The Solar Impulse has made significant progress toward its aim of being the first solar-powered aircraft able to fly at night. The team initiated by famous balloonist Bertrand Piccard began construction of the 200-foot-wingpsan prototype just four weeks ago. Flight tests should start next year.

The design has been significantly changed since an earlier presentation in 2005. There are now four electric motors instead of two. “One motor would be better in terms of efficiency but two, as we were planning in 2005, and now four, allow for better weight distribution over the wing,” Solar Impulse CEO Andre Borschberg explained during a press conference here on Monday.

Another benefit is safety. A motor failure with a two-motor design would have made the aircraft too asymmetric to maintain control. It would thus have called for shutting down the second motor and gliding. With four motors, if one fails, the pilot can still shut down the opposite one and fly symmetrically, Borschberg said. The combined power of the four motors is 11 hp, close to that of the Wright Brothers’ Flyer in 1903.

The prototype will be rolled out next summer. It will help refine the final design. According to Borschberg, Solar Impulse’s second aircraft, which is intended to fly around the world in four or five legs, needs an estimated 260-foot wingspan. But this is close to a physical limit. So having a smaller wingspan would be appreciated, he explained. The Solar Impulse team is still hoping to limit the wingspan to, say, 230 feet. The round-the-world flight has already been postponed from 2010 to 2011.

The single-seat Solar Impulse will fly at an average speed of 38 knots. During the day, it will store energy from 2,700 sq ft of solar cells on the wing. Recent progress in electrical batteries is making the team optimistic but further advancement is needed, Borschberg said. Another way to store energy during the day will be to climb to high altitudes, allowing for night descents. A low sink rate has been a design driver. The Solar Impulse’s wing load is close to that of a hang-glider,  at 1.6 pounds per square foot.