Flying to the moon on drops of fuel

European scientists at EPFL labs have designed an ultra-compact motor – dubbed MicroThrust – that will allow small satellites to journey beyond Earth’s orbit using just a tenth of a liter of fuel.

Indeed, the complete thruster weighs a few hundred grams and is specifically designed to propel small (1-100 kg) satellites, allowing them to voyage to more distant destinations.

According to EPFL exec Herbert Shea, MicroThrust prototypes are already slated for integration on CleanSpace One, a satellite tasked with removing space debris, as well as OLFAR, a swarm of Dutch nanosatellites that will record ultra-low radio-frequency signals on the far side of the Moon.

"At the moment, nanosatellites are stuck in their orbits," explained Shea. "Our goal is to set them free."

To be sure, small satellites are quite popular now because their manufacturing and launch costs are considered relatively low - about half a million dollars, compared to conventional satellites that run into the hundreds of millions. However, nanosatellites currently lack an efficient propulsion system that would render them truly autonomous and thus able to carry out exploration or observation missions.

As such, instead of a combustible fuel, the new mini motor runs on an "ionic" liquid, in this case the chemical compound EMI-BF4, which is used as both a solvent and an electrolyte. It is composed of electrically charged molecules (like ordinary table salt) called ions, except that this compound is liquid at room temperature. The ions are extracted from the liquid and then ejected by means of an electric field to generate thrust. This is the principle behind the ionic motor: fuel is not burned, rather, it is expelled.

In the motor developed at EPFL, the flow of ions is emitted from an array of tiny silicon nozzles – over 1,000 per square centimeter. The fuel is first guided by capillary action from a reservoir to the extremity of the micro-nozzles, where the ions are then extracted by an electrode held at 1,000 volts, accelerated, and finally emitted out the back of the satellite. The polarity of the electric field is reversed every second, so that all the ions - positive and negative - are ejected.

SystematIC Design, a MicroThrust project partner, designed the motor's electrical system. The ion ejection system requires a high electrical voltage, but the available energy aboard a 1-liter nanosatellite is limited to a few small solar cells - or about four watts of power. Fortunately, the Dutch company was able to develop a system that overcame this difficulty.

After six months of acceleration, the microsatellite's speed increases from 24,000 km/h, its launch speed, to 42,000 km/h. The acceleration is only about a tenth of a millimeter per square second, which translates into 0-100 km/h in 77 hours. But in space, where there is no friction to impede motion, gentle but steady acceleration is the preferred way to go.

"We calculated that in order to reach lunar orbit, a 1-kg nanosatellite with our motor would travel for about six months and consume 100 milliliters of fuel," said Muriel Richard, a scientist in EPFL's Swiss Space Center.

As noted above, the ionic motor will power CleanSpace One - a nanosatellite whose mission is to tidy up space by grabbing space debris and pulling it into the Earth's atmosphere to be safely incinerated. 

According to the Swiss Space Center, CleanSpace One will take two to three months and more than 1,000 terrestrial revolutions to reach one of its targets, the decommissioned Swisscube cubesat or Tlsat-1 cubesat. The target date leaves scientists with  just over a year to finalize their system.

"Our prototype still has a few flow problems at the nozzle extremities, which could cause short-circuits," Shea acknowledged.