Rolls-Royce Aerospace makes jet engines that power many of the world’s airplanes. While they are state of the art in terms of efficiency, they also leave a lot of greenhouse gas emissions in their wake.
The company has been a leader in developing electric propulsion systems for airplanes in order to help lead the world forward into a future of zero-emissions flight. Getting there will involve lots of innovations. The major hurdle is that batteries have a much lower energy density per unit of weight than jet fuel.
Let’s dig into that a bit. According to JetPack Aviation, a liter of jet fuel has an energy density of 9.6 kWh and weighs about 0.8 kilograms. That translates to 12 kWh per kilogram. In comparison, some of the best lithium-ion batteries have an energy density of 265 Wh per kilogram. The net result? Jet fuel has almost 50 times more energy available per kilogram to power an airplane than batteries do.
But the analysis doesn’t stop there. While jet fuel is energy dense, even the best combustion engines are not all that efficient at converting that energy into forward motion. JetPack says the inefficiency of internal combustion means that 1000 pounds (453.59 kg) of jet fuel yields only about 14 times more power than 1000 lb (453.59 kg) of batteries. A lot of that wasted energy goes out the back of the engine and into the atmosphere.
The net result is the “fuel load” for an electric airplane will be much greater than it would be for a conventional jet aircraft. In point of fact, cramming enough batteries into an airplane to make it fly leaves precious little carrying capacity for cargo and passengers, let alone a pilot.
Rolls-Royce Aerospace has been hard at work developing an electric airplane that can fly faster than 300 mph (that’s 483 km/h for those who insist on using the metric system), making it the fastest electric airplane ever built. This week, the finished plane made its first sustained flight. It was in the air for 15 minutes. I leave it to you to convert that to parsecs if you wish. And if you think 15 minutes isn’t very long, remember Wilbur and Orville Wright’s first powered flight at Kitty Hawk lasted a mere 12 seconds and that led to some pretty amazing things.
Rolls-Royce Aerospace says the test flight is “the beginning of an intensive flight testing phase in which we will be collecting valuable performance data on the aircraft’s electrical power and propulsion system,” according to Engadget. The company claims the single seat airplane has “the most power-dense battery pack every assembled for an aircraft,” but gives no specifics. It uses a 6,000 cell battery pack with a three-motor powertrain that currently delivers 400 kW (500+ horsepower).
The flight comes about a year after the originally scheduled takeoff and about six months after taxi trials. Rolls-Royce is developing an air taxi with aircraft manufacturer Tecnam with the goal of delivering an “all-electric passenger aircraft for the commuter market.” It has also worked with Siemens and Airbus on another e-plane concept.
The project is being funded by the Aerospace Technology Institute and the UK government as a preliminary step toward creating all-electric passenger planes. “This is not only about breaking a world record; the advanced battery and propulsion technology developed for this program has exciting applications for the Urban Air Mobility market and can help make ‘jet zero’ a reality,” says Rolls-Royce CEO Warren East.
Eviation has already created a production prototype of a 9 passenger electric commuter plane with a range of 440 nautical miles (815 km) and a cruising speed of 220 knots (407.44 km/h, 253 mph, or 317 feet per second, if you prefer). United Airlines has also invested in Swedish startup Heart Aerospace and ordered 100 of its electric short-haul passenger planes to be delivered by the end of this decade.
Electric flight is coming, maybe not tomorrow, but soon. Batteries with higher energy densities will be the key that unlocks their potential. Air travel accounts for about 7% of global emissions, so anything that reduces the amount of emissions from aircraft is welcome. Our children will surely fly in electric planes and find nothing remarkable about doing so.
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