Salt-based propellant may lead to dual-mode rockets

electric propulsion system testing
Associate Professor Joshua Rovey, left, talks about electric propulsion system testing with AE graduate students Nick Rasmont and Matt Klosterman
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electric propulsion system testing
Associate Professor Joshua Rovey, left, talks about electric propulsion system testing with AE graduate students Nick Rasmont and Matt Klosterman

Researchers led by the University of Illinois at Urbana-Champaign are studying salt-based propellants that have the potential to power dual-mode rockets that work as both combustion and ion engines. This opens up the possibility of rockets with the advantages of both liquid-fueled rockets and ion drives for space missions.

When planning a space mission, one of the most basic questions is what kind of propulsion is it going to use to get from point A to point B. At one extreme, there are the liquid-fueled rockets that can provide a large amount of thrust in a very short time, and at the other are the electrostatic or ion engines that provide a tiny thrust for a very long time.

Both of these have their advantages and disadvantages. Liquid rockets are good for getting a payload into orbit but aren't very efficient for covering long interplanetary distances. Meanwhile, ion engines are very good for powering a probe from Earth to Pluto, but they're rubbish for launching in any kind of gravity.

One way of getting around this is to create a dual-mode rocket, which can produce a large amount of thrust by burning two propellants, then using the same propellants as the engine switches to ion mode.

Ion engines work by giving propellant atoms an electric charge, then accelerate them in an electric field. The type used on current space missions rely on heavy atoms like xenon or cesium, but the Illinois study focused on an electrospray, or colloid, propulsion. This is a system that's derived from biology and chemistry lab techniques where droplets of liquid are sprayed as an aerosol, charged, and then accelerated.

To do this, a liquid is pumped through a capillary tube with a tiny diameter. When the liquid reaches the tube's tip, it encounters a strong electrostatic field, pulling away small droplets and spraying them out. The idea is that the liquids selected for the system could be used in a dual-mode rocket. By using two propellants, they could do double duty – both as ion propellant and by burning with each other in chemical rockets. The tricky bit is selecting the right ones.

"We need a propellant that will work in both modes," says Joshua Rovey, associate professor in the Department of Aerospace Engineering in The Grainger College of Engineering at the University of Illinois. "So, we created a propellant that is a mixture of two commercially available salts – hydroxylammonium nitrate and emim ethylsulfate. We have published other research papers showing that salt propellants work in the high-acceleration combustion mode. Now we know that this unique combination of salts will also work in the electric fuel-efficient mode.

"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."

According to Rovey, the study will provide a better understanding of now how these salts can interact for electrosprays as well as how this interaction can affect the output of the spray, which is important for more mature rocket designs.

The research was published in the Journal of Propulsion and Power.

Source: University of Illinois

1 comment
I thought that salt was either sodium chloride or potassium chloride. How does hydroxylammonium nitrate fall under that designation?