This photo is provided by NASA. One of the most remarkable and noble demonstrations of the power of chemistry is, without question, space exploration. From the first unmanned missions in the late 1950s to the Space Shuttle and now Artemis, various rocket engine technologies have relied on the invention of powerful fuel/oxidizer packages. To provide maximum thrust efficiency, rocket propellants are made from highly reactive chemical materials, making them potentially very toxic, corrosive, and difficult to store and transport. These constraints will give each propellant package a set of benefits and drawbacks, and the selection of the best package will depend on the rocket’s mission. The Liquid Oxygen/Liquid Hydrogen (LOX/LH2) cryogenic propellant is truly remarkable through its efficacy and reliability, but also fascinating because its combustion generates a completely green by-product: water! Cryogenic Propellants At room temperature, rocket propellants can come as solids, liquids, or gases. Gaseous propellants are not practical for large-scale rocket missions because they would require very large, sturdy, and, therefore, heavy rocket tanks to safely carry the needed quantity of compressed fuel and oxidizer gases. Because liquids contain a lot more molecules than gases per volume unit, the solution is to transform the gases into their corresponding liquid phases through compression and cooling. Some of these propellants have extremely low boiling points and, therefore, require extremely low-temperature cooling techniques to liquefy them. Gas liquefaction requiring temperatures below -150°C (-238°F) is defined as cryogenic cooling. With boiling points of -183°C (-297°F) and –253°C (-423°F), respectively, oxygen and hydrogen need to be cryogenically cooled to be liquified. This requirement is a significant constraint when it comes to using LOX/LH2 for rocket propulsion, but the LOX/LH2 package’s benefits well outweigh its disadvantages. Specific Impulse or How to Measure a Propellant’s Efficacy Oxidizers and fuels are stable elements at room temperature, but they will react in a violent exothermic reaction when mixed and triggered by a heat source. In rocket engines, the two reactants will be mixed in the main combustion chamber and ignited. Fuel and oxidizers aren’t always mixed in stochiometric proportions. Engineers may decide to run the engine “fuel rich” to reduce the combustion temperature and protect the engine from overheating. The resulting explosive reaction between the oxidizer and the fuel is what will provide the rocket its thrust. Fuel + Oxidizer + Ignition = High Heat + High Exhaust Energy + Nozzle Geometry = Thrust Two important characteristics, sometimes confused, define a rocket propellant’s potency. The first one, thrust, is measured in Newtons (or pounds) and expresses the propellant’s reaction force potential, as defined by Newton’s Third Law. The higher the thrust, the larger the payload the rocket will be able to lift. The second characteristic is specific impulse (Isp). Considered the most important parameter for a propellant, specific impulse defines how efficiently a propellant can convert its mass into thrust. More precisely, it measures the time (in seconds) that a certain quantity of propellant will be able to push a certain load. A high thrust propellant doesn’t mean a high specific impulse value. Engines using propellants with a high specific impulse will tend to have lower thrusts, but they will use their propellant’s mass more efficiently. In simple terms, they get greater gas mileage. Table 1 lists some of the most common LOX and fuel combinations. LOX/LH2 exhibits the highest Isp value. That efficiency is one of the reasons why the LOX/LH2 pair has been so abundantly utilized as a rocket propellant in the last five decades of space exploration. Table 1. Propellant packages: properties of LOX and various fuels. Note: *RP-1 (Rocket Propellant-1) is a highly refined form of kerosene and is widely used in liquid rocket engines. Hydrogen Combustion – Reaction Mechanisms While other propellants release large quantities of polluting chemicals after combustion, the LOX/LH2 package is particularly special because, in addition to its outstanding specific impulse, its by-product is water. This is an extraordinary benefit Mother Nature allows us to leverage! Hydrogen (H2) and Oxygen (O2) are stable elements, and they will not spontaneously react when mixed at room temperature. For a reaction to occur, H-H and O=O covalent bonds need to be broken. The energy necessary to break these bonds is called activation energy (Ea), as shown in Figure 1. When enough energy is supplied to overcome the H-H and O=O bonding energy, a chain reaction will occur until water is formed and a much lower, very stable energy level is reached. The transition reaction towards water’s very stable structure is what gives off large amounts of energy during H2 combustion with O2. The net energy released (DE) equals -482kJ/mol (Figure 1). Figure 1. Energy levels for H2, O2, H2O molecules and energy changes from reactants to the products. Despite the reaction’s apparent simplicity, H2 combustion with O2 is complex and involves several intermediary reactions involving extremely reactive H and O radicals. For this study, only the main reactions leading to the formation of water are listed (Figure 2). All reactions are reversible. The letter k represents the different reactions’ rate constants. When one radical generates two or more radicals (Figure 2 reactions numbered 3 and 4), the reactions are called chain-branching reactions. Because these reactions produce more reactive radicals than they consume, they accelerate very rapidly and explain the explosive nature of H2 combustion reaction with O2. The radical reactions aren’t always happening in the exact order displayed in Figure 2. Other radicals, not mentioned here, may be formed through other chain reaction schemes. Propellant mixture, pressure, and temperature also influence H2 combustion kinetic mechanisms. Figure 2. Main radical reactions involved in H2 combustion in O2. The Turbopump – A Fundamental Element of Modern Rocket Engines Various rocket designs with different liquid propellants have been proposed in the last century; however, WWII Germany’s V-2 (Figure 3) program probably advanced large rocket engineering in the most significant way. The V-2 design was the first iteration of what would become revolutionary large-scale rocketry capable of carrying
