Rocket Fuel

        This article deals with a theorized, substantial improvement to rocket science, particularly in relation to fuel. The theory is then explored in detail pertaining to how we would go about producing this new fueling method, the statistical improvements that would be made with this so far theoretical fuel, and how exactly it would be applied to the rockets themselves.

        This new component is an alternate phase of the most abundant element in our known universe: Hydrogen. Experimentally, hydrogen would be most effective in its metallic form; a phase that has yet to be reached for the element and its isotopes. In order to produce metallic hydrogen, relatively extreme levels of pressure and temperature would be needed. Previously, over 70 years ago, according to Wigner and Huntington [2], it was predicted that pressures of about 25 GPa (gigapascals) would be enough to transition solid molecular hydrogen to an atomic solid that would be metallic in nature. It has also been predicted that metallic hydrogen would be metastable, or essentially that it would maintain its metallic state even when the pressure was released. The reason that metallic hydrogen would be a revolutionary chemical rocket propellant if able to be economically produced is because of large energy of reaction, or recombination, and its very light mass. In rocketry, the article states that one of the most important attributes of a propellant is its specific impulse (Isp). Later, the article goes on to explain that specific impulse “represents the number of seconds that a thrust of one kilogram of force can be sustained by one kilogram mass of propellant” [1]. For scale, currently the most powerful propellant we have has an Isp of about 460 seconds which was used for the main engine of the United States’ Space Shuttle; metallic hydrogen is calculated to have an Isp of nearly 1400 seconds [3]. The higher the Isp of the rocket, the higher the payload. The payload describes how much weight can be carried by the rocket that is also practical and passes safety guidelines. In addition, metallic hydrogen is speculated to be about 10 times denser than molecular hydrogen (the propellant we use currently), and would not need cooling as a cryogenic fuel; therefore adding even more of a payload.

        The article then analyzed how we would integrate metallic hydrogen into our current system of rocket engineering. It proposed that the temperatures needed in the combustion chamber would be way too high for current construction materials to withstand. It was proposed that the metallic hydrogen be diluted in molecular hydrogen or water in order to lower the combustion temperatures. While this would potentially decrease the total specific impulse, it would still be a drastic improvement in rocketry. They suppose that a single stage to orbit would even be possible. The article then goes on to describe previous experiments attempting to create the metastable, or even just the metallic hydrogen.

        Although the pressure predicted to bring about metallic hydrogen was 25 GPa, recent experiments have all exceeded the pressure by magnitudes with no phase change proceeding after the non-metallic molecular phase. These experiments were also carried out on both hydrogen and its isotope, deuterium. Currently, there are two methods of producing such high pressures needed for the phase change: static and dynamic. The first, used the most by modern scientists, are studies run through diamond anvil cells. Shockwave experiments can also be used to achieve high pressures for a short period of time. In one experiment the article mentions by Loubeyre, Occelli, and LeToullec [4], the data found indicated that the transition pressure for molecular hydrogen to metallic hydrogen was above 4.5 Mbar (or 450 GPa). “Efforts to study hydrogen and its isotopes at still higher pressures, aimed at the transition to metallization, continue.” [1] The next section of the article reaches closer into the physics of a rocket and its engine, such as how it receives its thrust for acceleration; this part serves as a precursor to the article’s conclusion regarding the implementation of metallic hydrogen-based rockets.

        Ironically, the article opens the section by explaining that rocket science is not rocket science; “A liquid rocket engine is fundamentally a quite simple device.” [1] Liquid fuel and an oxidizer are put into a reaction chamber where combustion occurs and the release of energy and hot gas follows. The combustion exits the chamber at a high pressure, then the gas expands as it passes through a nozzle. The article says that as the gas expands the random motion (the chaotic, haphazard movement of atoms and molecules) of the gas particles gets changed to directed motion (the uniform movement of particles in one direction) via collisions with each other and the nozzle wall. The gas particles are directed out of the chamber in the opposite direction of the acceleration of the rocket, thus producing the thrust and initial velocity needed to get the rocket moving. The expansion of the gas can only be carried out by the speed of sound, which is a function of temperature; generally, as temperature increases, the speed of sound also increases. So, in practice, the expanding gases are pushing the gases in front of them, which are also expanding, to a maximum speed of the local speed of sound. As a result, the transfer of momentum to the physical rocket is supersonic (exceeds the speed of sound) itself. “The kinetic energy [energy that a body possesses by virtue of being in motion] from the exhaust represents the energy that can be obtained from the propellant by the engine for acceleration.” [1] Products with a lighter molecular weight give higher exhaust velocities; something that metallic hydrogen would be able to provide. As for current fueling techniques, the fuel and oxidizer must be injected into the chamber at a higher pressure than the container itself. This extra pressure is produced by pumps powered by turbines. The high-pressure exhaust is then injected with the fuel into the combustion chamber, and therefore this approach results with most of the pumps’ energy being recovered and improving thrust and Isp overall.