Every now and then something new comes along that causes you to want to dig down and find out a little more information. Tearing my attention away from Caitlin Jenner for a moment, I thought I'd take a closer look at the remarkable Solar Impulse 2 – an airplane powered solely by solar energy.
Hmm.... solely solar... alert an alliteration aficionado. But I digress.
At any rate, the SI 2 is engaged on a round-the-world trip, temporarily interrupted in June when the plane was stranded in Hawaii after suffering irreversible battery damage caused by overheating during the five-day flight from Japan. Assuming the money can be found to repair the damage and sustain the support team over the winter (€20M – a rounding error for The Donald, surely?) it will resume in April 2016.
While we wait, let's check out some of the technology that enables a human to traverse oceans aided by the sun, while avoiding the soggy fate of legendary aviatorIcarus.
Design: Including the Solar Impulse 1 prototype, the project has taken 50 engineers and technicians and 80 technology partners over 12 years from feasibility study to concept, design and construction. For the latter stages of the project. there are also 22 mission controllers. The principal partners include Solvay, Schindler, Omega and ABB.
Look Ma, it works! Solar Impulse 2 leaving Oman (source: Solar Impulse)
Construction: In order to maximize the aerodynamic performance, the plane has a wingspan of 236ft - greater than that of a Boeing 747 and only 3 feet less than the Airbus A380, the world’s largest passenger plane.
The airframe is made of composite materials: carbon fibre and honeycomb sandwich. The upper wing surface is covered by a skin consisting of encapsulated solar cells, and the lower surface by a high-strength, flexible skin. 140 carbon-fiber ribs spaced at 18-inch intervals give the wing its aerodynamic cross-section, and also maintain its rigidity. The whole plane weighs 5070 lbs., only about 100 lbs. more than a Chevy 1500 cargo van. Go (here to see an infographic about the structure.
Propulsion: Four sensorless BLDC motors, each generating 17.4 hp (13.5 kW), are mounted below the wings; they are fitted with a reduction gear limiting the rotation speed of the 13-ft diameter, two-bladed propeller to 525 rpm. The entire system is 94% efficient, setting a record for energy efficiency. Each engine's average power output over a 24-hour period is 15hp, about the same as a small outboard motor.
Such modest power gets the job done, albeit very slowly - Solar Impulse flies between 20 (23 mph) and 77 kts (88mph). This varies with altitude: at sea level, the minimum and maximum speeds are 20 kts and 49 kts (56 mph) respectively; at maximum altitude that increases to a minimum of 31.5 kts (36 mph) and a maximum of 77 kts.
Solar cells: The Solar Impulse 2 uses 17,248 monocrystalline silicon photovoltaic cells, each 135 microns thick, mounted on the wings, fuselage and horizontal tailplane, providing the best compromise between lightness, flexibility and conversion efficiency (23%). The cells take up an area of 2900 ft2 and can produce up to 340kWh of energy per day.
Batteries: The energy collected by the solar cells is stored in lithium polymer (soft-pack batteries), manufactured by Air Energy (Germany) with specific energy of 260 Wh/kg. The batteries are insulated by high-density foam and mounted in the four engine nacelles, with a system to control charging thresholds and temperature. Their total mass amounts to 1395 lbs., or just over a quarter of the aircraft’s gross weight.
In order to save energy, the aircraft climbs to 28,000 ft during the day to capture the maximum amount of solar energy, and descends to 5000 ft at night where it operates at reduced speed.
Cockpit: The cockpit is 134 cubic feet in size – just enough for the pilot plus essential equipment such as a parachute, life raft, and 6 oxygen bottles since it's unpressurized and unheated, with temperatures during flight ranging from -40oC to +40oC. There's also a fully-reclining seat for a cat nap or two; during a typical 24-hour flight cycle, the pilot rests 8 times, averaging between 5 and 20 minutes per session.
The cockpit must also contain 5 lbs. of food, 3 quarts of water, and a quart of sports drink per for each day of flight – up to six days total. I know you were wondering about bio-breaks on such a long flight (admit it); let’s just say that gravity plays a key role in the process.
There's a comprehensive suite of instruments, as shown in figure 2: from left to right the panel contains fuses, throttle, power management, autopilot, motor control, flight instruments and communications. Although the plane has only a single seat, piloting the plane for days at a time with almost no sleep is too much for a single pilot, so there are actually two - Bertrand Piccard and André Borschberg - who take turns.
You can find out more about the Solar Impulse project by visiting their web site. In the meantime, there's the little matter of that €20 million shortfall.
How about it, Donald? You might tie up the geek demographic.