Limited by the Sun

The solar energy that hits a square foot of panels on a solar plane in an hour is tiny compared to the energy in a gallon of jet fuel. Solar panels also convert less solar power -- between 10 and 20 percent -- to electrical power for turning the propellers, compared to the amount of power combustion in a jet engine applies to thrusting a jet forward. In the end, one square foot (0.09 square meter) of solar panels yields three to six times less power than you need to light a 60-watt light bulb, so you can see why engineers paper the plane with panels and try to keep it light [source: Del Frate].

Solar versus Traditional Airplanes

As mentioned earlier, solar airplanes are mostly surveillance craft. Boeing's 747-400, on the other hand, flies from Detroit to Tokyo, carrying hundreds of passengers on decent fuel mileage. And the F-22A Raptor, by contrast, is a fighter plane for the U.S. Air Force. It's designed to be fast, agile, quiet and almost invisible. These are the basic differences. Let's put these planes head-to-head, or wing-to-wing to find out even more.

Many solar planes are shaped like flying rulers. NASA's Helios plane, for instance, has a 247-foot (75-meter) wingspan but is only 12 feet (3.7 meters) long [source: NASA]. A 747's wingspan is shorter, at 211 feet (64.3 meters), and its fuselage is about the length of its wingspan [source: Boeing]. The F-22A Raptor is a spade-shaped, stubby plane, 44.5 feet (13.6 meters) across the wings and 62 feet (18.9 meters) long [source: Lockheed Martin].

Compared to the other planes, solar planes are practically kites. Some are launched by hand with a running toss into the air. The Helios is too heavy for that. It weighs 2,048 pounds (929 kilograms) at most, made of pricey, light and strong materials -- and Styrofoam [source: NASA]. Amazingly, the whole plane bends. (More on that later.) A Raptor weighs a formidable 83,500 pounds (37,875 kilograms) and is most definitely not bendable. The 747 weighs up to 875,000 pounds (396,893 kilograms), including all the luggage in the cargo hold.

You'll find a lot of electric propellers -- up to 14 -- on a solar airplane, and that's all of its propulsion. Of course, electric propellers wilt next to jet engines. The Raptor's jet engines shoot it forward with 70,000 pounds (311,500 newtons) of total thrust [source: Lockheed Martin]. A 747's two engines move it with up to 126,600 pounds of total thrust (563,145 newtons) [source: Boeing].

You wouldn't be surprised by which one would win in a race. While the environment smiles on pollution-free solar planes, the gods of speed do not. When cruising at low altitudes, the Helios travels no more than 27 miles per hour (43.5 kilometers per hour) [source: NASA]. A 747 cruises at 567 miles per hour (913 kilometers per hour), and the Raptor can reach close to Mach 2 [source: Boeing, Lockheed Martin].

The Raptor also wins on maneuverability. While the 747 and the Helios can turn, pitch and change their speeds, the Raptor can fly a spinning loop-the-loop.

So far, the 747 wins on distance. The farthest flight for a solar plane has been 163 miles (262 kilometers). The Raptor's maximum range is 1,841 miles (2,963 kilometers), while the 747 can fly 8,355 miles (13,446 kilometers) [source: Lockheed Martin, Boeing].

Solar planes win in a category you probably haven't considered -- longevity. Jets must land to refuel. Solar planes don't have to. They can stay aloft as long as their batteries are charged to get them through the night. By staying aloft for more than three days, solar planes have already surpassed jets, and many solar-plane makers share the goal of months to years.

Now that you know how a solar plane stacks up against other aircraft, let's take a closer look at its design.