Decarbonising long-range aviation is challenging. This study evaluates the performance of six low-carbon fuels and their realistic impacts on aircraft design for a large long-range passenger aircraft using Breguet’s range equation. Liquid hydrogen (LH2) and 100 % synthetic paraffin kerosene (SPK) are the only two alternative fuels found to be viable. Using present-day technology, we find that the design-point specific energy consumption (SEC, MJ/tonne-km) of tube-wing aircraft powered by LH2 and 100 % SPK are 11 % higher and 0.2 % lower relative to Jet-A, respectively. At off-design points, SEC of 100 % SPK and LH2 are always similar to and greater than Jet-A, respectively. LH2 aircraft SEC decreases with increasing range and is less sensitive beyond 10,000 km. In a first, we develop an equation that enables LH2 aircraft weight-sizing. Our results should inform studies on LH2 and 100 % SPK aircraft operating costs and lifecycle emissions.