New Salt-based Propellant Rockets

Researchers from the University of Illinois at Urbana-Champaign used a salt-based propellant that had already been proven successful in combustion engines, and demonstrated its compatibility with electrospray thrusters. This will make dual-mode rocket engines successful. It will function in both combustion and electric propulsion systems.

With electrospray or colloid propulsion, the thrusters electrostatically accelerate ions and droplets from these liquids. It’s a technique that started in the biology/chemistry community, then the propulsion community began looking at it about 20 years ago.

Liquid is fed through a very small diameter needle, or capillary tube. At the tip of the tube, a strong electric field is applied that interacts with the liquid in the tube because the liquid itself is a conductor. The liquid responds to that electric field. Small droplets and ions get pulled out of the liquid—spraying them out of the tube or needle.

In this study, in addition to showing that the propellant could be sprayed, Rovey said they were interested in learning what kinds of chemical species come out in the plume. “Because no one has ever tried this type of propellant before, we expected to see species that no one else has ever seen before and, in fact, we did.”

Propulsion Power Journal – Hydroxylammonium Nitrate Species in a Monopropellant Electrospray Plume

Rovey said they also saw a new swapping of the constituents that make up the two different salts.

“We saw some of the hydroxylammonium nitrate salt bonding with the emim ethyl sulfate salt. The two are mixed together inside the propellant, and are constantly bonding with each other and then detaching.

“There’s a chaotic nature to the system and it was unclear how those interactions within the liquid itself would propagate and show up in the spray. There are no chemical reactions happening. It’s just that we start with A and B separately and when they come out in the spray, A and B are bonded together,” he said.

Rovey said these findings shed a lot of light on what’s happening in these mixtures of salts that are possible propellants for electrosprays. But it also opens doors to a lot of other questions that will lead to fundamental studies that try to understand the interactions within these propellants and how that translates into what comes out in the spray itself.

Additional collaborative researchers were from Missouri University of Science and Technology, Boston College, and the Air Force Research Laboratory in New Mexico. Co-author Mitchell Wainwright used the apparatus at the Kirtland Air Force Base in Albuquerque, New Mexico for the study to take measurements. The team supplied the unique propellant.


A mixture of 1-ethyl-3-methylimidazolium ethyl sulfate ([Emim][EtSO4]) and hydroxylammonium nitrate (HAN) is an energetic monopropellant potentially suitable in a multimode chemical-electric microtube-electrospray micropropulsion system. In this work, electrospray plume mass spectra are compared between the monopropellant mixture and neat [Emim][EtSO4]. This comparison clearly indicates new and additional species present in the plume due to the addition of HAN. Mass spectra from 20 to 600amu/q were obtained over a variety of angles and flow rates from 2pL/s to 3nL/s in both cation and anion extraction modes. Mass spectra dependence on flow rate and angular orientation agree qualitatively well with literature. Results indicate the presence of HAN-based species in anion mode, but no HAN-based species in cation mode. Three of the four monomer species from the monopropellant are apparent in the plume; [Emim]+, [EtSO4]−, and [NO3]− are observed, whereas [HA]+ is noticeably absent. Results also show emission of both proton-transferred covalent forms of HAN: [HA-H] and [HNO3]. Swapping of anion and cation species between the constituents of the monopropellant mixture is also observed.