Rocket fuels are generally based around chemical reactions. In space there is no air, meaning no oxygen to combust a fuel with. So rockets must carry their own oxygen (O2), usually as a cryogenically cooled liquid (LOX). However there are monopropellants that carry oxygen atoms in the molecule and are separated by a catalyst to become combustible, but these introduce more flaws than they fix more often than not. Rocket fuels have 2 important properties. 1. Isp (specific impulse) This is the “Fuel economy” of rocket fuels. Isp is measured in seconds. The formula is the square root of the Combustion Temperature divided by the molecular mass. sqrt(Te/M) Lower mass makes Isp higher, and higher combustion temperature also makes Isp higher. Isp in seconds is how long it can make 1kg of fuel provide 9.81N (9.81 newtons) of force. Isp also has a second way to be calculated that results in the same as the first equation regardless. Thrust / (Mass-flow-rate x 9.81m/s) 2. Volume (m^3) This is important for major reasons, the highest Isp fuel is LH2 (Cryogenic Liquid Hydrogen), but it has a size so small it leaks out of even metal seals, and despite it being so small, its not very dense, 1kg of Hydrogen takes up vastly more volume than other gasses, sometimes this can actually kill the Isp gain in weight of the fuel tanks. For these reasons scientists had to find a way to make LH2 take up less volume, to do that, you must make a molecule with a secondary atom. To achieve the highest Isp, you must choose the atom with the highest result of dividing how many hydrogen atoms if can hold by its mass (Hydrogen-Bonds / Mass). Carbon has the highest result, and it can hold 4 hydrogen atoms, creating CH4, which is methane. H2 has an Isp of ~450 seconds in a vacuum, CH4 has ~380 seconds. It sounds like a massive loss until you look at the aerodynamic drag reduction, weight savings for the tanks, and even ability to keep liquid. CH4 boils at a higher temp than H2, making it easier to keep in its liquid state.
Written by me. Next Lesson: https://scratch.mit.edu/projects/1390521517/
