Oxidant may offer a boost to greener rockets

Ammonium perchlorate is widely used as an oxidant in the solid propellants that power rocket boosters and missiles, but it comes with serious environmental costs. Every time the Ariane 5 space launcher lofts a satellite into orbit, for example, the reaction between ammonium perchlorate and fuel in its boosters generates about 270 metric tons of concentrated hydrochloric acid, which destroys stratospheric ozone and causes acid rain.

Chemists seeking a halogen-free alternative to ammonium perchlorate now think they have a promising candidate: nitraminodiacetic acid bis(2,2,2-trinitroethyl ester) (NABTNE) (Energy & Fuels 2020, DOI: 10.1021/acs.energyfuels.0c02910). The researchers hope that the compound can overcome most of the drawbacks of other substitutes, which have included inadequate performance, instability, and high cost. “We’re trying to find more economical alternatives, from less costly starting materials,” says Burkhard Krumm, part of the University of Munich team behind the work.

In rocket propellants, ammonium perchlorate is often combined with aluminum and a polymer binder, which serve as the fuel. This blend has been popular for decades, in part because ammonium perchlorate boasts high thermal stability, high oxygen content, and low sensitivity to shock, all at a low cost. “It is such a good material in all aspects apart from the chlorine content,” says Niklas Wingborg, deputy research director at the Swedish Defense Research Agency, who works on rocket propellants and was not involved in developing NABTNE. Environmental concerns aren’t the only motivation, Wingborg adds: “From a military point of view, it has always been interesting to have solid propellants that do not generate any smoke” because it is more difficult to track missiles that do not leave a vapor trail of hydrochloric acid droplets.

Krumm, Thomas M. Klapötke, and colleagues made NABTNE in two steps from commercially available iminodiacetic acid. The new oxidant has a similar density to ammonium perchlorate, a high decomposition temperature of 180 °C, and is not particularly sensitive to friction or impacts that can cause other propellant materials to detonate. NABTNE also has the same specific impulse as ammonium perchlorate, a measure of how efficiently a rocket could use the propellant to generate thrust.

One benefit of solid propellants is that they have a longer shelf life than liquid propellants, and the researchers’ kinetic and thermodynamic calculations suggest that NABTNE should indeed be stable enough for long-term storage, with no decomposition expected after three years at 80 °C. “It is looking really promising,” says Cornelia C. Unger, part of the Munich team.

However, Wingborg is skeptical that the compound could be produced economically at industrial scale. He also points out that the second step of the NABTNE synthesis—a one-pot step involving two reactions—uses reagents, including oxalyl chloride and dimethyl formamide, which are not exactly green materials.

From 2015 to 2018, Wingborg led a project called GRAIL that investigated whether a mixture of ammonium dinitramide (ADN) and ammonium nitrate could usurp ammonium perchlorate. “We still think ADN is the best candidate,” Wingborg says. ADN has a low environmental impact and a high burning rate, which complements the properties of low-cost ammonium nitrate, but the combination cannot yet match the overall performance of ammonium perchlorate.

For now, Unger says that one of the next steps for NABTNE will be to combine it with aluminum and a polymer binder to test whether it is compatible with those fuels.