Boron and Rocket Fuel
In today’s podcast, we’re going to look at research into how boron can be used as rocket fuel. Boron has a high intrinsic energy density. It’s inexpensive, plentiful, and stable, making it a good choice for solid rocket fuels and an additive for jet fuels to increase energy output. However, the oxide shell and its impurities make boron unreliable at the ignition point. Research is ongoing into overcoming these impurities so that boron fuel may be used more widely in space travel.
Boron Rocket Fuel
Herman Krier, and Richard W. Kritzer, Professor of Mechanical and Industrial Engineering at the University of Illinois have stated that rocket performance is determined by energy per pound of propellant. If the rocket can carry less propellant, it has more payload. Liquid fuels or oxidizers offer more energy per pound than solid propellants. However, liquids must be cryogenically cooled, which adds to the cost and makes them difficult to store.
Solid propellants can be safely stored for many years, even in harsh environments, and are also cheaper to produce. Solid-fueled rockets today use propellants dating back from the 1960s and seventies.
Krier stated that these materials are safe and reliable but not very energetic. New propellants are likely to contain high energetic metals like magnesium and boron. Krier said that boron is a desirable additive for rocket fuels because of its low molecular mass and high combustion energy.
Specifically speaking, the bi-propellant composed of boron and nitrogen produces large quantities of hot gases in a short time. These gases are helpful for many purposes including providing the thrust necessary for rockets. Additionally, boron carbide has a high combustion enthalpy. Boron carbide can be easily manufactured using well-known electric furnace techniques and is also readily available at a low cost.
These boron carbide solid oxidizing agent mixtures can be stored for as long as one wants without deterioration or danger of premature ignition. These mixtures are so stable that an igniter is needed to start the reaction between the components.
Boron carbide has been considered a good fuel component for solid rocket propellant compositions. Rocket propulsion can also be achieved using a bi-propellant made of boron and nitrogen compounds. A nitrogen-containing combination that is only composed of carbon, hydrogen, and nitrogen, is also combined with a boron-containing compound composed of carbon, hydrogen, and carbon.
At least one reactant needs to contain hydrogen and at least one reactant needs to contain carbon. In a 2015 study published in Aerospace Research Central, a composition consisting of 80% polytetrafluoroethylene and 20% boron by weight was considered a potential high density, solid fuel mixture for mixed hybrid rocket propulsive applications.
What are the Challenges and Ongoing Research Related to Boron Combustion?
There has been extensive research on board and as solid rocket fuel and as an additive to jet fuels. However, this propellant has been difficult to tame due to its variable and complex combustion behavior.
Alla Pivnika, Nikita Muravyev, and colleagues from the Semonov Federal Research Center for Chemical Physics in Moscow have conducted a series of experiments to explain why this is. Pivnika says that boron particles cannot be ignited easily because the solid boron core has an oxide shell that acts as a protective coating, delaying or preventing ignition and combustion.
So, how to figure out the thermal behavior? The researchers discovered that different boron powders have different shell compositions because of various impurities, such as magnesium and aluminium, and their oxides during production.
Pivkina says we demonstrated that boron particles with differing impurities within the oxide shell have significantly changed thermal behavior. The researchers reported their findings in the journal Combustion, Explosion, and Shockwaves. They have now shown that impurities can either hinder or enhance combustion performance by thermal analysis of the powder.
Pivkina also reported that they found that aluminium and magnesium oxide impurities reduced evaporation of the boron oxide and thus decreased the activity of boron ignition.
In contrast, magnesium fluoride had the opposite effect. The oxide layer goes through melting, dehydration, and evaporation during heating. Each step takes place at a different temperature, depending upon the presence of impurities. Researchers discovered that magnesium and aluminium undergo metallic thermite reactions with boron oxide, which can cause problems with combustion and leave unwanted oxide residues.
However, magnesium fluoride forms boron fluoride gas at a thousand degrees centigrade. This promotes the evaporation and activation of the boron core at around 1300 degrees centigrade, which helps reduce troublesome ignition delay.
The conclusion for Pivkina and his team is to carry out ongoing investigation into a variety of binders to optimize the combustion of boron-based fuels. The results will open up new avenues for the ignition and combustion of boron particles in energetic formulations.
For more information on boron and rocket fuels please refer to the Borates Today website. And that’s all for today. Thanks for listening.