Why is Si2 such a Special Airplane?


We get asked this question a lot, now that Solar Impulse has completed the first round-the-world solar flight, 40'000km without fuel! If this airplane has achieved a first for energy as well as for aviation it is because it’s technology is unique. Let’s dig deeper!

Lit by ultralight LEDs, covered with 17'000 solar cells, the width of a 747 and the weight of a car…

For some for sure it’s Solar Impulse’s elegance that makes it stand out from other airplanes at first sight. Its whiteness, diaphanous skin, huge wings, smooth lines, and utter silence as it rises in the sky. You may find it looks more like a gigantic bird than a flying machine. And being one of a kind has allowed it to become the “first” in many regards:

“The first airplane to have the wingspan of a Boeing 747 and the weight of a family car. The first to fly for several days and night non-stop without a drop of fuel. The first solar plane to cross the Pacific and Atlantic, the two biggest oceans in the world. The first to fly around the world without fuel!” André Borschberg

When you dig deeper, you realize that Si2 is not only special on the surface, but also on the inside. Under its beautiful appearance lies a complex mind, made up of a wide array of clean technologies cleverly assembled by our engineering team led by André Borschberg. Here’s a selection

  • 1. Four energy-efficient electric engines improved with an additive by Solvay which decreases friction and thus allows an energy efficiency of 97% compared to 30% for normal thermal motors
  • 2. Four energy-dense batteries upgraded with a special binder by Solvay which reduces their weight while increasing their energy density and the number of charge and discharge cycles they can withstand
  • 3. 17,248 ultralight efficient solar cells made by Sunpower that convert solar energy into electricity with an efficiency of 22.7%, compared to 16% for regular cells. They’re 135 microns thick, like a human hair, which makes them ultralight
A cool 3D/VR model by our friends from Sketchfab
  • 4. A protective transparent resin developed by Solvay which covers the solar cells and protects them from harsh weather. It is UV resistant, waterproof and only 17 microns thin, so barely weighs anything.
  • 5. Intense and lightweight LEDs which have an incredible “watt to weight” ratio and are used to illuminate the landing area brilliantly at night. They are protected by the same resilient plastic found in Omega’s watches
  • 6. Ultra-lightweight high-density thermal insulation foam provided by Solvay and Covestro to insulate the cockpit and gondolas and thus protect the pilot and batteries from extreme temperatures. The foam’s pores are 40% smaller than usual, rendering it more rigid and strong while keeping it lightweight
  • Smart energy dispatcher systems developed by Omega which optimize the energy use on the plane and make use of bi-directional functionality to ensure that either battery can support the other on the same wing in the event that one engine fails
  • Composite materials, such as carbon fiber, engineered by Solvay, North TPT and Decision to lighten many parts of the plane. The carbon “bee-nest” structure used to build the spar for instance reduced the weight of a layer of carbon material from 80 grams per m² to 25 gram per m² (3 times lighter than paper!)
Solar Impulse arrives in Hawaii after 117h52 of flight without fuel! A first for energy thanks to clean technologies!

But Si2 is more than an airplane.

It is a message to encourage people to use existing clean technologies on the ground to ensure a cleaner future. With the technologies we have on this airplane, we could already cut our energy consumption, and thus polluting emissions, by two. Not to mention that they will create jobs and profit for developed and developing countries.

We want as many people to hear and spread the message: let’s improve the quality of life of present and future generations without sacrificing our comfort!

“Solar Impulse is more than an airplane, it is a message to encourage people to use existing clean technologies on the ground to ensure a cleaner future. With them, we could already cut our energy consumption, and thus polluting emissions, by two.” Bertrand Piccard

Over the years, Si2 has also become a friend with whom we’ve shared moments of happiness, sadness, anxiety, and so many adventures. And soon, a friend with whom we’ll have accomplished Bertrand Piccard’s dream of flying around the world with no fuel. Thank you dear companion for this hell of a ride!

The #FUTUREISCLEAN hashtag on the cockpit, to spread the word about clean technologies

And then what? Well, after having had to deal with all of our acronyms: Si2, MCC, PNR, RTW… here’s a new one you’ll soon have to get used to: ICCT.


Written by

Two pioneers flying around the world with a solar airplane to promote clean technologies #futureisclean ! @bertrandpiccard @andreborschberg

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