Study identifies trace metals in propellants, reports decomposition mitigation method

Samples containing 100 ppm iron with 1, 2, 3, 5, 7.5, and 10 times the number of moles of Bibi added respectively, from left to right in HNO3 solution. Credit: University of Illinois Department of Aeronautical Engineering

The salt used to make green rocket fuel is known to degrade metals – such as those found in metallic propellant storage tanks. Recent research at the University of Illinois Urbana-Champaign found that there are also trace metals in the fuel itself and looked at a way to slow decomposition using compounds that bind with metals.

“We knew from previous studies that metals act as a catalyst for the decomposition of ammonium hydroxyl nitrate — a fly-proven monoblock material that you don’t have to wear a HAZMAT suit around — but we didn’t know their rate of decomposition, especially when you’re an engineering student in the Department of Engineering,” said Emil Bromilcic, Ph.D. Aviation at UIUC.” The study I did focused on atmospheric pressure, so it’s more like storage conditions. Over time, the HAN in stock can melt the steel tank, which contains enough iron to stimulate the decomposition process.

“In the manufacturer’s HAN samples, we found trace amounts of metals – about the parts per million level – but even those trace impurities have a stimulating effect. In this study, I examined the temperature at which they decompose and how quickly they decompose. Then I used the additives to see if It could have been used to remove metal. And it worked.”

In a combustion chamber environment, a HAN-dependent propellant will degrade faster at much higher temperature and pressure, Broemmelsiek said. “According to my results, I wasn’t expecting the inherent impurity to do something significant, but it was motivating.”

He used thermogravimetric analyzer analysis—essentially an imaginary scale—to hold HAN at a constant temperature. He found that the substance decomposes slowly at 75 degrees Celsius, or 167 Fahrenheit.

“When heated at a constant rate, we can check the temperature at which it decomposes and get an idea of ​​the maximum temperature at which it is not expected to decompose. This is important because in a storage tank, it is not intentionally heated. It just sits there.”

In the second part of his research study, Broemmelsiek used additives known to bind – or chelate – with metals. An additive known to bind with copper, changed the HAN solution to blue. Another additive known to be related to iron, changed HAN to red.

Broemmelsiek said that the word chelate comes from a Greek word meaning “claw.” The added compounds capture the trace metals like a chelate and form a chelating compound, which is a very stable molecule that can act to isolate a metal.

“My hypothesis was that after adding these chelating agents, they would form a stable compound and no metal ions and would not react and catalyze the impulse. I found that they did, in fact, bind to the chelating agent and stop the ionic metal from reacting with HAN. It showed a blue-red color change in this HAN solution. interaction.”

About the research, co-author and Broemmelsiek consultant, Joshua Rovey, said, “Emil’s work is the first to demonstrate and quantify the effect of additives on the storability of a new propellant that we have developed and designed specifically for multimodal space propulsion. We learned that additives can reduce the rate of propellant decomposition. By sequestering or chelating metal ions that cause reactions that degrade the propellant. This helps keep the propellant stable for longer while in a storage tank on a spacecraft.”

Although Broemmelsiek measured some significant changes, he said it wasn’t a big enough difference to justify recommending the use of the plugins.

“We learned from this that metals exist and interact and that the rate of decomposition increases with increasing temperature. And we learned more about the properties of how the propellant degrades. This work is not only a good review of the main decomposition mechanisms it also sheds some light on how an additive can be used to slow decomposition.”

Under pressure, non-toxic salt-based fuel works well

more information:
Emil J. Broemmelsiek et al, Effect of mineral sequestration on the decomposition of ammonium hydroxyl nitrate, triggers (2021). DOI: 10.3390 / catal11121488

Presented by the University of Illinois at Urbana-Champaign

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Andrew Naughtie

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