Ion thrusters that self-assemble from fuel have promise for small spacecraft

Michigan Technological University researchers develop concept for ferrofluid-based propulsion

As the trend for miniaturisation of electronic technology continues, more spacecraft and satellites are becoming smaller, with typical sizes reducing from about that of a fridge or small car to approaching a shoebox or even a smartphone. But even these tiny craft need a way to manoeuvre and change their orientation to take measurements or send signals in the appropriate direction. But the types of plasma thrusters used on larger craft do not work well when scaled down. Graduate mechanical engineering student Brandon Jackson is leading development of a method that may be cheaper and more effective than current technologies.

Ferrofluid
In a magnetic field, ferrofluids change from a pool of tarry liquid to an array of spines

One way of propelling small spacecraft is known as electrospray and involves using electrostatic fields to eject fluids of charged particles from a hollow needle-like device in the opposite direction to the movement required. But needles are expensive to make and delicate. Jackson’s solution, which he is developing with Prof L Brad King, is to use a magnetic liquid salt as a fuel. This has an interesting property; it is a ferrofluid. That is, in a magnetic field, it spontaneously shapes itself into an array of spiky peaks, looking a little like a miniaturised hedgehog. Moreover, when a strong electric field is applied to those peaks, each one emits its own individual jet of ions.

Brandon’s contribution to the research is twofold: he has built the experimental model and devised a model of the physics of the instability that gives rise to the ion jet from a single peak, in particular the relationship between the magnetic and electric fields and the surface tension of the fluid, which was previously poorly understood.

If successful, the research could result in an electrospray thruster that requires no needles and is essentially indestructible. If the electrofluid spines are damaged, they heal and regrow themselves.

In a paper in the journal Physics of Fluids, Jackson and King explain that the magnetic field that forms the ferrofluid spines has a greater effect than expected on the surface tension of the fluid. King has now received more funding for the project from the Air Force Office of Scientific Research, and is investigating whether it is possible to obtain thrust from more than one peak in the spiky array. “Often in the

“Often in the lab we’ll have one peak working and 99 others loafing,” he said. “If we are successful, our thruster will enable small inexpensive satellites with their own propulsion to be mass produced. That could improve remote sensing for better climate modelling, or provide better internet connectivity, which three billion people in the world still do not have.” Electrospray technology also has potential applications in spectrometry, pharmaceutical production and nanofabrication, the University added in a press release.

According to the university, electrospray technology also has potential applications in spectrometry, pharmaceutical production and nanofabrication.