SINGAPORE (The Straits Times/Asia News Network): A local firm, together with a British company and academics, has built a satellite engine that runs on water – a cheaper and more environmentally sustainable alternative to conventional engines propelled by expensive rare gases.
The new AquaHET engine is an optimised version of Hall-effect thrusters commonly used to fly satellites in space by accelerating ions – usually of a noble gas – out of the engine to create a propulsive force.
For the new engine, oxygen ions that originate from water are accelerated to produce thrust.
This process is made possible by a device called a hollow cathode built by Singapore space-tech company Aliena. The company was roped in to the project in 2021 for its expertise in building the device, which produces electrons.
The oxygen is first produced by running electricity through water, and this produces hydrogen at the same time. The hydrogen is taken in by Aliena’s hollow cathode to create electrons.
The electrons then turn the oxygen atoms into ions, which shoot out of the engine to propel the satellite, said Aliena’s chief technology officer George-Cristian Potrivitu.
The oxygen arm of the engine, called an anode, was developed by British company URA Thrusters and researchers from Imperial College London. The anode takes in oxygen.
Dr Potrivitu said: “The force of the engine’s thrust is akin to the weight of two 10-cent coins in your palm – very little force. But in space, there is no air and therefore no friction, so the slightest and continual force from the thruster is enough to push the satellite to high accelerations.”
AquaHET can drive satellites that are 200kg and larger.
The team behind the engine is looking to commercialise it and test it on a satellite in a few years.
Aliena, a Nanyang Technological University spin-off, creates propulsion engines that allow satellites to fly closer to Earth and take higher-quality images.
Hall-effect thrusters are the most widely used satellite engines in space. They are found in SpaceX’s network of Starlink satellites, large telecommunication satellites and military satellites.
Conventional Hall thrusters currently are fuelled by rare gases such as xenon, an inert gas that is easily ionised.
Xenon makes up just 0.0000087 per cent of the planet’s atmosphere. It is energy-intensive to extract it from air, making the gas expensive, said Aliena chief executive Mark Lim.
“A lot of energy and resources are needed to distil and purify the gas. Since xenon is so rare in the atmosphere, even more amounts of air need to be processed for just a small quantity of xenon,” he said.
Deionising water to remove any ions or dissolved solids before the water is split into oxygen and hydrogen – the process used in the new engine – is nowhere as complex or expensive.
Producing 1kg of xenon requires up to 4,500 kilowatt-hour (kWh) of energy, noted scientists from Imperial College London’s Imperial Plasma Propulsion Laboratory, where the engine was first tested in December 2022.
In contrast, it takes less than 100kWh to produce 1,000kg of deionised water.
Dr Lim noted that water has fuelled some satellite engines in the past, but not Hall thrusters. But those water-based engines were not as fuel- and power-efficient.
Using water as a fuel, there is also the possibility of satellites or spacecraft – in future deep space missions – refuelling at celestial bodies with signs of water.
While water-fuelled satellites could make satellites greener, there is no escaping the fact that rockets are still required to launch satellites into space.
All that rocket fuel – including highly refined kerosene – being burned contributes to a warming planet.
The black carbon or soot from rockets contributes nearly 500 times more to global warming than all earthbound sources of soot, says a 2022 study published in the Earth’s Future journal under the American Geophysical Union.
That is because rockets directly inject soot into the upper layers of the atmosphere, where the pollutants linger for a longer period than earthbound emissions.
When asked if launch rockets will be able to use water as fuel one day, Dr Lim said it is unlikely because rocket launchers need a far higher thrust and a boost to lift off than what water-fuelled Hall thrusters can provide.
Some progress has been made to mitigate rockets’ emissions.
Propellants developed by local rocket and space launch company Equatorial Space produce mainly nitrogen and eliminate about 85 per cent of greenhouse gas emissions compared with traditional rocket fuel.
“(The propellants) do not emit dangerous effluent that some other traditional systems are infamous for,” said Equatorial Space chief executive Simon Gwozdz.
The recovering ozone layer is also at risk when dead satellites and reusable rockets heat up as they plummet back to Earth, causing chemical reactions that can deplete ozone.
The irony is that many satellites are launched into space to help combat climate change, from measuring the amounts of greenhouse gases in the atmosphere to monitoring sea-level rise.