MOF-based alternative could make for safer, cleaner rocket fuel

Hydrazine rocket fuel is highly toxic, carcinogenic and unstable, making it incredibly dangerous to work with. Scientists at McGill believe their new alternative, based on metal-organic frameworks, will lead to cleaner, safer, more controllable rockets.

Space flight relies upon a class of fuels known as hypergolic, meaning that they ignite spontaneously when they come into contact with an external oxidizer. Hypergolic propulsion is particularly handy once you’re already up in space, because there’s no need for an ignition system per se – you can simply pressure-feed your fuel and oxidizer into a combustion chamber at an easily controllable rate to get just as much thrust as you want, exactly when you want it, and it’ll also stop exactly when you want it to.

What’s more, you can easily store hypergolic fuels and oxidizers as liquids across the normal range of temperatures and pressures encountered in space flight.

The same can’t be said of the cryogenic liquid oxygen-based fuels that handle a lot of the heavy lifting at blast-off. SpaceX’s Raptor rockets, for example, use cryogenic liquid methane and liquid oxygen, which need to be stored at close to their freezing points.

Typically, though, when you talk about hypergolic rocket fuels, you’re talking about the most common ones: hydrazine-based fuels reacting with dinitrogen tetroxide oxidizers. And while they’ve been super handy in everything from the Titan II to Soviet ICBMs and the Space Shuttle, hydrazine-based fuels can be super nasty to deal with.

As well as being massive fire risks, they’re highly toxic. Hydrazine burns the skin, irritates the eyes, nose and throat, causes nausea and vomiting, shortness of breath, pulmonary edema, depression of the central nervous system, temporary blindness, seizure, comas, and organ damage to the nervous system, liver and kidneys. The International Agency for Research on Cancer goes on to rates hydrazine as group 2A: probably carcinogenic to humans.

Many of those dangers are exclusive to the poor souls who work with it, but the researchers claim the aerospace industry is responsible for around 12,000 tons of hydrazine fuels being accidentally released into the atmosphere each year. Hence the search for hypergolic rocket fuels that are safer and easier to deal with.

Which leads us to a study from a group of researchers split between the Department of Chemistry at McGill University in Montreal, Canada, and the School of Metallurgy and Materials at the University of Birmingham, UK.

This team has figured out how to induce hypergolic behavior in metal-organic frameworks (MOFs) based on zinc, cobalt and cadmium, using acetylene or vinyl substituents as the oxidizers. The team created six such MOF-based fuels, and demonstrated that they can be fine-tuned for a range of different properties, including the all-important ignition delay, or ID value. Several of these fuels showed ultra-short IDs below 5 milliseconds.

“The ability to achieve and fine-tune hypergolic performance, combined with the absence of hydrazine-based carcinogens or explosive components, should make hypergolic MOFs promising candidates for safer, environmentally friendly propellants,” reads the study.