In an initiative that could lead to carbon-free space launches, scientists at the University of California, Riverside, are experimenting with ammonia borane (NH₃BH₃) as a substitute to conventional carbon-based chemical rocket fuels.
Aside from everyday use in eye drops and washing powder, boron has a fascinating history in the aerospace field.
In the 1950s, the US Air Force and Navy were seeking ways to build aircraft that were faster and had longer range. One major hurdle to achieving this is fuel.
Ideally, the best fuel would be hydrogen, but even when cooled into a cryogenic liquid it still takes up far too much space, is extremely difficult and dangerous to handle, and introduces all sorts of logistical problems. Conventional aviation fuels based on hydrocarbons refined from fossil fuels are much more practical because the hydrogen is bonded with carbon atoms, making them much more compact and tractable.
Unfortunately, fuels like kerosene were still too bulky and contained too little energy per unit for the US military's need for supersonic jets that can fly halfway across the world. To overcome this, engineers wanted to create a new fuel that used some other element to store the hydrogen, yet could be cheap enough and easy enough to use to be practical.
The result was a new family of fuels based on boron, which sits right next to carbon in the periodic table. These hydro-boron compounds, or boranes, were developed under the code name Project Zip, hence they were nicknamed "zip fuels."
At first these fuels were very promising, producing almost twice the heat energy of conventional fuels. Aircraft like the XB-70 Valkyrie strategic bomber, XF-108 Rapier long-range interceptor, BOMARC missile, and others were designed to use them, and there were plans for converting jet engines already in use to burn boranes.
But borane fuels had major problems. Because they are solid at room temperature and burned at higher temperatures, they had to be mixed with hydrocarbon-based fuels. Worse, they produced a sticky waste that gummed up engines and could only be removed with a great deal of effort. As a result, the projects were cancelled in 1959.
Today, borane in the form of ammonia borane is used in fuel cells to power electric vehicles, where the hydrogen is released by way of catalysts.
Now a team at UC Riverside, led by Prithwish Biswas, is looking for a way to use boranes to power rockets capable of putting satellites into orbit.
The key to this is to develop a better understanding of how boranes burn and how to get them to do so efficiently, without the need for a base fuel or catalysts, which could result in a more environmentally friendly alternative to hydrocarbon-based fuels and, possibly, cheaper space launches.
Unlike hydrocarbons, boranes have a complex combustion process because they devolve into a number of different compounds as they burn. By forming the borane into nanoparticles and burning it with oxidizers potassium perchlorate (KClO₄) or ammonium perchlorate (NH₄ClO₄), the goal is to alter the combustion process and release more energy more quickly.
"This is analogous to the use of catalytic converters to enable the complete combustion of hydrocarbon fuels,"said Pankaj Ghildiyal, University of Maryland chemistry Ph.D. student. "Here, we were able to create more complete combustion of the chemicals and increase the energy of the entire reaction by using the chemistry of the oxidizer itself, without needing a catalyst."
Because the fine particles used in the study tend to degrade in humid environments, the next step is to find a way to give them a protective coating for storage.
"We’ve determined the fundamental chemistry that powers this fuel and oxidizer combination," said Biswas. "Now we are looking forward to seeing how it performs at large scale."
The research was published in The Journal of Physical Chemistry C.
