Solar Impulse 2 May Be an Exercise in Wishful Thinking

The Solar Impulse 2 may be an exercise in wishful thinking, or it might just be the next big step forward for aviation. Yesterday, like several hundred other media outlets, this publication covered the announcement of a planned round the world flight by the Solar Impulse team. Several of those news organizations were reporting that this was going to be a non-stop flight. Fox News actually stated the trip would take five consecutive days.

Please do the math. The Solar Impulse 2 has an average cruising speed of 43 mph. The Earth’s circumference is approximately 26,366, depending upon exactly which route is being used, because the planet is not a perfect sphere. Dividing the circumference of the earth by the speed of the aircraft, and it turns out that a single circumnavigation of the Earth in the Solar Impulse 2 would take 613 hours or 25 days to complete…but the aircraft may not be able to carry enough supplies to sustain the pilot for the duration of the flight.

With an empty curb weight of 4,600 pounds, and a projected maximum takeoff weight of around 5,500, the SI2 has a maximum payload weight of approximately 900 pounds, which has to cover the weight of the pilot and the pilot’s life support systems, including food, water, and air.

Water needs differ on the basis of size and weight. A 2oo pound person needs approximately 100 ounces of water per day, if sedentary, at sea level, and up to twice that amount if strenuously active. That number doubles, however, at higher altitudes. At 27,000 feet, the pilot of the Solar Impulse 2 would need a minimum of 200 ounces of water a day to avoid high altitude sickness or pulmonary edema. That works out to 39 gallons of water for a projected 25 day journey. That amount of water weighs in at 325 pounds.

Food is a different matter. Humans can live on very little food for very long periods of time, but their functionality decreases along with their caloric intake. There is a piece of folk wisdom that human beings eat half their body weight in the course of a month. A 200 pound person might, then, be expected to consume 100 pounds of food in the course of a month or around 83 pounds of food over a 25 day period. In the real world, a British hospital conducted a study indicating that, on average, people consume around 1,277 grams of food per day. Over a 25 day period, that would work out to 319.25 kg or 702 pounds of food. In this case, the real world trumps folk wisdom.

Between food and water, then, the Solar Impulse pilot would need up to 1,037 pounds of supplies to remain aloft for 25 days. Throw in the pilot’s hypothetical body weight of 200 pounds, and you get a payload requirement of 1,237 pounds, 237 pounds over the maximum payload capacity of the aircraft….and that is before you put any air on board the plane.

At sea level, air it is free for the taking and most people take it for granted. At 18,000 feet, however, humans are standing at death’s doorstep. At 27,000 feet (the operational ceiling for this aircraft), visitors have stepped over that threshold, into a territory only mountain climbers frequent, where no one survives without highly specialized acclimation and access to bottled air.

If the Solar Impulse pilot is going to hang around above 18,000 for any length of time, that pilot is going to need compressed air or oxygen. NASA estimates that an average human being needs .84 Kg of oxygen per day or 21 Kg (42 pounds) of air for a 25 days mission…but humans do not live on oxygen alone. In fact, breathing pure oxygen for more than 24 hours can actually cause pulmonary edema and can be fatal.

The oxygen must be mixed with an inert gas to create the volume of gas the lungs need to operate. Oxygen is 21% of the gas mixture at sea level, so 168 pounds of inert gas are needed along with the 42 pounds of oxygen for a total of 210 pounds of the breathing mixture, which is equal to 260 cubic feet of gas. The gas cylinder for the compressed air adds another 160 pounds, with the air regulator included for a total of 370 additional pounds. This brings the total payload requirement for this mission up to as much as 1,607 pounds, 707 pounds over the weight limit. Even if the lower food weight of 83 pounds is used, the payload still exceeds the plane’s capacity by 88 pounds.

The Solar Impulse could fly at lower altitudes, and reduce the amount of air and water needed for the pilot, but this is a very fragile aircraft and, as such, it needs to avoid air turbulence as much as possible. With a cruising speed of 43 mph, the SI2 cannot outrun bad weather, so it has to maintain the highest possible altitude to go over the weather it can’t outrun.

More importantly, the SI2 must fly as high as possible as long as possible during the daylight hours so that the pilot can “feather” his propeller and use the plane’s glider-like characteristics to maintain headway while the solar cells recharge the aircraft’s batteries for the next night’s flight. This finally brings us to the biggest problem facing the Solar Impulse team: endurance.

Endurance is the amount of time a solar-powered aircraft can remain aloft. The benchmark that aircraft designers are shooting for is 48 hours, which is considered the threshold for perpetual flight. The theory is that, if an aircraft can remain aloft for two daily cycles, then it has demonstrated the ability completely recharge its batteries in time to fly through the night….and remain aloft indefinitely.

In order to cross the Atlantic, the SI2 must traverse anywhere from 3150 to 3700 miles of open ocean (depending on the route) which, at 43 mph, will take anywhere from 73 to 89 hours around 80 hours. Solar Impulse 2 may be an exercise in wishful thinking, or the next big leap forward in aviation, but the SI flight team won’t know which until the plane completes flight testing.

By Alan M. Milner