MIT engineers are testing a new propulsion system that combines the power and speed of conventional chemical thrusters with the precision and fuel-efficiency of electrical thrusters. The system could enable the design of nimbler, more flexible small satellites, which could perform both fast, powerful manoeuvres and slower, precise adjustments, depending on the mission and moment at hand.
The key to the new system is a special propellant that can power both chemical and electrical thrusters, which traditionally have required separate, bulky fuel sources. ‘If you can have chemical and electrical propulsion in one small package, it’s the best of both worlds,’ said Amelia Bruno, a former postdoc in MIT’s Department of Aeronautics and Astronautics (AeroAstro). ‘This opens the door for small satellites to do even more science, more observations and more interesting missions, all on a smaller and cheaper platform.’
The engineers have demonstrated that a type of ‘green monopropellant’ originally developed by the US Air Force for use in chemical propulsion in space can also effectively power tiny ‘electrospray’ thrusters. Electrospray thrusters are rockets roughly the size of a five pence coin or a dime that use electric fields to charge up a liquid propellant’s particles, which are then shot into space as a thrust-generating spray.
Electrospray thrusters are extremely fuel-efficient and can perform slow and precise manoeuvres, such as pushing a small spacecraft bit by bit through a long, interplanetary journey. Chemical thrusters, in contrast, require a large fuel supply to perform short and fast bursts, for instance to quickly ascend and descend, or speed up and slow down.
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Now that the MIT group has found a propellant that can fuel both chemical and electrospray thrusters, they see big potential for small spacecraft. The team is working with NASA to launch the Green Propulsion Dual Mode mission – a briefcase-sized CubeSat that will carry a chemical thruster and four electrospray thrusters, all fuelled by a single propellant tank.
The mission will be the first to test such a two-in-one propulsion system for small spacecraft. If it’s successful, Bruno said the mission could pave the way for small satellites to explore beyond the Earth’s orbit. ‘We could send CubeSats to Mars, or the asteroid belt, where they could make the journey slowly, using electrospray thrusters,’ said Paulo Lozano, the Miguel Alemán Velasco professor of aeronautics and astronautics at MIT. ‘You could then use your chemical thrusters to quickly move to look at interesting features. You could have a lot more flexibility to do a lot more things.’
Lozano’s group at MIT designs, fabricates and tests electrospray thrusters for use in satellites that range from the size of a lunchbox to a small carry-on suitcase. Compared to conventional satellites, these microsatellites are significantly smaller and cheaper to launch into space.
But smaller spacecraft require smaller everything else, including propulsion systems. In that respect, electrospray thrusters are a good fit. The thrusters Lozano develops are about the size of a thumbnail. Each thruster sits atop a small reservoir of ionic liquid propellant. When the reservoir is connected to a battery, the battery supplies a voltage that electrically charges a corresponding amount of ions in the liquid. The charged particles are then channelled out of the reservoir, through the thruster’s tips and into space as a thrust-inducing spray.
Over the past decade, Lozano has tested many thruster designs, under varying conditions and with various types of ionic liquid propellant – a fuel that’s essentially made from salts that can remain in liquid form. ‘Ionic liquids are very stable and can even remain liquid in space, which not a lot of materials can do,’ Bruno said. ‘And it’s basically a sea of ions, which is why we base our technology around it, so we can pull those ions out into an electrospray.’
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Bruno and Lozano have collaborated with the US Air Force, which synthesised a new kind of ionic liquid propellant – the Advanced SpaceCraft Energetic Non-Toxic propellant (ASCENT) – which was being tested in chemical thrusters. Chemical thrusters are high-force propulsion systems typically associated with launching rockets and performing hard and fast manoeuvres once in space.
ASCENT was designed as a less toxic alternative to hydrazine, which has been the traditional fuel source for chemical propulsion and is extremely hazardous to handle. ‘ASCENT happens to be an ionic liquid mixture,’ Bruno said. ‘And we said, “Hey, that’s the stuff we typically use. Theoretically, this should work. Let’s go figure out how.”’
In their new study, Bruno, Lozano and Corrado tested the performance of electrospray thrusters that they fuelled with ASCENT. Each thruster they used was attached to a small cube-shaped reservoir about the size of a Lego brick. They filled each reservoir with a gram of ASCENT, a liquid that has a viscosity similar to baby oil. They then attached a thruster to opposite sides of a CubeSat, which they set on a MagLev stand – a custom testbed that’s designed to magnetically levitate a sample or device.
Over multiple experiments, the team remotely applied varying levels of voltage to activate the thrusters, which in turn produced a spray that spun the CubeSat around, like a floating, spinning top. The researchers measured the amount of thrust produced with each trial and calculated ASCENT’s fuel efficiency as they ran the thrusters continuously over periods lasting up to 100 hours.
In the end, they found that ASCENT was able to successfully fuel each electrospray thruster. What’s more, the propellant, which was originally intended for chemical propulsion, was just as efficient as other, conventional ionic liquids at propelling electric thrusters.
Now that ASCENT has been proven to work in both chemical and electrical propulsion, Bruno and Lozano say that a single tank of the fuel can be used to power both types of thrusters, all in a compact, two-in-one system that could fit within a small CubeSat. The team will test the idea with NASA’s Green Propulsion Dual Mode mission, which is scheduled to launch in November.
The research has been published in the Journal of Propulsion.