Using water as a propellant, Aquajet could allow future spacecraft to refuel at asteroids.
The UK Space Agency has awarded a consortium that includes ISIS Neutron and Muon Source (ISIS) a £560,000 grant to develop an innovative new spacecraft propulsion system that runs on the ultimate ‘green’ propellant: water. The Aquajet thruster, which uses technology found in the ISIS machine, opens up the possibility of low-cost, long-mission spacecraft that could refuel using water found on asteroids.
Since the first V-2 rockets shrieked their way into the heavens in the 1940s, space exploration has quite literally come a long way. We have landed on the Moon and sent robotic emissaries to every planet, several moons, and even a few comets and asteroids. We have even sent probes as far from Earth as 21 billion kilometres (and counting) but, despite our advances, all of our space-faring ambassadors are all limited by one intractable factor: propellant.
Whether they be pushed forward by powerful, but briefly-firing, chemical rockets or by the gentle, continuous thrust of an ion drive, all spacecraft must use propellant to fuel their interplanetary journeys. They also need propellant to power the thrusters they use to make course corrections. Whatever the propellant is, or however it is used, once it runs out and the tanks are dry, the mission is over. Sure, you can still take readings and make observations, but manoeuvring and making course corrections become a thing of the past and the craft becomes a slave to Newton’s laws of motion.
One possible solution to this problem might be to build a series of interplanetary refuelling stations that – like a family car on a weekend drive – a spacecraft could stop at to refill its tanks and perhaps grab a light snack (a Mars bar perhaps?). Unfortunately, aside from the prohibitive cost of building such a network, it would require the craft to expend a huge amount of fuel just to stop (let alone get going again) and the other option of getting the refuelling station to match the exact trajectory and velocity of its ‘customer’ would also be hugely impractical.
Another, more practical, solution might be to design a probe that could visit a passing asteroid, or comet, and extract the chemical ingredients required to make some fuel of its own. Water found on such bodies could be extracted and then the oxygen and hydrogen bound up in its molecules could be split via electrolysis to provide the raw materials the craft needs to fuel the next leg of its journey. Unfortunately this method is rather complex and very energy-intensive.
But what if you could use water as a propellant without having to mess with it at all? If you could propel a craft using nothing but good old H2O, you could, in theory, travel the cosmos forever – just pausing now and then to pick up some water from a passing asteroid. But to make this dream a reality, you first need a thruster that can run on water alone, which is where the designers of Aquajet thruster come in.
The Aquajet project is run by a consortium that is led by Added Value Solutions UK Ltd (AVS) and includes Surrey Satellite Technologies Ltd (SSTL), STFC’s ISIS Neutron and Muon Source (ISIS), Surrey Space Centre, VIPER RF Ltd – and funded by a grant from the UK Space Agency’s National Space Technology Programme.
Aquajet is a type of electric propulsion (EP) system called a plasma thruster, which in itself is nothing new – EP thrusters have been in use for decades and with great success. In 2013 alone, some 200 spacecraft were operating in the Solar System that used EP thrusters for station-keeping, orbital adjustments, or as their primary means of propulsion. What makes Aquajet unique is that it has the ability to use water as a propellant and that, thanks to its design, the thruster itself won’t experience the wear-and-tear that limits the operational lifetime of other thrusters.
Electric thrusters fall into two camps: ion drives and plasma thrusters – and, although the way they work differs, at their most basic they achieve thrust by sending a stream of particles out into space at high velocities. Compared to a conventional chemical rocket, the amount of thrust they provide is tiny – as little as the equivalent to the downward force exerted by a piece of paper held in the palm of your hand – but, in the vacuum of space, you don’t need much thrust to get moving. Because electric thrusters use very little fuel, unlike chemical rockets (that burn through their fuel in seconds) they can apply that thrust for months at a time and accelerate a craft to great speeds.
The main types of electric thrusters in use today consist of two electrodes (a positively-charged anode and a negatively-charged cathode) across which travels hundreds (or thousands) of volts of electrical current. This current accelerates the propellant and provides thrust but, over time, the electrodes erode, which causes the thruster to lose power and eventually fail.
AVS, who are also working on several alternatives to conventional EP systems, have designed Aquajet to be an electrodeless thruster. Instead of using electrodes to accelerate the propellant, Aquajet uses a combination of magnetic and electric fields - so there is no physical component to be worn down. This eliminates the erosion issue and means that, in theory, the thruster will never lose power.
Aquajet works by injecting a gas propellant (water), into a chamber where it is exposed to a magnetic field from a magnet and microwave energy from an antenna. This heats the gas and creates a high-density plasma – an ionised gas made up of negatively-charged electrons and positively-charged water molecules, or ions (water molecules that have lost an electron). Because they now carry an electric charge, these ions are picked up by an electric field (generated by electrons travelling along the magnetic field) and accelerated until they pass out of the thruster at 20,000 metres per second – generating thrust.
The consortium have tested a prototype of Aquajet, achieving ‘break-through results’ and number of ‘firsts’ for such a device. As well being the first to demonstrate that water can be used as propellant in a device of this kind, they have demonstrated that the thruster works at high power (more than three times higher than anyone else) and that, for the first time, the performance increases in line with the power increase. As a result of these demonstrations the project was recently awarded a £560,000 grant by the UK Space Agency. With the next phase of development funded, the team expect to achieve ‘dramatic improvements at all power levels’ before AVS bring the thruster to the market, which they hope to do by 2021.
Aquajet achieved the first ever demonstration of water propellant in an electrodeless thruster of this type
As well as the obvious appeal of using water as a fuel and the thruster’s potential for operating over long timescales, the design also promises to be much cheaper than existing electric propulsion systems. Current designs have to use high-purity gases like Argon and Xenon as propellants, which, along with their expense (Xenon can cost as much as £6,000 per kg), require high-pressure storage systems that add significantly to a craft’s launch weight. Aquajet avoids this added weight and complexity – potentially making launches significantly cheaper and safer. Also, the thruster’s electrodeless design gives spacecraft designers more flexibility because it can use Argon and Xenon, but also cheaper gases like carbon dioxide, ammonia and, of course, water.
With several thousand small satellites expected to be launched over the next five to ten years –potentially deployed as constellations of hundreds of spacecraft – the consortium expect there to be significant demand for a cheap, reliable and potentially flexible propulsion system like Aquajet.
Although Aquajet is likely to be used initially on small satellites and cubesats (even smaller satellites measuring ten cubic centimetres), it is possible that, by deploying multiple thrusters in large arrays, the system could be used to power much larger spacecraft.
Last updated: 22 May 2019