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...