The Solar Impulse HB-SIA, a solar-powered, battery-equipped airplane capable of flying at night, is being prepped to fly across the United States in five legs beginning May 1. The plane is seen here in the hangar at Moffett Field in Mountain View, Calif., on March 27.

The average power available to the airplane is just about that of a typical scooter, meaning that the inventiveness and feats of engineering which allow this plane to fly are pretty incredible -- consider, for instance, its ability to fly day and night for more than 26 hours at a time.

In its first public appearance after being reassembled in the U.S. since arriving about a month ago from Switzerland, the record-setting airplane with a battery capacity equivalent to that of the Tesla Model S, the Solar Impulse is set to make a five-city tour of the U.S. this summer.

In a hangar yesterday at Moffett Field, Calif., near the nexus of Silicon Valley's big thinkers, Solar Impulse pilot Andre Borschberg says it is fitting that the plane will begin its cross-country journey here. The main goal of the cutting-edge airplane, he said, is to inspire others to embrace the spirit of exploration and to participate in the innovative and creative spirit which went into the project.

More than 80 companies partnered on the project to tackle the challenges of solar- and battery-powered flight, in areas including chemical, electrical, and materials technologies.

The Solar Impulse HB-SIA is an oversized machine in relation to its weight, with a wingspan of 63.4 meters and a weight of 1,600 kilos. The ultra-thin photovoltaic cells atop the wings are just 150 microns thick. Some 12,000 of the mono-crystalline silicons photovoltaic cells cover the wings.

The flight will carry just one pilot, but many messages, says Bertrand Piccard, chairman and initiator of the Solar Impulse project -- for instance, that people can innovate, use resources more wisely, respect the environment, and also be profitable.

As the aircraft travels cross-country, with stops in Phoenix, Dallas-Fort Worth, then either Atlanta or Nashville and then Washington D.C. and into New York, transmissions live from the plane and partnerships with school will allow people to speak directly to the pilot and ask questions of the team.

Piccard speaks alongside the Solar Impulse plane inside the hangar at Moffett Field.

Solar Impulse has already had several record-breaking flights in the solar airplane category, including the first intercontinental flight from Europe to Africa across the Mediterranean Sea, which also set a world record in distance at 1,116 kilometers.

On July 8, 2010, for the first time in history, Solar Impulse succeeded in flying day and night without fuel, powered for 26 hours by stored solar energy alone.

The next big goal for the project is to fly around the world in 2015.

The main constraint today in alternative fuel flying is in storing the energy -- the batteries. With an energy density of 200Wh/kg, the accumulators needed for night flight weigh 400 kilograms -- more than one-quarter of the plane's total mass.

Under the port-side wing, we see one of the engine nacelles for the propeller-driven aircraft.

Solar Impulse pilot Andre Borschberg says it is fitting that the team will begin its cross-country journey in Silicon Valley, as he says the main goal of cutting-edge airplane is to inspire others to embrace the spirit of exploration and participate in the innovative and creative spirit which went into the project.

Each motor has a maximum power of 10 horsepower, with a gearbox reducing the rotation of each 3.5-meter-diameter twin-blade propeller to 200 to 400 rpm.

Beneath the wings sit four nacelles, each containing an electric motor, a battery consisting of 70 accumulators and a management system controlling charge/discharge and temperature.

The thermal insulation has been designed to conserve the heat radiated by the batteries and to keep them functioning despite an average temperature of minus-40 degrees Celcius at 8,500 meters.

It would have been impossible to fly through the night as the Solar Impulse plane does without significant improvements on the weight of the plane. The plane flies at just about 40 mph, and a specialized low wing profile maximizes the aerodynamic performance.

Here's a good overview of the plane in the hangar at Moffett Field in Mountain View, Calif., on March 27, 2013.

Another angle on the Solar Impulse HB-SIA in the hangar.

It's a long stretch from the cockpit to the boxy tail assembly.

Another view of the Solar Impulse in the Moffett Field hangar.

In its current design, every square meter of solar cells provides only 28 watts of continuous power to each of the four propellers -- a miniscule amount, really, just about that of a light bulb -- during each 24 hour period.