Tiny Ion Rockets Could Power Satellites and Spacecraft

The tip of an electrospray thruster forms problematic tree-like structures when exposed to radiation. (Image courtesy of Michigan Tech/Kurt Terhune.)

Imagine a small drop of molten salt that can propel a spacecraft all the way to another planet. It sounds like science fiction, right down to the name – electrospray thruster – but engineers have really developed these tiny rockets and hope to see them used for a number of future space applications.

To help realize that future, a team of researchers recently made significant strides in improving these tiny rockets by placing them under a Transmission Electron Microscope (TEM).

Electrospray Thrusters

Electrospray thrusters really aren’t much more than a drop of ionic liquid. Applying a strong electric field to the drop causes molecular ions to evaporate from the tip of the electrospray thruster. This spray of ions creates a tiny force (on the order of micronewtons) that’s less than the weight of a single human hair. Though that sounds unimpressive, in the vacuum of space it’s sufficient to propel a small object.

Mechanical engineering PhD student Kurt Terhune holds a nanosatellite thruster, a device which has attracted a lot of attention in aerospace research, but engineers are still working on the best way to use them to propel nanosatellites through space. (Image courtesy of Michigan Tech/Nathan Shaiyen.)

But there’s a problem with the ion rockets. Sometimes the tip of the thruster forms a tree-like structure that disrupts the flow of ions and turns the ionic liquid into a gel. To understand how to solve this problem, the research team from the University of Maryland set out to find the cause of the defects. However, as team member Lyon B. King reflects, this was no easy task.

"Getting a close look at these droplets is like looking through a straw to find a penny somewhere on the floor of a room,” said King. “And if that penny moves out of view, like the tip of the molten salt needles do—then you have to start searching for it all over again."

Finding a Needle in a Haystack

Using a TEM, which can see extremely small images at a high resolution, the team was able to capture the progression of the tree-like structures as they formed. Using these images, they were able to determine that high-energy electrons, such as those used in the TEM beam, cause damage to the molecular structure of the ionic liquid. This thickens the liquid into a gel and causes the disruption to the ion flow.

These TEM images illustrate the progression of the tree-like structures after 18, 21, and 27 minutes respectively. (Image courtesy of Kurt Terhune.)

"We were able to watch the dendritic structures accumulate in real time," said the team’s leader Kurt Terhune, commenting on the significance of the research. "The specific mechanism still needs to be investigated, but this could have importance for spacecraft in high-radiation environments."

Though more work remains to be done, the potential of the ion rockets is worth the effort. Combining several thrusters into a packed array could allow more massive spacecraft to eventually reap their benefit. Furthermore, the electrospray thrusters are well suited for Earth-orbiting nanosatellites. In fact, the ion rockets are currently being tested on the European Space Agency’s LISA Pathfinder as a mechanism to place objects precisely in space.

For more rocket-related news, check out NASA’s successful test fire of the SLS rocket.