Posted Nov 28, 2022 2:47 PM
Chasing the dream of Icarus, without burning your wings… For eight years, the Swiss explorer Raphaël Domjan has been preparing meticulously for the extraordinary expedition which will take him to familiarity with space at the controls of an airplane powered exclusively by solar energy. “Dreaming is what advances humanity. With this record flight, I want to demonstrate the extreme potential that can be drawn from this resource”, he defends, supporting figures. “Our societies burn 0.3 trillion kWh every day when the sun brings us 1,000 every day. Taming it is imperative for the survival of our civilization,” he argues.
Seen from the outside, the SolarStratos prototype that he designed with a team of fifteen engineers and scientists to push back the technological limits of aviation looks like a glider, only bigger: 25 meters wingspan, an ovoid fuselage to reduce aerodynamic drag, a cockpit for two, covered with a one-piece Plexiglas cover… Only a propeller and the two wings covered with 24 square meters of solar cells betray its prototype status.
More than 16 km of altitude
SolarStratos is a concentrate of innovations. “It’s a plane for science”, describes the explorer. Head for the stratosphere, a deep layer of our atmosphere where only rockets and weather balloons circulate at an altitude of between 16 and 50 km above our heads. Air and pressure are scarce there. Beyond is the universe of meteors and the vacuum of space. “This hostile environment requires technical responses that combine aeronautical and aerospace disciplines,” explains engineer Calin Gologan, the designer of the device at the head of Elektra Solar, a spin-off from the German Aerospace Center which made a name for itself with the design of a solar-powered single-seater in 2011.
Raphaël Domjan’s team does not count the number of technical challenges to be met. Chief among them is the flutter of the structure, what engineers call “flutter”. “This is a dangerous phenomenon encountered when flexible materials are subjected to significant aerodynamic stresses. With increasing wind speed on the wings, such tension can occur that the stiffness needed to hold them together is insufficient to compensate for the vibrations. The structure then enters into resonance. The intensity of the spasms increases and it is the rupture,” explains Calin Gologan. It is the same vibratory phenomenon which can surprise motorcyclists and which had precipitated in water a detachment of 74 soldiers marching in quick step on the suspension bridge of Broughton in 1831.
Vibration risks
At the altitude targeted by Raphaël Domjan, the risk is high because the amount of air available will not provide enough lift to allow his plane to stay in flight. It will therefore have to compensate for the lack of lift with a higher speed by blocking the speedometer at 160 km/h, twice the speed necessary at sea level. Endless calculations were therefore necessary to find the right balance between the rigidity of the carbon fiber structure and its lightness (the assembly should not exceed 500 kg in the long term). This fall, two flights validated the data taken from the Ground Vibration Test carried out by the National Office for Aerospace Studies and Research (Onera). “Objectives achieved”, concluded test pilot Gérald Ducoin.
The other challenge is that of the extreme temperatures, around minus 60 degrees, to which the equipment and the pilot will be subjected. A pressurized space suit, also powered by solar energy, is provided for the explorer. But to protect the electronics, the team still has to design systems capable of compensating for the absence of humidity necessary for their thermal regulation. “Neither too hot nor too cold. Still a delicate balance to find”, sighs Roland Loos, director of the project.
Seven hour flight
A hundred hours of test flight have already been performed during 68 outings. “We are clearing uncharted territory which forces us to move forward step by step, solving each problem as it arises”, explains Raphaël Domjan. In July 2018, for example, the plane suffered major damage during a resistance test on land, forcing the team to rethink the resistance of the structure in extreme flight situations.
After these adjustments, the aircraft should test its solidity at an altitude of 10,000 meters next year, the tropospheric layer of the atmosphere in which airliners circulate. This test will be decisive for programming the first stratospheric flight the following year. The mission will last approximately 7 hours: 3 to 4 hours to reach the required altitude, 15 minutes head in the stars, then 3 hours to return to earth.
Recreational aviation
Past the dense layer of the atmosphere that it will have to cross using batteries, the plane will have more energy than necessary to propel itself: thanks to the cold which increases the efficiency of the photovoltaic cells, and to the slightest filtering the sun’s rays, the panels will benefit from 30% more resources compared to sea level, unlike thermal planes which lose their efficiency at altitude as the oxygen necessary for fuel combustion decreases.
The propeller will also provide energy in the manner of electric cars when they brake. “The energy efficiency of SolarStratos is 95%, compared to 25% for fossil fuel engines”, compares the explorer.
To demonstrate this potential for future aviation, the expedition will require a total of 10 million euros, three times less than the PlanetSolar ship he piloted around the world in 2012, and barely 6% of the budget. brought together by Solar Impulse, Bertrand Piccard’s solar plane. No aircraft manufacturer is among its partners. But Raphaël Domjan already has his idea to promote the work of his team: leisure aviation. In France alone, the activity mobilizes 2,300 planes and 40,000 pilots.
Performance race
World record broken for the conversion efficiency of a solar cell into silicon. The feat was announced last week by the Chinese module manufacturer Longi, which presented a result of 26.81%, 0.1 point higher than the previous record. Even minimal, this performance maintains a race on which depends the speed of the rate of penetration of solar energy in the most demanding applications. The best cells today show a yield much lower than this score, around 20%. In addition to the progress of silicon-based cells, manufacturers are impatiently observing the work of scientists on perovskites, a new type of hybrid cell, half-organic, half-inorganic, and much cheaper. In the lab, they achieved 24.5% efficiency for over 2,000 hours with less than 1% loss in efficiency when subjected to continuous exposure to light at 60°C and 70% humidity. . Another record, longevity this time, for a yield of this level.
50 years of solar conquest
1974 The Sunrise 1 inaugurates the first solar-powered flight over a salt lake in California. It rotates for 20 minutes, without a pilot, at an altitude of 100 meters using 4,096 cells on its wing.
1978 Solar One equips a glider of the time with solar propulsion. He struggles to take off.
1980 The Gossamer Penguin takes its creator’s 13-year-old son on board to make the first manned solar flight. It has a wingspan of 22 meters and weighs only 31 kg.
1990 The Sunseeker 1 electric glider crossed the United States from east to west in 21 stages.
2001 NASA sends its Helios prototype 32 km above the Pacific. It has a wingspan of over 82 meters and is 180 m2 of solar panels. It crashed in 2003.
2009 The Sunseeker 2 crossed the Alps.
2015 Solar Impulse 2, a modernized version of an initial prototype designed five years earlier, takes Bertrand Piccard on a world tour in 17 stages.
2017 SolarStratos performs its first maiden flight in Switzerland.
