Although still in the early stages, researchers from McGill in Canada and the University of Birmingham in the UK have uncovered a potential new type of rocket fuel that is greener and safer than existing fuels (1). The class of porous solids known as metal-organic frameworks or MOFs can be fine-tuned to exhibit a range of properties including the ultra-short ignition delay required to propel rockets, according to a new study published on 5 April in Science Advances.
Jet fuel or hypergolic fuels currently power rockets and other spacecraft. Hydrazine-based fuels are the most common, composed of highly energetic materials that immediately ignite upon contact with an oxidizer. Since there is no oxygen outside the Earth’s atmosphere, a source of oxygen, such as dinitrogen tetroxide, is also needed.
The low ignition delay and high energy density of these hypergolic fuels provide the power required to launch rockets into space and keep satellites in orbit. Hypergolic fuels and oxidizers are also “ideal” as they can be stored as liquids within the range of temperatures and pressures typically encountered during space travel. Moreover, the thrust rate of a spacecraft can be easily controlled by the combustion process.
However, current jet propulsion technologies are also accompanied by significant environmental costs. Hypergolic fuels derived from hydrazine are highly toxic. Every year, it is estimated that spacecraft release nearly 11,000 tonnes of toxic propellants into the atmosphere causing irreparable damage to the environment.
Furthermore, contact with hydrazine can lead to a range of unpleasant symptoms such as dizziness, vomiting, seizures, organ damage, and blindness, moreover, the materials are currently classed as carcinogenic.
Environmentally friendly fuels
On the other hand, MOFs are non-toxic. They are composed of clusters of metal ions combined with an organic molecule called a linker. And as scientists have now discovered, can be designed to act like hypergolic fuels. The team of researchers, led by Prof Tomislav Friščić from McGill, investigated six of these hypergolic MOFs, based on zinc, cobalt, and cadmium, as potential new rocket fuels, free of highly energetic or carcinogenic components.
The scientists successfully induced “hypergolic behaviour” in the MOFs. In other words, they elicited a huge amount of energy from materials not typically considered energetic. To do this, they used so-called trigger functionalities and unlocked the latent energy within the MOFs. The fuels combusted rapidly, with 2 milliseconds, demonstrating ideal properties that could potentially be used to propel rockets.
More research is still needed but the findings suggest these fuels could lead to rockets that are environmentally friendly, and perhaps, even more controllable. Friščić is working with McGill and ACSYNAM, a McGill spinout that focuses on the clean manufacture of materials, to commercialise the technology.
(1) Titi, H.M. et al. Hypergolic zeolitic imidazolate frameworks (ZIFs) as next-generation solid fuels: Unlocking the latent energetic behavior of ZIFs. Science Advances (2019). DOI: 10.1126/sciadv.aav9044